Reuven Ramaty High Energy Solar Spectroscopic Imager - Scientific Objectives

Scientific Objectives

Researchers believe that much of the energy released during a flare is used to accelerate, to very high energies, electrons (emitting primarily X-rays) and protons and other ions (emitting primarily gamma rays). The new approach of the RHESSI mission is to combine, for the first time, high-resolution imaging in hard X-rays and gamma rays with high-resolution spectroscopy, so that a detailed energy spectrum can be obtained at each point of the image.

This new approach will enable researchers to find out where these particles are accelerated and to what energies. Such information will advance understanding of the fundamental high-energy processes at the core of the solar flare problem.

The primary scientific objective of RHESSI is to understand the following processes that take place in the magnetized plasmas of the solar atmosphere during a flare:

• Impulsive energy release,
• Particle acceleration,
• Particle and energy transport.

These high-energy processes play a major role at sites throughout the universe ranging from magnetospheres to active galaxies. Consequently, the importance of understanding these processes transcends the field of solar physics; it is one of the major goals of space physics and astrophysics.

The high energy processes of interest include the following:

• The rapid release of energy stored in unstable magnetic configurations,
• The equally rapid conversion of this energy into the kinetic energy of hot plasma and accelerated particles (primarily electrons, protons and ions),
• The transport of these particles through the solar atmosphere and into interplanetary space,
• The subsequent heating of the ambient solar atmosphere.

These processes involve:

• Particle energies to many GeV,
• Temperatures of tens or even hundreds of millions of degrees,
• Densities as low as 100 million particles per square cm,
• Spatial scales of tens of thousands of kilometers, and
• Magnetic containment times of seconds to hours.

It is impossible to duplicate these conditions in laboratories on the Earth.

The acceleration of electrons is revealed by hard X-ray and gamma-ray bremsstrahlung while the acceleration of protons and ions is revealed by gamma-ray lines and continuum. The proximity of the Sun means, not only that these high-energy emissions are orders of magnitude more intense than from any other cosmic source, but also that they can be better resolved, both spatially and temporally.