Transient events in the EUV transition region and chromosphere

Abstract

Rapid time cadence observations of the quiet Sun extreme ultraviolet emission, observed by the cds instrument on soho, are discussed. Numerous transient brightenings are observed in network features in both a transition region line (O V 629.73 Ä) and a chromospheric line (He I 584.33 Ä), indicating a dynamic coupling between the chromospheric and transition region network. Their durations are between 80 and 200 s and dimensions 6 000-10 000 km. A wavelet analysis reveals a tendency for semi-periodic behaviour, with excess power at a frequency of about 4 mHz. The variations are much less evident in the internetwork or cell regions, although they are again semi-periodic. Relative line-of-sight velocities have also been derived from the data, the cds spectral resolution allowing a precision of between 4.7 and 6 km s(-1) . There is a clear association of brightenings in the network with downflows of ~ 13 km s(-1) at 250 000 K with some events having velocities of up to ~ 20 km s(-1) , these being measured relative to the average quiet Sun emission. Within the internetwork, we also find a weak correlation between events seen in the He I (584.33 Ä) and the O V (629.73 Ä) lines. In this case, the events have a smaller size (<= 2 000 km), amplitude (both in terms of their intensity and velocity), and also show a higher frequency of about 6 mHz. The apparent differing properties of network and internetwork events implies that both these regions are heated by two distinct mechanisms. In the case of the internetwork, these results further confirm that acoustic waves propagating up from the photosphere and forming shocks in the overlying atmosphere are the most likely heating mechanism. For the network, it is apparent that the heating required must be in excess of that supplied by acoustic shocks. Our view is that the network events are produced by nanoflare-like magnetic reconnections in the corona, or possibly excitation due to a spicule-type event in which there is a repeated rebound.