Monitoring the Solar Wind Before It Reaches L1

TL;DR

This paper discusses the potential benefits of measuring the solar wind upstream of L1 to improve space weather predictions and interplanetary physics understanding, using real-time data from STEREO-A during a 2024 geomagnetic storm.

Contribution

It highlights the advantages of upstream solar wind measurements over traditional L1 observations for enhancing space weather forecasting accuracy.

Findings

Upstream measurements can extend lead time for space weather alerts.

Real-time data from STEREO-A improved prediction accuracy during the 2024 storm.

Upstream monitoring aids fundamental interplanetary physics research.

Abstract

Space weather predictions of the solar wind impacting Earth are usually first based on remote-sensing observations of the solar disc and corona, and eventually validated and/or refined with in-situ measurements taken at the Sun$-$Earth Lagrange L1 point, where real-time monitoring probes are located. However, this pipeline provides, on average, only a few tens of minutes of lead time, which decreases to $\sim$30 minutes or less for large solar wind speeds of $\sim$800 km/s and above. The G5 geomagnetic storm of 2024 May provided an opportunity to test predictions generated employing real-time data from the STEREO-A spacecraft, placed 13{\deg} west of Earth and 0.04 au closer to the Sun than L1 at the time of the event, as shown recently by Weiler et al. (2025). In this Commentary, we contextualise these results to reflect upon the advantages of measuring the solar wind in situ upstream of L1, leading to improvements in both fundamental research of interplanetary physics and space weather predictions of the near-Earth environment.