Evidence for the Formation of a Large-Scale Current Sheet in a Solar Flare

We present X-ray evidence for the formation of a large-scale current sheet in a flare observed by the Ramaty High-Energy Solar Spectroscopic Imager on 2002 April 15. The flare occurred on the northwest limb, showing a cusp-shaped flare loop in the rise phase. When the impulsive rise in hard X-rays (>25 keV) began, the cusp part of the coronal source separated from the underlying flare loop and remained stationary for about 2 minutes. During this time, the underlying flare loops shrank at ~9 km s -1 . The temperature of the underlying loops increased toward higher altitudes, while the temperature of the coronal source increased toward lower altitudes. These results indicate that a current sheet formed between the top of the flare loops and the coronal source during the early impulsive phase. After the hard X-ray peak, the flare loops grew outward at ~8 km s -1 , and the coronal source moved outward at ~300 km s -1 , indicating an upward expansion of the current sheet. About 30 minutes later, postflare loops seen in the Solar and Heliospheric Observatory ( SOHO ) EUV Imaging Telescope 195 Å passband rose at ~10 km s -1 . A large coronal looplike structure, observed by the SOHO Large Angle and Spectrometric Coronagraph C2 and C3 detectors, also propagated outward at ~300 km s -1 . These observations are all consistent with the continued expansion of the current sheet.