Using STEREO-A data from April to November 2023 as a sub-L1 monitor

Sub-L1 monitors are currently being researched in mission concepts for small satellites and may be deployed on distant retrograde orbits around the Earth in the future. Depending on the location of the sub-L1 monitor, the lead time for the arrival of coronal mass ejections (CMEs) and for determining their geo-effectiveness could be prolonged. If the sub-L1 monitor was to orbit the Earth at a distance of 0.05 AU, as is proposed for the MIIST mission, for example, Dst predictions could be made up to 5 hours in advance. The close encounter of STEREO-A and Wind from April 2023 to November 2023 represents such a constellation, and therefore allows us to investigate potential impacts of future sub-L1 missions. Following the method of Bailey et al. (2020), the data from STEREO-A are mapped to L1, taking into account an expansion of the CME. We then calculate the Dst of the temporally and spatially shifted data, and compare the result with the Dst calculated from L1 solar wind data and the observed Dst. In this way, we can analyse and quantify the implications of sub-L1 monitors on space weather forecasting.

The events included in our study are part of the HELIO4CAST lineup catalog v2.0 (https://helioforecast.space/lineups). The catalogue includes CMEs that were observed by at least two spacecraft such as Solar Orbiter, Parker Solar Probe, BepiColombo, STEREO-A, and Wind. In contrast to single in situ measurements, which do not adequately capture the vast structure of CMEs, the events in the catalogue allow us to study the temporal and spatial evolution of CMEs and improve our current understanding of the large-scale structure of their magnetic flux ropes. In view of the upcoming maximum of solar cycle 25, further multipoint events are expected to be continuously added to the catalogue.