Minimize RHESSI

RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager)

Spacecraft    Launch    Mission Status    Sensor Complement   Ground Segment   References

RHESSI is a NASA SMEX (Small Explorer) solar mission, selected in Oct. 1997, and managed for NASA/GSFC by the Space Science Laboratory (SSL) at the University of California, Berkeley (UCB). The overall objective is to explore the basic physics of particle acceleration and energy release in solar flares. The prime observations performed are simultaneous, high resolution imaging and spectroscopy of solar flares from 3 keV X-rays to 17 MeV gamma rays with high time resolution. RHESSI (the original name was HESSI) is a collaboration between the following institutions: GSFC, UCB (PI: Robert P. Lin), PSI (Paul Scherrer Institut, Villigen, Switzerland), and ETH Zürich (HESSI Experimental Data Center - HEDC). 1) 2) 3) 4)

Background: The former HESSI mission was formally renamed to RHESSI in April 2002. This renaming is in recognition of the enormous contribution that Reuven Ramaty made to gamma-ray astronomy in general and to the HESSI program in particular. Reuven Ramaty died in 2001, after a long and distinguished career in the Laboratory for High Energy Astrophysics at NASA/GSFC, Greenbelt, MD. Ramaty was a pioneer in the field of solar-flare physics, gamma-ray astronomy and cosmic rays. - RHESSI is also known as Explorer 81.



The S/C bus was designed and built by Spectrum Astro Inc. of Gilbert, AZ (note: in July 2004, General Dynamics acquired Spectrum Astro; Spectrum Astro is now part of General Dynamics C4 Systems of Scottsdale, AZ). RHESSI is a sun-pointing and spin-stabilized S/C spinning at 12-20 rpm (15 rpm nominal). The bus consists of the structure and mechanisms, the power system (including the battery, solar panels, and control electronics), the attitude control system (ACS), thermal control, command and data handling (C&DH), and telecommunications.

The S/C structure is 1.1 m in diameter (at base) and 2.1 m in length. Its attitude and control subsystem employs sun sensors (fine and coarse) and a magnetometer for attitude sensing and magnetic torque rods as actuators. The S/C is capable of performing autonomous sun acquisition and spin-up from any orientation. Sun pointing (precession) control is < 0.2º provided by SAS (Sun Aspect System). The on-orbit mass properties adjustment direct the sun pointing error measurement to about 0.05º.

The SAS, built by the Paul Scherer Institute, consists of three identical lens-filter assemblies mounted on the forward grid tray to form full-sun images on three 2048 x 13-µm linear diode arrays mounted on the rear grid tray. Simultaneous exposures of three chords of the focused solar images are made every 10 ms by each of the arrays. A digital threshold algorithm is used to select four pixels that span each solar limb for inclusion in the telemetry. These digitized pixel outputs allow six precise locations of the solar limb to be obtained on the ground by interpolation, thus providing knowledge of sun center in pitch and yaw to 1.5 arcsec (3σ).

The SAS provides several functional services to the ACS such as:

• High-resolution, high-bandwidth aspect information for image reconstruction

• Real-time aspect error signals for spacecraft pointing

• Monitoring of the relative twist of the two grid trays

• Full-sun white-light images for co-alignment with ground-based images.

The S/C mass is 293 kg, power = 400 W. The power is provided by four deployable solar wings; in addition there is an NiH2 battery energy of 15 Ah. The nominal S/C design life is two years with a goal of three years.


Figure 1: Artist's view of the RHESSI spacecraft viewing the sun (image credit: NASA, UCB)


Figure 2: Photo of the spacecraft bus (image credit: NASA)


Figure 3: Schematic showing the location of instrument and spacecraft components on the RHESSI spacecraft (image credit: NASA, UCB, Ref.42)

Legend to Figure 4:

- The acronyms in the top view are: FSS (Fine Sun Sensor), SSR (Solid State Recorder), CPC (Cryocooler Power Converter), IPC (Instrument Power Converter), IDPU (Instrument Data Processing Unit)

- The acronyms in the bottom view are: RAS (Roll Angle System ), PMT RAS (Photomultiplier Roll Angle System), IAD (Inertial Adjustment Device), SEM (Spacecraft Electronics Module).

The ACS (Attitude Control Subsystem) enables RHESSI to follow the sun over time autonomously with a 3σ pointing accuracy of 0.14º (8.4 arcmin). The primary attitude sensor is an Adcole Inc. FSS (Fine Sun Sensor) with a ±32º field of view and 0.005º resolution, that is mounted to the front of the imager tube. The pointing error measured by the FSS, together with local magnetic field measurements made by the spacecraft magnetometer, are inputs to the ACS control algorithms in the flight software. This runs on the RAD6000 flight processor in the C&DH (Command and Data Handling) subsystem to drive three orthogonally-mounted Ithaco Inc. 60 Am2 electromagnetic torque rods to maintain the spacecraft attitude. Finally, a set of eight coarse sun sensor cells (two mounted on each solar array wing) allow the ACS subsystem to acquire the sun from any initial attitude after separation from the launch vehicle (Ref. 42).


Figure 5: Block diagram of the RHESSI spacecraft bus (left) and the instrument (image credit: NASA, UCB)

C&DH (Command and Data Handling) subsystem: The SEM (Spacecraft Electronics Module) houses the CCB (Charge Control Board), the PCB (Power Control Board), and the ADB (Auxiliary Driver Board) for the EPS (Electrical Power Subsystem); and CIB (Communications Interface Board), the PACI (Payload and Attitude Control Interface) board, and the flight computer (CPU) board of the C&DH subsystem. A separate SSR (Solid State Recorder), built by SEAKR Engineering, provides 4 GB of solid-state memory for science data storage.

The IDPU (Instrument Data Processing Unit) provides formatted telemetry packets of science data directly to the SSR recording high-speed parallel interface. Science data are played back from the SSR for downlink via a high-speed parallel interface with the CIB, the command and telemetry interface for the SEM to the RF transponder. The CIB is powered from the essential bus and is operational at all times. It provides command decoding capability for critical functions including the reset and power control of the flight computer, control of the telemetry transmitter, and adjustment of the battery charge control parameters. This hardware command decoding capability of the CIB provides an operational backup for faults which result in the shutdown of CPU or software.

The PACI (Payload and Attitude Control Interface) board is responsible for telemetry encoding and data acquisition. It digitally encodes analog voltage, current and temperature data, and formats telemetry frames for downlink and on-board storage. It provides serial communications interfaces for control and monitoring of the SSR and the IDPU. The PACI board is powered by the essential power bus and is always producing hardware state of health telemetry packets; whenever the transmitter is powered on these packets are transmitted to the ground. This feature along with the CIB hardware command decoding, allows problems to be diagnosed and fixed from the ground, even without the CPU or software running.

The CPU board is a radiation-hardened RAD6000 processor made by BAE Systems. It contains 128 MB of DRAM for data memory storage and cache memory storage, and 3 MB of EEPROM for code memory storage. The CPU board controls the operation of all of the other boards in the SEM. The SEM also houses DC/DC power converters and an OCXO (Oven-Controlled Crystal Oscillator). The essential bus +28 V power provided by the power subsystem is used to generate secondary +5 V, and ±15 V services which power the SEM boards. The OCXO provides a stable clock signal at a frequency of 222 Hz, which is divided by the CIB to produce clock signals at 1 Hz and 220 Hz (approximately 1 MHz). These signals are distributed to the CIB, the PACI, and the IDPU, where they are used to time-stamp data acquisition and frame transmission times.

EPS (Electrical Power Subsystem): The EPS utilizes four triple-junction gallium arsenide (GaAs) solar array wings, each producing 133.5 W for a total of 534 W at 3 years end-of-life. Energy for eclipse operations is stored in a 15 Ahr battery comprised of eleven common pressure vessels, each containing two nickel hydrogen cells. The battery can operate at 50% depth-of-discharge for the full three year design life, and provide up to 280 W during the nominal 35 min eclipse duration. The CCB (Charge Control Board) uses a direct energy transfer system and is better than 95% efficient. The amount of current produced by the solar array is controlled by pulse-width modulating FET switches between the eight solar cell circuits and the power bus. Unused solar array power is dissipated in the solar array, not in the spacecraft. The CCB uses a temperature-compensated battery voltage algorithm to set the battery charge current. The PCB distributes power to the spacecraft components and provides switched power to those components requiring unregulated power at 28+7/-4 V. It also provides current sensors for telemetry monitoring and over-current protection for the power bus and under-voltage load shedding. The ADB (Auxiliary Driver Board) provides drive signals for the Inertia Adjustment Devices and the electromagnetic torque rods, and controls the solar array wing deployments.

RF communication is in S-band. The downlink frequency is at 2215 MHz with selectable data rates of 4 Mbit/s, 1 Mbit/s or 125 kbit/s, with NRZ-M and BPSK data modulation. The uplink frequency is 2039.6458 MHz, the data rate is 2 kbit/s. Continuous S/C operations are supported through a UCB ground station and the Mission/Science Operations Center. The data are distributed to SDAC (Solar Data Analysis Center) at GSFC and to HEDC at Zürich. There is also a complementary ground-based program supported by observatories throughout the world.


