Motion of Flare Footpoint Emission and Inferred Electric Field in Reconnecting Current Sheets

Abstract

A systematic motion of Hα kernels during solar flares can be regarded as the chromospheric signature of progressive magnetic reconnection in the corona, in that the magnetic field lines swept through by the kernel motion are those connected to the diffusion region at the reconnection point. In this paper, we present high-cadence and high-resolution Hα-1.3 A observations of an impulsive flare that exhibits a systematic kernel motion and relate them to the reconnecting current sheet (RCS) in the corona. Through analyses of X-ray and microwave observations, we further examine the role of the macroscopic electric field inside the RCS in accelerating electrons. We measure the velocity of the kernel motion to be 20-100 km s-1. This is used together with the longitudinal magnetic field to infer an electric field as high as 90 V cm-1 at the flare maximum. This event shows a special magnetic field configuration and motion pattern of Hα kernels, in that a light bridge divides a flare kernel into two parts that move in different manners: one moving into the stronger magnetic field and the other moving along the isogauss contour of the longitudinal magnetic field. The temporal variation of the electric field inferred from the former type of kernel motion is found to be correlated with 20-85 keV hard X-ray light curves during the rise of the major impulsive phase. This would support the scenario of magnetic energy release via current dissipation inside the RCS, along with the hypothesis of the DC electric field acceleration of X-ray-emitting electrons below 100 keV. However, there is no good temporal correlation between the hard X-ray emission and the inferred electric field from the other motion pattern. Furthermore, the microwave emission, which supposedly comes from higher energy electrons, shows a time profile and electron spectrum that differs from those of the X-ray bursts. We conclude that either the two-dimensional magnetic reconnection theory related to the Hα kernel motion is applicable to only some part of the flare region due to its special magnetic geometry, or the electron acceleration is dominated by other mechanisms depending on the electron energy.