94 research outputs found
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 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 among those reported after 2012,
indicating a possible rapid (approximately of the previous
value) pressure drop between 2016 (the latest reported estimate) and 2019.
However, this drop is detected at a 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
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
Southward propagating auroral structure in meso-micro scale obtained from ground-based multiple observations at Poker Flat Research Range
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
High-precision polarimetry of nearby stars (d < 50 pc): Mapping the interstellar dust and magnetic field inside the Local Bubble
Context:We investigate the linear polarization produced by interstellar dust aligned by the magnetic field in the solar neighborhood (d Aims: We aim to detect and map dust clouds which give rise to statistically significant amounts of polarization of the starlight passing through the cloud, and to determine the interstellar magnetic field direction from the position angle of the observed polarization.Methods: High-precision broad-band (BV R) polarization observations are made of 361 stars in spectral classes F to G, with detection sensitivity at the level of or better than 10−5 (0.001%). The sample consists of 125 stars in the magnitude range 6–9 observed at the 2.2 m UH88 telescope on Mauna Kea, 205 stars in the magnitude range 3–6 observed at the Japanese (Tohoku) T60 telescope on Haleakala, and 31 stars in the magnitude range 4–7 observed at the 1.27 m H127 telescope of the Greenhill Observatory, Tasmania. Identical copies of the Dipol-2 polarimeter are used on these three sites.Results: Statistically significant (>3σ) polarization is found in 115 stars, and >2σ detection in 178 stars, out of the total sample of 361 stars. Polarization maps based on these data show filament-like patterns of polarization position angles, which are related to both the heliosphere geometry, the kinematics of nearby clouds, and the Interstellar Boundary EXplorer ribbon magnetic field. From long-term multiple observations, a number (~20) of stars show evidence of intrinsic variability at the 10−5 level. This can be attributed to circumstellar effects (e.g., debris disks and chromospheric activity). The star HD 101805 shows a peculiar wavelength dependence, indicating size distribution of scattering particles different from that of a typical interstellar medium. Our high signal-to-noise measurements of nearby stars with very low polarization also provide a useful dataset for calibration purposes
Ion-dispersion and rapid electron fluctuations in the cusp: a case study
We present results from co-ordinated measurements with the low altitude REIMEI satellite and the ESR (EISCAT Svalbard Radar), together with other ground-based instruments carried out in February 2006. The results mainly relate to the dayside cusp where clear signatures of so-called ion-dispersion are seen in the satellite data. The cusp ion-dispersion is important for helping to understand the temporal and spatial structure of magnetopause reconnection. Whenever a satellite crosses boundaries of flux tubes or convection cells, cusp structures such as ion-dispersion will always be encountered. In our case we observed 3 distinct steps in the ion energy, but it includes at least 2 more steps as well, which we interpret as temporal features in relation to pulsed reconnection at the magnetopause. In addition, fast variations of the electron flux and energy occurring during these events have been studied in detail. The variations of the electron population, if interpreted as structures crossed by the REIMEI satellite, would map near the magnetopause to similar features as observed previously with the Cluster satellites. These were explained as Alfvén waves originating from an X-line of magnetic reconnection
High-precision broad-band linear polarimetry of early-type binaries. II. Variable, phase-locked polarization in triple Algol-type system λ Tauri
Aim. To study the binary geometry of the classic Algol-type triple
system λ Tau, we have searched for polarization variations over
the orbital cycle of the inner semi-detached binary, arising from light
scattering in the circumstellar material formed from ongoing mass
transfer. Phase-locked polarization curves provide an independent
estimate for the inclination i, orientation Ω, and the direction
of the rotation for the inner orbit. Methods: Linear polarization
measurements of λ Tau in the B, V , and R passbands with the
high-precision Dipol-2 polarimeter have been carried out. The data have
been obtained on the 60 cm KVA (Observatory Roque de los Muchachos, La
Palma, Spain) and Tohoku 60 cm (Haleakala, Hawaii, USA) remotely
controlled telescopes over 69 observing nights. Analytic and numerical
modelling codes are used to interpret the data. Results: Optical
polarimetry revealed small intrinsic polarization in λ Tau with
0.05% peak-to-peak variation over the orbital period of 3.95 d. The
variability pattern is typical for binary systems showing strong second
harmonic of the orbital period. We apply a standard analytical method
and our own light scattering models to derive parameters of the inner
binary orbit from the fit to the observed variability of the normalized
Stokes parameters. From the analytical method, the average for three
passband values of orbit inclination i = 76° +
1°/-2° and orientation Ω =
15°(195°) ± 2° are
obtained. Scattering models give similar inclination values i =
72-76° and orbit orientation ranging from Ω =
16°(196°) to Ω =
19°(199°), depending on the geometry of
the scattering cloud. The rotation of the inner system, as seen on the
plane of the sky, is clockwise. We have found that with the scattering
model the best fit is obtained for the scattering cloud located between
the primary and the secondary, near the inner Lagrangian point or along
the Roche lobe surface of the secondary facing the primary. The
inclination i, inferred from polarimetry, agrees with the previously
made conclusion on the semi-detached nature of the inner binary, whose
secondary component is filling its Roche lobe. The non-periodic scatter,
which is also present in the polarization data, can be interpreted as
being due to sporadic changes in the mass transfer rate.
The polarization data for λ Tauri are only available at the CDS
via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/611/A69
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Pi pulsations at substorm onset site
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27日(火) 国立極地研究所 2階大会議
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