Magnetic loops in the solar transition region

Abstract

Transition region (TR) loops are arcade-like features in the solar transition region, with temperatures roughly between $2\times10^4$ K and $6\times10^5$ K. They are a fundamental building block of TR, which are results of the coupling between the magnetic field and the TR plasma. Their dynamics is closely related to the transport of energy and mass through the TR. Studies on this class of loops since the launch of the Interface Region Imaging Spectrograph (IRIS) have revealed that they are distinct from coronal loops. Observations have revealed that they are associated with many small-scale dynamic phenomena in the TR, from which one can infer the physics behind the energy and mass transfer in a magnetically confined environment at TR temperature. This review summarises the observational results of TR loops, showing their morphology, dynamics, plasma parameters, their relationship with flux emergence, their heating properties, and their implication in the heating of the solar atmosphere. This class of magnetic loops is much less well understood than their coronal counterparts. This review also concludes with several critical questions that need to be answered in the coming era with more advanced observational techniques and more precise and realistic simulations.