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

Abstract

A systematic motion of Ha kernels during solar Ñares can be regarded as the chromospheric signature of progressive magnetic reconnection in the corona, in that the magnetic Ðeld lines swept through by the kernel motion are those connected to the di †usion region at the reconnection point. In this paper, we present high-cadence and high-resolution Ha[1.3 observations of an impulsive Ñare that exhibits a 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 Ðeld 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 Ðeld to infer an electric Ðeld as high as 90 V cm~1 at the Ñare maximum. This event shows a special magnetic Ðeld conÐguration and motion pattern of Ha kernels, in that a light bridge divides a Ñare kernel into two parts that move in di †erent manners : one moving into the stronger magnetic Ðeld and the other moving along the isogauss contour of the longitudinal magnetic Ðeld. The temporal variation of the electric Ðeld inferred from the former type of kernel motion is found to be correlated with 20È85 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 Ðeld acceleration of X-rayÈemitting electrons below 100 keV. However, there is no good temporal correlation between the hard X-ray emission and the inferred electric Ðeld from the other motion pattern. Furthermore, the microwave emission, which supposedly comes from higher energy electrons, shows a time proÐle and electron spectrum that di †ers from those of the X-ray bursts. We conclude that either the twodimensional magnetic reconnection theory related to the Ha kernel motion is applicable to only some part of the Ñare region due to its special magnetic geometry, or the electron acceleration is dominated by other mechanisms depending on the electron energy.

Non-LTE Calculation of the N[CLC]i[/CLC] [CSC]i[/CSC] 676.8 Nanometer Line in a Flaring Atmosphere

“…Recent investigations (e.g., Kosovichev & Zharkova 2001;Wang et al 2002;Qiu & Gary 2002) mostly employ magnetograms observed by the Michelson Doppler Imager (MDI) instrument on board the Solar and Heliospheric Observatory (SOHO), which uses the Ni i 676.8 nm line to measure the velocity field and magnetic field (Scherrer et al 1995). Formation of this line in the quiet-Sun atmosphere was investigated in detail by Bruls (1993), whose results confirm that the line is a good choice for study of helioseismology.…”

“…In some cases, there appears a transient sign reversal of the magnetic fields in sunspots (Patterson 1984;Qiu & Gary 2002). Qiu & Gary (2002) discovered that the transient sign reversal occurs in locations of strong hard X-ray emission and that the time profile of the reversed magnetic flux coincides well with the hard X-ray light curve. Patterson (1984) proposed that the magnetic sign reversal is produced by transient emission of the iron line used by the Big Bear Solar Observatory magnetograph.…”

“…Transient magnetic field changes, or magnetic transients, that are associated with solar flares have been observed by groundbased observatories (Patterson & Zirin 1981;Patterson 1984) and more recently by space instruments (Kosovichev & Zharkova 2001;Qiu & Gary 2002). In some cases, there appears a transient sign reversal of the magnetic fields in sunspots (Patterson 1984;Qiu & Gary 2002).…”

“…Patterson (1984) proposed that the magnetic sign reversal is produced by transient emission of the iron line used by the Big Bear Solar Observatory magnetograph. Qiu & Gary (2002) simulated in detail the influence of the line shape on the measurements and concluded that only an emission profile can lead to the observed magnetic sign reversal.…”

Quiescent Reconnection Rate Between Emerging Active Regions and Preexisting Field, with Associated Heating: NOAA AR 11112

“…Such studies of rapid reconnection during flares, in particular those using the evolution of flare ribbons overlaid on cotemporal magnetograms (Forbes and Priest, 1984;Poletto and Kopp, 1986;Fletcher and Hudson, 2001;Qiu et al, 2002;Qiu et al, 2007;Longcope et al, 2007;Qiu, 2009;Longcope et al, 2010;Kazachenko et al, 2010;Kazachenko et al, 2012), are the most direct antecedents of the present study. These two-ribbon flares generally follow the standard CSHKP flare model (See Priest and Forbes, 2002 for a review).…”