Potential benefits of future sub-L1 missions (HENON, SHIELD) for space weather forecasting

We investigate the feasibility and potential forecasting benefits of future sub-L1 missions. Spacecraft positioned sunward of the Sun–Earth L1 point offer a promising opportunity to increase forecast lead times for geoeffective solar-wind structures.  
ESA is currently preparing a sub-L1 mission, HENON, scheduled for launch by the end of 2026. HENON is a CubeSat mission on a distant retrograde orbit (DRO) at roughly 0.9 au. A larger follow-on mission, SHIELD, is being studied, consisting of a fleet of spacecraft with an orbit planned at about 0.86 au. Together, these efforts represent the first concrete steps toward operational sub-L1 monitoring. Compared to L1 monitoring, the forecast lead times for CME in situ structures and their geomagnetic impacts are increased by factors of roughly 10 and 14 for HENON and SHIELD, respectively. 

In our study, we evaluate key requirements for future sub-L1 missions. To this end, we analyse past observations from spacecraft that have crossed the Sun–Earth line at heliocentric distances of less than 0.95 au, including STEREO-A, Solar Orbiter, and Parker Solar Probe. We assess whether and how these data could be used to reliably reproduce observed geomagnetic storms at Earth. We develop a baseline methodology that continuously time-shifts sub-L1 measurements to Earth and hereafter applies the Temerin and Li solar wind-to-Dst model, enabling a direct comparison between predicted and observed geomagnetic indices.  
Exploiting the Sun–Earth line passage of STEREO-A from November 2022 to June 2024, we find that a radial separation to Earth of up to 0.05 au sometimes results in negative lead times, with structures being observed at L1 before STEREO-A. This implies that future sub-L1 monitors must be positioned closer to the Sun than 0.95 au. We also find that stronger geomagnetic events are reproduced best, with 82% of all intense storms being successfully modelled using sub-L1 data. Furthermore, we identify a possible east–west asymmetry in forecast lead time, with higher lead times eastward of the Sun-Earth line than westward. This could, however, be a trajectory effect of STEREO-A and should be systematically investigated by HENON.  
Using Solar Orbiter and Parker Solar Probe measurements at even smaller heliocentric distances, we aim to statistically determine an ideal trade-off between increased lead time and forecast accuracy.