Chromospheric counterparts of solar transition region unresolved fine structure loops

TL;DR

This study reveals chromospheric signals associated with low-lying transition region loops, showing strong Doppler shifts and magnetic reconnection evidence, which informs models of unresolved fine structures in the solar atmosphere.

Contribution

First observational evidence of chromospheric impact of transition region loops, highlighting their dynamic nature and potential magnetic reconnection processes.

Findings

Chromospheric signals detected in transition region loops.

Presence of inverse Y-shaped jets indicating magnetic reconnection.

Loops show strong Doppler shifts without significant chromospheric heating.

Abstract

Low-lying loops have been discovered at the solar limb in transition region temperatures by the Interface Region Imaging Spectrograph (IRIS). They do not appear to reach coronal temperatures, and it has been suggested that they are the long-predicted unresolved fine structures (UFS). These loops are dynamic and believed to be visible during both heating and cooling phases. Making use of coordinated observations between IRIS and the Swedish 1-m Solar Telescope, we study how these loops impact the solar chromosphere. We show for the first time that there is indeed a chromospheric signal of these loops, seen mostly in the form of strong Doppler shifts and a conspicuous lack of chromospheric heating. In addition, we find that several instances have a inverse Y-shaped jet just above the loop, suggesting that magnetic reconnection is driving these events. Our observations add several puzzling details to the current knowledge of these newly discovered structures; this new information must be considered in theoretical models.