Theory and simulations of relativistic particle motions in a magnetosonic shock wave

The motions of relativistic particles in a magnetosonic shock wave propagatingobliquely to an external magnetic field are studied. In the zeroth-ordertheory, particles continue to move nearly parallel to the external magnetic field inthe shock transition region, when the shock speed is close to c cos θ, where c is thespeed of light and θ is the propagation angle. Perturbations to this zeroth-ordermotion are also analyzed for positrons and ions. The perturbation frequency ofpositrons is ω ? Ωp0γ?1 and that of ions is ω ? Ωi0γ?1/2, where Ωp0 and Ωi0 arethe non-relativistic gyrofrequencies of positrons and of ions, respectively, and γis the Lorentz factor. These theoretical predictions are confirmed with numericalsimulations

Evidence for a rapid decrease in Pluto's atmospheric pressure revealed by a stellar occultation in 2019

We report observations of a stellar occultation by Pluto on 2019 July 17. A single-chord high-speed (time resolution $= 2\,$s) photometry dataset was obtained with a CMOS camera mounted on the Tohoku University 60 cm telescope (Haleakala, Hawaii). The occultation light curve is satisfactorily fitted to an existing Pluto's atmospheric model. We find the lowest pressure value at a reference radius of $r = 1215~{\rm km}$ among those reported after 2012, indicating a possible rapid (approximately $21^{+4}_{-5} \%$ of the previous value) pressure drop between 2016 (the latest reported estimate) and 2019. However, this drop is detected at a $2.4\sigma$ level only and still requires confirmation from future observations. If real, this trend is opposite to the monotonic increase of Pluto's atmospheric pressure reported by previous studies. The observed decrease trend is possibly caused by ongoing ${\rm N_2}$ condensation processes in the Sputnik Planitia glacier associated with an orbitally driven decline of solar insolation, as predicted by previous theoretical models. However, the observed amplitude of the pressure decrease is larger than the model predictions.Comment: 7 pages, 3 figures, accepted for publication in Astronomy and Astrophysic