S. Gburek and M. Siarkowski

RF15-I X-ray photometer aboard INTERBALL-Tail satellite has observed numerous flares of B GOES class with significant emission above 30 keV. In this paper we analyse these flares using Yohkoh SXT images and RF15-I data. We show that some of these flares occur in systems of relatively small low laying loop systems with Y type interaction. 

We shall focus on properties of the small solar flares recorded by the photometer part of RF15-I instrument. RF15-I is an instrument, consisting of a photometer and imager, developed in order to investigate solar flares and brightenings in soft and hard X-rays. The instrument, launched onboard INTERBALL-Tail satellite in summer 1995, has been collecting telemetric data since 6 August 1995. Detailed description of the instrument construction, and performance can be found in Sylwester, et al. (2000). The RF15-I photometer measures integral solar flux in three soft channels s1, s2, s3 and five hard h1, h2, h3, h4, h5 channels. Energy band for each channel is shown in Table I.

Table 1. Nominal energy range for RF15-I soft and hard x-ray channels.

 A large database of the photometer records is directly accessible at www site www.cbk.pan.wroc.pl/rf15-i_www/default.htm. The database forms a valuable source for analyse of X-ray solar events that appeared during the rise phase of 23 activity cycle During the first three years of observation, RF15-I has registered about 1800 well-observed flares (Siarkowski et al., 1999). Well-observed means that the entire flare profile has been followed and no signature of magnetospheric particle contamination is seen. The six of flares observed were of X, 48 of M and 576 of C GOES class. All flares with GOES class above C3 (256) have clear emission component seen in channel h2 and all flares above M3 class (20) have the emission in channel h3 noticeable. It is known that the GOES class of flares does not necessarily represents the flare high energy spectra properly (McDonald et al, 1999). This effect is especially well seen in RF15-I measurements. There are numerous cases where B class flare spectra are harder than C or even M ones class event spectra (Siarkowski et al., 1999).

A distinguishable class among these events is a class of 151 events that have very hard components in their spectrum (above 30 keV) and relatively low emission in soft X-rays (GOES class C5.0 or lower). A list of these events with their lightcurves recorded by RF15-I photometer can be found on page www.cbk.pan.wroc.pl/rf15-i_www/super_hard_RF_events/smallsuperhard.htm

An interesting question arises, how these little, short living, flares can produce so energetic photons that rather come to being in huge flares - hundreds or thousands times stronger in GOES classification. One of possible answers to this question can be suggested by studding the magnetic loop configuration in which these flares appeared.

Interaction of two loops can be roughly divided into three principal categories named: I-type, X-type and Y-type interactions. I-type interactions are interactions of two parallel loops. An Y-type interaction occur when loops have a certain common contact place, small in comparison with their length. X-type interaction happens when one loop intersects the second at almost right angle. (Sakai-Ichi and de Jager, 1996)

During an interaction of magnetic loops plasma particles can be accelerated by reconnection processes in current-sheets formed between oppositely directed magnetic field. Accelerated particles interact with ambient plasma, increase its temperature and cool down radiating their energy away. The higher density of surrounding plasma, the more effective the entire process is. As it was pointed by (McDonald et al. 1997), (Sakai and de Jager 1989) strong hard X-rays can be produced locally in this way.

Observed loop configuration for our selected flare class shows great variety in morphology and can range from very complex to relatively simple structures. For instance, consider 2 August 1999 event of GOES importance merely C3.0 and significant emission above 60 keV The event duration observed in soft X-rays was about 4 min; peak value of impulsive, hard X-ray burst appeared at 9:16. Magnetic loop configuration for this event, shown in Fig. 1, forms quite complicated system.

Fig. 1. Sun SXT image taken on 2-Aug-99 09:20:02 in AlMg Filter - the left panel. In the right panel is a full resolution SXT image, taken a bit earlier at 09:16:26 in Al.1 filter which shows the flaring region, enclosed by a rectangle in the left image. The centre of right image lies approximately at heliographic co-ordinates (-80, 340) arcsec. Observed loop configuration was similar before and after the flare.

On the other hand, 2 February 1997 and 27 December 1997 flares present very simple Y-type interactions of two loops with contact regions situated in vicinity of their footpoints. Booth of these flares are of GOES class B and have hard X-ray emission above 30 keV. Observed Y-type loop interactions near flare footpoints, where plasma is denser, provides a possible explanation for extraordinary hard x-ray spectral component released in these events. Below, we discus the 2 February 1997 event in a more detailed way. We propose a loop system morphology for the event exploiting Yohkoh SXT images. We describe the flare time behaviour and determine its physical parameters.

In soft X-rays channels of RF15-I photometer this event, (GOES class B9.0), started at 11:44:50 and lasted about 3 minutes. Thirty seconds long impulsive hard X-ray burst reached its maximal value at 11:45:56 in h3 channel. The system seen in Yohkoh SXT images consists of two loops which manifest Y-type interaction in later phase of the event development - during the time of hard X-ray burst occurrence. Preflare temperature T=6 [MK] and emission measure EM=45.7 10⁴⁷ [cm^-3] in loop interaction region were determined in loop using filter-ratio method for AlMg and Al12 SXT images with help SXT software (Yohkoh Analysis Guide 1994). Morphology of preflare loop system is suggested in Fig 2.

Fig. 2. A full resolution SXT preflare image for 2-FEB-97 event is shown to the left. The Image was taken at 11:43:53 in AlMg Filter. Proposed configuration of a loop system is depicted on the right with a loop interaction region encloses in a grey rectangle.

Fig. 3. RF15-I records for 2 February 1997 flare of GOES class B9.9. The 30 second long X-ray burst visible in h3 channel at about 11:46 is an unusual feature for such a tiny flares.

Fig. 4. Plots of temperature (T) and emission measure (EM) during impulsive phase of 2 February 1997 flare. Booth plots were obtained by exploiting RF15-I data shown in Fig. 3.

We have found in RF15-I photometer records 151 event with unusually hard components in their emission (above 30 keV) and relatively low emission in soft X-rays (GOES class C5.0 or lower). From inspection of SXT images it appears that these event rarely emerge as compact flares but rather are seen in systems of magnetic loops which can mutually interact. Among these events we have identified examples of small flares (GOES class B) with relatively simple morphology of Y-type interacting loops. For the 2 February 1997 event we proposed a loop system morphology exploiting Yohkoh SXT images. We described also the flare time behaviour and determined its physical parameters.

We thank the Yohkoh Team and the YDAC at Mullard Space Science Lab. for the SXT data. This paper was supported by KBN grant no. P03D.024.17.