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

A detailed case study of dayside diffuse aurora using GEOTAIL, FAST, and South pole all sky imager

第3回極域科学シンポジウム/第36回極域宙空圏シンポジウム 11月26日（月）、27日（火） 国立極地研究所 2階ラウン

Vertical emissivity profiles of Jupiter's northern H-3(+) and H-2 infrared auroras observed by Subaru/IRCS

We resolved the vertical emissivity profiles of H-3(+) overtone, H-3(+) hot overtone, and H-2 emission lines of the Jovian northern auroras in K band obtained in December 2011 observed by the IR Camera and Spectrograph of the Subaru 8.2m telescope with the adaptive optics system (AO188). The spatial resolution achieved was similar to 0.2 arcsec, corresponding to similar to 600 km at Jupiter. We derived the vertical emissivity profiles at three polar regions close to the Jovian limb. The H-3(+) overtone and H-3(+) hot overtone lines had similar peak altitudes of 700-900 km and 680-950 km above the 1 bar level, which were 100-300 km and 150-420 km lower, respectively, than the model values. On the contrary, the H-2 peak emission altitude was high, 590-720 km above the 1 bar level. It was consistent with the value expected for precipitation of similar to 1 keV electron, which favors a higher-altitude emissivity profile. We concluded that the lower peak altitudes of H-3(+) overtone and hot overtone lines were caused by the nonlocal thermodynamic equilibrium effect stronger than the model assumption. We could reproduce the observational emissivity profiles from the model by including this effect. It has been proposed that neutral H-2 and ionized H-3(+) emissions can have different source altitudes because of their different morphologies and velocities; however, our observed results with a general circulation model show that the peak emission altitudes of H-3(+) and H-2 can be similar even with different velocities