Kavli Affiliate: Robert Cameron
| First 5 Authors: Guanchong Cheng, Guanchong Cheng, , ,
| Summary:
Flux emergence is ubiquitous in the Sun’s lower atmosphere, where the
emerging magnetic flux can reconnect with the pre-existing magnetic field. We
investigate plasmoid formation and the resulting multi-thermal emissions during
three-dimensional magnetic reconnection in the lower solar atmosphere. We
performed 3D radiation magnetohydrodynamic simulations using the MURaM code,
which incorporates solar convection and radiative transfer. A flat magnetic
flux sheet was introduced into the convection zone to trigger flux emergence.
For comparison with previous observations, we used the RH1.5D code to
synthesize Halpha and Si IV spectral line profiles, and generated
ultraviolet images using the optically thin approximation. The simulations show
that flux emergence occurs as the imposed flux tube crosses the photosphere. In
the lower solar atmosphere, magnetic reconnection forms thin, elongated current
sheets, and plasmoid-like structures develop, producing numerous small twisted
magnetic flux ropes that are expelled toward both ends of the reconnection
region. This process results in the coexistence of hot plasma exceeding 20,000
K and cooler plasma below 10,000 K. Synthetic images and spectral line profiles
through the reconnection region exhibit features characteristic of Ellerman
bombs (EBs) and UV bursts. Cooler plasma associated with EBs can be found above
hot plasma at altitudes exceeding 2 Mm above the solar surface, while hot
plasma associated with UV bursts can extend downward into the lower
chromosphere, reaching approximately 0.7 Mm above the surface. These results
indicate that turbulent reconnection mediated by plasmoid instability can occur
in small-scale events such as EBs and UV bursts, and that the coexistence of
hot and cool plasma in such reconnection processes can account for UV bursts
that are temporally and spatially connected to EBs.
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