Evidence for a Transition Region response to penumbral microjets in sunspots

Abstract

Penumbral microjets are short-lived, fine-structured and bright jets that are generally observed in chromospheric imaging of the penumbra of sunspots. Here we investigate their potential transition region signature, by combining observations with the Swedish 1-m Solar Telescope (SST) in the Ca II H and Ca II 8542Å lines with ultraviolet imaging and spectroscopy obtained with the Interface Region Imaging Spectrograph (IRIS), which includes the C II 1334/1335Å, Si IV 1394/1403Å and Mg II h & k 2803/2796Å lines. We find a clear corresponding signal in the IRIS Mg II k, C II and Si IV slit-jaw images, typically offset spatially from the Ca II signature in the direction along the jets: from base to top, the penumbral microjets are predominantly visible in Ca II, Mg II k and C II/Si IV, suggesting progressive heating to transition region temperatures along the jet extent. Hence, these results support the suggestion from earlier studies that penumbral microjets may heat to transition region temperatures.

Accepted for publication in the Astrophysical Journal Letters. 6 pages, 5 figures

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