Abstract
Context. Ultraviolet bursts are transients in the solar atmosphere with an increased impulsive emission in the extreme UV lasting for one to several tens of minutes. They often show spectral profiles indicative of a bi-directional outflow in response to magnetic reconnection. Aims. To understand UV bursts, we study how motions of magnetic elements at the surface can drive the self-consistent formation of a current sheet resulting in plasmoid-mediated reconnection. In particular, we want to study the role of the height of the reconnection in the atmosphere. Methods. We conducted numerical experiments solving the 2D magnetohydrodynamic equations from the solar surface to the upper atmosphere. Motivated by observations, we drove a small magnetic patch embedded in a larger system of magnetic field of opposite polarity. This type of configuration creates an X-type neutral point in the initial potential field. The models are characterized by the (average) plasma-β at the height of this X point. Results. The driving at the surface stretches the X-point into a thin current sheet, where plasmoids appear, accelerating the reconnection, and a bi-directional jet forms. This is consistent with what is expected for UV bursts or explosive events, and we provide a self-consistent model of the formation of the reconnection region in such events. The gravitational stratification gives a natural explanation for why explosive events are restricted to a temperature range around a few 0.1 MK, and the presence of plasmoids in the reconnection process provides an understanding of the observed variability during the transient events on a timescale of minutes. Conclusions. Our numerical experiments provide a comprehensive understanding of UV bursts and explosive events, in particular of how the atmospheric response changes if the reconnection happens at different plasma-β, that is, at different heights in the atmosphere. This analysis also gives new insight into how UV bursts might be related to the photospheric Ellerman bombs.
Highlights
The solar atmosphere is in a highly dynamic state
The top panel a displays the kinetic energy integrated in a rectangle around the reconnection region as outlined by the dashed red lines in panels b–d
The density and magnetic field in our model are motivated by a UV burst observation, and we expect that the Alfvén speed in a UV burst is comparable to what we find in our model
Summary
The solar atmosphere is in a highly dynamic state. Using data from rocket flights, Brueckner & Bartoe (1983) isolated what they originally called turbulent events and what were later termed explosive events. They stood out as excessive broadening of spectral lines interpreted as nonresolved motions in response to heating of the plasma. Given the limited amount of observing time of the experiments on rockets and a Space Shuttle flight, these explosive events have been mostly seen in the quiet Sun where a clear connection could be established to the bright network structures and the magnetic field (Porter & Dere 1991). With the era of the Solar and Heliospheric Observatory
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