Distinguishing Between Thermal and Non-Thermal Electrons in Solar Flares Using X-Ray Observations

1. Science Summary A large variety of astrophysical objects are sites of powerful particle acceleration and impulsive energy release. The Sun provides us with an excellent nearby opportunity to study the physical mechanisms that drive these processes. Rapid changes in the highly-stressed magnetic fields of solar flares lead to large-scale energy release of up to ~10 ergs per second. These explosions accelerate electrons up to hundreds of MeV and ions up to tens of GeV. The accelerated electrons are non-thermal, possessing kinetic energies several times the thermal energy of the surrounding plasma. They travel along the field lines and interact with the ambient solar atmosphere, releasing part of their energy as bremsstrahlung X-ray photons. The remaining energy contributes to heating of the atmosphere, driving the plasma temperature to tens of millions of degrees – well above the pre-flare values. Thermal electrons interacting in the hot plasma also emit bremsstrahlung X-rays. We can directly observe the X-ray emission from both populations of electrons. The non-thermal electron bremsstrahlung observed in solar flares has a characteristic power-law spectrum. From the shape, amplitude, and low-energy extent of the photon spectrum we may directly calculate the total energy contained in non-thermal electrons (Brown 1971; Lin 1974) and determine their kinetic energy spectrum (Johns and Lin 1992). The hard X-ray flux observed in many flares implies that the total energy deposited by non-thermal electrons is of the same order as the total energy output of the flare (Lin and Hudson 1971, 1976; Brown 1973). This suggests a direct connection between flare-energy release and particle acceleration. Many flare models also predict that non-thermal X-ray emission should be linearly polarized due to beaming of the electrons. The degree of polarization depends on the directionality and amount of beaming (e.g. Brown 1972). By measuring polarization we can learn about the anisotropy of the electron acceleration and assess the validity of these models. Thermal electron bremsstrahlung has a characteristic quasi-exponential spectrum, in contrast to the non-thermal spectrum. From the shape and amplitude of the thermal continuum, along with an estimate of the source volume, we can calculate the plasma temperature and density, and thereby obtain the total thermal energy of the plasma. This, in turn, allows us to learn how energy is deposited into the solar atmosphere by non-thermal electrons. The high flare temperatures also ionize coronal atoms and excite atomic transitions that are characterized by spectral line emission. In particular, transitions of highly ionized iron (Fe) and nickel (Ni) generate numerous lines that comprise two “complexes” centered at ~6.7 keV and ~8 keV. Recent calculations by Phillips (2004) show that the equivalent widths and integrated fluxes of these complexes are strongly temperature-dependent. Observations of the Fe and Fe/Ni line complexes can thus provide confirmation of the continuum temperature measurement, and can also allow us to probe the Fe abundance in the solar corona.