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Yohkoh statistical studies

Yohkoh statistical studies. Michał Tomczak Astronomical Institute, University of Wrocław, Poland. Introduction.

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Yohkoh statistical studies

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  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. 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. 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. ISSI Workshop, October 3-6, 2006

  5. Comparison between surveys 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). ISSI Workshop, October 3-6, 2006

  7. L M1 Masuda 1994 ISSI Workshop, October 3-6, 2006

  8. August 18, 1998 Petrosian et al. 2002 ISSI Workshop, October 3-6, 2006

  9. Petrosian et al. 2002 Tomczak & Ciborski 2006 ISSI Workshop, October 3-6, 2006

  10. Masuda 1994 Footpoint sources Loop-top sources ISSI Workshop, October 3-6, 2006

  11. Petrosian et al. 2002 γFT = 4.9 ± 1.5 γLT = 6.2 ± 1.5 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. ISSI Workshop, October 3-6, 2006

  13. Alexander & Metcalf 1997, ApJ, 489, 442 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. 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 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. 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). 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 ISSI Workshop, October 3-6, 2006

  19. 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. ISSI Workshop, October 3-6, 2006

  20. ISSI Workshop, October 3-6, 2006

  21. ISSI Workshop, October 3-6, 2006

  22. Additional 16 partially occulted flares observed by Yohkoh in years 1997–1999 are preparing (Tomczak & Sokolnicki). ISSI Workshop, October 3-6, 2006

  23. 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. ISSI Workshop, October 3-6, 2006

  24. ISSI Workshop, October 3-6, 2006

  25. 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. ISSI Workshop, October 3-6, 2006

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