IRIS Observations of Short-term Variability in Moss Associated with Transient Hot Coronal Loops

Abstract

We observed rapid variability ( 60 s) at the footpoints of transient hot (∼ 8 − 10 MK) coronal loops in active region cores, with the Interface Region Imaging Spectrograph (IRIS). The high spatial (∼ 0.33 ) and temporal ( 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.

Impulsive coronal heating during the interaction of surface magnetic fields in the lower solar atmosphere

“…This would then give rise to the observed rapid variability and plasma flows observed in the low atmosphere. Such models produce Si iv Doppler shifts of about ±40 km s −1 and line widths of up to 20 km s −1 (Testa et al 2020). Provided one assumes an appropriate distribution for the energy flux carried by non-thermal particles accelerated at a coronal site, such a model can explain both the intermittency and the spectral properties of the UV emission at one footpoint in one of our examples (e.g.…”

“…In contrast, footpoint W1 exhibited brightenings that are consistent with rapid moss variability in response to the impulsive loop heating (e.g. Testa et al 2014Testa et al , 2020. When the slit crossed the region around 10:41 UT (around the time when the coronal signal has pulse-like enhancements), the IRIS profiles are markedly different.…”

“…This suggests a rapid temperature increase with height in the chromosphere (Pereira et al 2015). The rapid moss variability suggests energy deposition at that loop footpoint-W1 (Testa et al 2020).…”

“…In that case, numerical models suggest that a secondary instability could impulsively heat the loop when the magnetic misalignment angle in the corona exceeds 45 • (Dahlburg et al 2005). In addition, it has been suggested that the observation that both footpoints of the loop brighten near-simultaneously together with imaging and spectroscopic signatures of rapid moss variability is consistent with coronal braiding nanoflares (Testa et al 2020).…”

“…Observations of transient soft X-ray brightenings (Shimizu et al 1992), the detection of hot plasma over 10 MK in the core of an active region (Ishikawa et al 2017), and rapid moss variability on timescales below 60 s in the ultraviolet (UV) and extreme-UV (EUV) emission at the footpoints of hot loops with near-simultaneous brightening at both footpoints (Testa et al 2014(Testa et al , 2020Gupta et al 2018;Chitta et al 2018) are possible signatures of impulsive nanoflares. In the traditional view, it is generally thought that footpoints of impulsively heated nanoflaring loops end in regions of unipolar magnetic fields at the solar surface and that energy is loaded into the magnetic loop by the slow driving of footpoints (Klimchuk 2006).…”