IRIS observations of short-term variability in moss associated with transient hot coronal loops

TL;DR

IRIS observations reveal rapid, small-scale variability at the footpoints of transient hot coronal loops, providing insights into coronal heating mechanisms and the role of non-thermal particles through combined spectral analysis and modeling.

Contribution

This study combines high-resolution IRIS spectral observations with 1D RADYN modeling to diagnose coronal heating processes and detect non-thermal particles in small-scale solar events.

Findings

Detection of rapid variability in IRIS data at coronal loop footpoints.

Spectroscopic evidence for non-thermal electrons in small reconnection events.

Mg II triplet emission as a diagnostic for non-thermal particles.

Abstract

We observed rapid variability ($\lesssim 60$ s) at the footpoints of transient hot ($\sim 8-10$ MK) coronal loops in active region cores, with the Interface Region Imaging Spectrograph (IRIS). The high spatial ($\sim 0.33$ arcsec) and temporal ($\lesssim 5$-10 s) resolution is often crucial for the detection of this variability. We show how, in combination with 1D RADYN loop modeling, these IRIS spectral observations of the transition region (TR) and chromosphere provide powerful diagnostics of the properties of coronal heating and energy transport (thermal conduction and/or non-thermal electrons (NTE)). Our simulations of nanoflare heated loops indicate that emission in the Mg II triplet can be used as a sensitive diagnostic for non-thermal particles. In our events we observe a large variety of IRIS spectral properties (intensity, Doppler shifts, broadening, chromospheric/TR line ratios, Mg II triplet emission) even for different footpoints of the same coronal events. In several events, we find spectroscopic evidence for NTE (e.g., TR blue-shifts and Mg II triplet emission) suggesting that particle acceleration can occur even for very small magnetic reconnection events which are generally below the detection threshold of hard X-ray instruments that provide direct detection of emission of non-thermal particles.