Another cloud free day in Scotland let me catch almost 9 hours of this huge and lively prom. Taken with my home made 90mm modded Coronado PST and DMK21 camera. Software: CdC, Eqmod, DSSR, AutoStakkert!, Wavesharp, DVS, Shotcut and Gimp.
David Wilson on April 8, 2025 @ Inverness, Scotland
https://spaceweathergallery2.com/indiv_upload.php?upload_id=221951
To my understanding they do chance circumference. The opaque doubly ionized helium forms at high temperature, expands until temperature drops (change in circumference), drops to singly ionized after expansion, and gets doubly ionized again after contraction (another change in circumference). In Cepheids, it's uniform across the whole star.
Thus, your question makes me doubt my original speculation that it's helium changing ionization levels. The way some material "climbs up" into the arc in this video (from the right end, at one point of time) while other material "rains down" make a magnetic explanation (proposed by others here) seem more plausible.