Small Flares on Aug.12,1992 in NOAA7248 

fl070.takano01
Posted:  03-Sep-92
Updated: 06-Apr-93, 26-Nov-93, 28-Aug-94, 29-Jun-95, 26-Apr-96
Events specified: Flares on 12-Aug-92 at 01:46, 02:30 and 04:36 UT




PI and Collab.: T.Takano, S.Enome, Y.Hanaoka, H.Nakajima, M.Nishio,
                K.Shibasaki, Y.Irimajiri,
                and other members of the Radio Heliograph groupe
Category: Flares, Stracture of flaring loops

Motivation: On Aug.12,1992, 3 small flares have been observed in NOAA7248 

      with the Nobeyama Radio Heliograph. In this region many small flares
      occured during the visible period. The 3 flares occured at 0146UT,
      0230UT, and 0436UT. The duration of the flare at 0436UT was as short
      as about 1 min. Double peaks of radio emission appeared from very
      beginning of the flare, which correspond to the foot points of a
      magnetic loop visible on a SXT image. After rapid disapperance of
      the double peaks, the loop top brightened and decayed. We plan to
      analyze the radio data in detail as well as the SXT data.
Method and Required Data: We will derive physical parameters such as 

      temperature, density, etc. from SXT data and strength of magnedtic
      field, etc. from radio data. We need magnetograms and H alpha images
      as well as SXT images corresponding these 3 periods: 0146UT,0230UT,
      and 0436UT for the region NOAA7248.

Update 26-Apr-96

We have had following progress since the last update:

  1. Precise estimation on the sizes of Sub-Second Brightenings (SSB's) has 

        been done by using model fitting as well as visibility amplitude fitting
    to the Radioheliograph data, in order to improve reliability of it.
    
  2.  High dynamic range images have been used to search any counterparts
of SSB's.

Update 29-Jun-95

We have discoverd 50msec pulses named SSB's (Sub-Second Brightenings) in a radio flare at 0232UT on 1992 August 12 in NOAA7248 region as described in the last update of the proposal.

We have had following progress since the last update:

  1. Precise estimation on the sizes of SSB's has been done by using UV 

        amplitude fitting to the Radioheliograph data.
    
  2.  Similar SSB's has been discovered in a radio flare at 1992/09/07
0509UT in NOAA7276 and the pulses show good coincidence with those of Hard X-ray's obtained BATSE observations. We, therefore, would like to continue the research on this proposal.

Update 28-Aug-94

  1. Part of the results on this proposal appeared as a paper in PASJ Letter Vol.46, L21-25(1994). 

        
  2.  We have discoverd 50msec pulses in a radio flare at 0232UT on 1992
August 12 in NOAA7248 region. Four peaks of pulses had rapid rising and falling time with their intervals of 1-2 sec. The sizes of the sources is estimated to be less than 2" and the position did not change within 1", which corresponds to a foot point of a coronal magnetic loop observed with SXT. These observational results will give important information on acceleration mechanisms of high-energy electron in the flare. One of the important progress on this event is the coincidence of these pulses with those of Hard X-ray's obtained BATSE observations.

We now preparing a paper on these results. We, therefore, would like to continue the research on this proposal.

Update 26-Nov-93

1. We submit part of the results on this proposal to PASJ Letter instead of Ap.J.Suppl. with in a week. The abstract of the paper is followings.

