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Astron. Astrophys. 342, 279-284 (1999)

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1. Introduction

With the launch of SOHO new opportunities have become available for studying short-time scale variability phenomena, such as explosive events. The instrument that allows us to do so is SUMER (Solar Ultraviolet Measurements of Emitted Radiation), a stigmatic normal incidence spectrograph operating in the wavelength range 450 to 1610 Å (Wilhelm et al. 1997). In July 1996, we obtained data with SUMER at several locations on the solar disk using two different modes of operation; (i) a sit-and-stare mode and (ii) rastering. Here we report two raster datasets; the first taken in an active region on 10 July 1996 and the second taken on 14 July 1996 in a northern coronal hole region. The observing sequences involved the resonance lines C IV 1548 Å and O VI 1032 Å formed in the transition region between [FORMULA] and [FORMULA] K.

The phenomena underlying some explosive events has been interpreted as a bi-directional jet. Dere et al. (1991) first suggested such an analysis and recently Innes et al. (1997) have found new evidence to support this interpretation. Innes et al. (1997) found Doppler shifts changing from red to blue within a few arc sec (an offset of 8 arc sec along the slit was observed), and the two wings of the emission moving away from the center of the jet by a distance of 6 arc sec in one case. During the decay of these events the velocities remained large having a lifetime of approximately 4 min. On the other hand, Chae et al. (1998) showed a strong tendency for explosive events to occur repeatedly in bursts. In the present work both previously mentioned characteristics are observed.

The purpose for obtaining this data was to provide input for an ongoing explosive event modelling programme (Erdélyi et al. 1999). The scope of this programme is to study the relevance of the explosive events phenomena in the process of heating the solar corona.

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© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998