BepiColombo is a joint mission between the European Space Agency, ESA, and the Japanese Aerospace Exploration Agency, JAXA, to perform a comprehensive exploration of Mercury. Launched on
20
th
...October 2018 from the European spaceport in Kourou, French Guiana, the spacecraft is now en route to Mercury.
Two orbiters have been sent to Mercury and will be put into dedicated, polar orbits around the planet to study the planet and its environment. One orbiter, Mio, is provided by JAXA, and one orbiter, MPO, is provided by ESA. The scientific payload of both spacecraft will provide detailed information necessary to understand the origin and evolution of the planet itself and its surrounding environment. Mercury is the planet closest to the Sun, the only terrestrial planet besides Earth with a self-sustained magnetic field, and the smallest planet in our Solar System. It is a key planet for understanding the evolutionary history of our Solar System and therefore also for the question of how the Earth and our Planetary System were formed.
The scientific objectives focus on a global characterization of Mercury through the investigation of its interior, surface, exosphere, and magnetosphere. In addition, instrumentation onboard BepiColombo will be used to test Einstein’s theory of general relativity. Major effort was put into optimizing the scientific return of the mission by defining a payload such that individual measurements can be interrelated and complement each other.
We summarize Jupiter's ultraviolet (UV) auroral response to solar wind dynamic pressure variations during Juno's approach to Jupiter in 2016. The response time of Jupiter's aurora to external drivers ...has thus far been unknown owing to a sparsity of upstream in situ solar wind measurements. Combining the Juno solar wind observations with continuous UV aurora data obtained by Hisaki EXCEED (Extreme Ultraviolet Spectroscope for Exospheric Dynamics) and Juno UV spectrograph, the UV aurora brightenings in response to three major shock arrivals showed time lags of 10–15 hr. These time lags are longer than the time required for ballistic propagation of the shocks by the solar wind. In addition to that puzzle, while an enhancement in the UV auroral power was observed with an increase in dynamic pressure to ~0.03 nPa, no associated brightening was observed with a dynamic pressure elevation of >0.1 nPa. These imply that internal magnetospheric aspects need to be taken into consideration to fully resolve the issue.
Plain Language Summary
Jovian ultraviolet aurora are emitted from hydrogen molecules in Jupiter's atmosphere when energetic electrons precipitate from the magnetosphere to excite the atmospheric molecules. The Jovian magnetosphere is always under the influence of the solar wind. Variation in the solar wind affects magnetospheric dynamics and thus the Jovian aurora intensity. The solar wind‐magnetosphere interaction is well studied for Earth, and the issue of aurora response to the solar wind is also well studied for Earth, but the issue remains open for Jupiter. Here we obtain the response time of aurora brightening upon intensification of the solar wind, which is a very fundamental quantity, to find it to be too long to be explained by a simple propagating model that assumes the solar wind as the dominant driver. Furthermore, some small variations in solar wind shocks led to aurora brightenings, while larger variations did not trigger other events. The characteristics discussed in this paper provide good case studies to validate theories or numerical simulations of how Jovian aurora may respond to changes in the solar wind.
Key Points
We compare Jupiter's ultraviolet aurora variation observed by Hisaki with changes in the upstream solar wind conditions observed by Juno
Transient brightenings responded to major solar wind shocks with ~10 hr lag time, which is inconsistent with a solar wind propagation model
A brightening triggered by a dynamic pressure elevation of 0.03 nPa was detected, whereas a 0.1 nPa elevation did not trigger a brightening
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
The hydrogen exosphere constitutes the uppermost atmospheric layer of the Earth, and its shape may reflect the last stage of the atmospheric escape process. The distribution of hydrogen in the outer ...exosphere remains unobserved because outer geocoronal emissions are difficult to observe from within the exosphere. In this study, we used the Lyman Alpha Imaging Camera on board the Proximate Object Close Flyby with Optical Navigation spacecraft, located outside the exosphere, to obtain the first image of the entire geocorona that extends to more than 38 Earth radii. The observed emission intensity distribution can be reproduced using our analytical model that has three parameters: exobase temperature, exobase density, and solar radiation pressure, which implies that hot hydrogen production in the magnetized plasmasphere is not the dominant process shaping the outer hydrogen exosphere. However, the role of the magnetic effect in determining the total escape flux cannot be ruled out.
