Flows along penumbral flux tubes in sunspots

Abstract

The super-Alfvenic, undulating sea-serpent flow configurations along thin magnetic flux tubes, proposed by Schlichenmaier (2002, Astron. Nachr., 323, 303; 2003, ASP Conf. Ser., 286, 24) to explain both the Evershed flow and mov- ing penumbral grains in a sunspot, are shown to be gravitationally unstable. Any undulations that occur for super-Alfvenic flow speeds will form preferentially in a horizontal plane and hence will not explain Evershed downflows or outward-moving penumbral grains. Sub-Alfvenic, arched flow configurations, on the other hand, are gravitationally stable.