A study of extreme ultraviolet blinker activity

R. A. Harrison, J. Lang, D. Brooks, D. E. Innes

Abstract
In a previous paper (Harrison, 1997a), we reported on the existence of extreme ultraviolet (EUV) flashes, known as blinkers, which were identified in the quiet Sun network using the CDS instrument on board the Solar and Heliospheric Observatory. Since then a number of dedicated observations have been made and we report here on an analysis of 97 blinker events. We identify blinker spectral, temporal and spatial characteristics, their distribution, frequency and general properties, across a broad range of temperatures, from 20,000 K to 1,200,000 K. The most significant brightenings were found in the transition region temperature lines of O III, O IV and O V, with modest or no detectable increases at higher or lower temperatures. A typical blinker event has a duration of order 1000 s, though the detection of short-duration blinkers may well be limited by the observation methods. However, a long tail of longer-duration blinkers puts the average blinker duration at almost 2400 s. Comparisons to plasma cooling times establish firmly that there is a continuous energy input throughout the blinker event. The projected blinker onset rate for the entire solar surface is 1.24 s^-1 i.e. 3,000 blinker events may be in progress at any point in time. An examination of the line ratios shows a remarkable feature. Ratios of lines from O III, O IV and O V show no significant change throughout the blinker event and this indicates that the blinkers are predominantly caused by inceases in density or filling factor. The intensity signatures of the blinkers are modelled using a basic time dependent code and this confirms that the lack of a change in the oxygen line ratios is consistent with a density or filling factor increase in a plasma cooling from a temperature above 5 x 10⁵ K. We estimate the thermal energy content of an average blinker at 2 x 10²⁵ erg and consider this figure and the total blinker rate in the light of the energy required for coronal heating. The results are used to compare blinker activity to reported micro-flare, network flare and explosive event activity, and to discuss their potential role in coronal heating and solar wind acceleration processes. Finally, a blinker model is presented which consists of the merging of a closed

Astronomy and Astrophysics
Volume 351, Page 1115
November 1999