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The temperature map of the loop and coronal sources of an occulted flare  

Youping Li   Submitted: 2006-10-16 23:09

An occulted flare occurred at about 06:07 UT on 2002 November 2. The RHESSI X-ray images show two separate parts. The lower part consists of a complete loop and the upper part a coronal source which well extends above the solar limb. The loop source shrank for about 3 minutes with a speed of ??24 km s-1 during the early impulsive phase and then expanded at ??7 km s-1, while the coronal source presented an upward motion at about 6 km s-1. We got the temperature map of the two sources from RHESSI image spectrum. The temperature of the loop increases with altitude, while for the coronal source the temperature decreases with altitude. These results indicate that the reconnection X-point of this flare is located between the looptop source and the coronal source.

Authors: Y. P. Li, W. Q. Gan
Projects: RHESSI

Publication Status: cospar 2006 (submitted)
Last Modified: 2006-10-17 10:57
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On the motions of RHESSI flare footpoints  

Youping Li   Submitted: 2006-10-16 23:07

The footpoint motions of flare HXR sources are directly related to the reconnection scenario of a solar flare. In this work, we tried to extract the information of footpoint motions for a number of flares observed with RHESSI. We found that the RHESSI flare results of the footpoint motions strongly support the classification proposed from the observations of HXT/YOHKOH. Furthermore, it is found that a flare can consist of two types of footpoint motions. We discussed the connections of the footpoint motions with the two-dimensional reconnection models.

Authors: W. Q. Gan, Y. P. Li, L. I. Miroshnichenko
Projects: RHESSI

Publication Status: cospar 2006 (submitted)
Last Modified: 2006-10-17 10:57
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A STATISTICAL STUDY OF RHESSI FLARES  

Youping Li   Submitted: 2006-10-16 23:01

Astatistical analysis of RHESSI X-ray flares in the 12?25 keV band during the period from February 2002 to June 2005 is presented.We found that a power-law with an index of 1.80?0.02 can fit well the frequency distribution of the peak count rates. This power-lawdoes not change significantly with time. However, the frequency distribution of the flare durations cannot be fitted well by a single power-law. There is a weak correlation between the peak count rates and the characteristic times like rise times, decay times, or durations. But the correlation between the rise times and decay times seems to be strong. We discuss the results obtained and compare them with previous works. The frequency distribution of rise times for the sub-group events with a similar magnitude of peak count rates is also shown. In particular, we propose a new parameter Ra, the growth factor of the count rate, defined as the peak count rate divided by the rise time, to reflect the characteristics of the rising phases of flares. The distribution of Ra is shown and discussed.

Authors: YANG SU, W. Q. GAN and Y. P. LI
Projects: RHESSI

Publication Status: Sol. Phys. (in press)
Last Modified: 2006-10-17 18:52
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ON THE PEAK TIMES OF THERMAL AND NONTHERAL EMISSIONS IN SOLAR FLARES  

Youping Li   Submitted: 2006-10-16 22:58

We selected a sample of 859 flares observed in hard X-rays (HXR) by BATSE and in soft X-rays (SXR) by GOES to study the timing of the SXR and HXR emission. Each sample event presents a simple lightcurve, i.e., the HXR has only one short time peak and the SXR seems to be a response to the pulse HXR emission. The statistical study shows that the peak time differences between the SXR and HXR emissions conform a decay exponential function. This distribution, being similar to that for the flaring loop size discovered previously, suggests that the flaring loop size is the reason for the peak time difference between the SXR and HXR emissions. The longer the flaring loop is, the longer the time for chromospheric evaporation reaching the looptop is, i.e., the later the SXR peaks. Our result here provides a support for the traditional chromospheric evaporation model. The fact that the SXR can increase for some time after the end of HXR emission does not seem to be in contradiction with the Neupert effect, if one considers that the electron-driven evaporation needs some time to reach the looptop. Further hydrodynamic modeling is obviously necessary to explain the observations.

Authors: Y. P. Li and W. Q. Gan
Projects:

Publication Status: ApJ (in press)
Last Modified: 2006-10-17 18:52
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Abstracts by Author
The temperature map of the loop and coronal sources of an occulted flare
On the motions of RHESSI flare footpoints
A STATISTICAL STUDY OF RHESSI FLARES
ON THE PEAK TIMES OF THERMAL AND NONTHERAL EMISSIONS IN SOLAR FLARES

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