and corona to these MHD interactions in the photosphere. This cause-and-effect observing capability is the core rationale for OSL, leading to knowledge of:

  • How the interaction between the convection and the magnetic field drives the heating that sustains the chromosphere, corona, and solar wind.

  • How nonpotential energy is built up in large-scale metastable configurations, how these configurations evolve (and possibly become unstable), and how the stored energy is released (including possibly in flares and coronal mass ejections).

  • How magnetic flux in the photosphere is "processed" by turbulent convection, and caused to diffuse away from active regions. This apparently sows the seeds for a new magnetic cycle in the dynamo process that sustains the Sun's magnetic field.

OSL is an extremely well-defined and thoroughly-studied mission. Planning for the critical component, a large-aperture diffraction-limited visible-light telescope (the Solar Optical Telescope, or SOT), began 17 years ago. Indeed the SOT was approved for development as a Spacelab mission more than a decade ago, but the development was then postponed for programmatic reasons unrelated to the scientific merits of the mission. Since then, technical capabilities have increased greatly, so that today the OSL has scientific potential far transcending the original mission. OSL is the key mission for solar physics, and enjoys the highest possible endorsement of the community. The Solar Panel notes with satisfaction that the OSL is now at the top of NASA's Strategic Plan for Space Science, and urges in the strongest possible terms that its development be resumed as soon as possible.

3.3.2 The High Energy Solar Physics (Hesp) Mission

The scientific goals of HESP -- a new mission to study the active Sun and flares during the activity maximum toward the end of the decade -- center on the mechanisms and processes of explosive energy release and particle acceleration associated with solar activity. These processes are at the core of the solar flare problem, and they play a major role in all of astrophysics, particularly in objects dominated by high energy processes. HESP will observe the high-energy radiations (hard X-rays, gamma-rays, and neutrons) which are the most unambiguous signatures of accelerated particle interactions. Observations of these emissions with high spatial resolution will allow the localization of the sources of the particles and the tracing of their transport paths. Observations with high spectral resolution will allow the deciphering of the rich information encoded in gamma-ray lines, such as abundances in both the ambient gas and the accelerated particles, beaming of the accelerated particles, temperatures and states of ionization of the ambient gas, and the structure of the magnetic fields. The combination of high spatial and energy resolution, by providing diagnostics which are qualitatively different from anything available so far, offers unique opportunities for resolving some of the most complex issues in solar physics (flare mechanisms, particle acceleration, coronal heating). Understanding of these issues will also be of great benefit to high energy astrophysics. In particular, HESP will provide information on:

  • The nature of acceleration mechanisms, by determining the ratio of accelerated protons to electrons from observations of gamma-ray lines and continuum. An overabundance of protons would favor shock and stochastic mechanisms, while the overabundance of electrons would favor electric fields. The combination of high spatial resolution and energy resolution could distinguish sites where electron acceleration dominates from sites where ion acceleration is dominant.

  • Angular distributions and magnetic field structures, from direct imaging of hard X-ray bremsstrahlung in the active flux tubes. The shapes of the nuclear deexcitation gamma-ray lines also reveal the angular distribution of the interacting ions, which in turn depend on the nature of the acceleration and transport of the fast particles and on the structure of the magnetic fields. With the combined angular and spectral resolving power of HESP it will be possible to determine angular distributions accurately as a function of position in the atmosphere.

  • Abundances in both the ambient gas and in the accelerated particles, from the relative intensities of gamma-ray lines. These abundances include that of 3He in the photosphere. The high energy resolution capability of HESP will allow the separation of many more lines than was possible with SMM, thereby qualitatively enhancing the power of the technique. Abundance variations shed light on mechanisms of mass motion in the atmosphere and turbulent mixing in the interior.

  • Temperatures, densities and states of ionization of the ambient gas, from the shape of the 511 keV positron annihilation line. Recent galactic observations of this line with high energy resolution have yielded

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