Launch: An air launch on a Pegasus XL vehicle took place on Feb. 5, 2002, about 180 km east-southeast of Cape Canaveral, FL, at a launch height of about 11.8 km above the ocean (use of Orbital Sciences' Stargazer L-1011 aircraft).

Orbit: circular orbit, altitude = 600 km, inclination = 38º, period = 96.98 minutes.



Mission status:

• November 20, 2018: Every morning for the past 16 years, solar physicist Säm Krucker sat down at his desk to check the latest data from NASA's RHESSI. Had the solar observatory seen a flare overnight? If there was a new flare, Krucker, RHESSI principal investigator at University of California, Berkeley, since 2013, would pore over the data, each recorded X-ray telling him something about the giant explosion on the Sun.

Now, many years after launching on Feb. 5, 2002, the RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager) mission has ended; Krucker, and many other scientists, will no longer check the spacecraft's data returns each day. In anticipation of losing touch with the spacecraft's aging receiver, mission operators sent the spacecraft commands to decommission on Aug. 16, 2018. 5)

- "It does impact everyday life that way," Krucker said. Though it's appropriate timing for RHESSI to stop operations now, he said, while the Sun nears solar minimum, the lull in its activity over an approximately 11-year cycle. "The next two to three years would have been quite boring."

Figure 6: An artist's representation of RHESSI. Flying up above Earth's radiation-blocking atmosphere, RHESSI could observe X-rays and gamma rays from solar flares (image credit: NASA)

- RHESSI's job was to watch the Sun for solar flares, some of the most dramatic events on the Sun that can sometimes fling solar energy toward Earth.

- During a flare, gas in the Sun's atmosphere rapidly soars over 20 million degrees Fahrenheit (11.1 million ºC), sending particles flying at near-light speeds. In turn, the particles emit high-energy emissions like X-rays and even higher gamma rays, which we can detect from far away. These rays can't, however, penetrate Earth's atmosphere and be measured from the ground, so RHESSI observed them from space. In doing so, RHESSI aimed to understand how flares work, the physics underlying how the Sun generates such powerful bursts of energy.

- Ask any scientist who worked on RHESSI what their favorite flare is, and they'll easily rattle off a date, as if it's a birthday or holiday they'll always remember. Krucker said his favorite is either Jan. 20, 2005, or July 19, 2012. Mission scientist Brian Dennis at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said April 21, 2002, RHESSI's very first X-class flare — the strongest type. For them, these are special dates because RHESSI taught them something new that day.

- "When I first looked at the Jan. 20 flare and saw where the high energy was coming from, that brought my understanding to the next level," Krucker said. "One guy even has a most-hated flare. He had many arguments with colleagues over it, trying to explain it, which was not always pleasurable."

- Throughout the course of its mission, RHESSI saw more than 75,000 solar flares. These observations have helped scientists craft and refine a model of how solar eruptions form. Launching in early 2002, just after the Sun reached solar maximum in 2001, the spacecraft was poised to see many flares.

Figure 7: Observations from RHESSI and another NASA mission TRACE are combined in these images of a powerful flare on April 21, 2002 (image credit: NASA Goddard/RHESSI/TRACE)

- "High energies are always interesting in astronomy," University of Minnesota solar scientist Lindsay Glesener said. "They're the biggest explosions, the hottest plasma. But flares are going on at our Sun, which is right next to us and affects us in lots of ways. When I was a grad student, it took just one conversation with Robert Lin, the principal investigator from 2002 to 2012, to convince me flares are the most fascinating thing in the universe."

- To understand how solar flares erupt from the Sun, you have to know where their energy comes from. For this, RHESSI carried just one instrument, called an imaging spectrometer, capable of recording both X-rays and gamma rays. For scientists, these high-energy emissions are like fingerprints, showing them how each eruption unfolds: X-rays are associated with electron activity, while gamma rays come from protons and ions.

- The instrument combined images of the Sun with its spectroscopy to show all the different energy levels in the flare. This allows scientists to map out where energy comes from during an explosion, and what's producing that energy.

- "Previous mission designs had two different instruments, but RHESSI was able to study this large energy range in X-rays and gamma rays, all with the same instrument," said Albert Shih, RHESSI deputy mission scientist at Goddard. "It was an innovative design, and a lot of good science came of it."

- RHESSI's observation of so many flares over the years, from one solar cycle to the next, broke new ground in solar physics and enabled a much deeper understanding of flares — where they accelerate particles, and just how much they can vary in scale, from tiny nanoflares to massive superflares, tens of thousands of times bigger and more explosive.

- "RHESSI had several firsts," Dennis said. "No one had ever imaged X-rays at this high an energy level before, or imaged gamma rays at all. RHESSI made great strides by making energy measurements with higher resolution than had previously been possible."

• January 17, 2018: Solar flares perturb all layers of the solar atmosphere dramatically, but the most important domain is the lower solar atmosphere. We normally describe this as a upper photosphere slightly cooler than the surface of the Sun, overlain by a chromosphere with temperatures of order 10,000 K as regulated by hydrogen ionization. Under the drastically time-varying environment of a solar flare, these physical properties change radically. The physics has enough complexity to require solution by numerical simulation, for which radiative-hydrodynamic (RHD) simulations in 1D have been developed over the years. These simultaneously capture the basic physics of hydrodynamics and radiative transfer theory, each one complicated in its own right. Typically these models assume non-thermal particle beams originating in the solar corona, where magnetic energy can be stored and suddenly released via particle acceleration; usually electron beams are favored in this process. 6)

A new approach to modeling the lower flare atmosphere: The simulations have shown that high energy deposition rates from electron beams produce two flaring regions at T~104 K that develop in the chromosphere: a cooling "condensation" (downflowing compression) and heated non-moving (stationary) flare layers just below the condensation. The high beam flux simulations are computationally expensive in 1D, and the (human) timescales for completing flare models with adaptive grids in 3D will likely be unwieldy for some time to come. We have developed a prescription for predicting the evolved states and emergent near-UV and optical continuum spectral properties of RHD model flare atmospheres based on an atmospheric reference parameter at early times in the simulations.

This reference parameter is the column mass in the lower atmosphere where the flare temperature increases above T~10,000 K and hydrogen becomes nearly completely ionized. The reference column mass, or mref, determines the amount of the lower atmosphere that is compressed into the chromospheric condensation in evolved states in the RHD simulations. In these evolved states, the condensation has cooled to T~10,000 K and has descended to the height of the top of the T~10,000 K stationary flare layers. We have made a (publicly available) Python GUI (Graphical User Interface) that allows one to vary mref through a large possible range and calculates the emergent LTE (Local Thermodynamic Equilibrium) approximation near-UV and optical continuum radiation properties of the parameterized evolved states. The interesting parameter space can then be investigated more thoroughly with time-dependent, non-LTE models. Our parameterization has been tested over a large range of electron beam energy deposition rates (1011 to 1013 erg cm-2 s-1) and can be easily adapted for other flare-heating scenarios that produce two flare layers at pre-flare chromospheric heights.

Our tool can be used for a new generation of semi-empirical flare modeling that includes density stratifications of chromospheric condensations parameterized by mref. Figure 8 demonstrates why static, semi-empirical flare models (and semi-empirical models that invoke an ad-hoc velocity field) need to be improved with more realistic modeling. The figure shows the temperature and density profiles of Marcos Machado's widely-used hydrostatic, semi-empirical flare model ("F2") compared to an evolved state of an RHD flare model atmosphere (the "5F11" electron beam heating model calculated with the RADYN code). In the F2 model, the flare transition region (where T exceeds 40,000 K in these figures) is located at a higher column mass (and lower height) than the transition region in their non-flaring model atmosphere, thus giving brighter emission lines in the flare. In the RHD model, the flare transition region is located at a larger column mass and lower height than in the pre-flare state as well. However, the RHD model includes the compression of the atmosphere (i.e., the chromospheric condensation) just below the flare transition region, as indicated by the factor of ~ten increase in hydrogen density. The compression is due to the large pressure gradient (spike) in the flare transition region, which drives material toward the photosphere — the magnitudes of downward gas speeds in the chromospheric condensation are indicated by the relative vertical sizes of the grey arrows. By conservation of mass, a velocity gradient in an atmosphere causes mass to pile up in certain locations. Static semi-empirical models like F2 do not include the increase in gas density below the flare transition region and thus the chromospheric flare opacities are not consistent with a deeper flare transition region. The value of log10 mref = -2.75 for the RADYN model in Figure 8 is indicated with a purple vertical line, which occurs where downflowing material of the chromospheric condensation meets the top of the stationary flare layers.

