Secondary Rayleigh-Taylor Instabilities in the Reconnection Exhaust Jet: A Mechanism for Supra-Arcade Downflows in the Solar Corona

Thursday, 18 December 2014: 4:15 PM
Lijia Guo1, Amitava Bhattacharjee2, Yi-Min Huang3 and Davina Innes1,4, (1)Max Planck Princeton Research Center for Plasma Physics, Solar Physics, Princeton, NJ, United States, (2)Princeton Plasma Physics Laboratory, Princeton University, Princeton, NJ, United States, (3)Princeton University, Princeton Plasma Physics Laboratory, Princeton, NJ, United States, (4)Max Planck Insitute for Solar System Research, Solar Physics, Gottingen, Germany
Supra-arcade downflows (hereafter referred to as SADs) are low-emission, elongated, finger-like features usually observed in active-region coronae above post-eruption flare arcades. Observations exhibit downward moving SADs intertwined with bright, upward moving spikes. Whereas SADs are dark voids, spikes are brighter, denser structures. Although SADs have been observed for decades, the mechanism for formation of SADs remains an open issue. Using high-Lundquist-number three-dimensional resistive MHD simulations, we demonstrate that secondary Rayleigh-Taylor type instabilities develop in the downstream region of a reconnecting current sheet. The instability results in the formation of low-density coherent structures that resemble SADs, intertwined with high-density structures that appear to be spike-like. Using SDO/AIA images, we highlight features that have been previously unexplained, such as the splitting of SADs at their heads, but are a natural consequence of instabilities above the arcade. Comparison with siumlations suggest that secondary Rayleigh-Taylor type instabilities in the exhaust of reconnecting current sheets provide a plausible mechanism for observed SADs and spikes. Although the plasma conditions are vastly different, analogous phenomena also occur in the Earth's magnetotail during reconnection.