Solar Extreme Events of 2003:
Fundamental Science and Applied Aspects

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POLARIZATION OF HARD X-RAYS OBSERVED IN SOLAR EXTREME EVENTS IN OCTOBER-NOVEMBER, 2003
Zhitnik I.A., Logachev Yu.I., Bogomolov A.V., Denisov Yu.I., Morozov O.V., Myagkova I.N., Svertilov S.I., Ignat'ev A.P., Oparin S.N., Pertsov A.A.,

In the experiment with X-ray polarimeter SPR-N onboard satellite 'CORONAS-F' (altitude 500 km, inclination 82.5o, period ~94.5 min.) the hard (20-100 keV) X-ray emission from several Solar flares in October-November, 2003 was detected. This work presents the results of measuring of the X-ray polarization in the powerful Solar flares 28.10.2003, 29.10.2003 and 04.11.2003. Before this experiment the X-ray polarization was measured only in soft X-rays and only for few flares, the values of polarization varied from 2-3% up to 20-40%. Value of polarization gives the information about directions of beams of accelerated electrons at the Sun. Measuring of polarization in SPR-N is based on the Compton scattering of X-ray photons in a hexagonal prism of beryllium. The scattered photons are detecting by six CsI(Na) detectors, positioned in the prism sides. Effective area of each detector is S~0.3 cm2 for E&#61543;=20keV and S~1.5 cm2 for E&#61543;=100keV. Non-polarized emission scatters omnidirectionally, therefore the count rates of all detectors in this case will be equal. Polarized photons are mainly scattering normally to a polarization plane. Proportions between count rates of detectors give information about part of polarized photons and position of polarization plane. Scattered photons are detecting in the energy bands 20-40, 40-60 and 60-100 keV. Experimental data of SPR-N show that X-ray emission in October, 29, 2003 Solar flare (20:40 - 21:00 UT) was strongly polarized, mainly in the peaks of X-ray intensity (at ~20:44 UT and ~20:51 UT). In the energy band 40-60 keV >85% of photons were polarized in the first peak, and >75% in the second peak. The similar situation is in the band 60-100 keV. In the band 20-40 keV the X-ray emission is less polarized. For the Solar flare 28.10.2003 (11:00 - 11:20 UT) and for the first stage of the flare 4.11.2003 (~19:34 UT) significant values of the polarization were not detected. Only upper limits <25% for the event 28.10.2003 and <40% for 4.11.2003 were obtained. Value of polarization for the main part of Solar flare 4.11.2004 was not obtained because during this period the satellite crossed radiation belts. In other events the background situation was much better (geomagnetic equator for 29.10.2003 and polar cap for 28.10.2003. The errors of measurements in the events 28.10.2003 and 29.10.2003 are mostly determined by uncertainty in the detector sensitivities, which have to be equal to each other in the ideal case. As for the first stage 4.11.2003 the statistical errors play more important role.





Skobeltsyn Institute of Nuclear Physics, Moscow State University, 2004