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