The Importance of Electron-Electron Bremsstrahlung in Solar Flares
Solar flares, magnificent and powerful bursts of energy from our closest star, have fascinated scientists and astronomers for centuries. These explosive events release vast amounts of radiation, including high-energy X-rays. Understanding the mechanisms responsible for the production of X-rays during a solar flare is crucial for unraveling the mysteries of these extraordinary phenomena.
Traditionally, the role of electron-ion bremsstrahlung in generating X-rays during a solar flare has been the focus of study. This process involves the interaction between accelerated electrons and ions in the solar atmosphere, resulting in the emission of photons across a wide range of energies. However, an important aspect has been largely overlooked – electron-electron bremsstrahlung.
Electron-electron bremsstrahlung, as its name suggests, involves the interaction between accelerated electrons themselves, giving rise to X-ray emission. While electron-ion bremsstrahlung can produce photons of all energies up to the maximum electron energy involved, electron-electron bremsstrahlung has a different behavior. The maximum photon energy that can be produced depends on the angle between the direction of the emitting electron and the emitted photon.
This distinction between electron-ion and electron-electron bremsstrahlung opens up new possibilities for understanding and characterizing the X-ray emission from solar flares. By considering the inclusion of electron-electron bremsstrahlung, we can gain insights into the upper cutoff energy of photons and the beaming of the accelerated electrons.
To explore this concept further, we analyze a significant solar flare event that occurred on January 17, 2005. Using data from the RHESSI satellite, we investigate the observed hard X-ray spectrum and its connection to electron-electron bremsstrahlung. Our findings reveal a clear upward break in the spectrum around 400 keV, a feature that can be explained by the inclusion of electron-electron bremsstrahlung.
By employing a regularized inversion technique, we reconstruct the underlying electron spectrum responsible for the observed X-ray emission. This analysis, using a cross-section that incorporates both electron-ion and electron-electron terms, uncovers a relatively constant spectral index over a wide range of electron energies. However, the available data does not provide sufficient detail to determine the presence or absence of an upper cutoff energy in the electron spectrum.
Through this study, we highlight the significance of including electron-electron bremsstrahlung in the understanding of X-ray production in solar flares. The insights gained from this research have implications for our understanding of high-energy processes in astrophysics and can help refine models of solar flares, ultimately advancing our understanding of these complex phenomena.
Abstract: Although both electron-ion and electron-electron bremsstrahlung contribute to the hard X-ray emission from solar flares, the latter is normally ignored. Such an omission is not justified at electron (and photon) energies above $sim 300$ keV, and inclusion of the additional electron-electron bremsstrahlung in general makes the electron spectrum required to produce a given hard X-ray spectrum steeper at high energies.
Unlike electron-ion bremsstrahlung, electron-electron bremsstrahlung cannot produce photons of all energies up to the maximum electron energy involved. The maximum possible photon energy depends on the angle between the direction of the emitting electron and the emitted photon, and this suggests a diagnostic for an upper cutoff energy and/or for the degree of beaming of the accelerated electrons.
We analyze the large event of January 17, 2005 observed by RHESSI and show that the upward break around 400 keV in the observed hard X-ray spectrum is naturally accounted for by the inclusion of electron-electron bremsstrahlung. Indeed, the mean source electron spectrum recovered through a regularized inversion of the hard X-ray spectrum, using a cross-section that includes both electron-ion and electron-electron terms, has a relatively constant spectral index $delta$ over the range from electron kinetic energy $E = 200$ keV to $E = 1$ MeV. However, the level of detail discernible in the recovered electron spectrum is not sufficient to determine whether or not any upper cutoff energy exists.