Visibility conditions of cometary dust tail striae for ground-based observations: Geometry and magnitude

Striae are fine structures occasionally observed in the dust tails of comets, typically near perihelion, shortly after a comet begins receding from the Sun. In this study, we investigate the observational conditions under which striae appear by examining eleven comets in which striae were observed during their past apparitions. Publicly available images are primarily used, and the geometric configurations for ground-based observations are compared. Apparent total magnitudes are also considered. We find that, with the exception of the unusual case of C/1995 O1 (Hale–Bopp), striae are most likely to be observed from the ground when the heliocentric distance r < 0.7 \, au and the perihelion distance q < 0.6 \, au, within approximately 22 days after perihelion passage. Favorable conditions further include a phase angle 40^∘ < α < 120^∘, a line-of-sight angle to the comet’s orbital plane more than 20^∘ from edge-on, and an apparent total magnitude brighter than roughly 2–3. Although apparent total magnitude depends not only on geocentric distance but also on intrinsic physical properties such as nucleus size, dust-to-gas ratio, and dust production rate, these properties show complex dependencies on heliocentric distance. Therefore, we focus on geometric parameters and apparent total magnitude as practical indicators for the detection of striae in future apparitions. Based on these criteria, we present a simple flowchart for evaluating the likelihood of detecting striae in ground-based observations.