Authors: Noé Lugaz (University of New Hampshire), Bin Zhuang (University of New Hampshire), Nada Al-Haddad University of New Hampshire), Sahanaj Banu (University of New Hampshire), Charles J. Farrugia (University of New Hampshire), Christian Möstl (Geosphere Austria), Toni Galvin (University of New Hampshire)

Recent measurements by STEREO-A, Wind, ACE, Solar Orbiter and Parker Solar Probe of coronal mass ejections (CMEs) reveal that the angular extent of the magnetic ejecta at the core of CMEs is narrower than previously thought. In addition, the variations of the CME properties with longitudes are found to be complex and not following a simple trend line. For separations of less than 10-15°, the overall properties of CMEs may be relatively similar, even though the time variations of the magnetic field and plasma parameters show higher variance. This fact can be used to forecast CME properties using measurement sunward of L1 (from so-called sub-L1 monitors), even for moderate longitudinal separations. We show a proof of concept using STEREO-A measurements around the time of its passage in front of Earth in August 2023.