Photospheric Response to a Flare

Abstract

Flares produce sudden and permanent changes in the horizontal photospheric magnetic field. In particular flares generally produce increased magnetic shear in the photospheric field along the neutral line. Recent observations show also that flares can produce sudden photospheric motion. We present a model for the observed changes as the response of the photosphere to a large-amplitude shear Alfvén wave propagating down from the corona on either side of the neutral line. The Alfvénic front is assumed to impact the photosphere close to the neutral line first, and then successively further away with time, such that the line of impact coincides with the flare ribbon. The wave introduces magnetic shear and velocity shear. The magnetic shear introduced at the photosphere has the same sign on either side of the neutral line, while the velocity shear has the opposite sign. We discuss the possibility that this process is responsible for particle acceleration in flares.

Comparative study of electric currents and energetic particle fluxes in a solar flare and Earth magnetospheric substorm

“…For the interpretation of changes in the magnetic fields and vertical currents in the investigated flare of June 22, 2015, Wheatland et al (2018) have proposed a simplified analytical model based on the response of the photosphere to a large-amplitude Alfven wave propagating downward from the energy release cite in the corona. The wave brings magnetic and velocity shear into the photosphere, which can qualitatively explain the observed changes.…”

“…The wave brings magnetic and velocity shear into the photosphere, which can qualitatively explain the observed changes. Wheatland et al (2018) also raised the issue that electrons can be accelerated in the field-aligned electric field at the front of this large-scale Alfven wave. To fulfill the condition that this field-aligned electric field exceeds the Dreicer field (when electrons can be effectively accelerated in the runaway mode), the front width should be rather narrow, ≈ 10 m. However, the authors mentioned that in the presence of an anomalous resistivity due to turbulence or micro-scale structures, the required front thickness could be much larger.…”

“…To fulfill the condition that this field-aligned electric field exceeds the Dreicer field (when electrons can be effectively accelerated in the runaway mode), the front width should be rather narrow, ≈ 10 m. However, the authors mentioned that in the presence of an anomalous resistivity due to turbulence or micro-scale structures, the required front thickness could be much larger. Although large-scale Alfven wave transformation into kinetic/inertial Alfven waves was not considered by Wheatland et al (2018), the general issue of energy cascade from the large to small scales in Alfven wave turbulence is quite similar for both magnetotail and solar corona systems.…”

“…Various aspects of this flare have already been studied and discussed in more than a dozen works (e.g. Jing et al 2016;Liu et al 2016;Bi et al 2017;Vemareddy 2017;Wang et al 2017;Kuroda et al 2018;Liu et al 2018;Wang et al 2018a;Wheatland et al 2018;Xu et al 2018;Kang et al 2019). The flare attracted widespread attention, first of all, as it was very well observed by a number of modern instruments.…”

“…Due to presence of strong magnetic shear (it can be seen as the enhanced horizontal magnetic field component around the PIL in Fig. 8(c); see also Wang et al 2017;Wheatland et al 2018;Kang et al 2019), currents in the photosphere are not directly connected by cross-field currents (as opposed to the ionosphere, see Fig. 4(a)), but rather connected by currents flowing almost along the PIL.…”