Simulation of quiet-sun hard x-rays related to solar wind superhalo electrons

Wang, W., Wang, L., Krucker, S. and Hannah, I. (2016) Simulation of quiet-sun hard x-rays related to solar wind superhalo electrons. Solar Physics, 291(5), pp. 1357-1367. (doi:10.1007/s11207-016-0916-z)

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Abstract

In this paper, we propose that the accelerated electrons in the quiet Sun could collide with the solar atmosphere to emit Hard X-rays (HXRs) via non-thermal bremsstrahlung, while some of these electrons would move upwards and escape into the interplanetary medium, to form a superhalo electron population measured in the solar wind. After considering the electron energy loss due to Coulomb collisions and the ambipolar electrostatic potential, we find that the sources of the superhalo could only occur high in the corona (at a heliocentric altitude ≳1.9 R⊙ (the mean radius of the Sun)), to remain a power-law shape of electron spectrum as observed by STEREO at 1AU near solar minimum (Wang et al., 2012). The modeled quiet-Sun HXRs related to the superhalo electrons fit well to a power-law spectrum, f∼ε−γ, with an index γ ≈ 2.0 - 2.3 (3.3 - 3.7) at 10 - 100 keV, for the warm/cold thick-target (thin-target) emissions produced by the downward-traveling (upward-traveling) accelerated electrons. These simulated quiet-Sun spectra are significantly harder than the observed spectra of most solar HXR flares. Assuming that the quiet-Sun sources cover 5% of the solar surface, the modeled thin-target HXRs are more than six orders of magnitude weaker than the RHESSI upper limit for quiet-Sun HXRs (Hannah et al., 2010). Using the thick-target model for the downward-traveling electrons, the RHESSI upper limit restricts the number of downward-traveling electrons to at most ≈3 times the number of escaping electrons. This ratio is fundamentally different from what is observed during solar flares associated with escaping electrons where the fraction of downward-traveling electrons dominates by a factor of 100 to 1000 over the escaping population.

Item Type:Articles
Keywords:Energetic particles, electrons, corona, quiet, solar wind, flares, x-ray bursts, hard.
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Hannah, Dr Iain
Authors: Wang, W., Wang, L., Krucker, S., and Hannah, I.
College/School:College of Science and Engineering > School of Physics and Astronomy
Journal Name:Solar Physics
Publisher:Springer
ISSN:0038-0938
ISSN (Online):1573-093X
Published Online:26 May 2016
Copyright Holders:Copyright © 2016 Springer Science+Business Media
First Published:First published in Solar Physics 291(5): 1357-1367
Publisher Policy:Reproduced in accordance with the publisher copyright policy

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
617953The many scales of flaring energy release on the SunIain HannahRoyal Society (ROYSOC)UF120063S&E P&A - PHYSICS & ASTRONOMY