RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager)

A NASA SMEX 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 20 MeV gamma rays with high time resolution. RHESSI (the original name was HESSI) is a collaboration between the following institutions: GSFC, UCB (PI: Robert Lin), PSI (Paul Scherrer Institut, Villigen, Switzerland), and ETH Zürich (RHESSI Experimental Data Center - REDC). ^{1) 2) 3)}

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.

Figure 1: Photo of the spacecraft bus (image credit: NASA)

Spacecraft:

The S/C bus was designed and built by Spectrum Astro of Gilbert, 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, 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 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 S/C design life is two years with a goal of three years.

Figure 2: Line drawing of the spacecraft bus structure and solar wings (image credit: NASA, UCB)

Figure 3: CDHS (Command and Data Handling System) of the spacecraft (image credit: SSL/UCB)

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 REDC at Zürich. There is also a complementary ground-based program supported by observatories throughout the world.

Launch: A 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.

Figure 4: View of the deployed spacecraft (image credit: SSL/UCB)

Mission status: The RHESSI spacecraft and its instruments are operating nominally as of 2007. 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.

• 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.

• 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.

Sensor complement:

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 20 MeV) and to provide high resolution spectroscopy up to γ-ray energies of about 20 MeV].

The instrument employs two new complementary technologies: fine grids (molybdenum and tungsten grids with slits as fine as 20 mm 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.

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.

Figure 5: Illustration of the 9 Germanium detector assembly (image credit: SSL/UCB)

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. ^{4) 5) 6) 7) 8)}

Table 1: RHESSI instrument specification

Figure 6: Detector arrangement of the RHESSI spectrometer (image credit: NASA)

Figure 7: Schematic illustration of the RHESSI grid pair alignment

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 7 and 8). 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 20 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 8: 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 9: Schematic of the RHESSI imaging technique (image credit: NASA)

2) http://hesperia.gsfc.nasa.gov/hessi/

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

4) R. P. Lin, B. R. Dennis, G. J. Hurford, et al. "The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)," Solar Physics, Vol. 210, 2002, pp. 3-32

5) 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

6) 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

7) 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

8) http://hessi.ssl.berkeley.edu/instrument/germanium.html