Center-to-limb Variation of the Inverse Evershed Flow

Abstract

Abstract We present the properties of the inverse Evershed flow (IEF) based on the center-to-limb variation of the plasma speed and loop geometry of chromospheric superpenumbral fibrils in eleven sunspots that were located at a wide range of heliocentric angles from 12° to 79°. The observations were acquired at the Dunn Solar Telescope in the spectral line of Hα at 656 nm to determine chromospheric flows and the photospheric Si i line at 1082.7 nm to estimate the photospheric umbral magnetic field strength. All sunspots display opposite line-of-sight (LOS) velocities on the limb and center side with a distinct shock signature near the outer penumbral edge. We developed a simplified flexible sunspot model assuming axisymmetry and prescribing the radial flow speed profile at a known loop geometry to replicate the observed two-dimensional IEF patterns under different viewing angles. The simulated flow maps match the observations for chromospheric loops with 10–20 Mm length starting at 0.8–1.1 sunspot radii, an apex height of 1–3 Mm, and a flow speed of 2–9 km s−1. We find on average a good agreement of the simulated velocities and the observations on elliptical annuli around the sunspot. Individual IEF channels show a significant range of variation in their properties and reach maximal LOS speeds of up to 12 km s−1. Upwards or downwards directed flows do not show a change of sign in the LOS velocities for heliocentric angles above 30°. Our results are consistent with the IEF being caused by a siphon flow mechanism driving a flow at about sonic speed along elevated loops with a flattened top in the chromosphere.