Simulation of Quiet-Sun Hard X-rays Related to Solar Wind Superhalo Electrons

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 $\gtrsim 1.9$ R${}_{\odot }$ (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, $f\sim {\epsilon }^{-\gamma }$, with an index $\gamma$ $\approx$ 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 $\approx$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.

arXiv comment14 pages, 4 figures

References in corpus (9)

• The Nuclear Spectroscopic Telescope Array (NuSTAR) Mission
• Observations of Subarcsecond Bright Dots in the Transition Region above Sunspots with the Interface Region Imaging Spectrograph
• Collisional relaxation of electrons in a warm plasma and accelerated nonthermal electron spectra in solar flares
• The First Focused Hard X-ray Images of the Sun with NuSTAR
• Constraining the hard X-ray properties of the quiet Sun with new RHESSI observations
• The first X-ray imaging spectroscopy of quiescent solar active regions with NuSTAR
• Measurements of EUV Coronal Holes and Open Magnetic Flux
• Solar Flare Hard X-ray Spectra Possibly Inconsistent with the Collisional Thick Target Model
• Numerical Simulation of Superhalo Electrons Generated by Magnetic Reconnection in the Solar Wind Source Region

Cited by in corpus (4)

• Coronal microjets in quiet-Sun regions observed with the Extreme Ultraviolet Imager onboard Solar Orbiter
• Small electron acceleration episodes in the solar corona
• Multifrequency microwave imaging of weak transients from the quiet solar corona
• Moving Plasma Structures and Possible Driving Mechanisms of Solar Microflares Observed with High-Resolution Coronal Imaging