The Space Weather Research on Coronal Mass Ejections Required Before Operational Sub-L1 Monitors

In-situ measurements from the Sun-Earth Lagrangian L1 point typically provide a 20-minute to 1-hour advanced warning of incoming interplanetary (IP) shocks, magnetic clouds before impact at the nose of Earth's magnetopause. Sub-L1 monitors may provide measurements sunward of the L1 point to improve the lead times for such transients to several hours, and various mission architecture have been proposed for more than 25 years. Because CMEs and shocks do not propagate exactly radially, the location of such a monitor with respect to the Sun-Earth line is a key parameter to take into account when designing such missions. Here, we highlight some recent results and measurements of CMEs that show that small angular separations may result in drastic differences in the CME properties measured by two spacecraft, and examples showing that CME evolution over a few hours may differ significantly from the average evolution as obtained from statistical studies over several decades. We highlight how a pathfinder mission is required to better understand the variation of properties within CMEs on moderate scales and the evolution of CMEs over a few hours. Such an improved knowledge will then allow for a dedicated fleet of operational monitors that will improve the lead time of space weather forecasting without a loss of accuracy