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Quadrupolar Magnetic Reconnection in Solar Flares: I. 3D Geometry inferred from Yohkoh Observations View all abstracts by submitter

Markus J Aschwanden   Submitted: 2000-01-25 14:02

We analyze the 3-dimensional (3D) geometry of solar flares that show so-called {sl interacting flare loops} in soft X-ray, hard X-ray, and radio emission, as previously identified by Hanaoka and Nishio. The two flare loops that appear brightest after the flare are assumed to represent the outcome of a quadrupolar magnetic reconnection process, during which the connectivity of magnetic polarities is exchanged between the four loop footpoints. We parametrize the 3D geometry of the 4 involved magnetic field lines with circular segments, additionally constrained by the geometric condition that the two pre-reconnecti- pre-reconnection field lines have to intersect each other at the onset of the reconnection process, leading to a 10-parameter model. We fit this 10-parameter model to Yohkoh SXT and HXT data of 10 solar flares and determine this way the loop sizes and relative orientation of interacting field lines before and after reconnection. We apply a flare model of Melrose to calculate the magnetic flux transfer and energy released when two current-carrying field lines reconnect to form a new current-carrying system in a quadrupolar geometry. The findings and conclusions are: (1) The pre-reconnection field lines always show a strong asymmetry in size, consistent with the scenario of new-emerging small-scale loops that reconnect with pre-existing large-scale loops. (2) The relative angle between reconnecting field lines is near-collinear in half of the cases, and near-perpendicular in the other half, contrary to the anti-parallel configurati- configuration that is considered to be most efficient for magnetic reconnection. (3) The angle between interacting field lines reduces by approx 10°-50^- 10°-50° after quadrupolar reconnection. (4) The small-scale flare loop experiences a shrinkage by a factor of 1.31pm0.44, which is consistent with the scaling law found from previous electron time-of-flight measurements, suggesting that electron acceleration occurs near the cusp of quadrupolar configurations. (5) The large-scale loop is found to dominate the total induction between current-carrying loops, providing a simple estimate of the maximum magnetic energy available for flare energy release due to current transfer, which scales as Delta E^I approx 1029.63 (r_2/109 { m cm}) (I_2/1011 A)^2, (with r2 the curvature radius and I2 the current of the large-scale loop) and is found to correlate with observed flare energies deduced from soft X-ray and hard X-ray fluxes. Most of the energy is transferred to small-scale loops that have half of the large-scale current (I_1=I_2/2). (6) The quadrupolar reconnection geometry provides also a solution of {sl Canfield's dilemma} of the offset between the maximum of vertical currents and the HXR flare loop footpoints. (7) The quadrupolar geometry provides not only a framework for interacting double-loop flares, but can also be considered as a generalized version of (cusp-shaped) single-loop flares.

Authors: Aschwanden,M.J., Kosugi,T., Hanaoka,Y., Nishio,M., and Melrose,D.B.
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Publication Status: Astrophys.J. 1999, Vol. 526, p. 1026-1045.
Last Modified: 2000-01-25 14:02
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