ISSI Workshop, October 3-6, Yohkoh statistical studies Michał Tomczak Astronomical Institute, University of Wrocław, Poland

ISSI Workshop, October 3-6, 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. database

ISSI Workshop, October 3-6, 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 10 3 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, 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, Comparison between surveys SurveyPeriod Total number of flares Number of selected events Masuda (1994), Ph. D. thesis Oct 91 – Sep Petrosian et al. (2002), ApJ, 569, 459 Oct 91 – Aug Tomczak & Ciborski (2006), A&A preprint Oct 91 – Dec *

ISSI Workshop, October 3-6, 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, L M1 Masuda 1994

ISSI Workshop, October 3-6, Petrosian et al August 18, 1998

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

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

ISSI Workshop, October 3-6, Petrosian et al γ FT = 4.9 ± 1.5 γ LT = 6.2 ± 1.5

ISSI Workshop, October 3-6, 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, Alexander & Metcalf 1997, ApJ, 489, 442

ISSI Workshop, October 3-6, 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, 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, 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, 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, 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, Tomczak (2001), A&A, 366, 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,

ISSI Workshop, October 3-6,

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

ISSI Workshop, October 3-6, 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,

ISSI Workshop, October 3-6, 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.