Line profile characteristics of solar explosive event bursts

Abstract

The spatial structure and temporal evolution of explosive events are explored using spectral observations of the Si IV 1393 Å line obtained with the Solar Ultraviolet Measurements of Emitted Radiation spectrograph (SUMER). Three areas of quiet Sun near disk center, 8″ × 120″ each, were rastered with a cadence of less than 1 min for a period of over half an hour. Events were identified by a non-Gaussian excess in the blue and/or red wings of the line. We found 21 sites where events re-occur. The sites are clustered near regions of evolving network fields seen in low resolution Michelson Doppler Imager (MDI) magnetic field images. Events tend to expand and shrink across the surface with a speed ∼ 25 km s-1. Individual events were also seen to move at this speed across the solar surface. Consecutive events often have different line profile characteristics and sizes, suggesting changes in the structure of the accelerated plasma from event to event. In the majority of events, blue and red wing brightenings do not produce a simultaneous line intensity increase. Also, the red and blue wing emission is mostly cospatial implying either non-directed flow or jets orientated along the line-of-sight (i.e. out from and towards the solar surface). The explosive events within a burst are in some cases separated by 3-5 min, suggesting that oscillations, which are known to have such periods, may play a role in triggering the individual events of a burst.