ISSI Workshop, October 3-6, 2006 1

Yohkoh statistical studies

Michał Tomczak

Astronomical Institute,

University of Wrocław, Poland

ISSI Workshop, October 3-6, 2006 2

Introduction

Yohkoh Hard X-ray Telescope gave for the first time an opportunity for massive investigation of spatial distribution of hard X-ray emission in solar flares: the mission-long database (Oct 1, 1991 – Dec 14, 2001) contains 3071 events.

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Observational constraints

to obtain a valuable hard X-ray image about 100–200 cts/SC are needed flux limit.

to distinguish different sources a flare size should be larger than spatial resolution of the instrument (we cannot resolve events having h < 8–9 x 103 km) size limit.

to distinguish coronal and footpoint sources a contamination of their photons should be omitted: event should be seen ‘on-side’ geometry limit.

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Selection criteria (Masuda 1994)

peak count rate in the M2–band exceeding 10 cts s-1 SC-1: at least one image is available in the channel which records the radiation of purely non-thermal electrons.

heliocentric longitude exceeding 80º.

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Comparison between surveys

Survey Period

Total number of

flares

Number of selected events

Masuda (1994), Ph. D. thesis

Oct 91 – Sep 93

850 10

Petrosian et al. (2002), ApJ,

569, 459

Oct 91 – Aug 98

1307 18

Tomczak & Ciborski (2006),

A&A preprint

Oct 91 – Dec 01

3071 117*

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What have the surveys taught us about coronal sources?

They are a common feature of solar flares (M: 7/10; P: 15/18; TC: 45/117).

During the impulsive phase they are usually fainter than footpoint sources, a disproportion becomes more important at bursts maxima and for higher photon energies.

Their photon energy spectra are usually softer than the spectra of footpoint sources (an exception: the above-the-loop-top sources).

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L

M1

Masuda 1994

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Petrosian et al. 2002

August 18, 1998

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Petrosian et al. 2002 Tomczak & Ciborski 2006

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Masuda 1994

Footpoint sources Loop-top sources

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Petrosian et al. 2002

γFT = 4.9 ± 1.5γLT = 6.2 ± 1.5

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HXR imaging constraints

The finite dynamic range of the HXT estimated to be about 1 decade (Sakao 1994).

The generation of spurious sources by the reconstruction routines a false photometry of real sources.

Weak sources suppression in the presence of strong sources (Alexander & Metcalf 1997) the reconstruction routines treat them as a statistically insignificant.

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Alexander & Metcalf 1997, ApJ, 489, 442

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To study weak sources in the presence of strong sources is our case!HXR imaging constraints limit a possibility of investigation of coronal sources.Moreover, our results can be even false due to the light curve mimicking that of the footpoints.

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What can we do?

To wait for modern instruments with a better dynamic range as well as for clever reconstructions routines.

To investigate only examples in which the coronal sources dominate we obtain only a partial picture.

To eliminate somehow stronger footpoint sources

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Behind-the-limb flares

We use the solar limb as a screen which occults the lower part of the flaring structure (footpoint sources) and leaves emission of the higher part only (coronal sources):

+ in this way we can separate coronal sources of all type; our choice do not favour any particular physical mechanism,

- we loose a possibility of the comparison with the footpoint sources.

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The behind-the-limb configuration has been used in many different way:

for HXRs (e.g. Frost & Dennis 1971, stereo-scopic papers of Kane).

Yohkoh Bragg Crystal Spectrometer diagnostics of bright SXR loop-top kernels (e.g. Khan et al. 1995, Mariska et al. 1996, Mariska & McTiernan 1999).

BCS diagnostics of X-ray plasma ejections (Tomczak 2005).

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How can we find out such events?

Compare the GOES list of flares to the Hα list from the SGD; events present in the first one and absent in the second one can be behind-the-limb type a prompt selection.

Check manually soft X-ray images of the prompt-selected flare impulsive SXR brightenings should not be seen

Check a time of the limb passage for the active region in which the prompt-selected flare has occurred λ(t) extrapolation

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Tomczak (2001), A&A, 366, 294

14 behind-the-limb flares that occurred between 1991–1994 has been selected.

In this case to investigate time variation of coronal sources we need not actually HXR images! our temporal resolution becomes better.

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Additional 16 partially occulted flares observed by Yohkoh in years 1997–1999 are preparing (Tomczak & Sokolnicki).

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Masuda flare

Masuda et al.1994, Nature, 371, 495 – about 300 citations in the Smithsonian/NASA Astrophysics Data System.

Tomczak & Ciborski (2006): an additional argument confirming how unusual this event is.

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Explanation:

Different converging field geometry: events from branch A occurred in more converged loops than the events from branch B: the more converged loops from branch A correspond to flux tubes that are less helically twisted; the less converged loops from branch B are more twisted.

Anomalous electron scattering – absent for events from branch A, present for events from branch B.