Solar X-ray bursts at energies less than 10 keV observed with OSO-4 |
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| Authors: | J L Culhane K J H Phillips |
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| Institution: | (1) Mallard Space Science Laboratory, Dept. of Physics, University College, London, UK |
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| Abstract: | Using data from a proportional counter spectrometer, sensitive in the wavelength range 1–20 Å, on OSO-4, X-ray bursts in the energy band 3.0 to 4.5 keV have been studied. 150 events have been identified between October 27, 1967 and May 8, 1968, mostly of an impulsive nature. Some gradual rise and fall bursts occur, but there is a selection bias against such long-enduring events. A study of the profiles of these events reveals no basis for identifying different types of impulsive event.Single frequency radio bursts and H flares of class > 1F are almost always accompanied by X-ray enhancements. For the sample of X-ray events, only 25% are correlated with radio bursts and 46% with flares. Only 11% of the sample events are associated with type III radio bursts. Microwave burst peaks occur an average of two minutes earlier than the X-ray burst peak, but the first observation of X-ray activity is usually before the start of the corresponding microwave burst.Impulsive bursts, although differing widely in fall time, are due to the heating of a volume of plasma from a temperature of 10.0 to 30.0 × 106 K. Differences infall time probably indicate different electron densities in the source. Observation of an iron line at 1.9 Å suggests that a non-thermal mechanism may be operating during some of these events since the temperatures are too low to permit thermal excitation of the 1s
2-1s 2p transition in Fe+24. It is also possible that, in spite of the low temperature, most of the iron ions have been stripped to the Fe+24 stage. Collisional excitation and dielectronic recombination processes would then be able to provide the observed flux in the resonance line of Fe+24. A gradual rise and fall event and event  precursors have also been studied. |
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