Testing size-dependent Geminid activity profiles using BRAMS/RMZ radio echo durations

Previous radar and visual studies (e.g., Šimek & McIntosh, 1989; Zigo et al., 2009; Uchiyama, 2010) have shown that brighter Geminid meteors exhibit systematically later peak activity times and narrower activity profiles compared to fainter meteors. This is generally interpreted as evidence for size sorting within the meteoroid stream, driven by long-term dynamical evolution and Poynting–Robertson drag.

Here, we test whether these size-dependent activity profile effects persist when the Geminid shower is measured with forward-scatter radio observations from the BRAMS/RMZ network, using echo durations as an observationally accessible proxy tied to meteoroid mass. We carefully note that echo duration is not solely intrinsic to particle size, but also depends on observational geometry and the evolution of the ionized trail.

We apply the multi-observer inversion framework developed by De Keyser et al. (2026).  Radio meteor detections are categorized into duration classes, and for each class we fit the shower activity profile with a double-exponential model parameterized by the peak time, rise time-scale, and decay time-scale. We examine whether a single (universal) observability function can be applied across all duration classes, or whether duration-dependent observability effects must be incorporated to reconcile the inferred activity profiles.

We further assess whether the fitted profile parameters reproduce the size-dependent trends reported in prior literature. As an additional, independent constraint, we analyze cumulative echo-duration distributions to derive estimates of the Geminid mass index (Belkovich et al., 2005).

If successful, this study will clarify whether the apparent size sorting inferred from visual/radar brightness carries over to radio-based measurements after accounting for measurement biases, strengthening our physical understanding of Geminid meteoroid evolution.