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
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)
Characteristics: ・ impulsive time-behavior ・ relatively hard spectrum ・ located above the SXR loop → magnetic reconnection above the loop
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)
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.
18-Aug-1998 flare HXR spectra 10 (keV) 100 looptop impusive source L M1 M2 H footpoint
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
Alexander and Metcalf, 1997 PIXON analysis of the 13-January-1992 flare Temperatures derived from HXT and BCS BCS HXT
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.
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.
2. Relationship between the looptop impulsive source and footpoint sources (2) Reconnection siteLooptopFootpoint (1) Reconnection site Looptop Footpoint Flow of high-energy electrons
Time Variation Looptop impulsive source Footpoint sources 13-Jan-1992 flare (M2-band) 17:27:3017:29:30 Time res. = 5 sec
Time Variation 04-Oct-1992 flare (M2-band) Looptop impulsive source 22:18:40 22:19:40 Footpoint sources Time res. = 4 sec.
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.
3. Acceleration Site Where is the particle acceleration site? Time-of-flight analysis (Aschwanden et al. 1996) Low-frequency = trapping High-frequency = direct precipitation
Particle acceleration: Site Electron Time of Flight (CGRO/BATSE) Aschwanden et al. 1996
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
Turbulence (non-thermal line broadening) observed with Yohkoh/BCS Mariska et al. (1999) OccultedNon-occulted No difference → turbulence exists at the looptop portion
4. Relationship between looptop impulsive source and looptop gradual source 04-Oct-1992 flare HXT/L-band footpoint looptop (Masuda et al. 1995)
Tsuneta et al. 1997 HXR impulsive looptop source vs high-T region HXR source is located between the two high-T region.
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.
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)
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?
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
Homologous Flares Comparison between occulted-flare and non-occulted flare occulted 40 non-occulted 600 1101100 350010000 14 hours