Chromospheric Evershed flow^*

¹ Solar Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
² Electronics Laboratory, University of Patras, Patras 26110, Greece
³ School of Sciences and Technology, Hellenic Open University, 26222 Patras, Greece
⁴ Research Academic Computer Technology Institute, Riga Fereou 61, 26221 Patras, Greece
⁵ Institut d'Astrophysique de Paris, CNRS, 98 bis boulevard Arago, 75014 Paris, France

Corresponding author: A. A. Georgakilas, ageorg@caltech.edu

Received: 10 September 2002
Accepted: 27 February 2003

Abstract

We studied the chromospheric Evershed flow from filtergrams obtained at nine wavelengths along the Hα profile. We computed line-of-sight velocities based on Becker's cloud model and we determined the components of the flow velocity vector as a function of distance from the center of the sunspot, assuming an axial symmetry of both the spot and the flow. We found that the flow velocity decreases with decreasing height and that the maximum of the velocity shifts towards the inner penumbral boundary. The flow related to some fibrils deviates significantly from the average Evershed flow. The profile of the magnitude of the flow velocity as a function of distance from the spot center, indicates that the velocity attains its maximum value in the downstream part of the flow channels (assumed to have the form of a loop). This behavior can be understood in terms of a critical flow that pass from subsonic to supersonic near the apex of the loop, attains its higher velocity at the downstream part of the loop and finally relaxes to subsonic through a tube shock. We computed the average flow vector from segmented line-of-sight velocity maps, excluding bright or dark fibrils alternatively. We found that the radial component of the velocity does not show a significant difference, but the magnitude of the vertical component of the velocity related to dark fibrils is higher than that related to bright fibrils.