Observational evidence for breakout reconnection

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Authors: Henry Aurass and Gordon Holman

In solar eruptive events (SEE's, Holman 2012) the transition from slow to fast (eruptive) energy release leads to the interaction of a flux rope with the overlying magnetic field. If the flux rope has a field component opposite to that of the overlying field, magnetic reconnection between both fields can become observable due to the associated electron acceleration and radio emission. Such a breakout reconnection scenario is included in many numerical simulations of SEEs, but observational support for it has so far been weak.

Strong evidence for breakout reconnection has recently been identified in the combined meter-wave radio and RHESSI X-ray observations of a well-observed SEE (Aurass et al. 2013). Figure 1 shows the dynamic radio spectrum (AIP) and the RHESSI 150-300 keV flux curve. The onset of strong particle acceleration is indicated by a light-brown box in Figure 1, and the radio spectrum is enlarged and contrast-enhanced in Figure 2. It depicts the dynamic spectrum of just the feature (a black arrow points on it) shown spatially resolved in Figure 3. Several radio sources appear instead the previous faint one above the active region: we show the radio images at two frequencies (Nançay Multifrequency Radioheliograph, NRH, courtesy: The Radioheliograph Group), as well as the RHESSI hard X-ray images.

The solar eruptive event of November 03, 2003
Figure 1: The dynamic radio spectrum (AIP, top), and the RHESSI 150-300 keV flux (bottom). The light-brown frame points on the time interval enlarged in Figure 2, and presented in spatial resolution in radio (NRH) and X-rays (RHESSI 15-20 keV) in Figure 3.
Figure 2: This (seemingly) minute spectral feature is the first radio signature of the two coronal radio sources in this flare, shown radially elongated above the active region, in heights of roughly 0.18 and 0.41 solar radii above the photosphere in Figure 3.

The two observed coronal X-ray sources serve to locate the two jets above and below the indicated flare reconnection site. Radially above the active region and the X-ray sources (in projection on the plane of sky) two radio sources are located. The lower one is a counterpart of the upper X-ray source. The upper of these two sources (here at NRH 432 MHz) indicates the breakout reconnection together with two other sources seen at lower frequencies (here shown at NRH 236.6 MHz): these flank the upper radially oriented radio source as nicely seen in Figure 3 (solid isolines).

A plausible explanation for this arrangement is that the lower radio source, like the upper X‑ray source, was associated with the upward-directed flare reconnection jet. The upper radio source above the flaring active region remained stationary in space in the next minutes of the flare thus indicating the ongoing breakout reconnection process in that height level of the corona.

Figure 3: Evidence for breakout reconnection in the 2003 November 3 SEE from radio and X-ray data. Contours show radio sources at 432 MHz (dot-dash), and 236 MHz (solid black). The color insert shows RHESSI 15 – 20 keV sources. The two 236 MHz sources would then naturally be associated with the roughly horizontal jets from the breakout reconnection well above the flaring active region.

The configuration given by Figure 3 marks some progress in our understanding of X-ray and radio data during the very onset of eruptive energy release in SEEs: the excellent timing between the occurrence of the radially elongated radio source formation and the "above the HXR loop top source" (Sui et al. 2004) reveals that both sources belong to the same (the upper flare reconnection) hot and turbulent plasma jet. The observations described here for a near-limb event are reminiscent of the results about another SEE seen in projection on the disc where a radio source in a distance of about 0.3 solar radii from the flaring active region occurs simultaneously with the most energetic X-ray and Gamma-ray emission (see Aurass et al. 2011, 2006).

References:

H. Aurass, G. Holman, S. Braune, G. Mann, P. Zlobec: 2013, A&A submitted.

H. Aurass, G. Mann, P. Zlobec, M. Karlicky: 2011, ApJ 730, 57A

H. Aurass, G. Mann, G. Rausche, A. Warmuth: 2006, A&A 457, 681

Holman, G.D.: 2012, Physics Today 04, 56

Sui, L., Holman, G.D., Dennis, B.R.: 2004, ApJ 612, 546

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