In summary, the deep solar atmosphere is the layer in which the most intense flare energy appears, even though this energy apparently needs to have come originally from the much more tenuous corona. Our new work, 7) based on the RADYN simulation code plus sophisticated radiative transfer within its framework, features a simplified lower atmosphere that is determined by a single reference parameter.


Figure 8: The deeper flare transition region in the RHD model (solid lines) with electron beam heating calculated with the RADYN code generates an increase in density by an order of magnitude. This increase in density is not included in static flare models, such as the widely-used F2 model (dashed lines), that adjust the position of the flare transition region relative to the pre-flare state. Our parameterization of the evolved state of the RADYN model (t=3.97 s after the start of electron beam energy deposition with an energy flux density of 5 x 1011 erg cm-2 s-1) is based on the value of the reference column mass (mref): deeper values generate denser chromospheric condensations and brighter continuum radiation. The temperature (T=9500-10,000 K) of most of the mass within the chromospheric condensation is similar to that in the non-moving layers at heights below mref, but the average density below and above mref are a factor of 5-10 different. Most of the emergent near-UV continuum radiation (as would be observed by IRIS) originates from log m from -2.2 to -3.6. Note, the height of the flare transition region in the F2 is similar to the height in the RADYN model here. Thick grey arrows pointing to the left indicate downward gas speeds with the relative vertical size of the arrow proportional to the speed (largest is ~50 km/s), image credit: UCB/SSL

• September 17, 2017: The Sun has just produced two major X-class flares in the waning phase of Solar Cycle 24. This has happened in previous cycles: Cycle 22 saw SOL1996-07-29 (X2.2), which produced the first-observed sunquake. Then Cycle 23 produced SOL2006-12-13 (X3.4), with its white-light continuum remarkably well documented by Hinode. Now we have (probably) ended Cycle 24's major flare activity with SOL2017-09-06 (X9.3) and SOL2017-09-10 (X8.2), both of which produced long-duration gamma-ray events observed by the Fermi Gamma-ray Space Telescope. 8)

- RHESSI got excellent observations of SOL2017-09-10, as illustrated in Figure 9. The flare was almost exactly at the limb, apparently such that one ribbon was entirely occulted and the other partially so, but still nicely showing both white-light flare continuum and hard X-rays with the same precise coincidence seen in events on the disk, confirming the 3D nature of this identification.


Figure 9: Imaging observations of SOL2017-09-10 at high resolution. Upper panel, comparisons of RHESSI hard X-rays with the HMI (Helioseismic and Magnetic Imager) white-light flare observation (footpoint); bottom panel, same RHESSI images compared with AIA ()Atmospheric Imaging Assembly) 193 Å (looptop), image credit: UCB/SSL

- This flare also produced a textbook example of CME ejection and the generation of a global wave. Figure 10 illustrates the canonical plasmoid/current-sheet pattern most clearly, with the very linear current-sheet EUV enhancement pointing straight at the loops seen by RHESSI and AIA.


Figure 10: SOL2017-09-10 produced a massive, fast CME, and in its wake this textbook-clear example of plasmoid and current sheet trailing it. Note the beautiful diffraction spikes produced by AIA's grids. This can be exploited in principle to increase the image dynamic range and also to provide spectral information (image credit: UCB/SSL)

• On 5 February 2017, the RHESSI mission completed its 15th year in orbit. RHESSI was designed to study energy release and particle acceleration in solar flares through imaging spectroscopy observations of the X-ray and gamma-ray emissions of these energetic events, the most powerful explosions in the solar system. The accelerated electrons and ions carry a predominant part of the released energy in a flare and only RHESSI, at present, can provide imaging spectroscopy in this spectral domain. 9) 10)

- During this interval of roughly 2/3 of a 22-year Hale Cycle, RHESSI has recorded over 114,000 X-ray events, 41 of them with gamma-ray emission above 300 keV. Figure 11 displays the catalog as a mean image of the hard X-ray Sun, quite striking in the sense that the body of the Sun actually is dark for these high-energy photons, and only the transient activity appears (and with a highly non-spherical distribution).


Figure 11: Map of the RHESSI quick-look flare catalog, displayed as angular elongations (i.e., not heliographic coordinates). This huge sample, obtained from 15 years of hard X-ray imaging, has remarkable smoothness with 30 arcsecond bins (image credit: Paulo Simões)

- RHESSI's systems are ageing but still quite functional, and able to provide imaging spectroscopy routinely as before. The operational procedures have changed slightly in that we normally operate only two of the nine germanium detectors, in order to help the cryocooler's refrigerating performance. This still lets us make images and spectra such as those illustrated in Figure 12.


Figure 12: Observations of SOL2017-01-25 (B8.5), a very recent flare observed by RHESSI and EOVSA, the expanded Owens Valley Solar Array. This is a new major solar radio observatory, one of the wonderful new complementary data sources that has appeared during RHESSI's active life (image credit: UCB/SSL)

- RHESSI's other pastimes include analysis of TGFs (Transient Gamma-ray Flashes) from the Earth's atmosphere, produced by lightning. Also, data from its optical solar aspect system has been used for the most accurate measurements of solar oblateness and mean photospheric temperature distribution ever made, with continually incrementing precision as the mission continues. All of the data and the analysis software have been made immediately available to the scientific community resulting in over 1,000 refereed papers that utilize RHESSI observations, with more than 100 in the last year alone. They are now cited over 4,000 times per year.

- The RHESSI spacecraft and instrument continue to operate well, as described above. The mission has no expendables and reentry is not predicted to occur until 2021 at the earliest. The slowly rising detector temperatures, resulting from the gradually decreasing cryocooler efficiency, is a matter of concern and has led us to operate only two of the nine detectors during periods of low solar activity. This minimizes the temperature increase and still permits effective imaging spectroscopy of flare hard X-rays. When major activity reappears, RHESSI can readily operate with full capability on short notice. We expect to be able to maintain RHESSI's unique hard X-ray imaging spectroscopy capability for the foreseeable future.

• January 30, 2017: Hard X-ray emission of solar flares can enable insights into the non-thermal energetic particle properties created during these eruptive events. Depending on the coronal densities and plasma properties, a part of the non-thermal electron population can be trapped in coronal magnetic fields. Theory suggests that the coronal region at the top of magnetic loops may be the main acceleration site for electrons. At high energies, however, the bright footpoint emission from these flare loops can obscure weaker loop-top sources due to the limited dynamic range of instruments like RHESSI. Thus, flares close to the solar limb, where the footpoints are occulted, are interesting events to study because they can reveal the emission in this region in isolation. This method has revealed many things. Here, we report on recent findings of a statistical study of occulted events, comprising of about 120 flares during solar cycles 23 and 24. 11)

- It is interesting to also study the so-called Neupert effect in this context of coronal sources. 12) The Neupert effect is an empirical correlation in solar flares, which relates the changes (i.e. the time derivative) in the soft X-ray (SXR) flux to the temporal hard X-ray (HXR) variations. These HXRs tend strongly to originate in the footpoint regions, leading to the "thick target" interpretation of coronally accelerated electron beams hitting the transition region and chromosphere. This in turn can lead to heating processes, increasing the overall SXR and extreme ultraviolet (EUV) emission, as observed by the GOES photometers.

• In April 2016, RHESSI successfully completed its fifth detector 'anneal' after over 14 years of successful operation. From February 23, 02:19 UTC to April 29, 03:48 UTC, the detectors were heated up to repair accumulated radiation damage and then cooled back down to operating temperatures. During that time, no science observations were made. 13)

- RHESSI's detectors accumulate radiation damage, and require periodic re-annealing (Figure 13). This involves bringing the temperatures up to a certain point, whereupon the crystal defects that have built up will tend to heal themselves. Then a careful cool-down to the normal operating temperature restores the pre-anneal performance. At least that is the hope, but it is a delicate operation and surprises have happened in past episodes. The project previously annealed in 2007, 2010, 2012, and 2014.

- The annealing process involves raising RHESSI's detectors to an elevated temperature close to 100º C for about 10 days, during which the effects of accumulated radiation damage are significantly reduced. The data flow was turned off during the annealing operation but was restored starting on April 29, 2016. Since that time, each of the nine detectors have been turned on and their performance tested.

- As a result of this evaluation, we have implemented a new default mode of operation. Starting on May 18 at ~1900 UT, only detectors 3 and 8 are kept on to allow the lowest operating temperature to be maintained while preserving RHESSI's core capability of X-ray imaging spectroscopy above 3–6 keV. Figure 14 shows images of the C1.3 event, SOL2016-05-24T10:20 that illustrate this capability.


