Generation of a North/South Magnetic Field Component from Variations in the Photospheric Magnetic Field

TL;DR

This paper presents an analytic method to calculate the transverse magnetic field component in the solar wind by considering lateral field line motions caused by variations in the photospheric magnetic field, challenging the assumption of purely radial fields at the source surface.

Contribution

It introduces a novel analytic approach to estimate the transverse magnetic field component based on lateral field line motions, using observational data.

Findings

Transverse magnetic field is mainly influenced by high latitude supergranular motions.

The model successfully estimates the lateral translation speed of magnetic field lines.

The transverse field component is significant and varies with solar activity.

Abstract

We address the problem of calculating the transverse magnetic field in the solar wind outside of the hypothetical sphere called the source surface where the solar wind originates. This calculation must overcome a widely used fundamental assumption about the source surface -- the field is normally required to purely radial at the source surface. Our model rests on the fact that a change in the radial field strength at the source surface is a change in the field line density. Surrounding field lines must move laterally in order to accommodate this field line density change. As the outward wind velocity drags field lines past the source surface this lateral component of motion produces a tilt implying there is a transverse component to the field. An analytic method of calculating the lateral translation speed of the field lines is developed. We apply the technique to an interval of approximately two Carrington rotations at the beginning of 2011 using 2-h averages of data from the Helioseismic Magnetic Imager instrument on the Solar Dynamics Observatory spacecraft. We find that the value of the transverse magnetic field is dominated on a global scale by the effects of high latitude concentrations of field lines being buffetted by supergranular motions.