Solar Flares and Coronal Mass Ejections

Abstract

Attempts to clarify the nature of the terrestrial effects of solar activity and variability continue at an increasing pace. While mechanisms relating possible changes in terrestrial weather patterns to changes in solar lumin­ osity remain elusive, it has long been thought that intense geomagnetic storms and interplanetary disturbances can be traced directly to large solar flares. To describe the basic scenario in simple terms, a large release of energy first occurs in a region of strong magnetic field. The energy release results in a rapid heating of coronal and chromospheric material, which expands outward into the interplanetary medium. In the case of the most energetic events the expanding material produces an interplanetary shock wave. The most energetic aspect of the flare, the impulsive phase, is charac­ terized by the production of energetic (E > I MeV) electrons and protons, some of which can be observed as a solar energetic particle (SEP) event at I AU. Over the past half century attempts have been made to identify the solar flares and their particular properties that result in geomagnetic storms and SEP events. These extensive studies, of interest to both solar physicists and forecasters of effects on the terrestrial environment, seemed to lay a solid foundation for the idea that the flare itself was the cause of the subsequent activity observed in the interplanetary medium and at the Earth. About two decades ago large coronal eruptions, now known as coronal