The Strength and Radial Profile of Coronal Magnetic Field from the Standoff Distance of a CME-driven Shock

TL;DR

This study measures the coronal magnetic field strength from 6 to 23 solar radii during a CME event by analyzing shock standoff distances and flux rope curvature, providing a radial profile consistent with previous estimates.

Contribution

It introduces a method to determine the coronal magnetic field using shock and flux rope measurements during a CME, extending the radial profile analysis.

Findings

Magnetic field decreases from ~48 mG at 6 Rs to 8 mG at 23 Rs.

Radial profile follows a power law consistent with prior studies.

Method provides direct measurements during a CME event.

Abstract

We determine the coronal magnetic field strength in the heliocentric distance range 6 to 23 solar radii (Rs) by measuring the shock standoff distance and the radius of curvature of the flux rope during the 2008 March 25 coronal mass ejection (CME) imaged by white-light coronagraphs. Assuming the adiabatic index, we determine the Alfven Mach number, and hence the Alfven speed in the ambient medium using the measured shock speed. By measuring the upstream plasma density using polarization brightness images, we finally get the magnetic field strength upstream of the shock. The estimated magnetic field decreases from ~48 mG around 6 Rs to 8 mG at 23 Rs. The radial profile of the magnetic field can be described by a power law in agreement with other estimates at similar heliocentric distances.