ar028.shimizu03 Posted: 05-Aug-92 Updated: 11-Mar-93, 06-Nov-93, 28-Aug-94, 20-Nov-94, 27-May-95, 3-Feb-96, 12-Oct-96 Events specified: AR 7260
Collaboration: T.Shimizu, S.Tsuneta, H.Hudson and others
Motivation: Lin et al.(1983, 1990) found short-lived small energy releases in the corona called as "micro-flares" with their high sensitive hard X-ray detectors. Their observations suggested that the micro-flares may contribute to the coronal heating, especially the active region heating. On the other hand, Hudson(1991)'s work about the flare distribution suggested that the released energy supplied by micro-flares is not enough to contribute to the coronal heating. The SXT observation, which provides us the high sensitivity for faint events with high temporal and spatial resolution, is the best for surveying such small events. The SXT reveals that active regions show many transient brightenings which supply mass and energy into the active corona and that they are common phenomena in active regions (Shimizu et al. 1992). To know whether the active region transient brightenings can contribute to the active region heating, we will search how many events occur with the specified released energy. The final goal of this work is to make the relation diagram between the released energy and the number of brightenings in each active region and to discuss whether the transient brightening(micro-flares) can contribute to the coronal heating in the active region.
Observations:
PFI images will be mainly used.
FULL spatial resolution, HIGHer time resolution
2x2 mossaic size
Quiet mode
Update 12-Oct-96
A paper entitled "DEEP SURVEY OF SOLAR NANO-FLARES WITH YOHKOH" is being submitted to ApJ. The abstract of the paper is as follows:
Short time-scale variability fainter than transient brightenings (microflares) is found in the solar position-dependent light curves observed with the Yohkoh Soft X-Ray Telescope. The time variability is found almost everywhere in active regions and X-ray bright points, while no significant variability is found in quiet regions. An intensity correlation is found between the magnitudes of the time variability and the intensities of the persistent corona. The time variability is apparently related to the heating mechanism of the persistent active-region corona. The intensity correlation can be explained with the idea that the persistent corona is made of extremely numerous nanoflares, larger ones of which are observed as the time variability. The alternative explanation is that a common parameter controls both the persistent corona and the time variability.
Update 3-Feb-96
I've been still investigating what tiny brightness increases in soft X-ray light curves of macro-pixels are. I've used soft X-ray images of AR 7558 on 6 and 8 August 1993. By visual inspection of the movies, I studied what X-ray features correspond to the macro pixels showing the tiny brightness increases and how the increases are identified in the movies. This study shows that many of the increases are observed in the pixels where long-lived bright loops or fainter loop structures are observed. The brightness increases are observed either in a part of the structures or in the whole, although the boundary of the increases is not apparent unlike transient brightenings (microflares).
Update 27-May-95
The following is the progress report for ar028.shimizu03
1, The paper entitled "Energetics and Occurrence Rate of Active-Region Transient Brightenings and Implications for the Heating of Active Region Corona" was published recently: Shimizu, T. 1995, PASJ, Vol.47, p251-263.
2, In the above paper, we have concluded that we need weaker events with occurrence rate much higher than the extraporated power-law to explain the heating of active region corona with transient brightenings. This means that it's more important to investigate weaker events than transient brightenings for the study of coronal heating. Using "macro-pixel" light-curve analysis developed by Shimizu (1995), we are now studying the weaker events; Macro-pixel light curves show the existance of numerous tiny soft X-ray intensity enhancements, which are not identified as being transient brightenings by visual inspection. The nature of these tiny enhancements and their implications for the heating of actie region corona are now in investigation. We use time sequences of soft X-ray images of AR 7558 obtained on 6 and 8 August 1993, during which the exposure duration of the images are fixed at 168 msec.
Update 20-Nov-94
I submitted a full-length paper to PASJ. This paper is related to the proposal AR028. AR028 study will be still in progress: From the survey on the light curves of the macro-pixel brightness we found many tiny enhancements which are not detectable by the visual inspection. We wonder if these tiny enhancements may help the understanding of tiny energy releases (nanoflares). We will study the nature of these tiny events for the next a few months. The following is the abstract of the submitted paper. The copies of the paper will be sent by postail mail to SXT DUCs and others.
by Toshifumi SHIMIZU
Institute of Astronomy, The University of Tokyo,
Mitaka, Tokyo 181
submitted on 15 Nov, 94 to Publ. Astron. Soc. Japan.
