Center-to-Limb Variation of the Inverse Evershed Flow

TL;DR

This study investigates the inverse Evershed flow in sunspots by analyzing its center-to-limb variation, modeling the flow with a simplified sunspot model, and finding that the flow is consistent with a siphon mechanism along elevated chromospheric loops.

Contribution

It provides the first detailed analysis of the inverse Evershed flow's center-to-limb variation and introduces a simplified model that replicates observed flow patterns across different viewing angles.

Findings

Flow speeds reach up to 12 km/s.

Flow is consistent with a siphon mechanism.

Flow occurs along elevated loops with constant sonic speed.

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 deg. The observations were acquired at the Dunn Solar Telescope in the spectral lines of Halpha at 656nm, CaII IR at 854 nm and HeI at 1083 nm. 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 2-3Mm and a true constant flow speed of 2-9km/s. 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 12km/s. Upwards or downwards directed flows do not show a change of sign in the LOS velocities for heliocentric angles above 30 deg. Our results are consistent with the IEF being caused by a siphon flow mechanism driving a flow at a constant sonic speed along elevated loops with a flattened top in the chromosphere.