Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012apj...745...52h&link_type=abstract
The Astrophysical Journal, Volume 745, Issue 1, article id. 52 (2012).
Astronomy and Astrophysics
Astronomy
Acceleration Of Particles, Plasmas, Sun: Flares, Sun: X-Rays, Gamma Rays
Scientific paper
I obtain and examine the implications of one-dimensional analytic solutions for return-current losses on an initially power-law distribution of energetic electrons with a sharp low-energy cutoff in flare plasma with classical (collisional) resistivity. These solutions show, for example, that return-current losses are not sensitive to plasma density, but are sensitive to plasma temperature and the low-energy cutoff of the injected nonthermal electron distribution. A characteristic distance from the electron injection site, x rc, is derived. At distances less than x rc the electron flux density is not reduced by return-current losses, but plasma heating can be substantial in this region, in the upper, coronal part of the flare loop. Before the electrons reach the collisional thick-target region of the flare loop, an injected power-law electron distribution with a low-energy cutoff maintains that structure, but with a flat energy distribution below the cutoff energy, which is now determined by the total potential drop experienced by the electrons. Modifications due to the presence of collisional losses are discussed. I compare these results with earlier analytical results and with more recent numerical simulations. Emslie's conjecture that there is a maximum integrated X-ray source brightness on the order of 10-15 photons cm-2 s-1 cm-2 is examined. I find that this is not actually the maximum brightness and its value is parameter dependent, but it is nevertheless a valuable benchmark for identifying return-current losses in hard X-ray spectra. I discuss an observational approach to identifying return-current losses in flare data, including identification of a return-current "bump" in X-ray light curves at low photon energies.
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