4.2.3 SOLAR
As discussed in section 4.1.3, the Sun is capable of producing large
measurable gamma-ray fluxes at Earth with even moderate solar flares. One
might think that this obviates the need for sensitive instruments in investigating
solar phenomena. This is not so. Given that the Sun is resolvable at most
wavelengths we know much of the basic physics of solar flares. This increases
the demands on the hard X-ray and gamma-ray measurements as more and better
data are necessary to properly use the information obtained at other wavelengths.
Of utmost importance is the ability to image gamma-ray emitting features
on the solar disk. This would reveal the extent of the energetic particle
population as it evolves through the course of the flare. Also of considerable
importance are high-energy resolution measurements. These measurements
provide information about the composition of the solar atmosphere that
is being bombarded with energetic protons. Conceivably, there may inhomogeneities
in the composition of the lower corona and good spectral data coupled with
imaging data could reveal this. Expanding the size and dimensionality of
the data space in this manner means that solar gamma-ray instruments will
not be small. The ideal instrument should be large, able to handle a large
dynamic range of intensities, and have good spectral resolution as well
as being a gamma-ray camera. These are serious and demanding requirements
of an instrument or a suite of instruments even given the strong solar
signal. However, some aspect of these problems could be addressed with
instruments on mid-sized and small spacecraft. For example, an instrument
that images up to about 300 keV with standard scintillator energy resolution
would be of value in tracking the distribution of energetic electrons in
the solar atmosphere. Another good instrument would be one along the lines
of the Gamma Ray Spectrometer on SMM, but with Ge-type resolution. One
final possibility is to design an instrument to attack a specific problem
such as the polarization of the electron bremsstrahlung radiation. Any
of these missions would be scientifically interesting. Some general gamma-ray
spectral monitoring is necessary for proper interpretation of hard X-ray
image data, polarization measurements and high-energy measurements. A useful
and productive set of instruments could be flown on a small to mid-sized
platform.
4.3 SUBORBITAL PROGRAM
The future of gamma-ray astronomy will continue to depend in a major way on the balloon program for the development of new instruments and techniques. The role of the sub-orbital program has been particularly important for gamma-ray missions, with all the instruments on CGRO (for example) having balloon programs. The role of the balloon program in gamma-ray science has not been as widely appreciated. Important scientific advances include the discovery of galactic 511 line emission, 56Co line emission from SN1987A, the hard X-ray imaging study and identification of several galactic bulge sources, and the mapping and study of both the diffuse 511 keV and 26Al 1.8 MeV emission. With the newly revived, and highly promising, development of superpressure balloon technology, the long-sought goal of long-duration balloon flights of large payloads appears to be finally within reach. The development of 100-day balloon flights with 3000 lb payloads will provide a significant opportunity for new gamma-ray missions and science. The GRAPWG strongly encourages that the current push to develop the 100-day long-duration balloon capability receive the technical, engineering and science payload development support needed to capitalize on this new mission opportunity. To ensure the timely success of this effort, action is needed on several fronts: incorporation of upgraded and currently available electronics (e.g., power and telemetry systems) and mechanical (e.g., gondola shock isolation and controlled parachute systems) engineering into the balloon program; attention to innovative data recovery and payload control systems (e.g., using worldwide cellular phones); and attention to establishing and maintaining international overflight agreements.