On the Weakening of Chromospheric Magnetic Field in Active Regions

TL;DR

This study investigates how magnetic fields weaken from the photosphere to the chromosphere in active regions, revealing that strong fields diverge faster with height and are weaker in the chromosphere than expected.

Contribution

It provides simultaneous measurements of magnetic and velocity fields at different heights, highlighting the weakening and divergence of strong magnetic fields in the chromosphere.

Findings

LOS magnetic field weakens in strong field regions at chromospheric heights

Velocity gradients are larger where strong photospheric magnetic fields are observed

Strong fields diverge faster with height compared to weaker fields

Abstract

Simultaneous measurement of line-of-sight (LOS) magnetic and velocity fields at the photosphere and chromosphere are presented. Fe I line at $\lambda6569$ and $H_{\alpha}$ at $\lambda6563$ are used respectively for deriving the physical parameters at photospheric and chromospheric heights. The LOS magnetic field obtained through the center-of-gravity method show a linear relation between photospheric and chromospheric field for field strengths less than 700 G. But in strong field regions, the LOS magnetic field values derived from $H_{\alpha}$ 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 magnetic field observed in $H_{\alpha}$ from the point of view of observed velocity gradients. The bisector analysis of $H_{\alpha}$ Stokes $I$ profiles show 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.