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  • 1
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    Jupiter's X-ray and EUV auroras monitored by Chandra, XMM-Newton, and Hisaki satellite (2016)
    Wiley
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    Publication Date: 2016-02-06
    Description: Jupiter's X-ray auroral emission in the polar cap region results from particles which have undergone strong field-aligned acceleration into the ionosphere [ Cravens et al. , 2003]. The origin of precipitating ions and electrons and the time variability in the X-ray emission are essential to uncover the driving mechanism for the high energy acceleration. The magnetospheric location of the source field line where the X-ray is generated is likely affected by the solar wind variability. However, these essential characteristics are still unknown because the long-term monitoring of the X-rays and contemporaneous solar wind variability has not been carried out. In Apr 2014, the first long-term multi-wavelength monitoring of Jupiter's X-ray and EUV auroral emissions was made by the Chandra X-ray Observatory, XMM-Newton, and Hisaki satellite. We find that the X-ray count rates are positively correlated with the solar wind velocity and insignificantly with the dynamic pressure. Based on the magnetic field mapping model, a half of the X-ray auroral region was found to be open to the interplanetary space. The other half of the X-ray auroral source region is magnetically connected with the pre-noon to post-dusk sector in the outermost region of the magnetosphere, where the Kelvin-Helmholtz (KH) instability, magnetopause reconnection, and quasi-periodic particle injection potentially take place. We speculate that the high energy auroral acceleration is associated with the KH instability and/or magnetopause reconnection. This association is expected to also occur in many other space plasma environments such as Saturn and other magnetized rotators.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 2
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    Periodic variations of oxygen EUV dayglow in the upper atmosphere of Venus: Hisaki/EXCEED observations (2015)
    Wiley
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    Publication Date: 2015-10-29
    Description: Using the Extreme Ultraviolet Spectroscope for Exsospheric Dynamics (EXCEED) aboard Hisaki and the Solar Extreme Ultraviolet Monitor (SEM) on the Solar and Heliospheric Observatory (SOHO), we investigate variations of the extreme ultraviolet (EUV) dayglow brightness for OII 83.4 nm, OI 130.4 nm and OI 135.6 nm in the Venusian upper atmosphere observed in Mar-April (period 1), April-May (period 2) and June-July (period 3) in 2014. The result shows that characteristic periodicities exist in the dayglow variations other than the ~ 27-day solar rotational effect of the solar EUV flux: 1.8, 2.8, 3.1, 4.5, and 9.9-day in period 1, 1.1-day in period 2, and 1.0 and 11-day in period 3. Many of these periodicities are consistent with previous observations and theory. We suggest these periodicities are related to density oscillations of oxygen atoms or photoelectrons in the thermosphere. The cause of these periodicities is still uncertain, but planetary-scale waves and/or gravity waves propagating from the middle atmosphere, and/or minor periodic variations of the solar EUV radiation flux may play a role. Effects of the solar wind parameters (velocity, dynamic pressure and interplanetary magnetic field's (IMF) intensity) on the dayglow variations are also investigated using the Analyser of Space Plasma and Energetic Atoms (ASPERA-4) and magnetometer (MAG) aboard Venus Express. Although clear correlation with the dayglow variations is not found, their minor periodicities are similar to the dayglow periodicities. Contribution of the solar wind to the dayglow remains still unknown, but the solar wind parameters might affect the dayglow variations.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 3
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    Weakening of Jupiter's main auroral emission during January 2014 (2016)
    Wiley
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    Publication Date: 2016-01-29
    Description: In January 2014 Jupiter's FUV main auroral oval decreased its emitted power by 70% and shifted equatorward by ∼1 ∘ . Intense, low latitude features were also detected. The decrease in emitted power is attributed to a decrease in auroral current density rather than electron energy. This could be caused by a decrease in the source electron density, an order of magnitude increase in the source electron thermal energy, or a combination of these. Both can be explained either by expansion of the magnetosphere, or by an increase in the inward transport of hot plasma through the middle magnetosphere and its interchange with cold flux tubes moving outward. In the latter case the hot plasma could have increased the electron temperature in the source region and produced the intense, low latitude features, while the increased cold plasma transport rate produced the shift of the main oval.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 4
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    Key Role of the Kuroshio Current in the Formation of Frontal Structure of an Extratropical Cyclone Associated with Heavy Precipitation (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract In this study, the authors examined how the Kuroshio Current affected the frontal structure of an extratropical cyclone associated with heavy precipitation. We focused on an extratropical cyclone and related heavy precipitation on Miyake Island, which is situated near the Kuroshio Current, in January 2017. We first investigated the frontal structure and the attendant precipitation using observational data and objective analysis data. As the cyclone grew around the Kuroshio Current, a nonclassic front (outer front) intensified to the north of a warm front. A line‐shaped precipitation band developed along the outer front, contributing to the heavy precipitation on Miyake Island. The outer front lay along a sea surface temperature (SST) front associated with the Kuroshio Current in its early life stage. To clarify the impact of the SST front on the formation of the incipient outer front, we next conducted cloud‐resolving numerical experiments using SST distributions with and without the oceanic frontal structure. In the real SST experiment, sensible heat supply from the Kuroshio Current was large, while the amount from the ocean on the northern side of the warm current was small. This meridional difference in sensible heating caused frontogenesis along the SST front, which created the incipient front. Consequently, the real SST experiment reproduced the formation of the front. In contrast, the SST sensitivity experiments did not simulate such a sensible heating pattern or front formation. Thus, the Kuroshio Current played a key role in forming the incipient outer front and contributed to the heavy precipitation.
