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 atmospheric model of Pluto. We find the lowest pressure value at a reference radius of
r
= 1215 km among those reported after 2012. These reports indicate a possible rapid (approximately 21
−5
+4
% of the previous value) pressure drop between 2016, which is the latest reported estimate, and 2019. However, this drop is detected at a 2.4
σ
level only and still requires confirmation from future observations. If real, this trend is opposite from the monotonic increase of Pluto’s atmospheric pressure reported by previous studies. The observed decrease trend is possibly caused by ongoing 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.
Embedded deep inside the huge magnetosphere of Jupiter, the moon Io has active volcanos. Jovian magnetospheric dynamics are driven by the expulsion of Iogenic plasma in the strongly magnetized, ...fast‐rotating system and should vary in response to Io's volcanic activity. In early 2015 when various observations indicated an increase in volcanic activity, the EXCEED instrument onboard the Hisaki spacecraft continuously observed the Jovian magnetosphere via the aurora emission and the emission from the Io plasma torus. The plasma diagnosis of the enhanced Io plasma torus spectrum along with a physical chemistry model for deducing plasma parameters revealed a higher plasma density and a 2–4 times faster radial flow as compared with a volcanically quiet period. Aurora emissions reflecting midmagnetospheric activities showed multiple highly elevated brightness peaks about a month later. Long‐term and continuous monitoring by Hisaki enabled the first comprehensive observations of the Jovian magnetosphere in response to Io's enhanced volcanic activity.
Plain Language Summary
This article, based on the high‐resolution and continuous extreme ultraviolet spectroscopic data taken by the Japanese Hisaki satellite, shows that the plasma dynamics in the Jovian magnetosphere is highly influenced by Io's volcanic activity. The observations were made continuously from the beginning to end of a volcanic event. Through the detailed analysis of the spectra around Io's orbit and the Jovian aurora, and by fitting the data to a physical chemistry model, this first comprehensive study concludes that the plasma transport within the magnetosphere is enhanced by a factor of 2–4 by Io's volcanic activity.
Key Points
Continuous remote observations of the Io plasma torus were made by Hisaki from beginning to end of an Io volcanic event in January 2015
The plasma parameters and transport timescales were deduced through a detailed analysis of the spectra and physical chemistry model
Plasma transport timescale and source strength were enhanced and changed from 34 to 9.9 days and 0.7 to 3.0 tons/s by Io's volcanic activity, respectively
Abstract
The satellite Io, which has volcanoes and is located at 5.9
R
J
from the center of Jupiter, is a powerful plasma source in the magnetosphere. The heavy ions originating from Io form a ...torus‐like structure and emit radiation. The pickup energy and hot electrons are believed to power the Io plasma torus. Voyager data showed that a trace amount of hot electrons (at several hundreds of eV) exist in the torus. The origin of hot electrons, that is, plasma heating and/or transport mechanisms, have been mentioned in previous research. However, the contribution of each mechanism toward supplying hot electrons remains poorly understood. To address this issue, we explored the time variation and spatial structure of hot electrons by spectroscopic observations using the Hisaki satellite. In this study, the radial distributions of plasma densities and temperatures were derived from the emission line intensities in the extreme ultraviolet range of day of year (DOY) 331 in 2014 to DOY 134 in 2015, which includes the Io's volcanically active period. We found that hot electrons inside the torus began to increase particularly on the duskside ~40 days after the onset of volcanic activity. This result suggests that the mass increase in the torus with volcanic activity enhanced the plasma transport from the outside within a specific region or via a local heating process.
Key Points
Long‐term continuous monitoring revealed that the plasma environment around Jupiter fluctuated coincident with Io's volcanic activity
Around 40 days after the volcanic activity starts, hot electron components inside the torus began to increase especially on the duskside
We present evidence that enhanced mass production produces either increased inward transport of hot electrons or local heating
Jupiter's sodium nebula, which originates from Io's volcanic gas, shows variations in its brightness due to the volcanic activity on Io. Imaging observation of D-line brightness in the sodium nebula ...was performed from 2013 through 2015 in a conjunction with the HISAKI mission. The D-line brightness of the sodium nebula had been stably faint and dim until January 2015, but it showed a distinct enhancement from February through March, 2015. The brightness increased by three times during this enhancement. Details in variations of Jupiter's sodium nebula are shown in this paper.
