The observation of 10–50 KeV solar flare X-rays by the OGO satellites and their correlation with solar radio and energetic particle emission

Abstract

More than 70 cases have been observed of energetic solar flare X-ray bursts by large ionization chambers on the OGO satellites in space. The ionization chambers have an energy range between 10 and 50 KeV for X-rays and are also sensitive to solar protons and electrons. A study has been made of the X-ray microwave relationship, and it is found that the total energy released in the form of X-rays between 10 and 50 KeV is approximately proportional to the peak or total energy simultaneously released in the form of microwave emission. For a given burst the rise time, decay time and total duration are similar for the 10–50 KeV X-rays and the 3 to 10 cm radio emission. Roughly exponential decay phases are observed for both emissions with time constants between 1 and 10 min. All 3 or 10 cm radio bursts with peak intensity greater than 80 solar flux units are accompanied by an X-ray burst greater than 3 × 10−7 ergs cm−2 sec−1 peak intensity. The probability of detecting such X-ray events is low unless the radio spectrum extends into the centimetric range of wavelengths. The best correlation between cm-λ and energetic X-rays is observed for the first event in a flare. Subsequent structure and second bursts may not correspond even when the radio emission is rich in the microwave component. The mechanism for the energetic X-rays is shown to be bremsstrahlung probably of fast electrons on a cooler plasma. If the radio emission is assumed to be synchrotron radiation then a relationship is developed between density and magnetic field which meets the observed quantitative results. One finds, on the average, that 5 × 10−54 joules m−2 (CPS)−1 of microwave energy at the Earth are required per electron at the Sun to provide the radio emission for the various events.A strong correlation between interplanetary solar flare electrons observed by satellite and X-ray bursts is shown to exist. This correlation is weak for solar proton events. One may infer a strong propagation asymmetry for solar flare electrons along the spiral interplanetary magnetic field.