Latitudinal dependence of the solar wind during periods of high and low activity through interplanetary scintillation

TL;DR

This study uses interplanetary scintillation data from the Murchison Widefield Array to analyze how the solar wind's latitudinal dependence varies during different solar cycle phases, revealing spherical symmetry during high activity and latitudinal structure during low activity.

Contribution

It provides the highest density of IPS radio source sampling to date and demonstrates the latitudinal dependence of the solar wind during different solar cycle phases.

Findings

Solar wind is spherically symmetric during high activity.

Latitudinal dependence emerges during low activity.

The equator-to-pole density reduction ratio is 1.62±0.02.

Abstract

We present a study of the solar wind over different periods of the solar cycle, specifically focussing on the minimum between solar cycles 24 and 25, and the active, ascending phase of solar cycle 25. With the use of interplanetary scintillation (IPS) data taken by the Murchison Widefield Array (MWA) from mid-2019 and early 2023, we have sampled over a wide range of solar latitudes and elongations, probing a large section of the surrounding heliosphere from $\sim$90 to 140R$_\odot$. The MWA observations provide the highest density of sampled IPS radio sources to date, allowing for an investigation into the latitudinal dependence of the scattering effect caused by the solar wind on a radio source as observed throughout the solar cycle. We find our measurements during periods of heightened solar activity are consistent with a spherically symmetric solar wind. On the other hand, with a reduction in solar activity we find the solar wind density inherits a latitudinal dependence. As is consistent with prior studies, an elliptical function better represents the transition from poles to equator, although we find a more exaggerated sigmoid shaped curve is required to represent the low- to mid-latitude transition region during the minimum of solar cycle 24. We find for a heliospheric distance range of 108 - 123R$_{\odot}$ the reduction ratio between the equator and the southern pole is 1.62$\pm$0.02.