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Monitoring the Solar Wind Before It Reaches L1

Erika Palmerio

TL;DR

The article evaluates measuring the solar wind upstream of the L1 point to improve space weather forecasts, addressing the limited lead time provided by L1 in-situ observations. It discusses how Weiler et al. used real-time STEREO-A data at $0.044$ au upstream during the 2024 May G5 storm to extend forecast lead times by about $2.5$ hours and to estimate geomagnetic indices with predictions $Dst$ and $SYM-H$ within tens of nT of observed values, specifically $Dst$ $=-460\pm55$ nT and $SYM-H$ $=-479\pm8$ nT versus $Dst=-406$ nT and $SYM-H=-518$ nT. The piece reviews hindcast studies and mission concepts showing that closer-in solar wind data can reduce arrival-time errors and improve magnetic-field predictions, while highlighting the need for multi-spacecraft configurations to quantify uncertainties. It argues that while sub-L1 monitoring holds promise for both operational forecasting and scientific insights into solar wind evolution, practical implementation requires careful optimization of monitor locations, distances, and constellations, along with further data validation. Overall, the work frames upstream measurements as a promising path toward longer lead times and more reliable space weather predictions, contingent on feasible mission designs and broader data validation.

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° 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.

Monitoring the Solar Wind Before It Reaches L1

TL;DR

The article evaluates measuring the solar wind upstream of the L1 point to improve space weather forecasts, addressing the limited lead time provided by L1 in-situ observations. It discusses how Weiler et al. used real-time STEREO-A data at au upstream during the 2024 May G5 storm to extend forecast lead times by about hours and to estimate geomagnetic indices with predictions and within tens of nT of observed values, specifically nT and nT versus nT and nT. The piece reviews hindcast studies and mission concepts showing that closer-in solar wind data can reduce arrival-time errors and improve magnetic-field predictions, while highlighting the need for multi-spacecraft configurations to quantify uncertainties. It argues that while sub-L1 monitoring holds promise for both operational forecasting and scientific insights into solar wind evolution, practical implementation requires careful optimization of monitor locations, distances, and constellations, along with further data validation. Overall, the work frames upstream measurements as a promising path toward longer lead times and more reliable space weather predictions, contingent on feasible mission designs and broader data validation.

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 SunEarth 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 30 minutes or less for large solar wind speeds of 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° 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.

Paper Structure

This paper contains 4 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Catching a Storm Before It Impacts L1: The 2024 May Event
  3. Advantages and Challenges of Solar Wind Monitoring Upstream of L1
  4. Concluding Remarks

Figures (2)

  • Figure 1: Overview of the available in-situ observations near 1 au of the interplanetary structure(s) responsible for the 2024 May geomagnetic storm, showing data collected by (a) STEREO-A and (b) ACE. Each panel shows, from top to bottom: (i) magnetic field magnitude, (ii) Cartesian fields components in Geocentric Solar Ecliptic (GSE) coordinates, (iii) latitudinal and (iv) longitudinal angles of the magnetic field, (v) solar wind speed, (vi) proton density, (vii) proton temperature, and (viii) plasma beta. Note that plasma data from STEREO-A were not available in real time, whilst the remaining data sets (magnetic field from STEREO-A as well as magnetic field and plasma from ACE) are displayed in both (purple) real-time and (grey) science-level formats (except for panel (ii), where only science-level data are shown). In both panels, the orange and blue dashed lines mark the first shock arrival time at STEREO-A and ACE, respectively.
  • Figure 2: Visualisation illustrating the upstream monitor positions that characterised the "sub-L1 CME hindcasting" studies discussed in this Commentary. The positions are reported at the time of the corresponding CME shock arrival at 1 au, and each colour represents a different event laker2024. The larger green circle represents the location near 1 au considered in each work palmerio2025.