1. Masuda Flare: Remaining Problems on the Looptop Impulsive Hard X-ray Source in Solar Flares Satoshi Masuda (STEL, Nagoya Univ.)

2. More than 3000 flares were detected. 1991 2001

3. Introduction Imaging Spectrometer(Kosugi et al. 1991) Only four energy bands, but not one 14 - 23 - 33 - 53 - 93 keV L M1 M2 H bands Simultaneous observations with SXT accurate co-alignment between these two telescopes (Masuda 1994) HXT

4. Discovery of a Hard X-ray Source above the Corresponding Soft X-ray Loop looptop impulsive source above-the-looptop source (13-Jan-92: Masuda et al. 1994)

5. Characteristics: ・ impulsive time-behavior ・ relatively hard spectrum ・ located above the SXR loop → magnetic reconnection above the loop

6. Problems 1. Emission Mechanism 2. Relationship between looptop impulsive source and footpoint sources 3. Acceleration Site 4. Relationship between looptop impulsive source and looptop gradual source 5. Universality (next speaker)

7. Emission Mechanism Imaging spectroscopy is needed. 18 Aug 1998 X-class flare Looptop impulsive source is observed in the H-band. Footpoint sources are occulted by the solar limb.

8. 18-Aug-1998 flare HXR spectra 10 (keV) 100 looptop impusive source L M1 M2 H footpoint

9. Emission mechanism Thermal emission (T ～ 100 MK) How to confine such a hot plasma in the compact region Non-thermal emission Low density of the ambient plasma (no target) The source in the higher energy band is located at the higher altitude. NG simple thin target model

10. Alexander and Metcalf, 1997 PIXON analysis of the 13-January-1992 flare Temperatures derived from HXT and BCS BCS HXT

11. Lack of L-band emission Temperature is inconsistent with BCS/SXT Alexander and Metcalf, 1997 Nonthermal emission is more appropiate. Note that the EM of the above-the-looptop HXR source is only a few 10^45 cm^-3. That of the SXR flaring loop (20MK plasma) is about 10^48 cm^-3.

12. Fletcher 1995, Fletcher and Martens 1997 Nonthermal (thin-target) model A very high coronal density was assumed. The actual hard X-ray source is located at a low- density corona.

14. Thick target footpoint Thin target looptop Accelerated electrons soft hard

15. 2. Relationship between the looptop impulsive source and footpoint sources (2) Reconnection siteLooptopFootpoint (1) Reconnection site Looptop Footpoint Flow of high-energy electrons

16. Time Variation Looptop impulsive source Footpoint sources 13-Jan-1992 flare (M2-band) 17:27:3017:29:30 Time res. = 5 sec

17. Time Variation 04-Oct-1992 flare (M2-band) Looptop impulsive source 22:18:40 22:19:40 Footpoint sources Time res. = 4 sec.

18. Time Variation The hard X-ray flux from the looptop impulsive source reaches its peak almost at the same time of the peaktime of the flux from the footpoint sources, or slighjtly earlier (~ a few seconds) than that of the footpopint sources. The time resolution is not enough to determine the time lag bacause the electron traveling time from looptop to footpoint is only ~ 1 second.

19. 3. Acceleration Site Where is the particle acceleration site? Time-of-flight analysis (Aschwanden et al. 1996) Low-frequency = trapping High-frequency = direct precipitation

20. Particle acceleration: Site Electron Time of Flight (CGRO/BATSE) Aschwanden et al. 1996

21. Above the soft X-ray loop Relation between loop-length and TOF distance (Aschwanden et al. 1996) TOF distance Loop radius = looptop height = 2 ×looptop height Best fit = 1.43 × looptop height

22. Turbulence (non-thermal line broadening) observed with Yohkoh/BCS Mariska et al. (1999) OccultedNon-occulted No difference → turbulence exists at the looptop portion

23. 4. Relationship between looptop impulsive source and looptop gradual source 04-Oct-1992 flare HXT/L-band footpoint looptop (Masuda et al. 1995)

24. Tsuneta et al. 1997 HXR impulsive looptop source vs high-T region HXR source is located between the two high-T region.

25. Tsuneta et al. 1997 The impulsive looptop source is related to the reconnection downflow and/or fast shock. The two high-T regions are related to the slow shock.

26. Summary: remaining problems Emission mechanism accurate spectrum (RHESSI) Relationship with fooptpoint sources higher time resolution Acceleration site direct precipitation vs trapping component (radio observation with a high spatial resolution) Relationship with a looptop gradual source reveal the heating mechanism Universality statistical study using data with a higher dynamic range (RHESSI)

28. 5. Universality Why was the looptop impulsive source observed in only several flares during the 10-years observational period of HXT? Is the event which shows the looptop impulsive source, the very special/minor case in solar flares?

29. Statistical study (Petrosian et al. 2002) The looptop impulsive source is much weaker than the footpoint sources. The ratio is generally close to 10:1, the HXT dinamic range. Looptop Impulsive Source Footpoint Sources 10:1 1:1

30. Homologous Flares Comparison between occulted-flare and non-occulted flare occulted 40 non-occulted 600 1101100 350010000 14 hours

31. Homologous flares occulted non-occulted

32. 10 (keV) 100 Double footpoint sources Looptop impulsive source Factor 100 Homologous Flares