SPA-GRL Issue Highlights

Violent solar eruptions hurl billions of tons of magnetized hot plasma over a timescale of minutes. Though these eruptions occur a few times a day during the maximum of solar activity, only a small subset significantly impact the Earth. Data on these eruptive events from a large number of spacecraft and ground based instruments have shown that the events are characterized by flares, coronal mass ejections (CMEs) and radio bursts in the corona and in the interplanetary medium. The primary criterion in choosing these events for study was that they produce nonthermal radio emission in the spectral domain of 1-14 MHZ by the WAVES experiment on board the Wind spacecraft.

Because they can produce intense radio emission in the outer corona and interplanetary medium, the eruptive events are called Radio-rich Solar Eruptive Events by Gopalswamy et al. The authors investigate the radio events observed during January 1996 to June 1998 in the dekameter-hectometric (DH) radio window (1-14 MHZ). They find a global enhancement in extreme ultraviolet wavelengths over an area much larger than the flaring active region at the onset of many events. A majority of the radio bursts are associated with interplanetary shocks and kilometric type II bursts.

Using magnetograms from the SOHO/MDI instrument, Lara et al. compute the magnetic flux of seven active regions and one disappearing filament region associated with the eruptive events. Examining the flux variation with time over a period of a few days before, during, and after the CME event, they find significant changes in structures with sizes smaller than the active region. Flare onsets and the filament disappearance occur during periods of significant variations. Thompson et al. focus on the changes in the inner corona which may be indicative of an energetic eruption. They show that the inner coronal structure of CMEs corresponds well with the transients observed at larger distances; specifically, the extended dimming regions appear to represent the white light "footprint" of the CME. The location of the flaring region, however, can be either at the center or at the edge of the dimming region.

Raymond et al. report on a fast (1200 km/s) CME observed on June 11, 1998. Simultaneously, type II bursts were observed by the Wind/WAVES experiment and by ground-based instruments at metric wavelengths. An interesting aspect of the radio emission is that the metric and WAVES type II bursts occur nearly simultaneously, suggesting that the shock was very extended and may have emitted radio waves from coronal levels that differ in density by more than an order of magnitude.