Authors: NoeLugaz (SSC, EOS, UNH), Bin Zhuang (SSC, EOS, UNH), Sahanaj Banu (SSC, EOS, UNH), Charlie Farrugia (SSC, EOS, UNH)

Coronal mass ejections (CMEs) evolve as they move outward from the Sun, undergoing expansion, distortion, and erosion. In situ measurements in the inner heliosphere are typically compared with those at L1 to study how CME properties change with heliocentric distance. Changes in CME speed are often described as deceleration due to interaction with the solar wind during propagation. Part of this variation, however, may instead reflect CME aging, namely the intrinsic evolution of the structure as it passes over a spacecraft. These two effects are not equivalent when the CME crossing time at a spacecraft (around one day at 1 au) is a substantial fraction of the CME age (about three days at 1 au). Separating propagation from aging requires near-simultaneous measurements at different heliocentric distances, which is observationally demanding. Here we use CME events recorded concurrently by STEREO-A and spacecraft near L1, taking advantage of STEREO-A’s passage in front of the Sun–Earth line in August 2023. The radial offset between STEREO-A and Earth (0.05–0.06 au, roughly 25–30% of a typical magnetic cloud’s radial size) allows us to probe CME evolution on timescales of a few hours, shorter than in most previous studies. We use these observations to distinguish changes linked to aging from those caused by propagation and to assess which description better matches the data. Clarifying this distinction is important for space-weather applications that will rely on upstream measurements closer to the Sun, where the CME evolution over only a few hours can affect forecasts at Earth.