A study of extreme ultraviolet blinker activity

In a previous paper(Harrison, 1997a), we reported on the existence of extre me ultraviolet (EUV) flashes, known as blinkers, which were identified in t he quiet Sun network using the CDS instrument on beard the Solar and Helios pheric Observatory. Since then a number of dedicated observations have been made and we report here on an analysis of 97 blinker events. We identify b linker spectral, temporal and spatial characteristics, their distribution, frequency and general properties, across a broad range of temperatures, fro m 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 blin ker event has a duration of order 1000 s, though the detection of short-dur ation blinkers may well be limited by the observation methods. However, a l ong tail of longer-duration blinkers puts the average blinker duration at a lmost 2400 s. Comparisons to plasma cooling times establish firmly that the re 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 b linker events may be in progress at any point in time. An examination of th e 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 in dicates that the blinkers are predominantly caused by inceases in density o r filling factor. The intensity signatures of the blinkers are modelled usi ng 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 in crease in a plasma cooling from a temperature above 5 x 10(5) K. We estimat e the thermal energy content of an average blinker at 2 x 10(25) erg and co nsider this figure and the total blinker rate in the light of the energy re quired for coronal heating. The results are used to compare blinker activit y to reported micro-flare, network flare and explosive event activity, and to discuss their potential role in coronal heating and solar wind accelerat ion processes. Finally, a blinker model is presented which consists of the merging of a closed magnetic system with pre-existing open field lines in t he network.