Estimation and Assessment of the Solar Wind Propagation Time from the Lagrange point L1 to the Earth's Bow Shock

Samira Tasnim^1*Ying Zou²Claudia Borries³Carsten Baumann⁴Brian M Walsh⁵Connor O'Brien⁵Krishna Khanal⁶Huaming Zhang⁶

• ¹Institute for Solar-Terrestrial Physics, German Aerospace Center (DLR), Neustrelitz, Germany
• ²Johns Hopkins University Applied Physics Laboratory, Laurel, United States
• ³Deutsches Zentrum fur Luft- und Raumfahrt DLR Institut fur Solar-Terrestrische Physik, Neustrelitz, Germany
• ⁴WEMAG Netz GmbH, Schwerin, Germany
• ⁵Boston University, Boston, United States
• ⁶The University of Alabama in Huntsville, Huntsville, United States

The solar wind (SW) passing the Earth is an important driver of electrodynamic processes in the Earth’s Magnetosphere-Ionosphere-Thermosphere (MIT) system. Since SW observations near Earth (at the bow shock) are very sparse, research and operational applications typically rely on measurements of SW monitors at the Lagrange point L1. The data of these monitors, which provide almost continuous datasets, need to be propagated in time to the bow shock conditions in order to be most useful for MIT studies. The most widely used data source for propagated SW data is provided by OMNIWeb. The near-Earth SW observations are highly relevant for the validation of the propagated SW estimates. This work makes use of the near-Earth (NE) SW observations to propose a novel method for the estimation of the SW propagation delay. It is based on a careful data assessment and a complex combination of correlation analysis and validation metrics. The developed algorithm generates a big dataset of 53880 events in the period from December 22, 2017, to April 30, 2024, which provides the SW delay along with a list of metrics indicating the quality of the match between the SW structures at L1 and the bow shock. This dataset shows higher reliability in the SW delay estimates than the OMNIWeb data, because it focuses on the comparison of structures in SW. Using the dataset of the period December 2017 to February 2018, the statistically estimated delay in comparison with the OMNIWeb data reveals that about $50\%$ of the delays are computed very accurately with less than 5 minutes uncertainty, and $80\%$ of the OMNIWeb data delay is reasonably accurate with less than 10 minutes difference from the statistically estimated delay, providing the best match. However, more than $5\%$ of the OMNIWeb data shows rather large differences of more than 20 minutes from the dataset. Thus, it can be concluded that in many cases, the uncertainty in the OMNIWeb delay estimate is larger than the value provided along with the data. The generated dataset of SW delay estimates provides an ideal foundation for validating and improving the solar wind propagation models.