Snapshot imaging spectroscopy of the solar transition region: the multi-order solar EUV spectrograph (MOSES) sounding rocket mission
Fox, James Lewis
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We have developed a revolutionary spectroscopic technique for solar research in the extreme ultraviolet. This slitless spectrographic technique allows snapshot imaging spectroscopy with data exactly cotemporal and cospectral. I have contributed to the successful realization of an application of this technique in the Multi-Order Solar EUV Spectrograph, MOSES. This instrument launched 2006 Feb 8 as a NASA sounding rocket payload and successfully returned remarkable data of the solar transition region in the He II 304 Ångstrom spectral line. The unique design of this spectrometer allows the study of transient phenomena in the solar atmosphere, with spatial, spectral, and temporal resolution heretofore unachievable in concert, over a wide field of view. The fundamental concepts behind the MOSES spectrometer are broadly applicable to many solar spectral lines and phenomena and the instrument thus represents a new instrumentation technology. The early fruits of this labor are here reported: the first scientific discovery with the MOSES sounding rocket instrument, our observation of a transition region explosive event, phenomena observed with slit spectrographs since at least 1975, most commonly in lines of C IV (1548 Ångstrom, 1550 Ångstrom) and Si IV (1393 Ångstrom, 1402 Ångstrom). This explosive event is the first seen in He II 304 Ångstrom. With our 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, in contradiction to standard models of explosive events, which give collinear jets. The jets have sky-plane velocities of order 75km s -¹ and line-of-sight velocities of +75km s -¹ (blue) and -30km s -¹ (red). The core is a region of high non-thermal doppler broadening, characteristic of explosive events, 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 explosive events observed in other wavelengths.