Authors: A. P. Rasca (CU/CIRES), H. J. Singer (NOAA/SWPC), G. H. Millward (CU/CIRES), J. G. Luhmann (UC Berkeley), C. O. Lee (UC Berkeley), L. Jian (NASA), G. Tóth (University of Michigan), Z. Huang (University of Michigan), A. B. Galvin (University of New Hampshire), L. Ellis (University of New Hampshire), C. T. Russell (UCLA), X. Liu (UCLA), P. Schroeder (UC Berkeley), H. Wei (UCLA)

On August 12, 2023 the STEREO-A spacecraft made its first close approach with Earth since 2006, passing at a distance of ~8 million km along the Earth-Sun line. Downstream at the L1 Lagrange point (1.5 million km from Earth) lie the ACE and DSCOVR spacecraft, with solar wind plasma and magnetic field measurements used at the inflow boundary for the operational Geospace global magnetosphere model that provides a 30-60 minute lead time for incoming space weather activity. The brief passage of STEREO-A near the Sun-Earth line provides an opportunity to test the performance of the Geospace model using sub-L1 observations that can increase the lead time by several hours. We use data from STEREO-A as input for the Geospace model for three geomagnetic storms in April, August, and September of 2023 and compare predictions with the L1-driven Geosapce model results that use Wind and ACE/DSCOVR measurements.