What is the nature of transition region explosive events? J.G. Doyle, R. Erdelyi, B. de Pontieu & C. de Forest High resolution ultra-violet (UV) spectra taken with HRTS and SUMER show transient enhancements of the wings of lines formed in the solar transition region, often referred to as explosive events. Observations to date show: the observed velocities range between 50 - 250 km/s, (ii) a particular explosive event may present different time histories with distinct activity episodes (iii) the life time distribution ranges between 20 - 200 sec. We have recently starting a modelling programme whereby numerical simulations are compared to observational data. However, many outstanding questions remain, e.g. why is the upflow plasma not observed in the corona? (ii) why is the enhanced activity in the transition region not observable in the lower atmosphere? Perhaps it is observable but has been labelled under a different header under what can be termed the solar zoo. Using semi-circular magnetic flux tubes we find that thermal energy perturbations drive flows along the flux tube. The time evolution of our simulations first shows a sudden rise in temperature at the perturbation site followed by the ejection of cool dense gas bullets and the generation of sound waves. This is then followed by the appearance of `new' transition regions moving at different velocities. Our computational results are converted into UV line profiles in (non)-equilibrium ionization. Our simulations are only a first approximation, however to advance further requires additional observational data. Here we propose a JOP involving simultaneous H alpha (via Tenerife), SUMER, MDI, CDS & EIT. We propose to observe a quiet Sun region at disk center using a high cadence mode in each instrument. The ground based programme runs from 8:00 - 11:00 UT on the 4, 5, 6 & 7 August 1998. H alpha ======= Observations to be done in Tenrife from 8:00 to 11:00 on the 4, 5, 6 & 7 August 1998, observing in the blue, red and line center of H alpha. Observers: B. de Pontieu & R. Erdelyi SUMER ===== Require coverage of high-to-low temperature region lines in a high cadence mode (~20 sec) in the 'sit-and-stare' mode at disk center from 8:00 - 11:00 on the 4 -- 7 August inclusive. Observer: J.G. Doyle MDI === Require MDI in the high resolution mode pointing at disk center from 8:00 - 11:00 on the 4 -- 7 August inclusive. Observer: C. de Forest & J.G. Doyle CDS === Require CDS in a high cadence raster mode observing O V 630, Mg IX 368 & He I 584 from 8:00 - 11:00 on the 4 -- 7 August inclusive. Plan to use the 4x240 arcsec slit, with a 120 arcsec spatial window, 30 pixel wavelength window, an exposure time of 10 sec, stepping with a 4 arcsec step for 15 steps (i.e. a raster of 60 x120 arcsec in 214 secs.), repeat 60 times, i.e. a total duration of ~214 mins. Observer: J.G. Doyle EIT === Require EIT in its high cadence mode observing one of the Fe lines from 8:00 - 11:00 on the 4 -- 7 August inclusive. J.G. Doyle will be present at Goddard from the 2 - 8 August 1998.