H2-05: Sub-L1 Solar Wind Monitoring for Future Space Weather Predictions

Team ID: H2-05

Team Title:
Sub-L1 Solar Wind Monitoring for Future Space Weather Predictions

Team Leads:
Emma Davies (Geosphere Austria, Austrian Space Weather Office, Graz, Austria) emma.davies@geosphere.at
Noe Lugaz (University of New Hampshire, USA) noe.lugaz@unh.edu

Introduction:

Accurately forecasting the magnetic structure, plasma properties and arrival time of coronal mass ejections (CMEs) with an actionable lead time remains a major challenge in space weather forecasting. Increasing the lead time of forecast from the current 1 hour from L1 to several hours is essential for mitigating impacts to space and ground infrastructure at Earth. CMEs may undergo complex evolution during propagation, causing their global structure to deviate from the idealised magnetic flux rope structure often depicted. Their exact magnetic field configuration, including the shock and sheath regions of the CME, can thus only be determined through direct in-situ spacecraft measurements.

Deploying solar wind monitors closer to the Sun than current capabilities at L1 (sub-L1) presents significant advantages for space weather forecasting by providing earlier warnings and knowledge of the CME and sheath structure with significant lead-time to make more accurate predictions of potential geomagnetic impact. We aim at advancing our understanding of the requirements of sub-L1 measurements and how best to use this type of data to advance space weather forecasting.

+ Key questions:

How do current knowledge gaps in CME properties impact operational forecasting capabilities?
What implications do the identified gaps have for future sub-L1 mission designs?
How can the findings from current mission measurements influence future space weather monitoring strategies?
How can we best satisfy operational needs and improve scientific understanding in space weather research?
How can we prepare to get the most out of future sub-L1 missions, e.g. ESA HENON and SHIELD?

Objectives:

Knowledge gaps regarding the variation of CME properties with angular separation and evolution over moderate radial distances mean that the optimal orbit for an operational sub-L1 mission is not yet fully known. The goal of this action team is to use current and past mission data to demonstrate both the advantages and limitations of potential mission configurations, and develop how best to utilise data to make better space weather predictions in preparation for future sub-L1 missions including ESA HENON and SHIELD.

+ Detailed objectives and output:

1. Spacecraft Observations:

Compile periods of sub-L1 data from STEREO-A, Solar Orbiter and past missions (e.g. Helios) that can be used for training.

2. Location:

Determine the best strategies/procedures to "propagate" sub-L1 measurements to Earth location.
Determine how the angular separation from the Sun-Earth line affects the forecast.

3. Further usage:

How can sub-L1 measurements be combined with solar, coronal and heliospheric imaging?
How can sub-L1 measurements be used for data assimilation in MHD and ML models?

Publications:

+ Lugaz, N. et al., 2025, The Need for a Sub-L1 Space Weather Research Mission: Current Knowledge Gaps on Coronal Mass Ejections, Space Weather, 23, 2024SW004189, doi:10.1029/2024SW004189.

Optimal orbits for sub-L1 missions remain undetermined due to limited multi-spacecraft measurements. Notably, the May 2024 superstorm illustrated significant variations in CME properties over moderate angular scales (0.2°–15°) and distances (0.05–0.2 au). Therefore, a dedicated research mission phase is recommended.

+ Weiler, E. et al., 2025, First Observations of a Geomagnetic Superstorm With a Sub-L1 Monitor, Space Weather 23, 2024SW004260, doi:10.1029/2024SW004260.

The geomagnetic superstorm of May 10–12, 2024, the strongest since 2003, was driven by five interacting CMEs. Using STEREO-A as a sub-L1 monitor, positioned 0.043 AU closer to the Sun than L1, the geomagnetic storm onset could have been predicted 2.57 hours earlier, with a predicted geomagnetic impact within 8% of observed Dst values.

+ Lugaz, N. et al., 2024, The Need for Near-Earth Multi-Spacecraft Heliospheric Measurements and an Explorer Mission to Investigate Interplanetary Structures and Transients in the Near-Earth Heliosphere, Space Science Reviews, 220, doi:10.1007/s11214-024-01108-8.

The proposed Mission to Investigate Interplanetary Structures and Transients (MIIST) aims to address critical knowledge gaps, characterising spatiotemporal variations of solar wind phenomena and enhancing space weather forecasting capabilities. MIIST's design includes four spacecraft to enable simultaneous observations, improving predictive accuracy for geomagnetic storms.

+ Lugaz, N. et al., 2024, The Width of Magnetic Ejecta Measured near 1 au: Lessons from STEREO-A Measurements in 2021–2022, The Astrophysical Journal 962, doi:10.3847/1538-4357/ad17b9.

Analysing 21 CMEs, only four were detected by both STEREO-A and Wind, indicating a typical CME magnetic ejecta angular width of approximately 20°–30°, significantly less than the previously assumed values of 40°–80°. The findings suggest that CME coherence is limited at larger separations, impacting future multi-spacecraft mission designs.

+ Banu, S. et al., 2025, Investigating Coronal Mass Ejections through Multispacecraft Measurements: STEREO-A and L1 in 2022–2023, The Astrophysical Journal 982, doi:10.3847/1538-4357/adb60c.

Among 49 total CMEs measured by STEREO-A or Wind, only 15 were simultaneously detected at both spacecraft, indicating that only 31% of CMEs impacted both within a longitudinal separation of 0°–35°. While 80%–100% of CMEs were observed by both spacecraft within 10°, this percentage rapidly dropped to approximately 30% beyond 10°, and near 30°, it fell further to 10%.

Action Topics:
Assessment and improvement of capabilities to predict CME arrival and impact at L1.

Keywords (Impact):
Space weather forecasting, upstream monitoring, sub-L1 monitors

Keywords (Activity Type):
Understanding , Requirements , Forecasting , Data Utilization , New Instrumentation A

Clusters with overlapping topics:
S3: Solar eruptions, H1: Heliospheric magnetic field and solar wind, H2: CME structure, evolution and propagation through heliosphere

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