Evershed and Counter-Evershed Flows In Sunspot MHD Simulations

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Wednesday, December 13, 2017

There have been a number of reports in the literature of counter-Evershed flows in well developed sunspot penumbrae, i.e. flows directed towards the umbra. Here we investigate the driving forces of such counter-Evershed flows in a radiative magnetohydro- dynamic simulation of a sunspot and compare them with the forces acting on the normal Evershed flow.

Field line connectivity and associated pressure perturbation image
Field line connectivity and associated pressure perturbation for filaments hosting a counter-Evershed flow (flow towards the umbra of the sunspot). The color of the field lines indicates the pressure perturbation along the field lines from ±100,000 dyne/cm^2, where white is low and purple is high pressure. The left foot point of the filament is located towards the umbra, whereas the right foot point is near the outer boundary of the penumbra, leading to pressure gradient that is consistent with a siphon flow.

The simulation covers a timespan of 100 hr and generates an Evershed outflow exceeding 8 km/s in the penumbra along radially aligned filaments where the magnetic field is almost horizontal. Additionally, the simulation produces a fast counter-Evershed flow (i.e., an inflow near τ = 1) in some regions within the penumbra, reaching peak flow speeds of ∼12 km/s. The counter-Evershed flows are transient and typically last a few hours before they turn into outflows again. By using the energy equation and evaluating its various terms in the simulation box, we found that the Evershed flow occurs due to magneto-convection while the counter-Evershed flows can be well described as siphon flows.

Publication Name: Astrophysical Journal

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