Non-thermal Transition Region Activity of the Sun as a Star

The Full-Sun Ultraviolet Rocket Spectrometer (FURST) is a NASA suborbital sounding rocket mission scheduled to launch in spring of 2023 and will obtain the first high-resolution (R ≈ 30,000 or ~10 km/s), radiometrically-calibrated VUV spectrum (118–185 nm) of the Sun as a star. These VUV spectral irradiance measurements would allow us to better understand the physical processes associated with chromospheric and coronal heating, such as magnetic-reconnection-driven supersonic plasma flows. They could also inform solar-stellar studies by providing a link which places the Sun into context with other stars through direct comparison with measurements from the Hubble Space Telescope.

Inspired by FURST, we have been using monthly Si IV 1394 and 1403 Å full-disk mosaics from the Interface Region Imaging Spectrograph (IRIS) to estimate solar transition region (TR) activity in the Sun-as-a-star spectrum. This is accomplished by detecting signatures of magnetic reconnection across the disk in the form of explosive events (EEs), which are transient, non-thermal, Doppler-broadened (or shifted) enhancements in TR spectral line profiles. The EE spectra are then separated from the remainder of the disk and spatially-summed, providing an estimate of TR activity that can be compared to stellar spectra. With the large number of IRIS full-disk mosaics available, the EE component of the Sun-as-a-star spectrum can be characterized across varying levels of solar activity and also used to estimate any remaining unresolved supersonic emission. This empirically-derived characteristic spectral shape of the EE component may also be used to estimate TR activity in Sun-like stars by comparing it to high-resolution stellar data from instruments such as the Space Telescope Imaging Spectrograph and Cosmic Origins Spectrograph on the Hubble Space Telescope. Here we present some initial results for a characteristic EE spectral profile from the extensive IRIS full-disk mosaic dataset.