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 belts.

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 interferometry.

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