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
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)
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
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
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
• 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.
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)
3 keV to 20 MeV (soft X-rays to gamma-rays)
< 2 keV below 1 MeV to 5 keV at 20 MeV
- 2.3 arcseconds from 3 to 100 keV, 7 arcseconds to 400 keV,
- 36 arcseconds above 1 MeV
Instrument mass, power
130 kg, 148 W
- Grid support structure: 45 cm diameter, 1.7 m long
- Detector/cooler enclosure: 1 m diameter x 30 cm deep
Data storage capability
16 Gbit in 10 minutes (solid-state on-board recorder)
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)
3) Information provided by Brian R. Dennis of NASA/GSFC
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
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,
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
Research, Vol. 34, 2004, pp. 462-466
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,
This description was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" - comments
and corrections to this article are welcomed by the author.