Figure 13: The final flare in the current sequence, as shown in the RHESSI flare catalog (image credit: UCB, Browser)


Figure 14: Left: RHESSI X-ray images in two spectral bands of SOL2016-05-24T10:20 (C1.3). Right: GOES and RHESSI light curves and RHESSI spectrogram of the same flare (image credit: UCB)

• July 2015 - The RHESSI mission is extended to 2017: The RHESSI mission is approved by NASA to continue planning against the current budget guidelines. Any changes to the guidelines will be handled through the budget formulation process. The RHESSI mission will be invited to the 2017 Heliophysics Senior Review. 14)

- RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager) is an imaging spectrometer which provides high resolution X-ray and gamma ray imaging spectroscopic observations over the range of energy from 3 keV to 17 MeV. This range covers soft X-rays emitted by hot thermal plasma, hard X-ray and gamma-ray bremsstrahlung emitted by energetic electrons, and nuclear gamma-ray lines produced by energetic ions. RHESSI was launched in February 2002, has a few arcsec angular resolution, and 1–few keV spectral resolution. It is presently in a 510 km roughly circular orbit (at an inclination of 38º), with expected operations through at least 2017.

• June 2015: The 2015 Heliophysics Senior Review panel undertook a review of 15 missions currently in operation in April 2015. The panel found that all the missions continue to produce science that is highly valuable to the scientific community and that they are an excellent investment by the public that funds them. 15)

- For the next mission extension, the RHESSI team proposes to collaborate extensively with the other HSO (Heliophysics Systems Observatory) missions, in particular with the newer missions such as IRIS (Interface Region Imaging Spectrograph) Observatory. RHESSI's overall science goal of understanding solar flare energy release and particle acceleration is elucidated through four Primary Science Goals: (1) the evolution of SEEs (Solar Eruptive Events), (2) the acceleration of electrons, (3) the acceleration of ions, and (4) the origin of the thermal plasma. Two additional objectives are listed which take advantage of RHESSI's capabilities for observing (5) the optical Sun and (6) X-ray and gamma-ray sources of both terrestrial and astrophysical origin. As the sole mission to observe high-energy solar emissions, the RHESSI science plan focuses on the expectations of this declining solar cycle phase. Additionally, in the past three years RHESSI data have been used to challenge the basic thick target flare model, dating from the 1970s and heretofore the essential description of the dominant non-thermal processes in the impulsive phase of a flare. Evidence has emerged for bulk acceleration in the corona, and, unexpectedly, the presence of non-thermal particles in the lowest layer of the solar atmosphere, close to the photosphere itself. The proposal anticipates major revisions to this paradigm, facilitated by new complementary data becoming available.

- RHESSI science strengths: RHESSI observations are both unique and highly complementary to data sets from other instruments in the HSO; this includes not only SDO (Solar Dynamics Observatory), STEREO (Solar-Terrestrial Relations Observatory) and Solar-B/Hinode but most recently, IRIS; astrophysics missions such as Fermi/LAT have also been recently involved with coordinated observing efforts. The proposal additionally describes ample motivation to coordinate with ground-based facilities, as well—in particular with radio observations due to the very complementary diagnostics they provide.

- The maturity of the RHESSI mission does provide an extensive database of observations over more than a full solar cycle, and its continued operation should provide additional observations of larger events which sometimes occur in a cycle's declining phase, as in the next extension. As the first and only imaging spectroscopic instrument in its energy range, and especially now with a number of additional observational resources and facilities with which to collaborate (highlighted above), there is very high expectation of further breakthroughs in topics of high priority for Heliophysics research. While the overall theme of RHESSI research focuses on solar flares and energetic events, the proposal does outline additional non-flare (and non-solar) topics that again, will benefit from another mission extension due to the availability of new complementary observing facilities.

- RHESSI spacecraft / instrument health and status: Although the proposal demonstrates conclusively that RHESSI is currently capable of continued productive activity, it is unlikely that this will continue without severe degradation until the next review cycle. There is no clear and unambiguous set of criteria provided to determine objectively when degradation will render the data effectively useless. The primary issue is the need for detector annealing, which strains the cryocooler, and can probably not be repeated.

• 26 June - 13 August 2014 - Fourth RHESSI Anneal: During the anneal, RHESSI's germanium detectors are heated up from their operating temperature of ~115 K to ~100ºC (373 K), held at that temperature for ten days, and then cooled back down. The detectors degrade steadily over time due to radiation damage from charged particles, and heating up the germanium restores lost sensitivity and resolution. During the anneal, RHESSI does not collect X-ray or gamma-ray data. 16)

• The RHESSI spacecraft and its instruments continue to functions nominally in the summer of 2013 - in its present orbit of 554 km x 533 km (from an initial orbit of 600 km). Except for the gradual loss of efficiency of the cryocooler, RHESSI has no other expendables and the relatively low level of solar activity means that its orbital decay has been less than predicted; consequently, its useful life should extend well beyond the next two years. RHESSI's useful life should extend at least into 2018. 17)

RHESSI has been providing unique diagnostic observations of high-energy processes in solar flares for over 11 years. These observations address the key Heliophysics goal of understanding the fundamental processes of particle acceleration and energy release in solar eruptions, both flares and coronal mass ejections (CMEs). The resulting photon emissions and accelerated particles directly affect our Home in Space, and are especially important for the Journey Outward. 18)

RHESSI is designed for imaging spectroscopy of hard X-ray (HXR) and gamma-ray continua emitted by energetic electrons, and gamma-ray lines produced by energetic ions. The single instrument makes imaging and spectroscopy measurements with a few arcsecond angular resolution and one- to a few- keV energy resolution at energies from soft X-rays to gamma-rays (3 keV - 17 MeV). No other current or planned observatory has this ability to provide direct quantitative information on the energetic electrons and ions that carry such a predominant part of the released energy in a flare.

Over 70,000 events are included in the RHESSI Flare List, over 14,000 of them with detectable emission above 12 keV, and 35 above 300 keV. Twenty-seven events show gamma-ray line emission. All the data and the analysis software have been made immediately available to the scientific community.

The value of future RHESSI observations is now greatly enhanced by improved complementary observations compared with those available during the first eight years of RHESSI's operational lifetime. Groundbreaking observations of thermal plasmas, magnetic fields, and heliospheric effects are now being provided on a regular basis by instruments on the SDO (Solar Dynamics Observatory ), Hinode (Solar-B), STEREO, and other components of the HSO (Heliophysics System Observatory), and instruments on the Fermi astrophysics mission are providing X-ray and gamma-ray spectroscopy (limited imaging and modest energy resolution) from ~10 keV to GeV energies. Interpretation of data from RHESSI, especially in conjunction with data from other instruments, forms a key part of "developing a comprehensive scientific understanding of the fundamental physical processes that control our space environment and that influence our Earth's atmosphere" (Ref. 18).


• Feb. 15, 2012: On the tenth anniversary of RHESSI's launch, B. Dennis and R. Lin published a science nugget reviewing the scientific achievements to date, which are listed below. 19)

RHESSI's Scientific Legacy: February 5, 2012, marks the 10th anniversary of RHESSI's launch. At this time, it seems appropriate to review the scientific achievements to date, and we present here a top-ten list of the iconic results that we believe will constitute RHESSI's scientific legacy. It is important to remember that RHESSI combines both X-ray and γ-ray imaging spectroscopy in a single instrument, with the stated goal of investigating particle acceleration and energy release in solar flares. The top-ten list consists of items, in our priority order, that show RHESSI's unique contributions in this area of flare research.

1) Number 1 -Discovery of Gamma-Ray Footpoint Structures: Prior to RHESSI, very little was known about where and how energetic ions are accelerated in solar flares. This is important since these ions may contain as much energy as energetic electrons, both carrying of order ~10-50% of the total energy released in a flare. RHESSI has provided the first-ever imaging of energetic flare ions through their nuclear gamma-ray line emission (Hurford et al. 2003, 2006). In the largest flare, shown in Figure 15, double ion footpoints are detected straddling the loop arcade, closely paralleling the electron footpoints. These observations show that the energetic ions are located in small footpoint sources in the vicinity of the flare. This is inconsistent with acceleration over a broad region by a shock front propagating far from the flare as suggested for SEP (Solar Energetic Particle) acceleration by shocks driven by fast CMEs. This strongly suggests that flare ion acceleration is similar to flare electron acceleration, with both possibly related to the process of magnetic reconnection. As shown in the figure, however, the ion footpoints are displaced from the electron footpoints by ~10-20 Mm, for reasons unknown.

RHESSI has also provided the first high resolution spectroscopy of flare γ-ray lines (Smith et al. 2003). Detailed analysis revealed mass-dependent Doppler red shifts of order ~1%, indicating that the emitting ions were traveling downward at an angle of ~30 to 40º to the vertical - likely along tilted magnetic fields. This result, combined with the detection of gamma-ray footpoint emission, shows clearly that the ions must have been accelerated over a relatively small volume in the corona, on closed field lines in the primary flare energy-release region.


