Statistics of convective collapse events in the photosphere and chromosphere observed with the Hinode SOT

¹ Utrecht University, The Netherlands e-mail: [C.E.Fischer;C.U.Keller]@uu.nl
² High Altitude Observatory, National Center for Atmospheric Research, Boulder, USA e-mail: [dwijn;rce;lites]@ucar.edu

Received: 7 May 2009
Accepted: 5 June 2009

Abstract

Convective collapse, a theoretically predicted process that intensifies existing weak magnetic fields in the solar atmosphere, was first directly observed in a single event by Nagata et al. (2008, ApJ, 677, L145) using the high resolution Solar Optical Telescope (SOT) of the Hinode satellite. Using the same space telescope, we observed 49 such events and present a statistical analysis of convective collapse events. Our data sets consist of high resolution time series of polarimetric spectral scans of two iron lines formed in the lower photosphere and filter images in Mg I b₂ and Ca II H, spectral lines that are formed in the high photosphere and the lower chromosphere, respectively. We were thus able to study the implication of convective collapse events on the high photospheric and the chromospheric layers. We found that in all cases, the event was accompanied by a continuum bright point and nearly always by a brightening in the Ca II H images. The magnesium dopplergram exhibits a strong downflow in about three quarters of the events that took place within the field of view of the magnesium dopplergram. The physical parameters from the full Stokes profiles were obtained with the MERLIN Milne-Eddington inversion code. For each of the 49 events we determined the duration, maximum photospheric downflow, field strength increase and size. We found event durations of about 10 min, magnetic element radii of about 0.43″ and 0.35″, before and after the event, respectively, and field strengths of up to 1.65 kG.