The Magnetic Field of the Solar Corona from Pulsar Observations

We present a novel experiment with the capacity to independently measure both the electron density and the magnetic field of the solar corona. We achieve this through measurement of the excess Faraday rotation due to propagation of the polarised emission from a number of pulsars through the magnetic field of the solar corona. This method yields independent measures of the integrated electron density, via dispersion of the pulsed signal and the magnetic field, via the amount of Faraday rotation. In principle this allows the determination of the integrated magnetic field through the solar corona along many lines of sight without any assumptions regarding the electron density distribution. We present a detection of an increase in the rotation measure of the pulsar J1801$-$2304 of approximately 160 \rad at an elongation of 0.95$^\circ$ from the centre of the solar disk. This corresponds to a lower limit of the magnetic field strength along this line of sight of $> 393\mu\mathrm{G}$. The lack of precision in the integrated electron density measurement restricts this result to a limit, but application of coronal plasma models can further constrain this to approximately 20mG, along a path passing 2.5 solar radii from the solar limb. Which is consistent with predictions obtained using extensions to the Source Surface models published by Wilcox Solar Observatory