New Evidence that Magnetoconvection Drives Solar–Stellar Coronal Heating

Abstract

How magnetic energy is injected and released in the solar corona, keeping it heated to several million degrees, remains elusive. Coronal heating generally increases with increasing magnetic field strength. From a comparison of a nonlinear force-free model of the three-dimensional active region coronal field to observed extreme-ultraviolet loops, we find that (1) umbra-to-umbra coronal loops, despite being rooted in the strongest magnetic flux, are invisible, and (2) the brightest loops have one foot in an umbra or penumbra and the other foot in another sunspot's penumbra or in unipolar or mixed-polarity plage. The invisibility of umbra-to-umbra loops is new evidence that magnetoconvection drives solar-stellar coronal heating: evidently, the strong umbral field at both ends quenches the magnetoconvection and hence the heating. Broadly, our results indicate that depending on the field strength in both feet, the photospheric feet of a coronal loop on any convective star can either engender or quench coronal heating in the loop's body.

A Cancellation Nanoflare Model for Solar Chromospheric and Coronal Heating

“…We have proposed a ubiquitous way of creating nanoflares near the base of chromospheric and coronal loops with sufficient energy to power the chromosphere and corona, building on previous flux cancellation theory (e.g., Parnell & Priest 1995;Welsch 2006). In future, it will be interesting to develop the model further by means of computational experiments, in order to investigate the nature of the energy release and its propagation along magnetic loops from the reconnection source.…”

“…Tiwari et al (2014) and Huang et al (2018) discussed examples of flux cancellation triggering coronal brightening in apparently braided loops. Chitta et al (2017a) observed that coronal loops in an evolved active region respond to an underlying ultraviolet burst and bidirectional jets, which in turn are triggered by magnetic reconnection at heights of 500 km above the photosphere driven by magnetic interactions leading to flux cancellation (Tiwari et al 2017). Furthermore, Chitta et al (2018) observed flux cancellation near the footpoints of coronal loops hosting nanoflares in the core of an active region.…”

Fine-scale Explosive Energy Release at Sites of Prospective Magnetic Flux Cancellation in the Core of the Solar Active Region Observed by Hi-C 2.1, IRIS, and SDO

“…However a careful inspection reveals the presence of flux convergence along the sharp neutral line, plausibly driving cancellation, accompanied by the loop brightening. The presence of mixed-polarity field and/or flux cancellation has been recently reported to play an important role in coronal loop heating and is proposed to be present at least at one footpoint of a bright coronal loop (Tiwari et al 2014(Tiwari et al , 2017Chitta et al 2017b;Priest et al 2018). Here we show smaller loop events than earlier reported ones but some of these might share the heating mechanism with those coronal loops with mixed-polarity field at least at one foot.…”

Magnetic Field Dynamics and Varying Plasma Emission in Large-scale Coronal Loops

“…Until very recently magnetic field measurements were dominated by SOHO/MDI, SDO/HMI, and Hinode/SOT instruments and several studies based on these data (e.g., Del Zanna 2003;Ugarte-Urra et al 2009) reported that coronal loops are often connected to highly dynamic but nevertheless unipolar plage fields. However, recently Wang (2016) argued that HMI instrument does not resolve many small-scale structures so, that mixed polarity may be present Using a similar approach Tiwari et al (2017) also argued that coronal heating may be fueled by vigorous magneto-convection which can braid magnetic field lines and that the heating rate is directly dependent on the field strength in the loop. However, strong fields, such as those found in the sunspot umbra suppress magnetoconvection thus reducing the heating rate (e.g., Chen et al 2014;Tiwari et al 2017).…”

“…However, recently Wang (2016) argued that HMI instrument does not resolve many small-scale structures so, that mixed polarity may be present Using a similar approach Tiwari et al (2017) also argued that coronal heating may be fueled by vigorous magneto-convection which can braid magnetic field lines and that the heating rate is directly dependent on the field strength in the loop. However, strong fields, such as those found in the sunspot umbra suppress magnetoconvection thus reducing the heating rate (e.g., Chen et al 2014;Tiwari et al 2017). Chitta et al (2017Chitta et al ( , 2018 further noted that some bright AR loops are rooted in mixed polarity areas, while Tiwari et al (2017) suggested that interaction of opposite polarity fields may supply additional energy, in excess of that generated by loop braiding.…”