Coronal implosion

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Revision as of 17:20, 10 February 2009

Introduction

During flares and coronal mass ejections (CMEs), magnetic reconnection results in the rapid release to the plasma of free energy stored in the large-scale structure of the coronal magnetic field. Specifically, the free energy is believed to be stored in the strongly nonpotential, i.e., sheared or twisted, field of a filament channel. Eruptions ensue if the force balance between the upward magnetic pressure force of the sheared filament channel field, and the downward magnetic tension force of the overlying quasi-potential field, is disrupted. As Hudson (2000) first pointed out, a reduction of magnetic energy during the transients in the flaring region, therefore the reduction of the upward magnetic pressure, would inevitably result in the contraction of the overlying field. The large-scale motions observed in almost all flares or CMEs, however, are explosive rather than implosive, which poses an obvious observational dilemma.

On the other hand, the contraction of flaring loops themselves during the early phase of flares, which is manifested as the converging conjugate footpoints and descending looptop emission, have been reported in X-ray, EUV, H-alpha, and microwave observations. The contraction can be explained by the relaxation of the sheared magnetic field, a scenario consistent with the Hudson conjecture.

One may wonder how the coronal magnetic field responds to the flare loop contraction. In this Nugget, observations of a coronal implosion is presented, showing that the EUV coronal loops overlying an eruptive filament push inward during the early phase of a flare, which is associated with the converging motion of the conjugate HXR footpoints and the downward motion of the HXR looptop source.

Observation

Figure 1 TRACE 171 Å image prior to a GOES C8.9 flare, overlaid with centroid positions of the HXR looptop emission (in a '+' symbol) and of the HXR footpoints (in a 'x' symbol), observed by RHESSI during the flare. The time evolution of the centroid positions is color-coded, and the positions are differentially rotated to 2005 July 30 16:40:34 UT, the time when the TRACE image was taken. A collection of coronal loops, which first contracted, then expanded, and erupted during the flare, is labeled 'L'. A filament appearing in absorption is labeled 'F', which became in emission and erupted with the coronal loops during their expansion phase. A horizontal slit is placed across the group of coronal loops to study their contraction.

The (6.7MB QuickTime) movie shows a collection of criss-cross coronal loops (labeled 'L' in Figure 1) that experience contraction during the early phase of a GOES class C8.9 flare on 2005 July 30. These loops were overlying a dark filament (labelled 'F' in Figure 1). The flare exhibits a relatively long pre-heating phase (Figure 2c), during which RHESSI hard X-ray (HXR) count rates at lower energies (below 25 keV) as well as GOES soft X-ray (SXR) fluxes began to increase gradually as early as 16:40:00 UT, and the flare emission is dominated by a thermal looptop source. The impulsive phase started over 6 min later from 16:46:36 UT onward, during which impulsive HXR bursts (above 25 keV) are observed, and the flare emission is composed of a pair of conjugate footpoints in addition to the looptop emission.

Figure 2 Coronal implosion observed in EUV and X-rays. a) Slices of TRACE 171 Å images cut by the horizontal slit as defined in Figure 1 are rotated to a vertical direction and placed on the time axis. The slice width on the time axis reflects the exposure duration of each image and voids are due to data gaps. b) Time evolution of the height of the HXR looptop emission (in a '+' symbol) as well as the separation between two conjugate HXR footpoints (in an 'x' symbol). c) RHESSI count rates (s^-1) in the 6 - 15 keV (grey) and 25 - 50 keV (black) energy range (scaled by the y-axis on the left), and GOES 1 - 8 Å (0.5 - 4 Å) fluxes in dotted (dashed) line (scaled by the y-axis on the right). Note that the sudden transition in the 6 - 15 keV lightcurve was due to the change of the RHESSI attenuator state from A0 to A1 at 16:44:08 UT, and that the RHESSI observation was discontinued at 16:49:28 UT when RHESSI went into eclipse.

From Figure 1 and Figure 2 one can see that the collection of coronal loops of interest is mainly composed of three clusters of loops at different altitudes. In Figure 2a all three clusters of loops contracted at approximately the same speed (4 - 7 km s^-1), starting at approximately the same time (~16:42 UT). The higher and middle clusters of loops appear to expand first (~16:48 UT) while the lower cluster of loops were still contracting till about 16:53 UT, and then began to expand at about 60 km s^-1. The whole structure, including the filament underneath, was observed to erupt in TRACE 171 &Aring at about 17:02 UT, and resulted in a fast CME. The contraction of the overlying coronal loops is associated with the converging motion of the conjugate HXR footpoints during the impulsive phase, as well as the downward motion of the HXR looptop source toward the solar surface as early as the pre-heating phase (Figure 1 and Figure 2b)

Discussion