masuda flare: remaining problems on the looptop impulsive hard x-ray source in solar flares satoshi...
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Masuda Flare: Remaining Problems on the Looptop Impulsive
Hard X-ray Source in Solar Flares
Satoshi Masuda (STEL, Nagoya Univ.)
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More than 3000 flares were detected.
1991 2001
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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
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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)
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Characteristics:・ impulsive
time-behavior・ relatively hard
spectrum・ located above the
SXR loop
→ magnetic reconnection
above the loop
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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)
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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.
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18-Aug-1998 flare
HXR spectra
10 (keV) 100
looptop impusive source
L M1 M2 H
footpoint
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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
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Alexander and Metcalf, 1997
PIXON analysis of the 13-January-1992 flare
Temperatures derived from HXT and BCS
BCS
HXT
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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.
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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.
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Thick target
footpoint
Thin target
looptop
Accelerated electrons
soft hard
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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
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Time Variation
Looptop impulsive source
Footpoint sources
13-Jan-1992 flare (M2-band)
17:27:30 17:29:30
Time res. = 5 sec
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Time Variation04-Oct-1992 flare (M2-band)
Looptop impulsive source22:18:40 22:19:40
Footpoint sources
Time res. = 4 sec.
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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.
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3. Acceleration Site
Where is the particle acceleration site?
Time-of-flight analysis (Aschwanden et al. 1996)
Low-frequency= trapping
High-frequency= direct precipitation
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Particle acceleration: Site Electron Time of Flight (CGRO/BATSE) Aschwanden et al. 1996
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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
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Turbulence (non-thermal line broadening) observed with Yohkoh/BCS
Mariska et al. (1999)
Occulted Non-occulted
No difference → turbulence exists at the looptop portion
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4. Relationship between looptop impulsive source and looptop gradual source
04-Oct-1992 flareHXT/L-band
footpointlooptop
(Masuda et al. 1995)
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Tsuneta et al. 1997
HXR impulsive looptop source vs high-T region
HXR source is located between the two high-T region.
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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.
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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)
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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?
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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
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Homologous FlaresComparison between occulted-flare and non-occulted flare
occulted
40
non-occulted
600
110 1100
3500 10000
14 hours
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Homologous flares
occulted non-occulted
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10 (keV) 100
Double footpoint sources
Looptop impulsive source
Factor 100
Homologous Flares