Plain Language Summary
In this report, we show the first high‐quality, and wide‐field‐of‐view (FOV) image of Earth's hydrogen corona of 100 Earth radii (RE) obtained by the first interplanetary microspacecraft. Because hydrogen geocorona has not been observed since Apollo 16 in 1972, which observed only up to 10 RE of FOV. The field of view of our observation is ~10 times wider than that in past. Furthermore, since the advancement in deep UV detection technology in the last four decades is very large, the improvement in data quality is very large. In fact, our newly obtained data strongly support a different picture for geocorona distribution. More specifically, we found that the observed ecliptic north‐south symmetrical distribution can be reproduced by a simple analytic model and is not consistent with past results. Our result strongly suggests a combination between a compact science instrument and a flexible interplanetary microspacecraft allows us to measure important scientific observables not readily accessible with conventional large‐scale spacecraft missions.
Key Points
The first image of the outer hydrogen geocorona at <16 RE shows ecliptic north‐south symmetry
We reproduced the observed spatial distribution using a model with neither a magnetic effect nor a satellite component
Substantial contribution of the magnetic effect to the escape flux cannot be ruled out
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
In early 2014, continuous monitoring with the Hisaki satellite discovered transient auroral emission at Jupiter during a period when the solar wind was relatively quiet for a few days. Simultaneous ...imaging made by the Hubble Space Telescope (HST) suggested that the transient aurora is associated with a global magnetospheric disturbance that spans from the inner to outer magnetosphere. However, the temporal and spatial evolutions of the magnetospheric disturbance were not resolved because of the lack of continuous monitoring of the transient aurora simultaneously with the imaging. Here we report the coordinated observation of the aurora and plasma torus made by Hisaki and HST during the approach phase of the Juno spacecraft in mid‐2016. On day 142, Hisaki detected a transient aurora with a maximum total H2 emission power of ~8.5 TW. The simultaneous HST imaging was indicative of a large “dawn storm,” which is associated with tail reconnection, at the onset of the transient aurora. The outer emission, which is associated with hot plasma injection in the inner magnetosphere, followed the dawn storm within less than two Jupiter rotations. The monitoring of the torus with Hisaki indicated that the hot plasma population increased in the torus during the transient aurora. These results imply that the magnetospheric disturbance is initiated via the tail reconnection and rapidly expands toward the inner magnetosphere, followed by the hot plasma injection reaching the plasma torus. This corresponds to the radially inward transport of the plasma and/or energy from the outer to the inner magnetosphere.
Key Points
By monitoring of Jupiter with Hisaki and HST we discovered that dawn storm is followed by outer emission during transient aurora
The monitoring with Hisaki indicated hot electron injection in the plasma torus during the declining phase of the transient aurora
Energy for these disturbances is likely released via tail reconnection and transported to the plasma torus within a few Jupiter rotations
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
The geocoronal responses to the geomagnetic disturbances Kuwabara, M.; Yoshioka, K.; Murakami, G. ...
