Chae et al. (1998) showed there is a strong tendency for explosive events to occur repeatedly in bursts. In this case the persistence of supersonic flows along our raster series in the active region (i.e., the O VI data) could be due to bursts of explosive events in the area observed, which are not connected physically with each other. However, the picture is quite complex and not easy to analyse. The fact that the data are simple raster series makes a proper time evolution analysis impossible, but it still allows us to follow the spatial (temporal) expansion of the jet along the rastered region. Our analyses have shown the complex structure of the velocity fields produced by these jets. There are clearly asymmetric profiles with significative fluctuations in intensity. Also the presence of overlapping velocity fields with a consistent structure along the slit is apparent, often leading to supersonic flows.
For the C IV data in Figs. 5 and 6, it is not possible to identify the signature as a burst of explosive events given the fact that the supersonic flows are concentrated in a small area along the slit, in a relatively small rastered area. Also, against this idea is the apparent pattern that the flows follow in the E-W direction. The characteristics of this sequence of events can, instead, be compared with those discussed by Innes et al. (1997). If we suppose this sequence is produced by only one jet propagating away from its source, the area in which it is visible ( arc sec2) and the observed lifetime ( s) would be in coincidence with those previous results of Innes et al. However, the present event does not show clear Doppler shift changes from red to blue along the E-W direction, or any obvious offset of the red and blue-shifts along the slit. Although there are some indications in the last three raster positions of mainly blue-shifted plasma, we still measure weak supersonic flows in the red wing for some positions. For the rest of the frames, in general we find blue-shifted as well as red-shifted profiles for each position along the slit. The maximum velocities, though, correspond to blue-shifts. Globally the blue wing is m ore intense than the red one.
The C IV event can perhaps be explained as a jet of bi-directional nature if we consider a high latitude as a possible explanation for the apparent confusion between the two opposite flows. The jet can be within a plane that forms a short angle with that formed by the line of sight and the E-W direction, and the axis of this jet forms a relatively small angle with the line of sight. This angle could explain the apparent South-to-North motion of the maximum of the blue-shifted velocity fields. That occurs while we raster West-to-East. In these conditions we can explain why the first raster position shows the coincidence of blue-shifts and red-shifts, while the latter ones are mainly blue-shifted. If this is correct, we can assume that the distance that this event covers in our raster (8 arc sec in 160 s) corresponds with its increase of size. If that change in size is due to the propagation of the head of the jet at a velocity equal to its maximum Doppler velocity, approximately 150 km s-1, then we can estimate its length. We calculate an average transverse velocity of 35 km s-1 which leads to an inclination angle for the axis of our jet of . Then the actual jet length is approximately 35 arc sec or km, estimated from the apparent length of 8 arc sec ( km). This extension implies that the jet reaches the corona and travels down along most of the transition region.
An ongoing explosive event modelling programme allows us to convert computational results into UV line profiles in non equilibrium ionization as a function of time (see, e.g., Erdélyi et al. 1997, 1998, Sarro el al. 1997). The early results of the ongoing modelling indicate a sudden deposition of energy below the transition region on one side of the loop resulting in the ejection of cool, dense gas bullets, plus the generation of sound waves. Following, the interactions between the cool gas bullets and sound waves (which develop into shock waves), we have the appearance of `new' transition regions, moving at different velocities. Although the atomic physics aspect of the simulations are in good shape, the MHD section requires further development and thus detailed comparison with the data is deferred (Erdélyi et al. 1999).
© European Southern Observatory (ESO) 1999
Online publication: December 22, 1998