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There have been a few reports in the literature of counter-Evershed flowsobserved in well developed sunspot penumbrae, i.e. flows directed towards theumbra along penumbral filaments. Here we investigate the driving forces of suchcounter-Evershed flows in a radiative magnetohydrodynamic simulation of asunspot and compare them with the forces acting on the normal Evershed flow.The simulation covers a timespan of 100 solar hours and generates an Evershedoutflow exceeding 8 km s$^{-1}$ in the penumbra along radially alignedfilaments where the magnetic field is almost horizontal. Additionally, thesimulation 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$12km s$^{-1}$. The counter-Evershed flows are transient and typically last a fewhours before they turn into outflows again. By using the kinetic energyequation and evaluating its various terms in the simulation box, we found thatthe Evershed flow occurs due to overturning convection in a strongly inclinedmagnetic field while the counter-Evershed flows can be well described as siphonflows.

Evershed and counter-Evershed flows in sunspot MHD simulations