Thermal structuring of the quiet solar corona

¹ Max Planck Institute for Solar System Research, 37077, Göttingen, Germany
² Institut für Sonnenphysik (KIS), 79110, Freiburg, Germany
³ NASA Goddard Space Flight Center, Solar Physics Laboratory, Heliophysics Science Division, Greenbelt, MD, 20771, USA
⁴ Northumbria University, Physics and Electrical Engineering, Newcastle upon Tyne, NE1 8ST, UK

^⋆ Corresponding author: milanovic@mps.mpg.de

Received: 11 November 2024
Accepted: 8 June 2025

Abstract

Context. The quiet solar atmosphere is populated with plasma loops that are typically observed in the ultraviolet (UV) and extreme-ultraviolet (EUV) wavelengths. The coronal counterparts of these loops are traditionally referred to as coronal bright points. Although they are very compact, bright points reveal a high degree of multithermal complexity through different layers in the solar atmosphere.

Aims. We investigate the thermal structuring of these loop systems to gain further insights into the physical mechanisms that heat the plasma. To this end, we report on the multithermal characteristics of bright points in the quiet solar atmosphere through the transition region and the corona.

Methods. We combined spectral data from the EUV spectrometer SPICE on board Solar Orbiter and imaging data from AIA on board SDO to cover a broad temperature range (log T [K] ≈ 4.6–6.5). The bright points were observed simultaneously in all available spectral and imaging channels. We analyzed 14 features in total, computed their differential emission measure (DEM) distribution, and compared them with the emission measure from the (average) quiet Sun.

Results. We found common characteristics of the DEM in the bright points. In the upper transition region, above temperatures of log T [K] ≈ 5.2, the slope of the DEM toward higher temperatures (i.e., towards the corona) is significantly shallower than in the quiet Sun. The situation is different in the lower transition region, below log T [K] ≈ 5.2: The negative slope of the DEM is similar to that of the quiet Sun there, which implies that in response to the additional heating, the density in the bright point increases by the same factor at all temperatures.

Conclusions. Our finding of the very shallow gradient of the DEM toward higher temperatures is relevant for coronal heating models. Based on earlier studies, a shallower DEM slope would imply fewer heating events in the bright points than in the quiet Sun. The apparent dichotomy between the plasma at the lower and at the higher temperatures might also imply distinct heating mechanisms or thermally disconnected loops in the two temperature ranges. To confirm these, however, a more detailed analysis is required. In particular, UV and EUV spectroscopic time series combined with co-temporal imaging data are required to better capture the thermal evolution of bright points, which in turn will shed further light on the nature of thermal structuring and plasma heating in the corona.