Behavior of Accelerated Electrons in a Small Impulsive Flare on 1992 August 12 

      T.Takano, S.Enome, H.Nakajima, K.Shibasaki, M.Nishio, Y.Hanaoka,
        C.Torii, H.Sekiguchi, T.Bushimata, S.Kawashima, N.Shinohara,
   Y.Irimajiri, H.Koshiishi, T.Kosugi, Y.Shiomi, T.Sakurai, and K.Ichimoto
Abstract A GOES C1.0 class impulsive flare was observed with the new Nobeyama Radioheliograph on 1992 August 12 with 1sec temporal and 10" spatial resolutions at 17GHz. The radio flare consists of an impulsive phase of about 20sec and a decay phase of about 90sec. Radio images show clear double sources in the impulsive phase, whereas in the decay phase a single elongated source appeared which connects the double sources. Soft X-ray images with Yohkoh/SXT show that the radio double sources correspond to foot points of newly appearing coronal loops and the single source was located at the top of one of the loops. The radio emission for both phases is explained with the gyrosynchrotron radiation from accelerated electrons. These loops began to brighten at their intersecting point about 1min before the radio flare. These facts suggest that the reconnection of magnetic fields heated up the coronal loops and produced accelerated electrons, which ran through the loops, precipitated onto the foot points, and caused the radio flare. Less than 1\% of the electrons were mirrored at the foot points and trapped at the top of the loop. Lack of radio emission in the loop top area at the impulsive phase implies that the accelerated electrons were highly beamed.

2. We have discoverd 50msec pulses in a radio flare at 0232UT on 1992 

   August 12 in NOAA7248 region.   Four peaks of pulses had rapid rising and 
   falling time with their intervals of 1-2 sec.  The sizes of the sources 
   is estimated to be less than 2" and the position did not change within 
   1", which corresponds to a foot point of a coronal magnetic loop observed 
   with SXT.  These observational results will give important information 
   on acceleration mechanisms of high-energy electron in the flare.
We now preparing a paper on these results. We, therefore, would like to continue the research on this proposal.

Update 06-Apr-93

Title: Small Flares on Aug.12, 1992 in NOAA7248

Objective: A small radio flare in 0436UT on Aug.12,1992

Progress: There have been significant progress.

  1. Both radio and SXT images have a loop structure whose foot points burst out 

          in radio images.
    
  2.  Radio emission observed with the Nobeyama Radioheliograph cannot be explaned
by thermal gas emission by comparing with the radio and the SXT images. This means that the radio emission comes from nonthermal accelerated electrons. 3. The SXT images suggests that two intersecting loops became brighter and hotter after the radio flare. This implies that reconection of magnetic fields caused acceleration of electrons. 

   Part of the results on this proposal will be submitted to Ap.J. Suppl.
     as a draft of proceedings of IAU Coll.#142 "Particle Acceleration" 
     held at Maryland last January. 
   Abstract of this paper is followings. 


          High Time- and Spatial-Resolution Observations 
           of a Small Impulsive Flare on August 12. 1992 
                with the Nobeyama Radioheliograph 

Nonthermal Electron Trapping in a Magnetic Loop

T. Takano, S. Enome, H. Nakajima, K. Shibasaki, M. Nishio, Y. Hanaoka, C. Torii, H. Sekiguchi, T. Bushimata, S. Kawashima, N. Shinohara, Y. Irimajiri, Y. Choi, H. Koshiishi, T. Kosugi, Y. Shiomi, M. Sawa, K. Kai*, T. Sakurai+, and K.Ichimoto+

Nobeyama Radio Observatory, the National Astronomical Observatory +the National Astronomical Observatory

* Passed away in March, 1991.

Abstract

A C1.0 class impulsive flare has been observed with the new Nobeyama Radioheliograph on August 12. 1992 with 1 sec time- and 10" spatial-resolution at 17GHz. The radio flare consists of a main phase whose duration was about 20 sec and a decreasing phase of about 60 sec. Radio images show clear double sources separated 50" from each other in the main phase. Contrary, a single source appeared in the decreasing phase, which locates in the middle of the double sources. Soft X-ray images with YOHKOH show that the radio sources correspond to foot points of a magnetic loop and the radio single source locates at the top of the loop. The intensities of radio emission for both the main and decreasing phase of the flare can only be explained with gyrosynchrotron radiation from accelerated electrons. These facts show hat the radio flare started on the foot points of the magnetic loop and accelerated electorons have been trapped at the top of the loop after the main phase. Lack of radio emission at the beginning of the flare ath the loop top area may show that accelerated electrons are highly beamed.