Sunspot rotation, flare energetics, and flux rope helicity: The halloween flare on 2003 October 28

Abstract

We study the X17 eruptive flare on 2003 October 28 using Michelson Doppler Imager observations of photospheric magnetic and velocity fields and TRACE 1600 Å images of the flare in a three-dimensional model of energy buildup and release in NOAA 10486. The most dramatic feature of this active region is the 123° rotation of a large positive sunspot over 46 hr prior to the event. We apply a method for including such rotation in the framework of the minimum current corona model of the buildup of energy and helicity due to the observed motions. We distinguish between helicity and energy stored in the whole active region and that released in the flare itself. We find that while the rotation of a sunspot contributes significantly to the energy and helicity budgets of the whole active region, it makes only a minor contribution to that part of the region that flares. We conclude that in spite of the fast rotation, shearing motions alone store sufficient energy and helicity to account for the flare energetics and interplanetary coronal mass ejection helicity content within their observational uncertainties. Our analysis demonstrates that the relative importance of shearing and rotation in this flare depends critically on their location within the parent active region topology. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.