# Pulsars and neutron stars/The Sun and interplanetary medium

## Introduction

The Sun significantly affects pulsar observations. It causes a delay in the pulse arrival times (the Shapiro delay) and increases the dispersion measure and rotation measure of a pulsar when the line-of-sight to the pulsar passes close to the Sun. The Sun and solar wind can therefore be studied through observations of pulsars, but also it needs to be accounted for when predicting pulse arrival times as part of the pulsar timing method.

## Modelling the solar wind

The simplest model, which is used in the standard pulsar timing software packages such as tempo and tempo2, is to assume that the solar wind is constant in time and spherically symmetric.

$DM_{\odot }{\rm {[cm}}^{-3}{\rm {{pc]}=4.85\times 10^{-6}n_{0}{\frac {\theta }{\sin \theta }}}}$ where $n_{0}$ is the electron density at 1 AU from the Sun (in cm-3) and $\theta$ is the pulsar-Sun-observatory angle.

A review of solar wind physics is available from Schwenn (1996). It is usually approximated as having two parts. One part is quasi-static and co-rotates with the Sun. The second part is transient and has time scales of hours to days (such transient events include coronal mass ejections). You et al. (2007) showed how the quasi-static part could be modelled. This part is divided into two components: "fast" and "slow".

### The slow wind

The slow wind originates at low or middle solar latitudes. You et al. (2007) modelled the electron density in the slow wind as:

$n_{e}=2.99\times 10^{14}R_{\odot }^{-16}+1.5\times 10^{14}R_{\odot }^{-6}+4.1\times 10^{11}(R_{\odot }^{-2}+5.74R_{\odot }^{-2.7}){\rm {m}}^{-3}$ at a distance of $R_{\odot }$ solar radii.

### The fast wind

The fast wind originates in regions with open magnetic field geometry called "coronal holes". Guhathakurta & Fisher (1995, 1998) showed that the electron density in the fast wind is approximately

$n_{e}^{\rm {fast}}=1.155\times 10^{11}R_{\odot }^{-2}+32.3\times 10^{11}R_{\odot }^{4.39}+3254\times 10^{11}R_{\odot }^{-16.25}{\rm {m}}^{-3}$ at a distance of $R_{\odot }$ solar radii.