Electron Acceleration at a Coronal Shock Propagating Through a Large-Scale Streamer-Like Magnetic Field

Abstract

Using a test-particle simulation, we investigate the effect of large-scale coronal magnetic fields on electron acceleration at an outward-propagating coronal shock with a circular front. The coronal field is approximated by an analytical solution with a streamer-like magnetic field featuring a partially open magnetic field and a current sheet at the equator atop the closed region. We show that the large-scale shock-field configuration, especially the relative curvature of the shock and the magnetic field line across which the shock is sweeping, plays an important role in the efficiency of electron acceleration. At low shock altitudes, when the shock curvature is larger than that of the magnetic field lines, the electrons are mainly accelerated at the shock flanks; at higher altitudes, when the shock curvature is smaller, the electrons are mainly accelerated at the shock nose around the top of closed field lines. The above process reveals the shift of the efficient electron acceleration region along the shock front during its propagation. We also find that, in general, the electron acceleration at the shock flank is not as efficient as that at the top of the closed field because a collapsing magnetic trap can be formed at the top. In addition, we find that the energy spectra of electrons are power-law-like, first hardening then softening with the spectral index varying in a range of −3 to −6. Physical interpretations of the results and implications for the study of solar radio bursts are discussed.

The Acceleration and Confinement of Energetic Electrons by a Termination Shock in a Magnetic Trap: An Explanation for Nonthermal Loop-top Sources during Solar Flares

“…The magnetic structures in the TS region may affect the acceleration and transport of particles near the loop-top region. Especially, a concavedownward magnetic structure is found below the termination shock, which may trap electrons at the looptop, as it is more difficult for particles to travel transverse to the magnetic field than along it Kong et al 2015Kong et al , 2016. Although such a magnetic configuration has been shown in earlier MHD simulations (e.g., Magara et al 1996), its role in confining energetic electrons has not been investigated heretofore.…”

The Acceleration of High-energy Protons at Coronal Shocks: The Effect of Large-scale Streamer-like Magnetic Field Structures

“…In the laboratory frame, the plasma flows radially from the shock center, with the velocity in the shock upstream being zero, i.e., V 1 =0, in the downstream V 2 =V sh (1 − 1/X), and in the shock layer V=V sh −U. Following our previous studies (Kong et al 2015(Kong et al , 2016, the background coronal magnetic field is taken to be an analytical solution of a streamer-like configuration (Low 1986). A brief introduction to the analytical solution is presented in the Appendix.…”

“…Furthermore, recent works have shown that type II radio bursts, which are good observational manifestations of energetic electrons accelerated at shocks, reflect the interaction between shocks and streamers (e.g., Reiner et al 2003;Mancuso & Raymond 2004;Cho et al 2008;Feng et al 2012Feng et al , 2013Kong et al 2012Kong et al , 2015Chen et al 2014) or the shock passing through high-density coronal loops (e.g., Pohjolainen et al 2008;Cho et al 2013). Using test-particle simulations, Kong et al (2015Kong et al ( , 2016 studied the effect of streamer-like magnetic fields on electron acceleration at coronal shocks and found that the closed field lines can trap a significant amount of electrons close to the shock front and lead to an efficient acceleration.…”

Corrugated Features in Coronal-mass-ejection-driven Shocks: A Discussion on the Predisposition to Particle Acceleration

“…In this interaction it is possible to assume the curvature of the shock larger than the curvature of the closed field lines. For this condition, Kong et al (2016) suggest that particles (electrons in Kong et al study) are swept by shock toward the shock flanks where are accelerated.…”