Shear Response of a Mass-loaded Corona

Physics

Scientific paper

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7509 Corona, 7513 Coronal Mass Ejections, 7524 Magnetic Fields

Scientific paper

Coronal mass ejections (CMEs) involve the expulsion of some 1016 g of solar material into interplanetary space, at hundreds of kilometers per second. Most CMEs originate in coronal helmet streamers, which feature an overdense helmet surrounding an underdense cavity that often contains a magnetic flux rope. Helmet streamers appear to evolve quasistatically in the days before a CME eruption, often showing only modest structural change. In one common scenario, the energy that powers a CME is stored gradually during this quasistatic evolution, until a triggering event, instability, or loss of equilibrium initiates the mass ejection. Here we present a model for studying the buildup of energy in a helmet streamer. The model includes an overlying helmet region loaded with specified excess mass, and is evolved by shearing the magnetic field underneath the helmet. Unlike force-free fields, in which shearing generally results in an obvious ballooning of the field, we find that the presence of excess mass in the helmet holds down the cavity field as the latter evolves. As a result, changes in the streamer structure are minimized, even as substantial magnetic energy builds in the cavity. We present results showing how the stored energy varies with the shear profile and with the distributions of mass and magnetic flux. This work was supported by NASA grant NAG5-9733 to Middlebury College.

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