Figure 15: Overlay of the 50%, 70%, and 90% contours of 35 high resolution gamma-ray images made with RMCs 6and 9 on a TRACE 195 Angstrom image of the 2003 October 28 flare. The red plus signs indicate 200–300 keV footpoint locations for successive adjacent intervals of 100, 120, 180, and 240 s beginning at 11:06:20 UT. The X and Y heliographic offsets are positive west and north of Sun center (image credit: UCB) 20)

2) Number 2 - Energy Content & Spectrum of Flare Energetic Electrons: A crucial question for flares is, how much of the energy released goes into particle acceleration? For the first time, RHESSI was able to show unambiguously that the power-law spectrum of energetic electrons extends down to ~20 keV, and therefore these electrons must contain a large fraction, ~10-50%, of the total energy released in many flares. RHESSI's uniquely fine energy resolution (~1 keV FWHM) is sufficient to resolve the steep high-energy fall-off of the hot flare thermal continuum, allowing for the accurate determination of the energy above which the hard X-ray emission must be non-thermal (e.g., Holman et al. 2003). For bright solar flares, RHESSI's measurements of the hard X-ray continuum are the best ever obtained for an astrophysical source. They are precise enough for model-independent deconvolution using powerful mathematical techniques developed for RHESSI observations (e.g., Kontar et al. 2011) to obtain the spectrum of the bremsstrahlung-emitting source electrons. For flare hard X-ray spectra with a flattening at low energies, the derived electron source spectra appear to show a roll-off around 20 - 40 keV (Kasparova et al. 2005), but after correcting for albedo (the Compton scattering of the source hard X-rays by the solar photosphere), using a newly-developed Green's function method (Kontar et al. 2006), all the derived source electron power-law spectra extend with no roll-off down to <~20 keV and sometimes as low as ~12 keV (see also Sui et al. 2005, 2007), where the hot flare thermal emission dominates. Power-law spectra extending that low imply that the source energetic electrons must contain a large fraction of the energy released in many flares.


Figure 16: Source electron flux spectra corresponding to the angle θo, the angle between the observer and the electron beam direction, for the electrons filling a semi-angle of α about the beam direction. Thus, α = 180 is an isotropic electron distribution (image credit: UCB) 21)

3) Number 3 - Ubiquitous Nonthermal Emissions from the Corona, & Bulk Energization: RHESSI's excellent energy resolution, in combination with its imaging capability, made it possible for the first time to cleanly measure nonthermal emissions from the corona (e.g., Battaglia & Benz 2006). These observations reveal that a non-thermal component is present in essentially all flares (Krucker & Lin 2008). RHESSI discovered that coronal nonthermal emissions are found in a large variety of associations such as in the pre-flare phase (Lin et al. 2003), in the absence of footpoint emission (Veronig & Brown 2004), associated with jets (Bain et al. 2009), with Coronal Mass Ejections (Krucker et al. 2007), and also in the gamma-ray range (Krucker et al. 2008). These observations indicate that a significant fraction of the total accelerated electrons are in the corona at one time, clearly favoring an acceleration site in the corona. Some rare, extremely bright sources provide crucial tests of the maximal efficiency of different acceleration models. The extreme brightness suggests that all electrons within the source are accelerated in a bulk energization process (Krucker et al. 2010; Ishikawa et al. 2011). Simultaneous microwave observations indicate that the energy in accelerated electrons at the peak of the event is of the same order of magnitude as the magnetic energy (i.e., plasma beta near unity). This indicates an extremely efficient conversion of magnetic energy into kinetic energy.


Figure 17: Images taken during the flare on January 20, 2005 at the peak time at 06:43:32 to 06:46:40 UTC (left), and during the decay phase at 06:50:00 to 06:55:01 UTC (right). Both figures show a TRACE 1,600Å image taken at 06:45:11 UTC overplotted with 12–15 keV (red) and 250–500 keV (blue) contours. The 12–15 keV image is reconstructed using a MEM algorithm and the contour levels shown are at 30, 50, 70, 90% of the maximum, while the Clean algorithm is used for the reconstruction of 250–500 keV images and 50, 70, 90% contours are displayed. During the peak the gamma-ray emission comes from footpoints, while later an additional coronal source becomes visible (image credit: UCB) 22)

4) Number 4 - Double Coronal X-ray Sources: The detection of twin coronal X-ray sources, one at the top and the other above the top of the flare loops. For both sources, higher energy and, therefore, higher temperature emission is shifted toward the region between the sources. This provided the strongest evidence yet for energy release in the corona between the two sources as indicated in Figure 18. Several other similar events have been reported, e.g., Liu and Petrosian, but it is not known how common this phenomenon might be.


Figure 18: Twin coronal X-ray sources with footpoints and presumptive location of energy release site indicated (image credit: UCB) 23)

5) Number 5 - Microflares and the Quiet Sun: RHESSI could readily observe both hard and soft X-rays from microflares, determining both spectrum and location. Figure 19 shows the solar coordinates of some 25,000 of them. Their distribution in space shows that they all come from active regions within the active North and South latitude bands. This establishes conclusively that flares, and these microflares, do not heat the quiet corona. — Related to this, RHESSI has established the lowest upper limits on quiet-Sun hard X-ray emission in the absence of active regions, placing strong constraints on any non-thermal energy release by nanoflares or other phenomena, and even on axions produced in the core of the Sun.


Figure 19: Flare positions as observed plotted on the solar disk. Positions were found by finding the centroid of back-projection images. Flares are concentrated in the active region bands (image credit: UCB) 24)

6) Number 6 - Initial Downward Motion of X-ray Sources: Sui and Holman (2003) also reported the initial downward motion of the coronal X-ray source prior to the previously reported continuous upward motion (Figure 20) . This was further detailed by Sui et al. (2004), where it was shown that the rate of altitude increase correlated with the hard X-ray flux, suggesting that it was related to the energy release rate. This initial downward motion has been seen now in other flares (e.g., Ji et al. 2008) and at other wavelengths, and it appears to be associated with the propagation of reconnection along flare ribbons, but its interpretation is still unclear.


Figure 20: Altitude History of the coronal X-ray source observed during the 2002 April 14–15 Flare. The source altitude initially decreased by 10 - 20% for the first ~3 minutes of the flare, and then increased at a speed of up to 40 km/s that is correlated with the hard X-ray flux (image credit: UCB) 25)

7) Number 7 - HXR Flare Ribbons: With RHESSI's unprecedented (~2 arcsec) spatial resolution, detailed hard X-ray imaging of flare ribbons finally became possible. This showed elongated structures along the ribbon that closely match the white light sources, but only partially match the EUV sources (Liu et al. 2007; Dennis & Pernak 2009; Krucker et al. 2011, Figure 21). The width of the hard X-ray sources, however, was often unresolved even by RHESSI, yielding an upper limit of ~1 arcsec for the dimension. This indicates that the number density of precipitating electrons is even larger than previously thought. These observations challenge our current understanding of the standard thick-target model and contribute new insights to our understanding of electron acceleration and transport.


Figure 21: Hard X-ray (green contours) and white light (G band) imaging of a flare ribbon. The left image shows the two ribbons. On the right is an enlargement of the brightest part of the southern ribbon (image credit: UCB) 26)

8) Number 8 - Location of Superhot X-ray Source: Superhot (defined as >30 MK) flare plasmas were discovered about three decades ago. (Previous instrumentation could only unambiguously measure flare plasma temperatures up to 10-20 MK). RHESSI's high spectral and spatial resolution has allowed the soft X-ray emission during the 2002 July 23 X4.8 flare (Figure 22) to be separated into two spatially distinct isothermal coronal X-ray sources, a superhot (>30 MK) source high in the corona, and a normal ~20 MK source located at a lower altitude (Caspi & Lin 2010). The super-hot source was present at a high altitude even during the flare pre-impulsive phase when no HXR footpoint emission was detected. This suggests a coronal origin for the superhot component, while the normal ~20 MK plasma originates primarily from chromospheric evaporation. The superhot and hot plasmas thus arise from fundamentally different physical processes.


Figure 22: X-ray spectrum and image of the 2002 July 23 X4.8 flare. Photon flux spectra (black), model fit (Fe and Fe-Ni lines: olive; super-hot: brown; hot: magenta; non-thermal: green; total model: blue), and normalized residuals during the RHESSI SXR peak (~00:31:30 UT), when the super-hot component is strongest. Inset: 50% and 90% contours of 6.3-7.3 (olive solid), 9-12 (blue dotted), 17-18 (brown dashed), and 60-100 keV (green dot-dashed) images at the same time; the crosses denote the derived centroid locations (and uncertainties) of the super-hot (brown; left) and hot (magenta; right) components (image credit: UCB) 27)

9) Number 9 - Photosphere as a Compton or "Dentist's" Mirror: With RHESSI's excellent spectral resolution, we can use the albedo contribution to the measured hard X-ray flux to our advantage. Kontar and Brown (2006) showed that, by considering the Sun's surface to act as a "Compton mirror," we can look at the emitting electrons both directly and from behind the source, providing vital information on the directionality of the propagating particles. Using this technique, they determined simultaneously the electron spectra of downward- and upward-directed electrons for two solar flares observed with RHESSI. The results reveal surprisingly near-isotropic electron distributions, which contrast strongly with the expectations from the standard model that invokes strong downward beaming, including a collisional thick-target model. Some would say that this result provides strong evidence against electron beam models in that simple particle beams aren't present in the energetically important particle distributions.


