Direct Measurements of Magnetic Twist in the Solar Corona

TL;DR

This study provides observational evidence linking magnetic helicity flux through the photosphere to its evolution in the solar corona, using EUV data and a novel linear force-free field matching method.

Contribution

It introduces a method to estimate coronal magnetic twist without assuming a strict force-free field, and compares helicity flux with coronal helicity changes during active region emergence.

Findings

Helicity flux rate in the corona is similar to flux through the photosphere.

The estimated coronal helicity change rate is about 0.021 rad/hr.

Method validates the connection between photospheric flux and coronal helicity evolution.

Abstract

In the present work we study evolution of magnetic helicity in the solar corona. We compare the rate of change of a quantity related to the magnetic helicity in the corona to the flux of magnetic helicity through the photosphere and find that the two rates are similar. This gives observational evidence that helicity flux across the photosphere is indeed what drives helicity changes in solar corona during emergence. For the purposes of estimating coronal helicity we neither assume a strictly linear force-free field, nor attempt to construct a non-linear force-free field. For each coronal loop evident in Extreme Ultraviolet (EUV) we find a best-matching line of a linear force-free field and allow the twist parameter alpha to be different for each line. This method was introduced and its applicability was discussed in Malanushenko et. al. (2009). The object of the study is emerging and rapidly rotating AR 9004 over about 80 hours. As a proxy for coronal helicity we use the quantity <alpha_i*L_i/2> averaged over many reconstructed lines of magnetic field. We argue that it is approximately proportional to "flux-normalized" helicity H/Phi^2, where H is helicity and Phi is total enclosed magnetic flux of the active region. The time rate of change of such quantity in the corona is found to be about 0.021 rad/hr, which is compatible with the estimates for the same region obtained using other methods Longcope et. al. (2007), who estimated the flux of normalized helicity of about 0.016 rad/hr.