AN INVESTIGATION INTO THE NON-THERMAL VELOCITY OF B CLASS FLARES OBSERVED BY THE BRAGG CRYSTAL SPECTROMETER ON YOHKOH. 

fl193.harra_murnion01
Posted:  05-Dec-94
Updated: 22-Aug-95, 7-May-96, 27-Jan-97
Events specified: N/A

L.K. Harra-Murnion (MSSL), T. Watanabe (NAOJ), J.L. Culhane (MSSL), S. Yuda (NAOJ), M. Takahashi (NAOJ).

Very little work has been carried out regarding non-thermal velocities during the lifetime of the Yohkoh mission. Part of the reason for this is due to the instrumental problem of line narrowing which affects the measurements of line widths, especially in channel 4. This affect increases with increasing counts. To avoid this problem we intend to derive the non-thermal velocity for B class flares observed during the year of 1994. During this period, the solar activity is much reduced with the quiescent level of the GOES 1-8 A channel ranging between A and low B level. Hence background contamination and line narrowing which affect the line widths is lessened. We also will investigate the hard X-ray emission using results from the Hard X-ray Telescope on Yohkoh. An investigation into the relative timings of soft and hard X-rays will be carried out (if the HXR emission is significant). Our purpose is to determine whether subflares have the same characteristics of non-thermal velocities and HXR emission as 'normal' more energetic flares.

27-Jan-97

The work from proposal fl193_harra_murnion01 has been accepted to ApJ. The abstract follows;

Previous examinations of line broadening in solar flares have tended to concentrate on large GOES ({\it Geostationary Operational Environmental Satellites}) classification events. Due to the sensitivity of the \yohkoh\ Bragg Crystal Spectrometer, it is possible to observe small GOES flares. We derive the non-thermal velocity for 45 small GOES flares observed during the year of 1994 observed by \yohkoh\ concentrating on the peak and rise phase of the non-thermal velocity (\vnt). We examine the variation of \vnt\ with electron temperature, GOES classification, duration of event, rise time, and the source size of the event. The results demonstrate that the broadening is independent of the flare size, complexity and intensity of hard X-ray bursts. There is a weak dependence on duration and rise time: as the durations and rise times become longer, the value of \vnt\ decreases. There is also a trend of increasing \vnt\ with increasing temperature for the same ion.

Update 7-May-96

Regarding our proposal fl193, we are putting the finishing touches to a journal paper. It shall be submitted to ApJ in the next few weeks. The abstract for this paper is as follows;

L.K.Harra-Murnion, K.Akita & T.Watanabe

Previous examinations of line broadening in solar flares have tended to concentrate on large GOES ( Geostationary Operational Environmental Satellites) classification events. Due to the sensitivity of the yohkoh Bragg Crystal Spectrometer, it is possible to observe small GOES flares. We derive the non-thermal velocity for 45 small GOES flares observed during the year of 1994 observed by yohkoh concentrating on the peak and rise phase of vnt We examine the variation of vnt with electron temperature, GOES classification, duration of event, rise time, and the source size of the event. The results demonstrate that the broadening is independent of the flare size and complexity. There is a weak dependence on duration and rise time: as the durations and rise times become longer, the value of vnt decreases. There is also a trend of increasing vnt with increasing temperature for the same ion.

Update 22-Aug-95

Bflare update;

Looked at events with both background removed and not removed. To compare, as the actual temp is probably between the 2 values. The largest difference is ~ 2MK, and for the non-thermal velocity it can make a difference of ~20km/sec

Following discussions with Kyo Akita (additional author) ;

32 B flares have been analysed looking at their Vnt, Te in Sxv. Unfortunately there are no available counts in HXT/BATSE to enable us to study the timings of the soft and hard X rays for such small events. The results from these suggest; 

  1.  no dependence of Vnt on the duration of the flare     
    
  2.  no dependence of Vnt on the rise time of the flare     
    
  3.  no dependence of Vnt on the intensity of the flare     
    
  4.  no dependence of Vnt on the temperature of the flare, but we know     
there is a dependence on ion - hence this would suggest a dependence
on Z and not temperature.
  • hint of a dependence of the difference between the time of the peak of the count rate and the peak of the Vnt, and the rise time of the flare.

    We shall extend the study to C class flare, so that it will be possible to study the timings and variations of HXRs and Vnt.

    In summary, the work to be carried out consists of;

    1. variation of Vnt with Te/Z - looking at other channels in BCS should determine this
    2. variation of Vnt with flare size - nothing as far as I know has been done on this - usually larger flares have been studied
    3. variation of Vnt across limb - Mariska studied the moment delta
    lambda ^2 and found no variation. However we want to re-look at this after subtracting instrumental, thermal, and source size affects. Don't expect a different result to him, but there have been surprises lately!
    1. variation of Vnt with the duration of the event - is it different for compact flares or LDEs? Suggestion so far is not - more events need to be looked at.
    2. variation of Vnt with HXR counts
    3. variation of Vnt with the power-law index

    The initial results will be presented in the Cool star meeting in Florence in October.