The Io plasma torus, situated in the Jovian inner magnetosphere (6–8 Jovian radii from the planet) is filled with heavy ions and electrons, a large part of which are derived from Io's volcanos. The ...torus is the key area connecting the primary source of plasma (Io) with the midmagnetosphere (>10 Jovian radii), where highly dynamic phenomena are taking place. Revealing the plasma behavior of the torus is a key factor in elucidating Jovian magnetospheric dynamics. A global picture of the Io plasma torus can be obtained via spectral diagnosis of remotely sensed ion emissions generated via electron impact excitation. Hisaki, an Earth‐orbiting spacecraft equipped with an extreme ultraviolet spectrograph Extreme Ultraviolet Spectroscope for Exospheric Dynamics, has observed the torus at moderate spectral resolution. The data have been submitted to spectral analysis and physical chemistry modeling under the assumption of axial symmetry. Results from the investigation are radial profiles of several important parameters including electron density and temperature as well as ion abundances. The inward transport timescale of midmagnetospheric plasma is obtained to be 2–40 h from the derived radial profile for the abundance of suprathermal electrons. The physical chemistry modeling results in a timescale for the outward transport of Io‐derived plasma of around 30 days. The ratio between inward and outward plasma speed (~1%) is consistent with the occurrence rate of depleted flux tubes determined using in situ observations by instruments on the Galileo spacecraft.
Key Points
Hisaki enables EUV spectral diagnosis of the Io torus for both S and O ions by eliminating the geocoronal contamination
Radial profiles are derived for the density of electrons and various ion species, plus electron temperature
Timescales of inward and outward plasma transport are estimated to be 2–40 h and 30 days, respectively
The innermost Galilean satellite, Io, supplies a large amount of volcanic gasses to the Jovian magnetosphere. The fast rotation of Jupiter and the outward transport of ionized gasses are responsible ...for forming a huge and rotationally dominant magnetosphere. The plasma supply from the satellite has a key role in the characterization of the Jovian magnetosphere. In fact, significant variations of the plasma population in the inner magnetosphere caused by the volcanic eruptions in Io were found in early 2015, using a continuous data set of the Io plasma torus obtained from an extreme ultraviolet spectroscope onboard the Hisaki satellite. The time evolution of the Io plasma torus radial distribution showed that the outward transport of plasma through 8 RJ from Jupiter was enhanced for approximately 2 months (from the end of January to the beginning of April 2015). Intense short‐lived auroral brightenings––which represent transient energy releases in the outer part of the magnetosphere—occurred frequently during this period. The short‐lived auroral brightenings accompanied well‐defined sporadic enhancements of the ion brightness in the plasma torus, indicating a rapid inward transport of energy from the outer part of the magnetosphere and the resultant enhancement of hot electron population in the inner magnetosphere. This evidently shows that the change in a plasma source in the inner magnetosphere affects a large‐scale radial circulation of mass and energy in a rotationally dominant magnetosphere.
Plain Language Summary
We present the first continuous and long‐term monitoring of both ultraviolet aurora activity and ionized gas around Jupiter obtained by the Earth‐orbiting spectroscope satellite, Hisaki. The innermost Galilean satellite, Io, is the volcanically most active body in our solar system. The volcanic gasses are ionized in the magnetosphere, the region manipulated by the planetary magnetic field, and obtain angular momentum from Jupiter's fast rotation through the magnetic field connecting with Jupiter. When Io's volcanic activity increased in early 2015, Hisaki observed that the Jovian magnetosphere was filled with iogenic ionized gasses for over 2 months and Jupiter's powerful auroral breakups occurred very frequently. This is contradictory to the terrestrial magnetosphere in which the aurora breakup occurs as a result of the solar wind‐energy penetration into the magnetosphere. Although Io occupies only a very small region in the vast Jovian magnetosphere, it releases significant amounts of material around the space near Jupiter, extracts energy from Jupiter's rotation, and affects activation of the powerful aurora of the giant planet.