Figure 23: Top: Electron anisotropy defined as the ratio of the downward to the upward directed fluxes with confidence values within the shaded area. — Bottom: Recovered mean electron flux spectra (thick lines) for the 2002 August 20 flare (accumulation time interval 08:25:20–08:25:40 UT) for the downward-directed (solid line) and observer-directed (dashed line) fluxes with corresponding errors (thin lines), image credit: UCB 28)

10) Number 10 - Broadened 511-keV Positron Annihilation Line: RHESSI's high resolution spectroscopy of the flare 511-keV positron annihilation line emission showed a line width of typically >~5 keV, indicating that the temperature of the accelerated-ion interaction region was above 105 K. Later, in some flares, the width of the line narrows to ~1 keV, consistent with annihilation in ionized H at <104 K and ≥1015 cm-3. The full implications of these observations are still unclear but they bring into question the energy source of the heating. We also do not know whether or how the ions alone can produce such a highly dynamic flaring atmosphere, at chromospheric densities that can reach transition-region temperatures with large column depths, and then cool to less than 104 K in minutes while remaining highly ionized.


Figure 24: RHESSI count spectra of the solar 511 keV annihilation line (instrumentally broadened) derived by subtracting bremsstrahlung and nuclear contributions during the 2003 October 28 flare when the solar line was broad (11:06–11:16 UT) and narrow (11:18–11:30 UT). The solid curve is the best-fitting model that includes a Gaussian line and positronium continuum (image credit: UCB) 29)

Non-flare-related Items:

- Solar Oblateness: RHESSI's Solar Aspect System has provided the most precise measurements of the shape of the Sun, showing an unexpectedly large flattening compared to what is predicted from solar rotation. Fivian et al. (2008) were able to show that this effect was likely due to magnetic elements in the enhanced network producing emission preferentially at lower latitudes. Once this contribution was removed, the corrected oblateness of the nonmagnetic Sun was determined to be 8.01 ± 0.14 marcsec, which is near the value expected from rotation of 7.8 marcsec, or ~0.001%. This result may explain the variation that had been reported with solar cycle, and it is the most accurate measure of the true oblateness ever made.


Figure 25: Comparison of oblateness measurements from space. Here, we compare the RHESSI oblateness measurement, representing data from 29 June to 24 September 2004 (red crosses), with the best earlier values, namely the balloon-borne SDS (Solar Disk Sextant) experiment (green diamonds) (13) and the MDI instrument on board SOHO (blue triangles). The surface rotation rate predicts the value shown with the dotted line. The histogram (scaled to the uniform rotation oblateness at solar minimum and to the higher MDI data point) shows the radio flux index F10.7, a good indicator of the solar cycle. All errors are ±1σ (image credit: UCB) 30)

- Magnetar Timing and Spectroscopy: RHESSI serendipitously detected a huge flare from the SGR (Soft-Gamma-ray Repeater) 1806–20 that was just 5.25° from the Sun at the time of the observations. SGRs are thought to be magnetars - isolated, strongly magnetized neutron stars with teraGauss exterior magnetic fields and even stronger fields within, making them the most strongly-magnetized objects in the Universe. In the first 0.2 s, the flare released as much energy as the Sun radiates in a quarter of a million years. This observation suggested that a significant fraction of the mysterious short-duration gamma-ray bursts may come from similar extragalactic magnetars.

Later work by Watts and Strohmayer (2006) revealed quasi-periodic oscillations (QPOs) in the RHESSI timing data directly related to the "ringing" modes of the neutron star which, when interpreted as arising from vibrations in the neutron star crust, offer a novel means of testing the neutron star equation of state, crustal breaking strain, and magnetic field configuration. 31)


Figure 26: a: RHESSI 20−100-keV X-ray time history plotted with 0.5-s resolution. The flare began with an intense spike followed by the oscillatory phase. Inset, time history of the precursor with 8-ms resolution; b: Spectral blackbody temperature versus time derived from RHESSI and Wind particle detectors for the spike and the RHESSI X-ray detectors for the oscillatory phase (image credit: UCB) 32)

- TGFs (Terrestrial Gamma-ray Flashes): RHESSI has detected over 1000 TGFs, which were first discovered with BATSE on the Compton Gamma Ray Observatory and associated with terrestrial lightning. RHESSI showed that TGFs are much more common and luminous than previously thought, and that they extend up to gamma-ray energies beyond 20 MeV, with just the spectral shape predicted by the relativistic runaway model. RHESSI's observations also show that the TGFs come from altitudes of around 15 km, and that they are associated with intracloud lightning and not cloud-to-ground lightning or sprites.


Figure 27: Accumulated spectrum of 85 RHESSI TGFs, with theoretical relativistic-runaway spectra. The only difference between the models is the altitude of the average TGF, which is constrained to be between roughly 15 and 21 km (image credit: UCB, Ref. 19) 33)


• The RHESSI SMEX spacecraft and its instruments are operating nominally in 2012. On Feb. 5, 2012, the RHESSI spacecraft was 10 years on orbit. During this time, RHESSI has observed more than 40,000 X-ray flares, helped craft and refine a model of how solar eruptions form, and fueled additional serendipitous science papers on such things as the shape of the sun and thunder-storm-produced gamma ray flashes. 34) 35)

- RHESSI's monitoring of gamma rays throughout the sky also made it a prime tool to measure what are called terrestrial gamma-ray flashes (TGFs), bursts of gamma rays emitted from high in the Earth's atmosphere over lightning storms. The first of these had been spotted before, but RHESSI showed that they are more common and more luminous than previously thought. - With RHESSI's help, scientists soon realized they occurred upwards of 50 times/day. Indeed, current numbers suggest there may be as many as 400 TGFs daily from thunderstorms at different locations around the world.

- The original mission was only for two years and we quickly achieved our initial science goals - but RHESSI didn't stop there. The mission has been extended several times, and this small mission just keeps going and going, collecting great data. In 2009, NASA extended the mission yet again. Now, scientists are working to integrate RHESSI flare observations with data from other solar telescopes such as STEREO (Solar TErrestrial RElations Observatory), SDO (Solar Dynamics Observatory), SOHO (SOlar and Heliophysics Observatory), and Solar-B/Hinode as they watch the sun's activity rise toward yet another solar maximum, currently predicted for 2013 (Ref. 34).

• The RHESSI spacecraft and its instruments are operating nominally in 2011.

• The RHESSI spacecraft and its instruments are operating nominally in 2010 (after completing 8 years of observations). - Already in Feb. 2004, the mission had reached its design life of 2 years. So far, mission extensions were granted by NASA. It is expected that observations can be obtained through the next solar maximum which is expected between 2010 and 2012.

• As of May 2009, RHESSI has detected over 800 terrestrial gamma-ray flashes, providing the largest statistical database for characterizing these events.

• The RHESSI spacecraft and its instruments are operating nominally as of 2008. All detectors were annealed at ~90ºC for 7 days in November 2007. As a result, about half the effects of radiation damage on the energy resolution and sensitive volume was removed. 36)

• A number of "first time" observations of solar processes have been obtained (hard X-ray imaging spectroscopy, high resolution spectroscopy of solar gamma-ray lines, etc.). Early observations with RHESSI have revealed information on flare energetics, timing and spatial structure which stimulated renewed efforts to model and understand flares and magnetic reconnection on the sun.
On Dec. 27, 2004, a gamma ray flare, in fact the brightest explosion of high-energy X-rays and gamma rays recorded so far, was detected by at least 15 satellites and spacecraft between Earth and Saturn, swamping most of their detectors. Some of the best observations were recorded by the RHESSI instrument. 37)

• In 2003, the NASA Senior Review Panel gave RHESSI the highest rating of any of the 14 SEC (Sun-Earth Connection) missions. The rating indicates that the eleven voting panelists regarded RHESSI as "clearly superior" with "compelling science and relevance to the SEC mission." 38) 39)

• RHESSI was the first satellite to accurately measure terrestrial gamma-ray flashes that come from thunder storms, and RHESSI found that such flashes occur more often than thought and the gamma rays have a higher frequency on average than the average for cosmic sources.

• RHESSI can also see gamma rays coming from off-solar directions. The more energetic gamma rays pass through the spacecraft structure, and impact the detectors from any angle. This mode is used to observe GRBs (Gamma Ray Bursts).

• Many emission processes that can generate gamma-ray photons can also result in the linear polarization of those photons. The level of polarization, however, may depend on the precise emission geometry. In addition, the energy-dependence of the polarization can provide clues to the emission mechanisms that may be operating.

• On Dec. 6, 2002, RHESSI caught an extremely bright gamma-ray burst in the background, over the edge of the sun, revealing for the first time that the gamma rays in such a burst are polarized. The result indicates intense magnetic fields may be the driving force behind these awesome explosions. 40)


Figure 28: The deployed spacecraft with some component allocations (image credit: SSL/UCB)



Sensor complement: (RHESSI)

RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager):

The instrument name is identical to the spacecraft name. The objective is to obtain high fidelity color movies of solar flares in X-rays and gamma rays [imaging of solar flares in energetic photons from soft X-rays (about 3 keV) to gamma-rays (up to about 17 MeV) and to provide high resolution spectroscopy up to γ-ray energies of about 17 MeV].

