The relaxation of sheared magnetic fields: A contracting process

Document Type

Article

Publication Date

5-1-2007

Abstract

In recent years, several authors have reported a contracting motion for solar flaring loops. That is, during the rising phase of solar flares, hard X-ray (HXR) loop-top sources or radio/extreme-ultraviolet (EUV) flaring loops have a descending motion and, at the same time, Hα ribbons or HXR footpoints (FPs) are converging. The usual expansion motion of flaring loops occurs only after the contraction. So far, the contracting motion cannot be fully explained in a two-dimensional flare model. The recent high-cadence Hα observation of an M-class flare made by Ji et al. at GanYu Solar Station of Purple Mountain Observatory may provide a clue leading to a proper understanding. The observations show that the flare shear decreases steadily during both the contraction and expansion phases. In this paper, we provide supporting observational evidence by giving a detailed analysis of the M6.8-class flare of 2003 June 17. For this flare, the EUV flaring loops, Hα ribbons, and HXR loop-top source show well-correlated contraction and subsequent expansion. The flare shear of this event, indicated by Hα ribbons, HXR FPs, and EUV flaring loops, decreases steadily throughout. The observations apparently imply that the contracting motion offlaring loops may be the result of the relaxation of the sheared magnetic field. In the framework of sheared linear force-free arcades, we establish a quantitative model to show that the release of magnetic energy will reduce magnetic shear of the arcades and less sheared arcades will have smaller height and span. © 2007, The American Astronomical Society. All right reserved.

Identifier

34248994888 (Scopus)

Publication Title

Astrophysical Journal

External Full Text Location

https://doi.org/10.1086/513017

e-ISSN

15384357

ISSN

0004637X

First Page

893

Last Page

900

Issue

1 I

Volume

660

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