ar085.hartl01 Posted: 17-Jul-95 Updated: 3-Sep-95, 24-Feb-96, 10-Nov-96 Events specified: N/A
M. D. Hartl, D. E. Gary, and T. Shimizu
Recent radio observations at 17 GHz showed no evidence of non-thermal emission in active region transient brightenings (ARTBs) identified by T. Shimizu from October 91 SXT data. We wish to check this null result by looking at the broader range of frequencies available from
OVRO data. Since OVRO data includes much lower frequencies, which are more sensitive to the presence of non-thermal electrons, the chances will be increased of observing non-thermal emission if it is present. Otherwise we will be able to set a better upper bound on non-thermalemission than has previously been possible with this data.
Shimizu has provided a list of times for the ARTBs he studied, and there is some overlap between the times he considered and OVRO coverage. We already have one good candidate for non-thermal emission from 25 October 91. We would like the SXT data from those times to compare them more directly with the OVRO time sequences.
Unfortunately, the overlap between Shimizu's ARTBs and OVRO data is not great. Since Yohkoh has data from other times that overlap better with OVRO coverage, we would like to look for additional ARTBs at other times and search for corresponding events in microwaves. We would like to consider as many events as possilble to add statistical weight to our study. We will probably select further time periods for study later, but will start with the following dates: 25, 26, 28, and 29 October 1991. The time range of interest is limited to about 16-24 hours UT on each day.
fl145.gopalswamy01 Search for Counterparts of Radio Microbursts in YOHKOH soft X-ray Data.
The following project has been completed. The paper is titled Nonthermal Radio Emission from Solar Soft X-ray Transient Brightenings and is scheduled to appear in ApJ, the Mar 10 issue.
Update 24-Feb-96
This work will shortly be available in preprint form. Briefly, a total of 34 SXT transient brightenings were identified during OVRO observations, and the OVRO data have been searched for corresponding radio events. We find that up to 85% of transient brightenings are either positively or possibly identified with radio events. Only 5 of the 34 events had no radio counterpart in the frequency range 1-18 GHz. In most cases the radio time profile shows a "neupert-like" relationship with the x-ray transient brightening, with the radio peaking during the initial rise of the transient brightening. The radio spectra show a wide variety of spectral shapes, similar to those found in flares and in most cases clearly due to nonthermal electrons. We conclude that soft x-ray transient brightenings are microflares, with all of the properties of flares.
Update 3-Sep-95
Dale Gary and I have completed a study of transient brightenings in NOAA 7172 using Yohkoh data from 20-29 May 1992. An abstract from the report written for my summer research fellowship appears below.
Transient Brightenings in Solar X-rays and Microwaves
Abstract: We have identified 40 transient brightenings (TBs) in the solar active region observed from 20-29 May 1992 using soft X-ray data from the Yohkoh satellite. A comparison with radio data from the Owens Valley Radio Observatory yielded the following new results: 1) radio peaks occur shortly before soft X-ray peak emission in several events; 2) the radio power spectra indicate a nonthermal electron distribution; 3) 70% are smaller TBs too weak to appear in the radio signal. Together these results suggest that TBs are the analogue of solar flares extended below the subflare energy range.
This is work is still continuing. We plan to collaborate with Toshifumi Shimizu to produce a journal article on this research.