Why Study Solar Flares in Hard X-Rays? 

In solar flares electrons are heated to high temperatures. In other words, energy is deposited into the flare plasma in such a way that the mean energy of the plasma electrons is substantially increased, but they remain a part of the thermal plasma. A portion of the electrons is not simply heated, but is accelerated out of the flare plasma to high energies. That is, some of the electrons leave the thermal plasma and attain energies that are much higher than the mean energy of the electrons in the thermal plasma. This raises several questions:

We don't know the answers to any of these questions. The most direct tracer of these electrons is the x-ray emission they produce.

A charged particle will produce radiation whenever its velocity is changed (that is, whenever it is accelerated or decelerated). Both electrons and ions radiate, but the radiation from the ions is generally negligible because of their higher mass. In solar flares the interaction of the energetic electrons with thermal protons provides the deceleration. This radiation is known as bremsstrahlung. As electrons stream through the plasma, they are attracted to the oppositely charged protons. X-ray photons with energies less than or nearly equal to the electron energy are produced. These x-ray photons are the emitted radiation signatures detected by scientific instruments.

X-ray Bremsstrahlung Image Picture of how an x-ray photon is produced

X-ray spectra provide a way to distinguish the radiation emitted by hot, thermal electrons from that emitted by accelerated, nonthermal electrons. The shape of the x-ray spectrum emitted by thermal electrons is distinct from that emitted by the nonthermal electrons. The radiation observed at soft x-ray energies is typically from thermal plasma with a temperature on the order of 10 - 30 million degrees Kelvin. The spectrum consists of a combination of thermal bremsstrahlung and spectral lines from the elements in the hot plasma. The hard x-ray spectrum is dominated by bremsstrahlung from the accelerated electrons and, at the lower hard x-ray energies, thermal bremsstrahlung from plasma with a temperature above around 30 million degrees Kelvin.

Observations of hard x-rays allow us to study the accelerated electrons and the hottest plasma in flares. Theoretical fits to the x-ray spectra provide the quantitative information required to answer the above questions. The relationship between the nonthermal electrons and the hottest thermal electrons can be studied by observing the time evolution of both components during a flare. Likewise, the relationship between these energetic components and somewhat cooler plasma can be studied by comparing the hard x-ray observations with the evolution of the soft x-ray emission. Images of both hard and soft x-rays are crucial for determining where the flare energy is released and sorting out the relationships among the thermal and nonthermal components.


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