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Coalescence of Macroscopic Magnetic Islands and Electron Acceleration from STEREO Observation

Hong-Qiang Song (宋红强)1, Yao Chen (陈耀)1, Gang Li (李刚)2,1,*, Xiang-Liang Kong (孔祥良)1, and Shi-Wei Feng (冯士伟)1

  • 1Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, School of Space Science and Physics, Shandong University at Weihai, Weihai, Shandong 264209, China
  • 2Department of Physics and CSPAR, University of Alabama in Huntsville, Huntsville, Alabama 35899, USA

  • *Corresponding author. gang.li@uah.edu
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Phys. Rev. X 2, 021015 – Published 27 June, 2012

DOI: https://doi.org/10.1103/PhysRevX.2.021015

Abstract

Magnetic reconnection is a fundamental plasma process. Recent theoretical studies and numerical simulations have suggested that electrons can be efficiently accelerated in contracting magnetic islands and when magnetic islands coalesce. Using data from the STEREO spacecraft, we report in this article the first observation of macroscopic magnetic-island coalescence and a possible splitting, and the associated electron acceleration. On 24 May 2010, two magnetic islands were observed by both the STEREO-A and STEREO-B spacecraft to propagate out along the current sheet behind a coronal mass ejection (CME) and merge. Electron acceleration to above 10 keV is inferred through the observation of a type-III-like radio burst. The acceleration process occurred at a macroscopic scale, likely during the merging of the two magnetic islands. Our observation of the magnetic-island coalescence is supported by a 2.5-D axisymmetric magnetohydrodynamic simulation of CME in which the merging of post-CME magnetic islands is clearly identified.

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Popular Summary

Despite extensive observation and modeling of explosive astrophysical events that occur in magnetized plasma such as solar flares, the exact mechanisms by which charged particles are accelerated in them remain unclear. Much of the evidence points to magnetic reconnection, a phenomenon that releases large amounts of energy when field lines are bent and stretched to the point where they snap and rejoin. A recently proposed mechanism for electron acceleration at a reconnection site is the so-called magnetic-island-contraction acceleration, where electrons or ions move in contracting magnetic islands, gaining energy by reflecting at the two ends of the magnetic island. In a slightly different scenario where these magnetic islands merge, electrons and ions are also accelerated. Direct detection of electron acceleration in contracting and merging magnetic islands has recently been reported in nearby events occurring in Earth’s magnetosphere. In solar flares and related coronal mass ejections, however, confirmatory observations of contracting and merging magnetic islands and the associated particle acceleration are still lacking. In our paper, we reveal, by combining observational data on a coronal mass-ejection (CME) event with numerical simulations, the merging or coalescence of magnetic islands propagating in the trailing current sheet of the CME that should lead to electron acceleration.

The data we have used comes from the two Solar Terrestrial Relations Observatory (STEREO) spacecraft that were launched in 2006 to study coronal mass ejections, which are immense explosions of matter and radiation from the sun. From the data, we find that, in one such event, blobs of bright high-density material ejected from the sun split and coalesced. Shortly following this coalescence, one of the spacecraft detected radio bursts with a frequency signature characteristic of the plasma emission near the coalescence height. We have also carried out magnetohydrodynamic simulations to further explore the mechanisms occurring in the coalescence. Multiple blobs and blob coalescence are evident in the calculations, and these agree well with the spacecraft observations. Taken together with the simulation results, the data show three distinct reconnection processes: splitting of a blob, coalescence of two blobs, and reconnection of the merged blob field lines with overlying field lines. This last reconnection is believed to have led to the observed radio emission from which electron acceleration is inferred.

Our study provides a valid basis for further theoretical and numerical investigations of blob formation and particle acceleration in energetic magnetic-island interactions.

Article Text

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Phys. Rev. X 2, 021015– Published 27 June, 2012
  • Received 30 October 2011
Article part of CHORUS
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This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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