Evidence for a Transition Region response to penumbral microjets in sunspots

TL;DR

This study combines high-resolution solar observations to provide evidence that penumbral microjets in sunspots heat to transition region temperatures, revealing a spatial and thermal progression along the jets.

Contribution

It offers the first direct observational evidence linking penumbral microjets to transition region heating using multi-wavelength data.

Findings

Penumbral microjets show a clear transition region signature in IRIS data.

Jets are spatially offset in different spectral lines, indicating heating progression.

Results support microjets heating to transition region temperatures.

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 8542{\AA} lines with ultraviolet imaging and spectroscopy obtained with the Interface Region Imaging Spectrograph (IRIS), which includes the C II 1334/1335{\AA}, Si IV 1394/1403{\AA} and Mg II h & k 2803/2796{\AA} 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.