On the Weakening of the Chromospheric Magnetic Field in Active Regions

Abstract

Simultaneous measurement of line-of-sight (LOS) magnetic and velocity fields at the photosphere and chromosphere are presented. The Fe I line at λ6569 and Hα at λ6563 are used for deriving the physical parameters at photospheric and chromospheric heights, respectively. The LOS magnetic field obtained through the center-of-gravity method shows a linear relation between the photospheric and chromospheric fields for field strengths less than 700 G. But in strong field regions, the LOS magnetic field values derived from Hα are much weaker than what one gets from the linear relationship, and also from those expected from the extrapolation of the photospheric magnetic field. We discuss in detail the properties of the magnetic field observed in Hα from the point of view of observed velocity gradients. The bisector analysis of Hα Stokes I profiles shows larger velocity gradients in those places where strong photospheric magnetic fields are observed. These observations may support the view that the stronger fields diverge faster with height compared to weaker fields.