The radial profile of the inner heliospheric magnetic field as deduced from Faraday rotation observations

TL;DR

This study uses Faraday rotation and polarized brightness data to determine the radial profile of the inner heliospheric magnetic field during solar minimum, finding a power-law decay and validating the PFSS model assumptions.

Contribution

It provides a novel empirical determination of the radial magnetic field profile near the Sun using combined Faraday rotation and polarization data.

Findings

Radial magnetic field follows a power-law of approximately r^{-2.29}.

The PFSS model with a source surface at 2.5 solar radii is validated.

The magnetic field strength at 5 solar radii is about 3.76 G.

Abstract

Faraday rotation measures (RMs) of the polarized emission from extragalactic radio sources occulted by the coronal plasma were used to infer the radial profile of the inner heliospheric magnetic field near solar minimum activity. By inverting LASCO/SOHO polarized brightness (pB) data taken during the days of observations on May 1997, we retrieved the electron density distribution along the lines of sight to the sources, thus allowing to disentangle the two plasma properties that contribute to the observed RMs. By comparing the observed RM values to those theoretically predicted by a power-law model of the radial component of the coronal magnetic field, using a best-fitting procedure, we found that the radial component of the inner heliospheric magnetic field can be nicely approximated by a power-law of the form B_r = 3.76 r^{-2.29} G in a range of heights from about 5 to 14 solar radii. Finally, our analysis suggests that the radial computation of the potential field source surface (PFSS) model from the Wilcox Solar Observatory (WSO), assuming a radial field in the photosphere and a source surface located at R_{ss} = 2.5 solar radii, is the preferred choice near solar minimum.