Evidence for Vortex Shedding in the Sun’s Hot Corona

Vortex shedding is an oscillating flow that is commonly observed in fluids due to the presence of a blunt body in a flowing medium. Numerical simulations have shown that the phenomenon of vortex shedding could also develop in the magnetohydrodynamic (MHD) domain. The dimensionless Strouhal number, the ratio of the blunt body diameter to the product of the period of vortex shedding and the speed of a flowing medium, is a robust indicator for vortex shedding, and, generally of the order of 0.2 for a wide range of Reynolds number. Using an observation from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we report a wavelike or oscillating plasma flow propagating upward against the Sun’s gravitational force. A newly formed shrinking loop in the postflare region possibly generates the oscillation of the upflow in the wake of the hot and dense loop through vortex shedding. The computed Strouhal number is consistent with the prediction from previous MHD simulations. Our observation suggests the possibility of vortex shedding in the solar corona.

Figure 1

The supra-arcade region observed in the SDO/AIA 94 Å (a) and 131 Å (b) filters at $11:55\mathrm{UT}$. The big and small rectangular boxes indicate the field of view in Figs. 2, 3 and 3, respectively. An animation is associated with this figure.

Figure 2

Measurement of the physical parameters of the wavelike upflow. (a) The upward propagating flow observed in the SDO/AIA 94 Å channel at $11:55\mathrm{UT}$. (b) The space-time (S-T) diagram for the wide slit marked as $S$ (rectangular box) in (a). (c) The background image is the same as (b). The red diamonds mark the edge or ridge of the upflow in the S-T diagram. The upflow speed was estimated from the slope of the slanted straight line, which represents a linear fit to the red diamonds. (d) The S-T diagram for the cut $S1$ shown in (a). (e),(f) Enlarged view of the white box in (d). The black diamonds and red bars represent the central positions of the upflowing feature and the associated $1\sigma$ errors at different times, respectively. The period of the associated oscillation was determined through fitting with a sinusoidal function [green line in (f)]. An animation is associated with this figure.

Figure 3

Association of the wavelike upflow with a SAD. (a) Temporal evolution of the upflow in AIA 94 Å. (b) Temporal evolution of the associated SAD in AIA 131 Å. The AIA 131 Å images have been enhanced by subtracting a smoothed (over 50 pixels along the $X$ axis) background at each $Y$ pixel. The red circles mark the tip of the downward moving SAD. The blue dashed line represents a linear fit of the locations of these red circles. The same line is also overplotted in the AIA 94 Å images in (a). (c),(d) AIA 94 Å image of the upflow and 131 Å image of the SAD at $11:23\mathrm{UT}$. The two red dashed lines (vertical and horizontal) are overplotted to demonstrate the coincidence of the upflow and the SAD. (e) AIA 131 Å intensity (diamond) and the associated measurement error (red vertical bar) along the cut $S1$ shown in (d). The red line shows the Gaussian fit (with a linear background).