The NANOGrav 11-year Data Set: Solar Wind Sounding Through Pulsar Timing

TL;DR

This study uses a decade of pulsar timing data to measure and analyze the solar wind's electron density, developing techniques to extract its signature from dispersion measure variations across multiple pulsars.

Contribution

We introduce methods to isolate the solar wind's imprint in pulsar dispersion measures and provide the first measurement of electron density at 1 AU using this data.

Findings

Electron density at 1 AU is 7.9 ± 0.2 cm⁻³.

Little evidence of long-term variation in solar wind density.

Techniques can potentially map large-scale latitudinal structure of the solar wind.

Abstract

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has observed dozens of millisecond pulsars for over a decade. We have accrued a large collection of dispersion measure (DM) measurements sensitive to the total electron content between Earth and the pulsars at each observation. All lines of sight cross through the solar wind which produces correlated DM fluctuations in all pulsars. We develop and apply techniques for extracting the imprint of the solar wind from the full collection of DM measurements in the recently released NANOGrav 11-yr data set. We filter out long time scale DM fluctuations attributable to structure in the interstellar medium and carry out a simultaneous analysis of all pulsars in our sample that can differentiate the correlated signature of the wind from signals unique to individual lines of sight. When treating the solar wind as spherically symmetric and constant in time, we find the electron number density at 1 A.U. to be $7.9\pm0.2$ cm$^{-3}$. Our data shows little evidence of long-term variation in the density of the wind. We argue that our techniques paired with a high cadence, low radio frequency observing campaign of near-ecliptic pulsars would be capable of mapping out large-scale latitudinal structure in the wind.