Presentation #107.03 in the session Coronal Heating: Present Understanding and Future Progress II.
The two primary candidates for coronal heating are small-scale reconnection (nanoflares) and plasma wave energy deposition. Our previous work (Bahauddin, Bradshaw, and Winebarger, Nature Astronomy 2021) marked the first time scientists captured the full life-cycle of a nanoflare — from bright origin to decaying demise in solar transition region. In this talk, I will cover how we were able to reveal that the small, low-lying loops residing in the solar transition region are in fact multi-stranded and that their observed electromagnetic spectra are a consequence of the dense plasma undergoing non-equilibrium ionization. One of the central findings of the work was that, in such a plasma, heavier ions (e.g. Si over O) are preferentially accelerated, consistent with ion-cyclotron heating mediated by reconnection. Recently we have extended that work with both detail studies of the individual events and the estimation of the spatiotemporal statistics of many events. An interesting finding of this work is the coexistence of multiple time scales in the dynamics: while the apex of the low-lying loops are impulsively heated by reconnection (< 10 s), their foot-points are heated on a much slower, periodic timescale, showing spectral signatures similar to magneto-acoustic wave-energy deposition. Thus both wave and nanoflares heating may be found at different locations in the same small scale structures in this region. A complementary investigation on resolving the individual sources of acoustic emissions (Bahauddin and Rast, ApJ 2021) in a simulated photosphere reveals that the emissions are clustered at a spatial scale of several Mm, consistent with the spatial scale of the footpoints of low-lying loops. Thus it is becoming possible to both observe and model how the photosphere supplies acoustic energy, how that energy is converted to various MHD modes in the chromosphere and how these waves drive the small-scale loops as precursors for reconnection mediated heating — reopening the paradigm of a unified atmospheric model. With DKIST coming online with its multi-instrument abilities, we should be able to observe these multi-scale, multi-height processes in action.