Context. The Evershed effect, a nearly horizontal outflow of material seen in the penumbrae of sunspots at the photospheric layers, is a common characteristic of well-developed penumbrae, but is still not well understood. Even less is known about photospheric horizontal inflows in the penumbra, also known as counter Evershed flows. Aims. Here we present a rare feature observed in the penumbra of the main sunspot of AR NOAA 10930. This spot displays the normal Evershed outflow in most of the penumbra, but harbors a fast photospheric inflow of material over a large sector of the disk-center penumbra. We investigate the driving forces of both, the normal and the counter Evershed flows. Methods. We invert the spectropolarimetric data from Hinode SOT/SP using the SPINOR 2D inversion code, which allows us to derive height-dependent maps of the relevant physical parameters in the sunspot. These maps show considerable fine structure. Similarities and differences between the normal Evershed outflow and the counter Evershed flow are investigated. Results. In both, the normal and the counter Evershed flows, the material flows from regions of weak (∼ 1.5 kG to ∼ 2 kG) to stronger fields. The sources and sinks of both penumbral flows display opposite field polarities; with the sinks (tails of filaments) harboring local enhancements in temperature, which are nonetheless colder than their sources (heads of filaments).Conclusions. The anti-correlation of the gradients in the temperature and magnetic pressure between the endpoints of the filaments from the two distinct penumbral regions is compatible with both the convective driver and the siphon flow scenarios. A geometrical scale of the parameters is necessary to determine which the dominant force driving the flows is.

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 τ = 1) in some regions within the penumbra, reaching peak flow speeds of ∼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.

Context. Recently, there have been some reports of unusually strong photospheric magnetic fields (which can reach values of over 7 kG) inferred from Hinode SOT/SP sunspot observations within penumbral regions. These superstrong penumbral fields are even larger than the strongest umbral fields on record and appear to be associated with supersonic downflows. The finding of such fields has been controversial since they seem to show up only when spatially coupled inversions are performed. Aims. Here, we investigate and discuss the reliability of those findings by studying in detail observed spectra associated with particularly strong magnetic fields at the inner edge of the penumbra of active region 10930. Methods. We apply classical diagnostic methods and various inversions with different model atmospheres to the observed Stokes profiles in two selected pixels with superstrong magnetic fields, and compare the results with a magnetohydrodynamic simulation of a sunspot whose penumbra contains localized regions with strong fields (nearly 5 kG at τ = 1) associated with supersonic downflows. Results. The different inversions provide different results: while the SPINOR 2D inversions consider a height-dependent singlecomponent model and return B>7 kG and supersonic positive v LOS (corresponding to a counter-Evershed flow), height-dependent 2-component inversions suggest the presence of an umbral component (almost at rest) with field strengths ∼ 4 − 4.2 kG and a penumbral component with v LOS ∼ 16 − 18 km s −1 and field strengths up to ∼ 5.8 kG. Likewise, height-independent 2-component inversions find a solution for an umbral component and a strongly redshifted (v LOS ∼ 15 − 17 km s −1 ) penumbral component with B∼ 4 kG.According to a Bayesian Information Criterion, the inversions providing a better balance between the quality of the fits and the number of free parameters considered by the models are the height -independent 2-component inversions, but they lie only slightly above the SPINOR 2D inversions. Since it is expected that the physical parameters all display considerable gradients with height, as supported by MHD sunspot simulations, the SPINOR 2D inversions are the preferred ones. Conclusions. According to the MHD sunspot simulation analyzed here, the presence of counter-Evershed flows in the photospheric penumbra can lead to the necessary conditions for the observation of ∼ 5 kG fields at the inner penumbra. Although a definite conclusion about the potential existence of fields in excess of 7 kG cannot be given, their nature could be explained (based on the simulation results) such as the consequence of the extreme dynamical effects introduced by highly supersonic counter-Evershed flows (v LOS > 10 km s −1 and up to ∼30 km s −1 according to SPINOR 2D), which would be much faster and more compressive downflows than those found in the MHD simulations and therefore could lead to the field intensification up to considerably stronger fields. Also, a lower gas density would lead to a deeper depression of the τ = 1 surfa...