Earth's atmosphere absorbs radiation over a large portion of the electromagnetic spectrum. It so happens that the atmosphere is completely opaque to X-ray radiation, i.e. to photons with energy levels above about 100 eV. Hence, observations of incoming X-ray radiation can only be done by instruments on spacecraft.

The instrument employs two new complementary technologies: fine grids (molybdenum and tungsten grids with slits as fine as 20 μm wide) to modulate the solar radiation, and germanium detectors to measure the energy of each photon very precisely (about 1 keV FWHM). The ITA (Imaging Telescope Assembly) consists of the telescope tube, grid trays, SAS (Solar Aspect System), and RAS (Roll Angle System). It was constructed, assembled, aligned, and tested at the Paul Scherrer Institut in Switzerland.


Figure 29: Illustration of the 9 Germanium detector assembly (image credit: SSL/UCB)

The spectrometer contains nine germanium detectors that are positioned behind the nine grid pairs on the telescope. These artificially grown crystals, pure to over one part in a trillion, were manufactured by Ortec of Perkin Elmer Instruments. When they are cooled to cryogenic temperatures (~75 K) and a high voltage is put across them (up to 4000 V), they convert incoming X-rays and gamma-rays to pulses of electric current. The amount of current is proportional to the energy of the photon. Germanium provides not only detections by the photoelectric effect, but inherent spectroscopy through the charge deposition of the incoming ray.

RHESSI is a FTS (Fourier Transform Spectrometer) device using a set of 9 RMCs (Rotational Modulation Collimators) or grid pairs (as opposed to conventional mirrors and lenses in the optical spectrum). Each RMC consists of two widely-spaced, fine-scale linear grids, which temporally modulate the photon signal from sources in the field of view as the S/C rotates about an axis parallel to the long axis of the RMC. The modulation can be measured with a detector having no spatial resolution placed behind the RMC. The modulation pattern over half a rotation for a single RMC provides the amplitude and phase of many spatial Fourier components over a full range of angular orientations but for a small range of spatial source dimensions. Multiple RMCs, each with different slit widths, can provide coverage over a full range of flare source sizes. An image is reconstructed from the set of measured Fourier components in exact mathematical analogy to multi-baseline radio interferometry. 41) 42) 43) 44) 45) 46) 47)

Energy range

3 keV to 17 MeV (soft X-rays to gamma-rays)

Energy resolution (FWHM)

< 1 keV at 3 keV, increasing to ~5 keV at 5 MeV

Angular resolution

- 2.3 arcseconds from 3 to 100 keV, 7 arcseconds to 400 keV,
- 36 arcseconds above 1 MeV

Temporal resolution

2 s for detailed image, tens of ms for basic image

FOV (Field of View)

full Sun (~ 1º)

Effective area (photopeak)

~ 10-3 cm2 at 3 keV, ~32 cm2 at 10 keV (with attenuators out), ~60 cm2 at 100 keV, ~15 cm2 at 5 MeV


9 germanium detectors (7.1 cm diameter x 8.5 cm), cooled to < 75 K with Stirling-cycle mechanical cooler


9 pairs of grids, with pitches from 34 µm to 2.75 mm, and 1.55 m grid separation

Aspect system

Solar Aspect System: Sun center to < 1 arcsec, Roll Angle System: roll to ~1 arcmin

Number of flares expected

~1000 imaged to >100 keV, ~ tens with spectroscopy to ~10 MeV

Instrument mass, power

131 kg, 143 W

Instrument size

- Grid support structure: 45 cm diameter, 1.7 m long
- Detector/cryocooler enclosure: 1 m diameter x 30 cm deep

Data storage capability

16 Gbit in 10 minutes SSR (Solid State Recorder)

Table 1: RHESSI instrument specification


Figure 30: Detector arrangement of the RHESSI spectrometer (image credit: NASA, UCB)

Legend to Figure 30: A cutaway view of the Spectrometer, showing the location of the germanium detectors under each grid (by number). The Sunpower Stirling-cycle mechanical cooler is below the cold plate holding the detectors. The thermal radiator faces anti-sunward to reject the heat of the cryocooler. The attenuators are automatically moved in when the counting rate exceeds thresholds (commandable from the ground).


Figure 31: Schematic illustration of the RHESSI grid pair alignment (image credit: B. R. Dennis, NASA/GSFC)

The detectors are the largest currently available (as of 2000) hyperpure (n-type) germanium detectors of size: 7.1 cm in diameter and 8.5 cm in length. They are cooled to 77 K by a single stage electro-mechanical cryocooler (an integral counterbalanced Stirling cycle cooler (built by Sunpower, Inc.) which provides up to 4 W of cooling at 77 K, with an input of 100 W). The cryocooler houses the 9 germanium detectors. The Ge detectors are segmented, with both a front and rear active volume (Figures 31 and 32). Low-energy photons (below about 100 keV) can reach a rear segment of a Ge detector only indirectly, by scattering.

The detectors cover the entire X-ray to gamma-ray energy range from 3 keV to 17 MeV. The keV spectral resolution of germanium detectors is necessary to resolve all of the solar gamma-ray lines (with the exception of the neutron deuterium line, which has an expected FWHM of only 0.1 keV).

The critical alignment requirement for the metering structure is to maintain the relative twist of the finest grid pair to within one arcminute. The metering structure is based on the TDU (Telescope Demonstration Unit) provided by GSFC.


Figure 32: Illustration of the forward grid tray (right) and the aft grid tray (left), image credit: SSL/UCB

RHESSI achieves the alignment feat by using tungsten and molybdenum grids with extremely fine slits, some as fine as 20 µm wide. The manufacture of these grids has been made possible by newly developed microfabrication techniques.


Figure 33: View of the single-stage Stirling cryocooler for the detector array (image credit: NASA)


Figure 34: Schematic of the RHESSI imaging technique (image credit: NASA)



GDS (Ground Data System):

RHESSI is operated from the highly integrated and automated MOC (Mission Operations Center) located at SSL/UCB (Space Sciences Laboratory of the University of California at Berkeley). The MOC also supports the FAST (Fast Auroral Snapshot Explorer). Co-located with the multi-mission MOC are the RHESSI and FAST SOC (Science Operations Center) and the BGS (Berkeley Ground Station), the primary ground station to support RHESSI on-orbit (Ref. 42).


Figure 35: Overview of the RHESSI ground data system (image credit: NASA)

RHESSI is operated in store-and-dump mode. The spacecraft transmitter is turned on and off by time sequence commands stored on-board. These commands and many others related to configuring instruments for various phases of the orbit are part of an ATS (Absolute Time Sequence) load generated with the MPS (Mission Planning System). Command loads are uploaded to the spacecraft every two days and cover 4–5 days in advance.

The spacecraft command and control system for RHESSI is the ITOS (Integrated Test and Operations System). Since ITOS was also used during mission integration and testing, members of the Berkeley Flight Operations Team were trained early on operating the spacecraft. This approach allowed for a smooth transition from spacecraft integration and testing to normal on-orbit operations.

Flight dynamics and mission planning products are generated by the Berkeley Flight Dynamics System, which is based on the SatTrack Suite V4.4. SatTrack also has heritage with various NASA missions and is used to generate all flight dynamics products such as ground station view periods, link access periods, terminator, high-latitude region, and SAA (South Atlantic Anomaly) crossings, and other orbit events needed as input to MPS. Other tools in the SatTrack Suite are employed to distribute real-time event messages to various ground data system elements such as ITOS and the BGS in an autonomous client/server network environment. SatTrack also provides a multitude of related automation functions as well as 2-D and 3-D real-time orbit displays.

All RHESSI space and ground systems are tied into the SERS (Spacecraft Emergency Response System),which is a data base system that regularly parses through log files and automatically checks for yellow or red limit violations. It also acts on warning and error messages received from various GDS subsystems via electronic mail. In case an anomaly is detected, the on-call operations team member is alerted via 2-way email pager in order to assess and resolve the situation. SERS completes the autonomous ground system and adds a high degree of reliability.




