Transcript Remaining Problems on the Looptop Impulsive Hard X

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
HXT
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)
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
footpoint
HXR spectra
L
10
M1
M2
(keV)
looptop impusive source
H
100
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
HXT
BCS
Alexander and Metcalf, 1997
Lack of L-band emission
Temperature is inconsistent with BCS/SXT
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 lowdensity corona.
Thin target
Thick target
Accelerated electrons
soft
hard
looptop
footpoint
2. Relationship between the looptop
impulsive source and footpoint sources
Flow of high-energy electrons
(1)
Looptop
Reconnection site
Footpoint
(2)
Reconnection site
Looptop
Footpoint
Time Variation
13-Jan-1992 flare (M2-band)
Footpoint sources
Time res.
= 5 sec
17:27:30
17:29:30
Looptop impulsive source
Time Variation
04-Oct-1992 flare (M2-band)
Footpoint sources
Time res.
= 4 sec.
22:18:40
22:19:40
Looptop impulsive source
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)
= 2 ×looptop height
TOF distance
Best fit =
1.43 × looptop height
= looptop height
Loop radius
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
looptop
footpoint
04-Oct-1992 flare
HXT/L-band
(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?
(Petrosian et al. 2002)
1:1
The looptop
impulsive
source is much
weaker than the
footpoint
sources.
The ratio is
generally close
to 10:1, the
HXT dinamic
range.
Footpoint Sources
10:1
Statistical study
Looptop Impulsive Source
Homologous Flares
Comparison between occulted-flare and non-occulted flare
non-occulted
occulted
3500
10000
110
1100
600
40
14 hours
Homologous flares
occulted
non-occulted
Homologous Flares
Double footpoint sources
Factor 100
Looptop impulsive source
10
(keV)
100