Center-to-limb Variation of the Inverse Evershed Flow
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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 2020
- DOI:
- 10.3847/1538-4357/abb3bf
- arXiv:
- arXiv:2008.12748
- Bibcode:
- 2020ApJ...902...30B
- Keywords:
-
- Solar chromosphere;
- Sunspots;
- Solar photosphere;
- 1479;
- 1653;
- 1518;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 14 pages, 12 figures