Masuda Flare: Remaining Problems on the Looptop Impulsive
Hard X-ray Source in Solar Flares
Satoshi Masuda (STEL, Nagoya Univ.)
More than 3000 flares were detected.
1991 2001
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 mechanismThermal 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 emissionTemperature 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.
Thick target
footpoint
Thin target
looptop
Accelerated electrons
soft hard
2. Relationship between the looptop impulsive source and footpoint sources
(2)
Reconnection site Looptop Footpoint
(1)
Reconnection site
Looptop
Footpoint
Flow of high-energy electrons
Time Variation
Looptop impulsive source
Footpoint sources
13-Jan-1992 flare (M2-band)
17:27:30 17:29:30
Time res. = 5 sec
Time Variation04-Oct-1992 flare (M2-band)
Looptop impulsive source22:18:40 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)T
OF
dis
tanc
e
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)
Occulted Non-occulted
No difference → turbulence exists at the looptop portion
4. Relationship between looptop impulsive source and looptop gradual source
04-Oct-1992 flareHXT/L-band
footpointlooptop
(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 mechanismUniversality
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
Foo
tpoi
nt S
ourc
es
10:1 1:1
Homologous FlaresComparison between occulted-flare and non-occulted flare
occulted
40
non-occulted
600
110 1100
3500 10000
14 hours
Homologous flares
occulted non-occulted
10 (keV) 100
Double footpoint sources
Looptop impulsive source
Factor 100
Homologous Flares