HESSI mission consists of a single spin-stabilized spacecraft in a
low-altitude orbit inclined 38 degrees to the Earth's equator. The
only instrument on board is an imaging spectrometer with the ability
to obtain high fidelity color movies of solar flares in X rays and
gamma rays. It uses two new complementary technologies: fine grids
to modulate the solar radiation, and germanium detectors to measure
the energy of each photon very precisely.
HESSI's imaging capability is achieved with fine tungsten
and/or molybdenum grids that modulate the solar X-ray flux as
the spacecraft rotates at ~ 15 rpm. Up to 20 detailed images can
be obtained per second. This is sufficient to track the electrons
as they travel from their acceleration site, believed to be in the
solar corona, and slow down on their way to the lower solar atmosphere.
The high-resolution spectroscopy is achieved with 9 cooled germanium crystals
that detect the X-ray and gamma-ray photons transmitted through the grids over the broad
energy range of 3 keV to 20 MeV. Their fine energy resolution of about 1 keV is more than
sufficient to reveal the detailed features of the X-ray and gamma-ray spectra, clues to
the nature of the electron and ion acceleration processes.
A spinning spacecraft pointing at or near Sun center provides a simple and reliable way
to achieve the rotation required for the HESSI imaging technique. A low-altitude
equatorial orbit that can be reached with a Pegasus launch vehicle is chosen to minimize
damage to the germanium detectors from the charged particles in the Earth's radiation
Context observations from ground-based observatories and a theory program are also
integral parts of the HESSI mission. Ground-based optical and radio telescopes will
provide complementary data on the magnetic fields, electric currents, hot plasma, and the
energetic electrons in the flaring regions where the X-ray and gamma-ray emissions are
generated. Also, it is hoped that other spacecraft will provide additional simultaneous
observations of the thermal and dynamic environment to further enhance our knowledge of
the conditions in the flaring region.
High Energy Imaging Spectroscopy
HESSI is designed to image solar flares in energetic photons
from soft X rays (~3 keV) to gamma rays (up to ~20 MeV) and to provide
high resolution spectroscopy up to gamma-ray energies of ~20 MeV.
Furthermore, it has the capability to perform spatially resolved
spectroscopy with high spectral resolution, thus allowing the full
diagnostic power of hard X rays and gamma rays to be applied on
a spatial point-by-point basis within solar flares.
HESSI will have the finest angular and the spectral resolution
of any hard X-ray or gamma-ray instrument ever flown in space. Relative
to previous instruments, HESSI, with its total effective
area of up to 100 square centimeters, will be a factor of 10 more
sensitive than SXT on Hinotori, and more than 100
times more sensitive than HXIS on SMM.
Compared to HXT on Yohkoh, HESSI will extend from 3 keV to 1 MeV
rather than from 15 to 100 keV, and will have an angular resolution of two arcseconds
compared to >5 arcseconds. Furthermore, the HESSI imaging technique using
rotational modulation collimators is inherently much less susceptible to systematic errors
due to calibration uncertainties so that HESSI will provide much better image
quality and dynamic range than HXT.
HESSI will be the first imaging spectrometer in orbit with high-resolution
germanium detectors. Thus, while it has less sensitive volume for the detection of gamma
rays than do the BATSE and OSSE instruments on CGRO, it will have a
factor of 25 superior energy resolution at 1 MeV.
INTEGRAL is also planned to include germanium detectors and is scheduled to fly
around the year 2000, but it will never be able to observe the Sun directly because of
spacecraft constraints and it has only multi-arcminute imaging capability.
The imaging capability of HESSI is based on a Fourier-transform technique using
a set of 9 Rotational Modulation Collimators (RMCs). Each RMC
consist of two widely-spaced, fine-scale linear grids, which temporally modulate the
photon signal from sources in the field of view as the spacecraft 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
HESSI will provide spatial resolution of 2 arcseconds at X-ray energies below
~40 keV, 7 arcseconds to 400 keV, and 36 arcseconds for gamma-ray lines and continuum
above 1 MeV. The chosen spacecraft rotation rate of 15 rpm provides a complete image
with the maximum number of Fourier components in 2 s, but spatial information from fewer
Fourier components is still available on time scales down to 10's of ms, provided the
count rates are sufficiently high.
The detectors baselined for HESSI behind the RMCs are the largest
currently available hyperpure (n-type) germanium detectors (HPGe), 7.1 cm in
diameter and 8.5 cm long. They will be cooled to their operating temperature of 75 K by a
single electro-mechanical cryocooler. Such detectors can cover the entire X-ray to
gamma-ray energy range from 3 keV to 20 MeV with the highest spectral resolution of any
presently available detector (<2 keV below 1 MeV to 5 keV at 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). It is also required to resolve the detailed features of the X-ray continuum spectrum
such as the steep super-hot thermal component and the sharp breaks in the nonthermal
component at higher energies.
Germanium detectors with two electrically-independent segments
will be used so that the front 1-cm thick segment will measure hard
X rays up to 200 keV with low background while the rear 7-cm thick
segment will provide undistorted high-resolution gamma-ray line
measurements, even in the presence of very intense hard X-ray fluxes
in large flares. The cumulative radiation dose to the germanium
detectors in a three-year mission lifetime is low enough in a low-Earth
orbit to avoid noticeable radiation damage to the detectors. Thus,
it is not necessary to add a thick, and necessarily heavy, shield
in this orbit, making the lightsat approach feasible.
Summary of the HESSI Mission