3) Information provided by Brian R. Dennis of NASA/GSFC

4) RHESSI Receiving Review, May 6, 2002, URL:

5) "NASA Retires Prolific Solar Observatory After 16 Years," NASA, 20 November 2018, URL:

6) Adam F. Kowalski, Joel C. Allred, "Parameterized Flare Models with Chromospheric Compressions," Nugget No 315, 17 Jan. 2018, URL:

7) Adam F. Kowalski, Joel C. Allred, "Parameterizations of Chromospheric Condensations in dG and dMe Model Flare Atmospheres," The Astrophysical Journal, Volume 852, Issue 1, article id. 61, 19 pp. (2018), doi: 10.3847/1538-4357/aa9d91

8) Säm Krucker, Hugh Hudson, "The Last Best Flare of Cycle 24?,"Nugget No 306, 11 September 2017, URL:

9) Brian Dennis, Säm Krucker, Albert Shih, "RHESSI's 15th Anniversary," Nugget No 292, 14 February 2017, URL:

10) "RHESSI celebrated its 15th anniversary on February 5, 2017," NASA, URL:

11) Frederic Effenberger , Fatima Rubio da Costa, "Hard X-ray Emission from Partially Occulted Solar Flares," UCB/SSL, Nugget No 291, 30 Jan. 2017, URL:

12) Werner M. Neupert, "Comparison of Solar X-Ray Line Emission with Microwave Emission during Flares," Astrophysical Journal, vol. 153, p.L59, 07, 1968, doi: 10.1086/180220, URL:

13) "RHESSI's 5th Anneal," UCB, April 11, 2016, URL:'s_5th_Anneal

14) "NASA Response to the 2015 Senior Review for Heliophysics Operating Missions," NASA, July 10, 2015, URL:

15) "The 2015 Senior Review of the Heliophysics Operating Missions, NASA, June 11, 2015, URL:

16) "RHESSI Status," NASA, June 26 to Aug. 13, 2014, URL:

17) Information provided by Gordon D. Holman of NASA/GSFC, Greenbelt, MD, USA

18) Samuel Krucker, Brian Dennis, Manfred Bester, Laura Peticolas, "Heliophysics Senior Review 2013, The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)," April 24, 2013, URL:

19) Brian Dennis, Bob Lin, "RHESSI's Tenth Anniversary," UCB, Feb. 15, 2012, URL:

20) G. J. Hurford, R. A. Schwartz, S. Krucker, R. Lin, D. M. Smith, N. Vilmer, "First Gamma-Ray Images of a Solar Flare," The Astrophysical Journal, Volume 595, Issue 2, pp: L77-L80, October 2003

21) Anna MariaMassone, A. Gordon Emslie, Eduard P. Kontar, Michele Piana, Marco Prato, John C. Brown, "Anisotropic Bremsstrahlung Emission and the Form of Regularized Electron Flux Spectra in Solar Flares," The Astrophysical Journal, Volume 613, Issue 2, pp. 1233-1240, October 2004

22) Säm Krucker, G. J. Hurford, A. L. MacKinnon, A. Y. Shih, R. P. Lin, "Coronal γ-Ray Bremsstrahlung from Solar Flare-accelerated Electrons," The Astrophysical Journal Letters, Volume 678, Issue 1, article id. L63, May 2008

23) Linhui Sui, Gordon D. Holman, "Evidence for the Formation of a Large-Scale Current Sheet in a Solar Flare," The Astrophysical Journal, Volume 596, Issue 2, pp: L251-L254, October 2003, doi: 10.1086/379343

24) S. Christe, I. G. Hannah, S. Krucker, J. McTiernan, R. P. Lin, "RHESSI Microflare Statistics. I. Flare-Finding and Frequency Distributions," The Astrophysical Journal, Volume 677, Issue 2, pp: 1385-1394, April 2008

25) Linhui Sui, Gordon D. Holman, Brian R. Dennis, "Evidence for Magnetic Reconnection in Three Homologous Solar Flares Observed by RHESSI," The Astrophysical Journal, Volume 612, Issue 1, pp. 546-556, September 2004

26) Säm Krucker, H. S. Hudson, N. L. S. Jeffrey, M. Battaglia, E. P. Kontar, A. O. Benz,A. Csillaghy, R. P. Lin, "High-resolution Imaging of Solar Flare Ribbons and Its Implication on the Thick-target Beam Model," The Astrophysical Journal, Volume 739, Issue 2, article id. 96, October 2011, doi: 10.1088/0004-637X/739/2/96

27) A. Caspi, R. P. Lin, "RHESSI Line and Continuum Observations of Super-hot Flare Plasma," The Astrophysical Journal Letters, Volume 725, Issue 2, pp. L161-L166 (2010), December 2010, doi: 10.1088/2041-8205/725/2/L161

28) Eduard P. Kontar, John C. Brown, "Stereoscopic Electron Spectroscopy of Solar Hard X-Ray Flares with a Single Spacecraft," The Astrophysical Journal, Volume 653, Issue 2, pp. L149-L152, December 2006, doi: 10.1086/510586

29) Gerald H. Share, Ronald J. Murphy, David M. Smith, Richard A. Schwartz, Robert P. Lin, "RHESSI e+-e- Annihilation Radiation Observations: Implications for Conditions in the Flaring Solar Chromosphere," The Astrophysical Journal, Volume 615, Issue 2, pp. L169-L172, November 2004, doi: 10.1086/426478

30) Martin D. Fivian, Hugh S. Hudson, Robert P. Lin, H. Jabran Zahid, "A Large Excess in Apparent Solar Oblateness Due to Surface Magnetism," Science, Volume 322, Issue 5901, pp. 560-, October 2008, doi: 10.1126/science.1160863

31) Anna L. Watts, Tod E. Strohmayer, "Detection with RHESSI of High-Frequency X-Ray Oscillations in the Tail of the 2004 Hyperflare from SGR 1806-20," The Astrophysical Journal, Volume 637, Issue 2, pp: L117-L120, February 2006

32) K. Hurley,S. E. Boggs,D. M. Smith, R. C. Duncan, R. Lin, A. Zoglauer, S. Krucker, G. Hurford, H. Hudson, C. Wigger, W. Hajdas, C. Thompson, I. Mitrofanov, A. Sanin, W. W. Boynton, C. Fellows, A. von Kienlin, G. Lichti, A. Rau, T. Cline, "An exceptionally bright flare from SGR 1806-20 and the origins of short-duration γ-ray bursts," Nature, Volume 434, Issue 7037, pp. 1098-1103, April 2005, doi: 10.1038/nature03519

33) J. R. Dwyer, D. M. Smith, "A comparison between Monte Carlo simulations of runaway breakdown and terrestrial gamma-ray flash observations," Geophysical Research Letters, Volume 32, Issue 22, CiteID L22804, November 2005, doi: 10.1029/2005GL023848

34) Karen C. Fox, "NASA Small Explorer Mission Celebrates Ten Years and Forty Thousand X-Ray Flares," Space Daily, Feb. 13, 2012, URL:


36) "RHESSI Detectors Successfully Annealed," Jan. 16, 2008, URL:

37) R. Sanders, "RHESSI satellite captures giant gamma-ray flare," Feb. 18, 2005, URL:

38) "Senior Review Rates RHESSI Highest," NASA, URL:

39) Wolfgang Baumjohann, David S. Evans, Priscilla Frisch, Philip R. Goode, Bernard V. Jackson, J. R. Jokipii, Stephen L. Keil (Chair), Joan T. Schmelz, Frank R. Toffoletto, Raymond J. Walker, William Ward, "Senior Review of the Sun-Earth Connection Mission Operations and Data Analysis Program," NASA, August 3, 2003, URL:

40) D. Savage, B. Steigerwald, R. Sanders, "RHESSI's Lucky Break May Lead To Secret Of Ultimate Explosions," May 28, 2003, URL:

41) R. P. Lin, B. R. Dennis, G. J. Hurford, G. J. Hurford, D. M. Smith, A. Zehnder, P. R. Harvey, D. W. Curtis, D. Pankow, P. Turin, M. Bester, A. Csillaghy, M. Lewis, N. Madden, H. F. Beek, M. Appleby, T. Raudorf, J. MyTierman, R. Ramaty, E. Schmahl, R. Schwartz, S. Krucker, R. Abiad, T. Quinn, P. Berg, M. Hashii, R. Sterling, R. Jackson, R. Pratt, R. D. Campbell, D. Malone, et al. "The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)," Solar Physics, Vol. 210, 2002, pp. 3-17, URL:

42) R. P. Lin, B. R. Dennis, et al., The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)," Solar Physics, Vol. 210, 2002, pp. 18-32, URL:

43) M. L. McConnell, J. M. Ryan, D. M. Smith, R. P. Lin, A. G. Emslie, "RHESSI as a hard X-ray polarimeter," Solar Physics, Vol.. 210, 2002, pp.125-142

44) M. L. McConnell, D. M. Smith, A. G. Emslie, G. J. Hurford, R. P. Lin, J. M. Ryan, "Hard X-ray solar flare polarimetry with RHESSI," Advances in Space Research, Vol. 34, 2004, pp. 462-466

45) M. L. McConnell, P. F. Bloser, "Status and Future Prospects for γ-ray Polarimetry," arXiv:astro-ph/0508315 v1, Aug 14, 2005, published in: Chinese Journal of Astronomy and Astrophysics, Supplement, Volume 6, Issue S1, 2006, pp. 237-246



The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (

Spacecraft    Launch    Mission Status    Sensor Complement   Ground Segment   References   Back to Top