What are supra-arcade downflows?
Supra-arcade downflows (SAD) were first observed on the Sun in 1999 by the Yohkoh spacecraft. They are dark, finger-like plasma streams that occur during eruptive flares, are located above groups of coronal loops (arcades), and move towards the Sun. Thought to be related to magnetic reconnection, they often move at speeds much slower than those predicted by theories.
They tried to explain them by models containing a pulsating jet, Rayleigh-Taylor type instabilities, or slowing down due to aerodynamic drag, but scientists failed to come to a full agreement between the theory and observational data.
We waited 20 years for an explanation
Astronomers led by Chengcai Shen of the Harvard-Smithsonian Center for Astrophysics reported that they had succeeded in explaining the nature of SAD structures. The scientists built a three-dimensional magnetohydrodynamic model of a solar flare caused by magnetic reconnection and then compared the simulation results with observations from the Atmospheric Imaging Assembly (AIA) instrument aboard the SDO solar probe.
The researchers determined that supra-arcade downflows are not located in the reconnection current sheet, characterized by high current density and a flat radiation intensity profile, but are located in a turbulent region above the arcade, characterized by a weak and highly inhomogeneous current density, as well as a sharply decreasing radiation intensity profile.
In it, fast downward reconnection outflows collide with closed coronal loops. Such a region is analogous to a highly turbulent region sandwiched between forward and backward shock waves in supernova remnants, where finger-like structures are also observed.
The scientists concluded that supra-arcade downflows are associated with, but are not reconnection effluxes. Instead, they are self-organized structures formed by Rayleigh-Taylor and Richtmyer-Meshkov instabilities, which usually arise at the interface between two substances with different densities due to vortices created by gravity or impacts. The plasma density in finger-like structures is less than in the environment, so they appear dark.
The simulation also shows that some observed plasma outflows over the arcades, including supra-arcade downflows, have velocities much lower than the Alfvén velocity, which can lead to a situation of overestimation of the reconnection rate (the ratio of the velocity of the plasma moving towards the reconnection current sheet to the local Alfvén velocity).
In addition, turbulent flows and fast and slow shock waves in the region above the arcades can lead to plasma compression and heating, and the complex magnetic configuration and turbulent plasma can contribute to efficient particle acceleration and small-scale magnetic reconnection. Such phenomena must be taken into account when interpreting spacecraft observational data.
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• Luntz, S. (2022, January 27). Solar flares have strange dark ‘fingers’ and now we may know why. IFLScience.
• O’Neill, M. (2022, January 31). In 1999 mysterious finger-like features were spotted on the sun – now scientists have an explanation. SciTechDaily.
• ScienceDaily. (2022, January 27). Scientists explain mysterious finger-like features in solar flares.
• Shen, C., Chen, B., Reeves, K. K., Yu, S., Polito, V., & Xie, X. (2022, January 27). The origin of underdense plasma downflows associated with magnetic reconnection in solar flares. Nature News.
• Starr, M. (n.d.). The source of Ghostly shadows in the Sun’s atmosphere could finally be identified. ScienceAlert.
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