Evershed and counter-Evershed flows in sunspot MHD simulations

TL;DR

This study uses 3D radiative MHD simulations to analyze the driving mechanisms of Evershed and counter-Evershed flows in sunspot penumbrae, revealing overturning convection and siphon flows as key processes.

Contribution

It provides the first detailed simulation-based analysis of counter-Evershed flows, comparing their driving forces with those of the normal Evershed flow.

Findings

Evershed flow driven by overturning convection in inclined magnetic fields.

Counter-Evershed flows are transient and resemble siphon flows.

Simulation shows flows exceeding 8 km/s and 12 km/s in the penumbra.

Abstract

There have been a few reports in the literature of counter-Evershed flows observed in well developed sunspot penumbrae, i.e. flows directed towards the umbra along penumbral filaments. Here we investigate the driving forces of such counter-Evershed flows in a radiative magnetohydrodynamic simulation of a sunspot and compare them with the forces acting on the normal Evershed flow. The simulation covers a timespan of 100 solar hours and generates an Evershed outflow exceeding 8 km s$^{-1}$ 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 $\tau = 1$) in some regions within the penumbra, reaching peak flow speeds of $\sim$12 km s$^{-1}$. The counter-Evershed flows are transient and typically last a few hours before they turn into outflows again. By using the kinetic energy equation and evaluating its various terms in the simulation box, we found that the Evershed flow occurs due to overturning convection in a strongly inclined magnetic field while the counter-Evershed flows can be well described as siphon flows.