Inferring the Magnetic Structure of a Sunspot from Decomposition of Photospheric Vertical Current Density into Twist and Shear Components

Abstract:

The electric current density, derived from the curl of the vector magnetic field, can be decomposed into the so-called twist and shear components. We examined this decomposition for 2 highly flare productive active regions undergoing magnetic flux emergence, viz. AR 10930 observed by SOT aboard Hinode and AR 11158 observed by HMI aboard SDO. We found the following common evolutionary behaviour in both the cases:

1. The percentage of variation of the spatially averaged value of the ratio of the shear current density to the twist current density is much smaller than the percentage of variation of the twist current throughout the evolution.

2. The average ratio was negative in sign, viz. the shear current density was opposite in sign to the twist current density in most of the pixels.

3. The magnitude of the average ratio increased with increase of pixel size.

The above 3 results were examined for two extreme kinds of magnetic structure: a) a monolithic continuous didtribution of magnetic flux and b) an intermittent cluster of magnetic fibrils separated by field free regions. We arrive at the conclusion that the above mentioned observations tend to favor the monolithic model of sunspot magnetic field over the fibril model, although a fibril structure at deeper layers cannot be ruled out. Observations of vector magnetic fields of sunspots using the infrared FeI line around 1560 nm could perhaps resolve this issue.