Spectroscopic and imaging observations of transient hot and cool loops by IRIS and SDO

Abstract

Coronal loops are the basic building blocks of the solar atmosphere and are observed on various length scales. However, their formation mechanism is still unclear. In this paper, we present spectroscopic and imaging observations of small-scale transients and the subsequent formation of transient loops. For this purpose, we have utilized the multiwavelength observations recorded by the Atmospheric Imaging Assembly (AIA) and the Interface Region Imaging Spectrograph (IRIS) slit-jaw imager (SJI), along with spectroscopic measurements provided by IRIS. For the photospheric magnetic field data, we obtained line-of-sight magnetogram data provided by the Helioseismic and Magnetic Imager (HMI). Small-scale transients are simultaneously observed with several EUV and UV passbands of AIA and IRIS/SJI. The HMI magnetogram provides evidence of negative flux cancellations beneath these transients. Differential emission measure (DEM) analysis shows that one of the transients attains temperatures up to 8 MK whereas another one only reaches 0.4 MK. These transients further lead to the formation of small-scale loops with similar temperature distributions, thus termed hot and cool loops respectively. During the course of events, the IRIS slit was rastering the region and thus provided spectroscopic measurements at both transients and associated loops. This enabled us to perform in-depth investigations of the hot and cool loops. Using a density-sensitive O IV line pair, we obtained average electron densities along the hot and cool loops to be 1011.2 and 1010.8 cm-3 respectively. Energy estimates suggest that flux cancellation could easily power the hot transient, while it is insufficient for the cool transient. Lifetime estimates and magnetic field extrapolation suggest the presence of small-scale and fine structures within these loops. The results provide crucial ingredients for the physics of loop formation and the thermodynamics involved.