Studies of soft x-ray emission during solar flares




Anandaram, Mandayam Nayaka

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Solar flare soft x-ray emission from 0.5 Å to 8.5 Å was observed during 1967-68 by U.S. Naval Research Laboratory Bragg crystal (LiF and EDDT) spectrometers aboard the OS0-4 satellite and also by NRL broad band ionization detectors aboard the OG0-4 satellite. In this work, instrumental parameters for the LiF crystal spectrometer based on experimental values have been determined and used in the data analysis. The source continuum spectra between 1 Å and 3.8 Å have been obtained for selected flares from OS0-4 spectrometer scans. As these spectra are each affected by time variations over 14 minutes, they are compared with the thermal continuum (free-free and free-bound) spectra predicted in the following manner. The instantaneous electron kinetic temperature and emission measure (equal to the product of the square of the electron number density and the total volume) of the flare plasma are determined from the available OG0-4 broad band data. The expected continuum flux is calculated by using these parameters. The comparison shows that there is good agreement between 2 Å and 3.8 Å. Thus it has been concluded that reliable values of the average electron temperature can be determined from the OG0-4 flare data. The earlier wavelength assigrunent and line identification list published by Meekins et al (1970, Solar Physics 13, 198) has been substantially improved in this work by separately summing a large number of OS0-4 spectrometer scans of the flaring and active sun. All identified wavelengths are found to agree with the more accurate theoretical valuesto within + 0.01 Å. Identifications of several weak lines as due to hydrogen-like and helium-like ions of chlorine and phosphorus have also been suggested. The temporal behaviour of selected ion line intensities (due to Fe, Ca, Si, S, Al, and Mg) indicates that they follow the expected temperature variations during the flare. The total continuum emission in the 0.5 to 3 Å and the 1 to 8 Å broad band segments has been determined from OG0-4 data for 21 flares. In doing this, a simple and approximate method of converting the total emission based on the gray body approximation (in which the OG0-4 data are reported) to one based on the thermal continuum spectrum has been developed. This study shows that the total energy lost in the 0.5 to 8 Å soft x-ray channel is of the order of 10²⁸ to 10³⁰ ergs depending on the Hα importance of the flare (from sub- to class 2); it is also shown that this energy is comparable with that emitted in higher wave length segments estimated by other authors.



Sun, Solar flares, Solar radiation, X-rays