Helioseismically Determined Near‐Surface Flows Underlying a Quiescent Filament

Abstract

The extended filaments seen in Hα images of the solar disk, and the corresponding prominences when viewed at the solar limb, are one of the great hallmarks of solar magnetism. Such arches of magnetic field and the coronal plasma structures they support are both beautiful and enigmatic. Many models of filament formation and maintenance invoke the existence of surface plasma flows, which are used to drive the magnetic reconnection needed to form twisted loops of flux held down by a coronal arcade. These flows are typically composed of a converging flow, which brings flux elements of opposite polarity together, combined with a tangential shear that stresses the coronal arcade. In this paper we present helioseismic measurements of near-surface flows underlying a single quiescent filament lying within a decayed active region. Newly devised high-resolution ring analyses (HRRA) with both 2° and 4° spatial resolution were applied to Doppler imaging data provided by the Michelson Doppler Imager (MDI) instrument on the SOHO spacecraft. A long-lived filament appearing in 2002 May and April was studied. We find that the filament channel is a region of vigorous subphotospheric convection. The largest observed scales of such convection span the region of weak magnetic field separating the active region's two polarities. Thus, the magnetic neutral line that forms the spine of the filament channel tends to lie along the centers of large convection cells. In temporal and spatial averages of the flow field, we do not find a systematic converging flow. However, we do detect a significant shearing flow parallel to the neutral line. This shear takes the form of two oppositely directed jets, one to either side of the neutral line and within 20 Mm of the line. The jets produce a net shear in the flow speed of 30 m s-1 occurring over a distance of 20 Mm.