A Transition Region Explosive Event Observed in He II with the MOSES Sounding Rocket

Abstract

Transition region explosive events (EEs) have been observed with slit spectrographs since at least 1975, most commonly in lines of C IV (1548 Å, 1550 Å) and Si IV (1393 Å, 1402 Å). We report what we believe to be the first observation of a transition region EE in He II 304 Å. With the Multi-Order Solar EUV Spectrograph (MOSES) sounding rocket, a novel slitless imaging spectrograph, we are able to see the spatial structure of the event. We observe a bright core expelling two jets that are distinctly non-collinear, in directions that are not anti-parallel. The jets have sky-plane velocities of order 75 km s^-1 and line-of-sight velocities of +75 km s^-1 (blue) and -30 km s^-1 (red). The core is a region of high non-thermal Doppler broadening, characteristic of EEs, with maximal broadening 380 km s^-1 FWHM. It is possible to resolve the core broadening into red and blue line-of-sight components of maximum Doppler velocities +160 km s^-1 and -220 km s^-1. The event lasts more than 150 s. Its properties correspond to the larger, long-lived, and more energetic EEs observed in other wavelengths.