Electron Density Variations during Ultraviolet Transient Events

High-resolution temporal observations performed with the SUMER spectrometer on SoHO provide an opportunity to investigate the electron density variations in thèquiet Sun' solar transition region due to UV transient events. Two datasets obtained in the density sensitive lines belonging to the O iv 1400 A multiplet were searched for such events, leading to the identiication of two explosive events, on 1996 July 10 and 1997 May 31. In both cases, the O iv 1401:16/1404:81 density sensitive line intensity ratio shows a clear variation, corresponding to enhancements in the electron density by factors of 3. This is fully consistent with recent 2.5D MHD simulations. The 1996 July 10 dataset also provided us with the opportunity to monitor the behaviour of the electron density through an UV blinker. Despite an increase of a factor of two in the line intensities, no variation of the electron density was found. This suggests that the intensity enhancement is due to an increase in the lling factor.