Above-the-Looptop HXR Source

(Masuda et al., Nature, 371, 495-497, 1994.)

Observations of a solar flare on 13 January, 1992 (17:25 UT) with the Hard X-ray Telescope (HXT) and the Soft X-ray Telescope (SXT) aboard Yohkoh clearly show an impulsive hard X-ray source well above a soft X-ray flaring loop. With no doubt this source reveals some process occurring in the primary energy release of the flare, maybe the magnetic reconnection.

(Masuda et al., Nature, 371, 495-497, 1994.)

It is most likely that this ``loop-top'' hard X-ray source represents the site where the downward plasma outflow, ejected from a reconnection point above the hard X-ray source, collides with an underlying closed magnetic loop. This observation thus shows for the first time where the high-energy electrons are energized.

Related to this event, more detailed studies were achieved.
(1) The 13-Jan-92 flare was analyzed, using precise timing of hard X-ray variability detected by large-area hard X-ray detectors aboard the Compton Gamma-Ray Observatory. Time-of-flight localization of the acceleration site (labeled with a cross) is consistent with the above-the-looptop location of the hard X-ray source observed with Yohkoh/HXT.

(Aschwanden et al., Astrophys. J., 464, 985-998, 1996.)

(2) A temperature map was generated from soft X-ray images and a hard X-ray image (contours) was overlaid on it. This figure shows that the above-the-looptop hard X-ray source is located in the high-temperature (> 20MK) region.

(Tsuneta et al., Astrophys. J., 478, 787-798, 1997.)