Spectrum of Magnetic Dissipation and Horizontal Electric Currents in the Solar Photosphere

TL;DR

This study investigates the spectrum and structure of horizontal electric currents in the solar photosphere, revealing their generation during active region emergence and decay, and proposing mechanisms for their formation.

Contribution

It introduces a new proxy for horizontal electric currents in the photosphere and analyzes their evolution, providing insights into their role in solar active regions.

Findings

Horizontal currents are generated during active region emergence.

Large-scale horizontal currents decay faster than small-scale ones.

Horizontal currents have densities comparable to vertical currents in active regions.

Abstract

A proxy for horizontal electric currents in the solar photosphere was suggested. For a set of evolving active regions (ARs) observed with {\it Solar and Heliospheric Observatory (SOHO)} Michelson Doppler Imager (MDI) in the high resolution mode, the dissipation spectrum, $k^2E(k)$, and the spatial structure of dissipation, i.e., the Stokes dissipation function $\epsilon(x,y)$, were calculated from the observed $B_z$ component of the magnetic field. These functions allowed us to calculate (a part of) the horizontal electric current density in the photosphere. It was shown that as an active region emerges, large-scale horizontal electric currents are gradually generated and determine a bulk of dissipation. When an active region decays, the large-scale horizontal currents decay faster than the small-scale ones. The density of horizontal currents in active regions is in the range of $<j_h > \sim (0.008 - 0.028)$ A/m$^2$, that is compatible with the density of vertical currents in active regions. We suggest two possible mechanisms for generation of such horizontal currents in the photosphere. One of them is the drift motions of charged particles in the medium of varying plasma pressure gradient in a horizontal plane at the periphery of a sunspot. Such a drift can produce quasi-circular closed horizontal currents around sunspots. Another possibility could be an existence of horizontal axial current inside a highly twisted horizontal magnetic structure laying in the photosphere along the magnetic neutral line. The horizontal currents may contribute significantly to the dynamics of the photosphere/corona coupling, as well as the estimation of non-potentiality of ARs.