Journal of geophysical research. Space physics,
January 2017, 2017-01-00, 20170101, Letnik:
122, Številka:
1
Journal Article
Recenzirano
Atomic hydrogen atoms in the terrestrial exosphere resonantly scatter solar Lyman alpha (121.6 nm) radiation, observed as the hydrogen geocorona. Measurements of scattered solar photons allow us to ...probe time‐varying distributions of exospheric hydrogen atoms. The Hisaki satellite with the extreme ultraviolet spectrometer (EXtreme ultraviolet spectrosCope for ExosphEric Dynamics: EXCEED) was launched in September 2013. EXCEED acquires spectral images (52–148 nm) of the atmospheres/magnetospheres of planets from Earth orbit. Due to its low orbital altitude (~1000 km), the images taken by the instrument also contain the geocoronal emissions. In this context, EXCEED has provided quasi‐continuous remote sensing observations of the geocorona with high temporal resolution (~1 min) since 2013. These observations provide a unique database to determine the long‐term behavior of the exospheric density structure. In this paper, we report exospheric structural responses observed by EXCEED to geomagnetic disturbances. Several geomagnetic storms with decreases of Dst index occurred in February 2014 and the Lyman alpha column brightness on the night side of the Earth increased abruptly and temporarily by approximately 10%. Hisaki reveal that the time lag between the peaks of the magnetic activity and the changes in the Lyman alpha column brightness is found to be about 2 to 6 h during storms. In order to interpret the observational results, we evaluate quantitatively the factors causing the increase. On the basis of these results, a coupling effect via charge exchange between the exosphere and plasmasphere causes variations of the exospheric density structure.
Key Points
Hisaki provides a unique database to determine the long‐term behavior of the exospheric density structure
Quasi‐continuous observation with high temporal resolution revealed the geocoronal response to the geomagnetic variation take 2 to 6 h
The increase of exospheric hydrogen density during a geomagnetic storm is caused by the suppression of the charge exchange
Jupiter's auroral emissions reveal energy transport and dissipation through the planet's giant magnetosphere. While the main auroral emission is internally driven by planetary rotation in the steady ...state, transient brightenings are generally thought to be triggered by compression by the external solar wind. Here we present evidence provided by the new Hisaki spacecraft and the Hubble Space Telescope that shows that such brightening of Jupiter's aurora can in fact be internally driven. The brightening has an excess power up to ~550 GW. Intense emission appears from the polar cap region down to latitudes around Io's footprint aurora, suggesting a rapid energy input into the polar region by the internal plasma circulation process.
Key Points
Energy is rapidly supplied to Jovian aurora during the solar wind quiet period
Auroral morphology suggests a global change in the auroral process
This suggests an internally driven disturbance during the quiet period
We present the first comparison of Jupiter's auroral morphology with an extended, continuous, and complete set of near‐Jupiter interplanetary data, revealing the response of Jupiter's auroras to the ...interplanetary conditions. We show that for ∼1–3 days following compression region onset, the planet's main emission brightened. A duskside poleward region also brightened during compressions, as well as during shallow rarefaction conditions at the start of the program. The power emitted from the noon active region did not exhibit dependence on any interplanetary parameter, though the morphology typically differed between rarefactions and compressions. The auroras equatorward of the main emission brightened over ∼10 days following an interval of increased volcanic activity on Io. These results show that the dependence of Jupiter's magnetosphere and auroras on the interplanetary conditions are more diverse than previously thought.
Plain Language Summary
Jupiter's auroras (northern lights) are the brightest in the solar system, over a hundred times brighter than the Earth's. Auroras on Earth are driven by the solar wind, a million mile‐per‐hour stream of charged particles flowing away from the Sun, hitting the Earth's magnetic field, and stirring it around, but it is not known whether the solar wind causes any significant auroras on Jupiter. The main reason for this uncertainty is a lack of observations of the planet's auroras obtained while spacecraft have been near Jupiter and able to supply a full and continuous set of measurements of the solar wind and its accompanying magnetic field. In early mid‐2016 Juno approached Jupiter, providing such an interplanetary data set, and we obtained over a month's worth of observations of Jupiter's auroras using the Hubble Space Telescope. We saw several solar wind storms, each causing auroral fireworks on Jupiter. We captured the most powerful auroras observed by Hubble to date, brightened main oval emissions, and flashing high‐latitude patches of auroras during the solar wind storms. These results indicate that Jupiter's auroral response to the solar wind is more diverse than we previously have thought.
Key Points
We present the first comparison of Jupiter's auroras with an extended and complete set of near‐Jupiter interplanetary data
During compressions, the well‐defined sector of Jupiter's emission and the dusk poleward region brightened, the latter pulsating
The power emitted from the noon active region did not exhibit dependence on any interplanetary parameter, though the morphology changed
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