    Print ISSN: 2169-897X
    Electronic ISSN: 2169-8996
    Topics: Geosciences , Physics
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  • 5
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    Azimuthal Variation in the Io Plasma Torus Observed by the Hisaki Satellite From 2013 to 2016 (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract 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.
    Print ISSN: 2169-9380
    Electronic ISSN: 2169-9402
    Topics: Geosciences , Physics
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  • 6
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    Hot electron component in the Io plasma torus confirmed through EUV spectral analysis (2011)
    Wiley
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    Publication Date: 2011-09-03
    Description: The Io plasma torus is composed mainly of sulfur and oxygen ions and their compounds, together with a background of electrons. In addition to those basic components, several in situ observations have shown that a small percentage of the electrons there have been excited to be as much as 100 times hotter than the background electrons. They have a significant impact on the energy balance in the Jovian inner magnetosphere. However, their generation process has not yet been clarified. One difficulty is that the available data about the hot electrons all come from in situ observations which cannot explore the temporal and spatial distributions explicitly. Therefore, remote sensing which can take a direct picture of the plasma dynamics is necessary in order to clarify the hot electron problem. In this study, a plasma diagnosis method was used for the Io plasma torus EUV spectra taken from the Cassini spacecraft. Agreement with previous observations confirmed the background electron temperature and ion compositions as determined by our model. In addition, the available data are matched even better when the model is run with a hot electron component. This consistent confirmation by remote sensing is a first. Because of the limited temporal resolution and observational coverage, the results could not be used to explain the generation process of the hot electrons. However, we expect that this method will be useful in studying the hot electron generation process when data from future missions with better temporal resolution and more complete coverage become available.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 7
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    On the relation between Jovian aurorae and the loading/unloading of the magnetic flux: simultaneous measurements from Juno, HST and Hisaki (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract We present simultaneous observations of aurorae at Jupiter from the Hubble Space Telescope and Hisaki, in combination with the in‐situ measurements of magnetic field, particles and radio waves from the Juno Spacecraft in the outer magnetosphere, from ~ 60 RJ to 80 RJ during March 17 to 22, 2017. Two cycles of accumulation and release of magnetic flux, named magnetic loading/unloading, were identified during this period, which correlate well with electron energization and auroral intensifications. Magnetic reconnection events are identified during both the loading and unloading periods, indicating that reconnection and unloading are independent processes. These results show that the dynamics in the middle magnetosphere are coupled with auroral variability.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 8
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    Radial variation of sulfur and oxygen ions in the Io plasma torus as deduced from remote observations by Hisaki (2017)
    Wiley
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    Publication Date: 2017-02-21
    Description: 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 mid-magnetosphere (〉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 EXCEED, 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 mid-magnetospheric plasma is obtained to be 2-40 hours from the derived radial profile for the abundance of supra-thermal 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.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 9
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    Response of Jupiter's Aurora to Plasma Mass Loading Rate Monitored by the Hisaki Satellite During Volcanic Eruptions at Io (2018)
    Wiley
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    Publication Date: 2018-02-28
    Description: The production and transport of plasma mass are essential processes in the dynamics of planetary magnetospheres. At Jupiter, it is hypothesized that Io's volcanic plasma carried out of the plasma torus is transported radially outward in the rotating magnetosphere and is recurrently ejected as plasmoid via tail reconnection. The plasmoid ejection is likely associated with particle energization, radial plasma flow, and transient auroral emissions. However, it has not been demonstrated that plasmoid ejection is sensitive to mass loading because of the lack of simultaneous observations of both processes. We report the response of plasmoid ejection to mass loading during large volcanic eruptions at Io in 2015. Response of the transient aurora to the mass loading rate was investigated based on a combination of Hisaki satellite monitoring and a newly-developed analytic model. We found the transient aurora frequently recurred at a 2–6-day period in response to a mass loading increase from 0.3 to 0.5 ton/s. In general the recurrence of the transient aurora was not significantly correlated with the solar wind although there was an exceptional event with a maximum emission power of ~10 TW after the solar wind shock arrival. The recurrence of plasmoid ejection requires the precondition that amount comparable to the total mass of magnetosphere, ~1.5 Mton, is accumulated in the magnetosphere. A plasmoid mass of more than 0.1 Mton is necessary in case that the plasmoid ejection is the only process for mass release.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 10
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    Impacts of SST patterns on rapid intensification of Typhoon Megi (2010) (2017)
    Wiley
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    Publication Date: 2017-12-15
    Description: Typhoon Megi (2010), a very intense tropical cyclone with a minimum central pressure of 885 hPa, was characterized by especially rapid intensification. We investigated this intensification process by a simulation experiment using a high-resolution (0.02° × 0.02°) three-dimensional atmosphere–ocean coupled regional model. We also performed a sensitivity experiment with a time-fixed sea surface temperature (SST). The coupled model successfully simulated the minimum central pressure of Typhoon Megi, whereas the fixed SST experiment simulated an excessively low minimum central pressure of 839 hPa. The simulation results also showed a close relationship between the radial SST profiles and the rapid intensification process. Because the warm sea increased near-surface water vapor and hence the convective available potential energy, the high SST in the eye region facilitated tall and intense updrafts inside the radius of maximum wind speed and led to the start of rapid intensification. In contrast, high SST outside this radius induced local secondary updrafts that inhibited rapid intensification even if the mean SST in the core region exceeded 29.0°C. These secondary updrafts moved inward and eventually merged with the primary eyewall updrafts. Then the storm intensified rapidly when the high SST appeared in the eye region. Thus, the changes in the local SST pattern around the storm center strongly affected the rapid intensification process by modulating the radial structure of core convection. Our results also show that the use of a high-resolution three-dimensional atmosphere-ocean coupled model offers promise for improving intensity forecasts of tropical cyclones.
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