We report on the discovery of unambiguous regions of low dust number density in amateur images of the dust tails of long period comets C/2014 Q1 (PanSTARRS) and C/2002 F1 (Utsunomiya). In both cases, one such gap appeared in the dust tail post-perihelion. These gaps present themselves as a wedge-shaped region devoid of dust, with dust being present on either side of it. Initially, it was thought by observers that this was a commonn separation of the dust tail and dust trail that lies along the comet’s path.
The dust tails were simulated using the Finson-Probstein model . The simulated tails were then overlaid onto images of the dust tails for comparison. Further analysis was performed by transforming the image and plotting it in dust beta vs. ejection time coordinate space, a novel temporal mapping method developed by Price and co-workers . The image dataset for C/2014 Q1 was the most extensive, covering July to September 2015. The gap was visible throughout this observation period, and its shape and structure remained constant. This also applies to the gap present in C/2002 F1, although the dataset only ranged between 27th March and 3rd April 2002.
The results of the C/2014 Q1 study show that none of the dust on either side of the gap lay along the comet’s orbital path, confirming that both sections of dust were part of the main dust tail, rather than the dust trail. The edges of the gaps were bounded accurately by lines of constant dust ejection time (synchrones), corresponding to dust that should have been ejected between 6th-12th July 2015. Analysis of C/2002 F1 found similar results, with the edges of its dust gap bounded accurately by synchrone lines corresponding to a dust ejection time between 18th - 24th April 2002. Thus, the dust gaps in both comets correspond to dust that should have been ejected during their respective perihelion passages on 6th July 2015 (C/2014 Q1) and 22nd April 2002 (C/2002 F1).
To the authors’ knowledge, this is the first time such features have been identified in cometary dust tails, and their cause(s) are still uncertain. The comparison of two comets that display this feature has allowed for a more accurate characterization of these dust gaps, however the lack of an extensive dataset for both comets has hindered the investigation of the gap’s formation mechanism. We shall briefly present potential formation mechanisms, the respective viability of which will improve through the identification and analysis of more comets which display these intriguing features.
 Finson, M. and Probstein, R. (1968) The Astrophysical Journal, 154, p.327  Price et al. (2019) Icarus 319: 540-557.