Abstract: Frequent transient brightenings are discovered in solar active regions by the Yohkoh Soft X-ray Telescope. We examine the possibility that these transient brightenings contribute to the heating of active region corona. The imaging observations provide the following physical conditions; temperature 4 - 8 MK, emission measure 10^44.5 - 10^47.5 cm^-3, electron density 2 x 10^9 - 2 x 10^10 cm^-3, gas pressure 5 x 20 dyne cm^-2, loop length 5 x 10^3 - 4 x 10^4 km, loop width 2 x 10^3 - 7 x 10^3 km, and duration 2 - 7 min. The energy involved in the transient brightenings is estimated to range from 10^25 to 10^29 ergs. The frequency distribution as a function of energy can be represented by a single power-law with an index 1.5 - 1.6 in the energy range greater than 10^27 erg, although the distribution deviates from the power-law in the energy range smaller than 10^27 erg due to the instrument dead time and/or the obscuration by bright coronal features. The single power-law of the frequency distribution continues from the solar flare range down to the energy of 10^27 erg. The total energy supplied by the transient brightenings, assuming that the power-law continues to lower energy, is estimated to be a factor of 5 smaller than the heating rate required for active region corona. We need weaker events with occurrence rate much higher than the extraporated power-law to explain the heating of active region corona with transient brightenings.
Update 28-Aug-94
Energetics and occurrence rate of active-region transient brightenings are studied with ~300 events appeared in NOAA 7260. Now we are preparing a full-length paper of this topics (now the paper draft is in check by one of coauthors), which will be submitted to PASJ at the beginning of this September.
I informed you on 28-Aug-94 that we would submit a full-length paper regarding to this project to PASJ by the beginning of the September. I, however, do not submit it until now. This is due to the following reason: In the paper, we use the visual inspection and macro pixel methods to search transient brightenings. On the preparation of the paper, I re-examined the occurrece rate of TBs with the macro-pixel method. The occurate rate obtained by this method is slightly steeper power-law distribution than that obtained previously. So that, further investigations are now in progress with the modification of the macro- pixel method to minimize the event-splitting effect. I will submit a paper as soon as possible after completing the investigations.
Update 06-Nov-93
By using the sequence of PFI images obtained from Aug 15 to 20, 1992, which target is mostly NOAA 7260, we have derived differential distributions of occurrence rate of transient brightenings. To pick up events without missing faint ones, we applied two independent methods: Visual Inspection Method and Macro Pixel Method. Our result in the first confirms that almost the same slope (-1.5) of the power-law distribution falls down to the range of transient brightenings (about 10^24 to 10^28 ergs) and that there is no significant increase in the occurrence rate of transient brightenings. It means that transient brightenings or microflares have no significant contribution to the heating of active region corona. This result has been presented by me on the following meetings: in the international meeting held at Kofu city on Sep. 1993 and in ASJ fall meeting held at Kagoshima Univ. on Oct. 1993. We are now writing a paper to be submitted to one of journals.
Update 11-Mar-93
To know the distribution function of occurrence rate of active-region transient brightenings, we used a smaple of 142 events in NOAA6891, which were used in statistical analysis from viewpoints of morphology and morphological evolution (AR027 topic). The derived distribution shows that intense brightenings have power-law distribution with the slope about -1. The slope of their distribution, however, becomes flatter in smaller peak X-ray flux apparently because of the following difficulities; 1), not enough temporal resolution makes long dead-time which loses small short-lived events, 2) changing exposure time controlled by AEC gives different sensitivities in individual image frames, 3) bright background structures may obscure faint events. To avoid the above difficulites, NOAA7222 region was used to decide the distribution function. Until now, we examined 22 events appearred during the period from 8:24 to 8:56 on 14 Jul,1992. The distribution between soft X-ray peak intensity and the integrated number of event shows clearly power-law distribution with the slope of -0.7. This result suggests that the power-law distribution of solar flares continues with the same slope in the range of transient brightening (microflare). As the SXT gives us T/EM of transient brightenings, we estimated the energy input from each transient brightening. Then we plotted the relation between the energy input and the integrated number of events, showing two power-law components, which are the slope of -0.7 in intense events and that of -0.5 in faint events. This results indicate intense brightenings are different in physical condition from faint events. Since we cannot find the component with the slope of more than -1 as the prediction of Hudson (1991), the result also suggests that transient brightening or microflare is not major source of active-region coronal heating. In the next step, we will confirm what is two power-law components. The above results were presented in ASJ autumn meeting in 1992 (see news.asj_9210).