Key Points
Evolution of Io plasma torus radial distribution caused by volcanic eruptions in Io was observed in early 2015
Outward plasma transport from the Io plasma torus through 8 RJ from Jupiter enhanced for approximately 2 months
An inner magnetosphere plasma source is shown to affect large‐scale mass/energy radial circulation in rotationally dominant magnetosphere
In the Jovian magnetosphere, sulfur and oxygen ions supplied by the satellite Io are distributed in the so‐called Io plasma torus. The plasma torus is located in the inner area of the magnetosphere ...and the plasma in the torus corotates with the planet. The density and the temperature of the plasma in the torus have significant azimuthal variations. In this study, data from three‐year observations obtained by the Hisaki satellite, from December 2013 to August 2016, were used to investigate statistically the azimuthal variations and to find out whether the variations were influenced by the increase in neutral particles from Io. The azimuthal variation was obtained from a time series of sulfur ion line ratios, which were sensitive to the electron temperature and the sulfur ion mixing ratio S3+/S+. The major characteristics of the azimuthal variation in the plasma parameters were consistent with the dual hot electron model, proposed to explain previous observations. On the other hand, the Hisaki data showed that the peak System III longitude in the S3+/S+ ratio was located not only around 0°–90°, as in previous observations, but also around 180°–270°. The rotation period, the System IV periodicity, was sometimes close to the Jovian rotation period. Persistent input of energy to electrons in a limited longitude range of the torus is associated with the shortening of the System IV period.
Key Points
Persistent azimuthal variations in the Io plasma torus were confirmed from three years of Hisaki observations
The characteristics of the azimuthal variation were consistent with the dual hot electron model but different behaviors were also found
The System IV period decreased 3 times. Two of these decreases were associated with increased volcanic activity on Io
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.
We report narrow band‐filtered imaging observations of the Jovian H3+ 3.4‐μm emission using the IRCS (infrared camera and spectrograph) on the Subaru telescope taken on 25 May 2016. Approximately ...1 hr of data was taken at intervals of 45–110 s, with high spatial resolution (~0.2 arcsec) using adaptive optics. In the northern polar region, we found bright patch‐like emissions on the poleward side of the main oval. One of them had a pulsation period of ~10 min. We utilized an H3+ emission model to investigate the response time of the H3+ emission to abrupt and periodic variations of the precipitating electron flux. The model showed that the H3+ emission could pulsate with this timescale due to a modulated flux of the precipitating electrons in the kilo‐electron‐volt to tens of kilo‐electron‐volt energy range.
Plain Language Summary
We made a movie of the Jovian infrared aurora for the first time. The high spatial resolution images were observed for ~1 hr with the time interval of 45–110 s using the infrared camera of the Subaru‐8‐m telescope. This movie showed that the infrared aurora from the hydrogen ion molecule H3+ had patchy structures on the northern auroral region and a pulsation period of ~10 min. Our model analysis proved that such a fast variation could be driven by the modulation of the kilo‐electron‐volt to tens of kilo‐electron‐volt electrons coming into Jupiter.
Key Points
We took infrared images of the Jovian H3+ aurora for ~1 hr at time intervals of 45–110 s with the assistance of adaptive optics
The aurora images showed a patchy structure on the polar side of the northern main oval with a pulsating interval of ~10 min
A model analysis showed that such a fast variation can be driven by the modulated electron flux with energy in the kilo‐electron‐volt to tens of kilo‐electron‐volt range
We resolved the vertical emissivity profiles of H3+ overtone, H3+ hot overtone, and H2 emission lines of the Jovian northern auroras in K band obtained in December 2011 observed by the IR Camera and ...Spectrograph of the Subaru 8.2 m telescope with the adaptive optics system (AO188). The spatial resolution achieved was ~0.2 arcsec, corresponding to ~600 km at Jupiter. We derived the vertical emissivity profiles at three polar regions close to the Jovian limb. The H3+ overtone and H3+ 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 H2 peak emission altitude was high, 590–720 km above the 1 bar level. It was consistent with the value expected for precipitation of ~1 keV electron, which favors a higher‐altitude emissivity profile. We concluded that the lower peak altitudes of H3+ 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 H2 and ionized H3+ 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 H3+ and H2 can be similar even with different velocities.
Key Points
Vertical emissivity profiles of Jovian IR aurora were resolved in K band
H3+ overtone and hot overtone lines had lower peak altitudes than the model
This lower peak was caused by the non‐LTE effect stronger than the model
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