ISSI Workshop, October 3-6, 20061 Yohkoh statistical studies Michał Tomczak Astronomical Institute,...
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ISSI Workshop, October 3-6, 2006 1
Yohkoh statistical studies
Michał Tomczak
Astronomical Institute,
University of Wrocław, Poland
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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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ISSI Workshop, October 3-6, 2006 3
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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ISSI Workshop, October 3-6, 2006 4
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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ISSI Workshop, October 3-6, 2006 5
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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ISSI Workshop, October 3-6, 2006 6
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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ISSI Workshop, October 3-6, 2006 7
L
M1
Masuda 1994
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ISSI Workshop, October 3-6, 2006 8
Petrosian et al. 2002
August 18, 1998
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ISSI Workshop, October 3-6, 2006 9
Petrosian et al. 2002 Tomczak & Ciborski 2006
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ISSI Workshop, October 3-6, 2006 10
Masuda 1994
Footpoint sources Loop-top sources
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ISSI Workshop, October 3-6, 2006 11
Petrosian et al. 2002
γFT = 4.9 ± 1.5γLT = 6.2 ± 1.5
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ISSI Workshop, October 3-6, 2006 12
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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ISSI Workshop, October 3-6, 2006 13
Alexander & Metcalf 1997, ApJ, 489, 442
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ISSI Workshop, October 3-6, 2006 14
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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ISSI Workshop, October 3-6, 2006 15
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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ISSI Workshop, October 3-6, 2006 16
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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ISSI Workshop, October 3-6, 2006 17
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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ISSI Workshop, October 3-6, 2006 18
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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ISSI Workshop, October 3-6, 2006 24
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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.