Transition region loops in the very late phase of flux-emergence in IRIS sit-and-stare observations

TL;DR

This study uses IRIS sit-and-stare spectroscopic observations to analyze the dynamics, interactions, and heating processes of transition region loops during the late phase of flux emergence, revealing ongoing expansion, reconnection, and periodic brightenings.

Contribution

It provides new insights into the behavior of transition region loops in the late flux emergence phase, highlighting their expansion, interactions, and heating mechanisms through detailed spectroscopic analysis.

Findings

Loops are still expanding and moving upward at a few km/s.

Magnetic reconnection is driven by loop interactions, causing brightenings and explosive events.

Quasi-periodic brightenings at the loop apex suggest oscillation-modulated reconnection.

Abstract

Loops are one of the fundamental structures that trace the geometry of the magnetic field in the solar atmosphere. Their evolution and dynamics provide a crucial proxy for studying how the magnetized structures are formed and heated in the solar atmosphere. Here, we report on spectroscopic observations of a set of transition region loops taken by the Interface Region Imaging Spectrograph (IRIS) at Si IV 1394 \AA\ with a sit-and-stare mode. The loops are corresponding to the flux emergence at its very late phase when the emerged magentic features in the photosphere have fully developed. We find the transition region loops are still expanding and moving upward with a velocity of a few kilometers per second ($\lesssim$10 km/s) at this stage. The expansion of the loops leads to interactions between themselves and the ambient field, which can drive magnetic reconnection evidenced by multiple intense brightenings, including transition region explosive events and IRIS bombs in the footpoint region associated with the moving polarity. A set of quasi-periodic brightenings with a period of about 130 s is found at the loop apex, from which the Si IV 1394 \AA\ profiles are significantly non-Gaussian with enhancements at both blue and red wings at Doppler velocities of about 50 km/s. We suggest that the transition region loops in the very late phase of flux emergence can be powered by heating events generated by the interactions between the expanding loops and the ambient fields and also by (quasi-)periodic processes, such as oscillation-modulated braiding reconnection.