A Cancellation Nanoflare Model for Solar Chromospheric and Coronal Heating. III. 3D Simulations and Atmospheric Response

Abstract

Inspired by recent observations suggesting that photospheric magnetic flux cancellation occurs much more frequently than previously thought, we analytically estimated the energy released from reconnection driven by photospheric flux cancellation, and proposed that it can act as a mechanism for chromospheric and coronal heating (Priest et al. 2018). Using two-dimensional simulations we validated the analytical estimates and studied the resulting atmospheric response (Syntelis et al. 2019). In the present work, we set up three-dimensional resistive MHD simulations of two cancelling polarities in a stratified atmosphere with a horizontal external field to further validate and improve upon the analytical estimates. The computational evaluation of the parameters associated with the energy release are in good qualitative agreement with the analytical estimates. The computational Poynting energy flux into the current sheet is in good qualitative agreement with the analytical estimates, after correcting the analytical expression to better account for the horizontal extent of the current sheet. The atmospheric response to the cancellation is the formation of hot ejections, cool ejections, or a combination of both hot and cool ejections, which can appear with a time difference and/or be spatially offset, depending on the properties of the cancelling region and the resulting height of the reconnection. Therefore, during the cancellation, a wide spectrum of ejections can be formed, which can account for the variety of multi-thermal ejections associated with Ellerman bombs, UV bursts and IRIS bombs, and also other ejections associated with small-scale cancelling regions and spicules.

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

“…The resulting heating and jet acceleration can occur in the photosphere, chromosphere, transition region, or corona in a way that is evaluated in the model and depends on the sizes of the magnetic fluxes, their separation, and the strength of the overlying magnetic field. The height of the separator and the rate of energy release are calculated analytically and verified in 2D and 3D computational experiments (Syntelis et al 2019;Syntelis & Priest 2020). Then a second cancellation phase occurs as the opposite-polarity fragments undergo actual cancellation and thus drive reconnection at the photospheric footpoints and in the overlying atmosphere.…”

“…Inspired by recent discoveries that flux cancellation is much more common than had previously been recognised and that bright loops often have footpoints in regions where oppositepolarity magnetic flux at the solar surface is cancelling, a theoretical model for the creation of nanoflares by flux cancellation rather than magnetic braiding has been developed (Priest et al 2018;Syntelis et al 2019;Syntelis & Priest 2020). It considers 2018 June 17 UT 00:00 to 2018 June 18 UT 10:00 14 12733…”

“…Then a second cancellation phase occurs as the opposite-polarity fragments undergo actual cancellation and thus drive reconnection at the photospheric footpoints and in the overlying atmosphere. This "cancellation nanoflare model", conditions for which could be set by previous flux emergence events, is able to qualitatively account for chromospheric and coronal heating and for a wide variety of dynamic jet-like phenomena throughout the atmosphere (Priest et al 2018;Syntelis et al 2019;Syntelis & Priest 2020).…”

ALMA observations of transient heating in a solar active region

“…Numerical simulations have shown that the magnetic topology that determines the height of reconnection along with the line of sight to the reconnection site play a role in the visibility of these small-scale heating events across the wavelength spectrum (Hansteen et al 2019;Peter et al 2019;Ortiz et al Movie associated to Fig. 5 is available at https:// www.aanda.org 2020; Syntelis & Priest 2020). For example, strong emission in the wings of Hα would imply reconnection in the lower atmosphere, whereas bright Mg ii h and k, Si iv 1393.7, 1402.8 Å or even extreme-ultraviolet (EUV) emission would originate in higher layers.…”

Formation of solar coronal loops through magnetic reconnection in an emerging active region

“…The difference is that EBs and UBs are more likely related to magnetic reconnection occurring in the lower-atmosphere part of the U-shape structures, whereas the reconnection in our observations appears to occur in a relatively higher part (possibly the lower corona) of the U-shape structures. We noticed that a similar process has been theoretically and numerically investigated by Priest et al (2018) and Syntelis & Priest (2020), respectively.…”