A remote-sensing framework for interpreting coronal mass ejection transit effects

This proof-of-concept study develops a remote-sensing methodology to identify coronal mass ejection (CME) transit effects in the inner heliosphere, using an ideal self-similar reference shell together with a compression model to place observed density falloffs in context. We apply the method to assess how pile-up shapes the density structure, and to what extent weaker, localized departures, potentially consistent with magnetic erosion, can be isolated once compression and line-of-sight projection effects are accounted for. We analyzed the 27 January 2012 CME using STEREO/SECCHI COR2-A, together with COR2-B and SOHO/LASCO C3. Observed electron-density profiles were extracted along the CME front in position-angle (PA) sectors and compared with synthetic profiles from a forward-modeled, self-similar graduated cylindrical shell (GCS). The synthetic profiles were normalized to the observed density at a selected reference height, establishing a common baseline for comparison. We quantified radial density evolution using sector-wise power-law fits that accounted for measurement scatter. These trends were compared with a pile-up baseline, and residual differences were used to identify localized departures. Sector-wise density indices reveal nonuniform evolution across the CME front. In COR2-A, western sectors exhibit pronounced flattening relative to the synthetic shell, approaching the snowplow-like compression benchmark, while several lower and central PA sectors show steeper-than-synthetic falloffs. COR2-B and LASCO C3 also show sector-dependent departures, with PA patterns varying with viewing geometry. Using a conservative excess criterion, clear pile-up is detected only in a subset of COR2-A sectors and timestamps, indicating spatially structured compression. Snowplow comparisons depend strongly on the assumed upstream wind: the fast-stream case shows no density deficit, whereas the slow-stream case yields deficits across all PA sectors. These results indicate that compression alone is insufficient to explain the observed evolution, which combines compression-dominated regions with localized steepening potentially reflecting depletion, material redistribution, or erosion-like evolution. This study shows that coronagraph-derived CME-front density profiles can be used as a quantitative diagnostic by comparing an idealized self-similar expansion reference with snowplow-based compression benchmarks. Applied to the 27 January 2012 CME, the results show a mixed front rather than a single global behavior: some sectors are compression-dominated and flatten toward the snowplow-like benchmark, while others fall off more steeply than the synthetic shell and therefore identify candidate regions of depletion or erosion-like evolution. Applied to larger samples and combined with event-local in situ constraints and modeling, this framework provides a practical route to separate the physical processes shaping CME evolution during transit.