Observational evidence for breakout reconnection
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|1st Author:||Henry Aurass|
|2nd Author:||Gordon Holman|
|Published:||2013 March 18|
|Next Nugget:||Antipodal Flares|
|Previous Nugget:||Burst-on-Tail (BOT)|
In solar eruptive events (SEEs; see Ref. ) the transition from slow to fast (eruptive) energy release leads to the interaction of a flux rope with the overlying magnetic field of the large-scale corona. Such a SEE combines the properties of a solar flare and a coronal mass ejection. If the flux rope has a magnetic-field component opposite to that of the overlying field, magnetic reconnection between the flux rope and this overlying field can occur. Such a breakout reconnection scenario, first proposed in 1999 , has been studied in many numerical simulations of SEEs, but observational support for it has so far been weak. Nevertheless, this reconnection should be observable, due to associated acceleration of electrons which produce radio emission.
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 . Figure 1 shows the dynamic radio spectrum (from 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 a seemingly minute feature (the black arrow points to it) shown spatially resolved in Figure 3. Several radio sources appear, instead of a previous faint one, above the active region. We show the radio images at three frequencies (from the Nançay Multifrequency Radioheliograph, NRH), as well as the RHESSI hard X-ray image.
The two observed coronal X-ray sources serve to locate the two jets above and below the indicated flare reconnection site. In projection on the plane of sky, two radio sources are located radially above the active region (432 MHz, red in Figure 3, coinciding with NRH 410 MHz, not shown) and the X-ray sources. The lower one is a counterpart of the upper X-ray source.
The upper of these two sources indicates the breakout reconnection. Two other sources, shown at the lower frequency of 236.6 MHz (green in Figure 3), flank the upper radio sources. The upper radio source at the intermediate frequency of 327 MHz (cyan in Figure 3) is inclined toward North. The observations show that 20 s later (in association with the main pulse of HXR emission; see Figure 1, bottom) the upper red (432 MHz) and the cyan (327 MHz) sources coincide at a location between the positions shown in Figure 3.
A plausible explanation for this arrangement is that the lower radio sources, like the upper X‑ray source, were associated with the upward-directed flare reconnection jet. It is important that the upper radio sources above the flaring active region remained stationary in space in the next minutes of the flare. They indicate the ongoing breakout reconnection process at that height level in the corona. The two 236.6 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 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"  reveals that both sources belong to the same (upper flare reconnection) hot and turbulent plasma jet. The observations described here for a near-limb event are reminiscent of the results obtained for another SEE seen in projection on the disc, where a radio source at a distance of about 0.3 solar radii from the flaring active region was observed simultaneously with the most energetic X-ray and Gamma-ray emission .
 [Radio evidence for breakout reconnection in a solar eruptive event]