Other
Scientific paper
Jan 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996phdt........36p&link_type=abstract
PhD Dissertation, Rept-24 Metsahovi Radio Research Station
Other
Solar Flares, X Rays, Radio Bursts, Statistical Analysis, Bremsstrahlung, Goes Satellites, Microwave Emission, Millimeter Waves, Synchrotron Radiation, Gamma Ray Observatory, Time Series Analysis, Solar Magnetic Field, Radio Telescopes, X Ray Detectors, Plasma Temperature, Time Lag
Scientific paper
This doctoral thesis presents observations and analysis of solar flares and the corresponding active regions, recorded at radio and X-ray wavelengths during 1989-1993. The flare data are used both in a statistical sense and in analyzing individual flares. The radio data in microwaves/millimeter waves have been measured at the Metsahovi Radio Research Station, Finland, and consist of nearly 400 recorded solar events. Parts of this large data set were used in the analysis together with other wavelength data. Statistical analyses were done using the radio data and the simultaneously observed soft X-ray data, provided by the GOES satellites. The gradual rise and fall type radio burst fluxes were found to have almost linear correlation with soft X-ray fluxes. Thermal bremsstrahlung fluxes were also calculated from the soft X-ray data, assuming the same emitting volumes in radio and soft X-rays. The calculated fluxes showed a very good correlation with the observed ones in the case of gradual type flares. Even when there were flux differences the temporal evolution, i.e., the start of flux rise and the time of flux maximum, was similar. In the case of impulsive type solar flares most of the microwave events started and peaked just before the soft X-ray maximum. The majority of impulsive type events had some amount of thermal bremsstrahlung present, often as a thermal 'tail' in the case of big events. Contrary to many previous studies, several microwave/millimeter wave flares were found without a visible component in soft X-rays. No common denominator could be found for these flares. Some flares were found with only a signature at the higher soft X-ray band (no calculated thermal bremsstrahlung as a consequence), and these cases could be correlated with bursts in hard X-rays. For many impulsive flares, the main emission mechanism could be determined to be gyrosynchrotron radiation by non-thermal electrons. The BATSE hard X-ray detectors on the Compton Gamma Ray Observatory have been operating since April 1991. Some of the solar events recorded in radio and soft X-rays were also observed with BATSE and these events were analyzed in detail. The results include defining the time delays between radio and hard X-ray flux peaks, and the hard X-ray burst differences in the case of gradual and impulsive radio flares. One case study presents a flare which connects a non-thermal microwave flare (with no observed soft X-ray emission) with a faint hard X-ray flare and a decimetric type 3-like event. The central meridian passage times of the active regions in the Metsahovi major flare data between 1989 and 1993 were studied with non-parametric methods to search for periodicity in a weighted time point series. Two significant synodic periods were found (22.07 and 26.72 days). It is proposed that the longer period may represent the synodic surface rotation rate close to the solar equator, while the shorter period could be a signature of a long-lived nonaxisymmetric component of the solar magnetic field below the visible surface, which is then observed as an active longitude on the solar surface.
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