Anomalous resistivity in beam-return currents and hard-X ray spectra of solar flares

Abstract

Context. Observations of hard-X ray (HXR) spectra from solar flares show that there is noncollisional energy loss when energetic beam electrons are transported along the flare loop from their acceleration site above the looptop in the corona to the loop footpoints in the chromosphere. Aims. This paper investigates anomalous (i.e., noncollisional) resistivity due to the effective collision by the wave-particle interaction in the beam-return current system of a flare and its effect on the HXR spectral evolution between the looptop and footpoint sources. Methods. To attribute the noncollisional energy loss to an induced electric field by the beam current, the induced electric field is estimated by the spectral evolution between the looptop and footpoint sources, which is deduced from the standard thin-thick target model. To include collisional and anomalous resistivity caused by the ion-acoustic wave turbulence excited by the return current, the necessary excited level and the excited condition are discussed for the steady-state case in which the return current density driven by the induced electric field in terms of Ohm's law is required to be equal to the beam current density. Results. The results show that including the anomalous resistivity can reasonably remove the discrepancy between observations and predictions. Meanwhile, the necessary excited level for the ion-acoustic turbulence is tens times of the thermal noise of electrostatic fluctuations in the background plasma, which is an ordinary and low excited level that is easily satisfied. Conclusions. This indicates that the microscopic kinetics of plasma particles possibly play an important and critical role in understanding the dynamics of beam-return current systems in the solar atmosphere and in the physics of solar flares.

A Unified Computational Model for Solar and Stellar Flares

“…These drifting electrons form the return current, which heats the atmosphere through Ohmic (Joule) dissipation. Many aspects of the beam-return current-atmospheric system have been studied, including pitch angle modifications of the beam (Emslie 1980), return current collisional rates (Karlický et al 2004), return currentbeam instabilities and subsequent particle acceleration (Karlický & Kontar 2012;Pechhacker & Tsiklauri 2014), and turbulent effects on the beam-return current system (Kontar et al 2014;Xu et al 2013). The return current modifications on the classical thick target model (Brown 1971) have been used to explain the difference between looptop and footpoint hard X-ray spectral indices (Battaglia & Benz 2008;Xu et al 2013).…”

A self‐consistent mechanism for electron cyclotron maser emission and its application to type III solar radio bursts

“…This return current can significantly compensate the charge and current carried by FEB. The electrodynamics of the beam‐return current system have been investigated by many authors [ Spicer and Sudan , ; Brown and Bingham , ; van den Oord , ; Xu et al , ; Chen et al , ]. Whatever the specific details of the electrodynamics are, the scenario of the beam‐return current system can be described as follows.…”

“…Moreover, the analysis of energy budget for energetic particles during flares also implies that flares are caused by a current‐carrying circuit system associated with mass‐loaded prominences or with emerging magnetized fluxes but not by only a local reconnection current sheet [ Spicer et al , ]. The formation of the return current and the electrodynamics of the beam‐return current system have been investigated by many authors [ Hammer and Rostoker , ; Spicer and Sudan , ; Brown and Bingham , ; van den Oord , ; Xu et al , ; Chen et al , ]. These theories and observations indicate that the return current can compensate the majority of the beam current in a beam‐return current system.…”

Understanding Breaks in Flare X-Ray Spectra: Evaluation of a Cospatial Collisional Return-current Model

“…Holman (1985) studied the effect of a runaway population of electrons and derived thresholds for producing ion acoustic, electrostatic ion cyclotron, and Buneman instabilities. Following Battaglia & Benz (2008), who concluded that the return current would be unstable to wave growth for one of the two flares they studied, Xu et al (2013) suggested that the return current could be unstable to ion acoustic instability resulting in enhanced (anomalous) resistivity. In this paper we derive a coronal resistivity from the parameters of the spectral fits to the measured x-ray photon spectra using Ohm's law, of which the validity is tested.…”