Transient events in the EUV transition region and chromosphere

Rapid time cadence observations of the quiet Sun extreme ultraviolet emissi on, observed by the CDS instrument on SOHO, are discussed. Numerous transie nt brightenings are observed in network features in both a transition regio n line (O V 629.73 Angstrom) and a chromospheric line (He I 584.33 Angstrom ), indicating a dynamic coupling between the chromospheric and transition r egion network. Their durations are between 80 and 200 s and dimensions 6 00 0-10 000 km. A wavelet analysis reveals a tendency for semi-periodic behavi our, with excess power at a frequency of about 4 mHz. The variations are mu ch less evident in the internetwork or cell regions, although they are agai n semi-periodic. Relative line-of-sight velocities have also been derived f rom 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 netw ork with downflows of similar to 13 km s(-1) at 250 000 K with some events having velocities of up to similar to 20 km s(-1), these being measured rel ative to the average quiet Sun emission. Within the internetwork, we also f md a weak correlation between events seen in the He I (584.33 Angstrom) and the O V (629.73 Angstrom) lines. In this case, the events have a smaller s ize (less than or equal to 2 000 km), amplitude (both in terms of their int ensity and velocity), and also show a higher frequency of about 6 mHz. The apparent differing properties of network and internetwork events implie s that both these regions are heated by two distinct mechanisms. In the cas e of the internetwork, these results further confirm that acoustic waves pr opagating up from the photosphere and forming shocks in the overlying atmos phere are the most likely heating mechanism. For the network, it is apparen t 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 spicu le-type event in which there is a repeated rebound.