Astronomers found first direct observational evidence for a shock wave reflected by a coronal hole.
Salvatore Mancuso and colleagues reported the first observational evidence in the radio range of the reflection of a coronal shock wave at the boundry of the coronal hole. The event occured above an active region located at the northwest limb of the sun and was characterized by an eruptive prominence and an extreme-ultraviolet (EUV) wave steepening into a shock. Their study recently appeared in Arxiv.
Solar type II radio bursts are the signature of particle acceleration by shock waves in the solar corona and interplanetary medium. The shocks originate in solar eruptions involving coronal mass ejections (CMEs) moving at super-Alfvenic speeds. Type II bursts exhibit a relatively slow drift from frequencies ranging from hundreds of MHz to tens of kHz. There are very less investigation on the type II radio bursts with reverse frequency drift. Within the framework of the plasma emission hypothesis, the generation of type II radio bursts takes place at frequencies close to the Langmuir frequency of the plasma and its second harmonic. Assuming generation of type II emission by shock waves, the observed reverse frequency drift would imply shock propagation toward the solar surface or at some angle in the direction of a local enhancement of the coronal electron density. However, since type II radio bursts with reverse frequency drift in the metric band are uncommon, a likewise extraordinary physical mechanism must be at work to account for their production.
Now, Salvatore Mancuso and colleagues provided evidence of production mechanism of type II solar bursts by analyzing imaging data from the two space-based EUV instruments and the Nancay Radioheliograph.
They showed that the reverse-drifting type II emission was produced at the intersection of the shock front, reflected at the coronal hole boundary, with an intervening low-Alfven-speed region characterized by an open field configuration.
“This is for the first time, anyone provided direct observational evidence in the radio band of the sunward reflection of a portion of the coronal shock wave surface at the boundary of a coronal hole.”
Reference: S. Mancuso, A. Bemporad, F. Frassati, D. Barghini, S. Giordano, D. Telloni, C. Taricco, “Radio evidence for a shock wave reflected by a coronal hole”, Arxiv, pp. 1-6, 2021. https://arxiv.org/abs/2107.05931
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