Fast photospheric flows and magnetic fields in a flaring active region

¹ Observatoire Midi-Pyrénées, 57 avenue d'Azereix, BP 826, 65008 Tarbes Cedex, France
² Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305, USA e-mail: sasha@khors.stanford.edu

Corresponding author: N. Meunier, meunier@bagn.obs-mip.fr

Received: 25 June 2003
Accepted: 3 September 2003

Abstract

We present new results from the coordinated observations between the THEMIS telescope (in the multi-line spectropolarimetric mode) and Michelson Doppler Imager (MDI) on SOHO obtained in November 2000 for active region NOAA 9236 which was the source of several X-class flares. The goal of these observations was twofold: to verify MDI measurements of the line-of-sight components of flow velocity and magnetic field, and to obtain more information about the photospheric flows and magnetic fields in flaring regions. Using the simultaneous observational data in several lines we have analyzed the structure and dynamics of this active region at the photospheric level before and after a X4.0 flare of November 26, the last major flare produced by this very active region. Vector magnetic field maps are computed from the THEMIS data by full inversion of the Stokes line profiles. In the Doppler velocity maps from THEMIS and MDI, we observe fast photospheric flows which appear to be supersonic in two regions located close to the region where the flare occurred. These flows seem to be long-lived (several hours at least). In one position, we observe a supersonic downflow strongly inclined with respect to the vertical (by 51°), while in another position, a flow suggesting a strong shear with a supersonic component as well, although almost horizontal upflows and downflows cannot be ruled out in that case. These flows seem to be present at least 8 hours before the flare, and the amplitude in the second case appeared to be modified during the flare, especially, during the first minutes. In the MDI data, we observed strong permanent changes of the longitudinal magnetic flux, associated with the flare. The role of the strong flows and their interaction with the magnetic field in the development of the active region and the flare is not understood yet.