A New Feature of the Quiet Sun Corona During Solar Minimum.

Abstract

Quiet Sun (QS) loops have received much less attention than active region loops, partly due to the relative difficulty in identifying individual QS loop structures. However, understanding the physical processes that heat the quiet corona is critically important, as the quiet corona overlies most of the Sun's surface (especially during solar minimum). To study QS loops, we developed a novel technique called Michigan Loop Diagnostic Technique (MLDT). MLDT combines Differential Emission Measure Tomography and a potential field source surface model to obtain the electron temperature and density at each point along a QS loop.

We applied MLDT to study QS loops using EUVI/STEREO and MDI/SOHO observations taken during Carrington Rotation (CR) 2077. The MLDT identified two types of QS loops: ``Up Loops" (UL) in which the temperature increases with height, and ``Down Loops" (DL) in which the temperature decreases with height. DLs are ubiquitous in the low-latitude quiescent corona, while ULs dominate at higher latitudes. ULs have been widely observed and studied in active regions. The identification of DLs was a surprise in solar physics and these loops constitute a new class of plasma structures populating the solar corona.

The discovery of DLs is not widely accepted as the community considers DLs are unstable against thermal instabilities. To address the thermal stability, we performed time-accurate 3D MHD simulations, first creating DLs by increasing the heating near the footpoints, and then imposing a small pressure perturbation near the apex. The evolution of the DL returned to its steady state solution about three hours after the perturbation, thus confirming that DLs are thermodynamically stable against small pressure perturbations.

We further studied DLs properties in 11 CRs to reveal more properties of DLs. DLs are always found to be located at low latitudes and are anti-correlated with sunspot numbers; moreover, they had systematically larger values of plasma beta than ULs.

Down loops are a newly discovered structure of the quiet Sun corona during solar minimum that may shed light on the physics of coronal heating: the coronal heating is enhanced near the footpoints of DLs while the heating is more uniform in ULs.