Chromospheric Heating due to Cancellation of Quiet Sun Internetwork Fields

Abstract

The heating of the solar chromosphere remains one of the most important questions in solar physics. Our current understanding is that small-scale internetwork (IN) magnetic fields play an important role as a heating agent. Indeed, cancellations of IN magnetic elements in the photosphere can produce transient brightenings in the chromosphere and transition region. These bright structures might be the signature of energy release and plasma heating, probably driven by magnetic reconnection of IN field lines. Although single events are not expected to release large amounts of energy, their global contribution to the chromosphere may be significant due to their ubiquitous presence in quiet Sun regions. In this paper we study cancellations of IN elements and analyze their impact on the energetics and dynamics of the quiet Sun atmosphere. We use high resolution, multiwavelength, coordinated observations obtained with the Interface Region Imaging Spectrograph (IRIS) and the Swedish 1-m Solar Telescope (SST) to identify cancellations of IN magnetic flux patches and follow their evolution. We find that, on average, these events live for ∼3 minutes in the photosphere and ∼12 minutes in the chromosphere and/or transition region. Employing multi-line inversions of the Mg II h & k lines we show that cancellations produce clear signatures of heating in the upper atmospheric layers. However, at the resolution and sensitivity accessible to the SST, their number density still seems to be one order of magnitude too low to explain the global chromospheric heating.

The multi-thermal chromosphere

“…Some of these assumptions relate to the specificities of the STiC code which assumes a 1D, plane-parallel atmosphere in hydrostatic equilibrium that is interpolated in between pre-defined nodes. Nevertheless, this approach has been shown to provide reasonable results on real data (e.g., de la Cruz Rodríguez et al 2016;Gošić et al 2018;Vissers et al 2019a;Esteban Pozuelo et al 2019). Other assumptions relate to the technicalities of the ALMA interferometric data that are composed of a discrete set of spatial scales unlike the UV data which show all scales down to the IRIS spatial resolution.…”

A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS)

“…Ramos, 2016), which allows quantitative determination of chromospheric conditions based on the extensive spectroscopic diagnostics of IRIS from the photosphere to the transition region, including the sensitive Mg II lines (e.g. to estimate heating from canceling granularscale internetwork fields, Gošić et al, 2018). STiC inversions of IRIS Mg II h and k spectra combined with ALMA mm radiation are particularly promising in terms of constraining the chromospheric temperature and turbulent motions over a wide range of heights (da Silva Santos, de la Cruz Rodríguez, and Leenaarts, 2018;da Silva Santos et al, 2020).…”

“…An example of the inversion approach is the work by Gošić et al (2018) who used STiC inversions to estimate the impact on chromospheric thermodynamics from the release of magnetic energy through cancelation of the pervasive weak magnetic fields in the internetwork. They found that while the local heating is significant, it likely does not play a dominant role in the average energy balance of the quiet-Sun internetwork chromosphere.…”

“…Weak magnetic fields continuously emerge in the quiet-Sun internetwork, and are considered a potential contributor to the heating of the QS chromosphere. Gošić et al (2018) used IRIS and SST observations and found that cancelations of such elements are unlikely to play a significant role in bulk chromospheric heating at the sensitivity levels of highcadence SST measurements, although coordination with deeper magnetograms from SST or DKIST are required to definitively settle this issue. On the other hand, the flux emerging in small magnetic loops may play an important role.…”

Exploring mutual information between IRIS spectral lines. II. Calculating the most probable response in all spectral windows

“…Although the intensity of certain lines may carry vital information about the thermal structure of the atmosphere, (e.g. Leenaarts et al 2013a;Gošić et al 2018), it is often the case that the majority of the atmospheric variation is encoded within the line shapes themselves. For instance, the width of optically thin lines such as O I 1355.6 Å are excellent diagnostics of non-thermal chromospheric velocities (Lin & Carlsson 2015).…”