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
Corotation of Bright Features in the Io Plasma Torus Suzuki, F.; Yoshioka, K.; Hikida, R. ...
Journal of geophysical research. Space physics,
November 2018, 2018-11-00, 20181101, Letnik:
123, Številka:
11
Journal Article
Recenzirano
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Electron energy distribution in the Io plasma torus (IPT) is non‐Maxwellian. The “hot” components induce extreme ultraviolet radiation, although their energy source remains unknown. One potential ...mechanism that may preserve the energy of hot electrons is inwardly directed plasma motion in the Jovian magnetosphere. Therefore, understanding the high‐energy component of the electron energy distribution is important. The extreme ultraviolet spectrometer onboard the HISAKI satellite has started the observation of the IPT. We show that bright transient features in one ansa of the IPT correlate with those in the other ansa after 5 hr. Because it takes 5 hr (one half of the rotation cycle of Jupiter) for a batch of plasma to move from one ansa to the other, the correlation indicates that the transient features are identical and that they survive for greater than 5 hr. Since the time scale of the radiative cooling process is ~3 hr, this fact suggests that injected hot electrons survive against cooling via Coulomb collision with ambient electrons for greater than 2 hr. Assuming the relationship with the cooling time, we can deduce the hot electron temperature from the brightening duration. Here we report the occasional hot electron injections, presumed to exceed 150 up to 650 eV, into the IPT (approximately 15% out of all events). For the most of events, the temperature of injected electron is lower than 150 eV.
Key Points
Continuous and long‐term observation by HISAKI led to the discovery of the corotation of bright spots in the Io plasma torus
Most of the spots (~85%) survived for shorter than 5 hr, suggesting that “warm” (<150 eV) electrons are supplied into the Io plasma torus
Approximately 15% were long‐lived (>10 hr), indicating that the hot electron temperature reaches 650 eV
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.
Key Points
Jupiter's auroral power decreased by 70% over 2 weeks of observations by the Hubble Space Telescope
Could be caused by expansion of the magnetosphere or increase in hot plasma transport
Aurora is variable without enhanced Io volcanism or solar wind pressure implications for Juno
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
In order to reveal variations of days to weeks in the brightness distribution of Jovian Synchrotron Radiation (JSR), we made simultaneous radio and ultraviolet observations using the Giant Metrewave ...Radio Telescope (GMRT) and the Hisaki EXtreme ultraviolet spectrosCope for ExosphEric Dynamics (EXCEED). It is known from visible and ultraviolet observations that Io plasma torus (IPT) has dawn-dusk asymmetry, and that this asymmetry is believed to be due to the dawn-dusk electric field. Continuous ultraviolet observation by Hisaki reveals that dawn-dusk asymmetry of IPT changes in days to weeks, therefore, if this global electric field around Io's orbit (5.9 Jovian radii) could penetrate the radiation belt region (<2 Jovian radii), the variations in brightness distribution of JSR and IPT are expected to be correlated. The GMRT observations were made from 2013 December 31 to 2014 January 16 at 610 MHz and 2016 March 14-June 23 at 1390 MHz, while Hisaki continuously monitored IPT. The statistical analysis indicates that JSR and IPT do not have a significant correlation. Although these results do not support our hypothesis, we cannot rule out the possibility that the dawn-dusk electric field was masked by some other process, including the conductivity variation and/or the time-variable longitudinal asymmetry of JSR.
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 that the transient aurora frequently recurred at a 2–6 day period in response to a mass loading increase from 0.3 to 0.5 t/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 an amount comparable to the total mass of magnetosphere, ~1.5 Mt, is accumulated in the magnetosphere. A plasmoid mass of more than 0.1 Mt is necessary in case that the plasmoid ejection is the only process for mass release.
Key Points
Response of Jupiter's aurora to mass loading from Io was investigated with a newly developed model and data from the Hisaki satellite
The estimated mass loading rate indicated increase and decay during volcanic eruptions at Io
During volcanic eruptions at Io, impulsive variation of aurora responded to the mass loading rate rather than the solar wind
The energetic particles in the Earth's radiation belt are known to fluctuate over various timescales. Although observations using satellites have been made for more than 50 years, there are few ...examples of continuous and long‐term observations at low altitude (<2,000 km) and in low L‐value (L < 2) regions, which are at the bottom of the inner radiation belt. This is because the orbits of satellites that are designed to cover large areas of the magnetosphere are not suitable for long‐term continuous observations at low altitudes. In this study, we focused on data from a space telescope that usually follows a low‐altitude circular orbit. The Hisaki space telescope, launched in 2013, continuously observes the planets from an altitude of ∼1,000 km (L‐value 1–2). By using the noise component counted on the photodetector of Hisaki as a radiation monitor, the flux variation of the high‐energy protons (energy > 30 MeV) in this orbit can be observed. The results show a clear dependence on solar activity. At around L = 2, it is found that the variation in the radiation belt proton flux is controlled by both the flux of the galactic cosmic rays and the neutral density of the thermosphere. The former one is the source process of high‐energy charged particles in the inner radiation belt, and the latter is the loss process due to the Coulomb collision. It is also found that the influence of galactic cosmic ray fluctuations becomes smaller as the L‐value moves closer to 1.
Key Points
The space telescope Hisaki made long‐term monitoring of high‐energy protons at ∼1,000 km above the Earth’s surface (L = 1–2)
The high energy proton flux generated through the cosmic ray albedo neutron decay process shows evident L‐value dependence and yearly variation
There are significant amounts of high‐energy proton flux at ∼1,000 km altitude which corresponds to the solar activity
Mercury is valuable to us because we can see the interactions between the planet and its space environment. This research aims to clarify how Mercury's neutral Na exosphere was produced. Data from ...MErcury Surface, Space ENvironment, GEochemistry, and Ranging/Mercury Atmospheric and Surface Composition Spectrometer (MESSENGER/MASCS) and model calculations that examine possible generation, transportation, and dissipation processes were compared. First, the seasonal variability in the amount of Na exosphere was analyzed for each local time (LT) using MASCS data. Previous research has shown that the amount of Na above LT12 reaches its maximum at the aphelion and that this maximum is recorded only at LT12. Following this result, we constructed a 3‐D Na exosphere model to understand the key seasonal variability processes occurring around LT12. The numerical calculation produced results that were consistent with the MASCS observations regarding the vertical profile and the seasonal variability at LT06 and LT18. However, the peak that occurs around the aphelion at LT12 could not be reproduced. However, the model produced results suggesting that less than 108 kg of comet stream dust particles per Mercury year could be the local and short‐term source of Na.
Plain Language Summary
Mercury is valuable because we can see the interaction between the planet and its space environment. In this study, the seasonal variability of Mercury's Na exosphere was investigated using optical observations by MESSENGER spacecraft and a 3‐D Monte Carlo model. An analysis of the observations showed that the amount of Na exosphere increased at noon around the aphelion. We first attempted to reproduce this feature with a theoretical model considering four well‐known outgassing processes of the atmosphere. However, when we add comet dust streams to our model as a new outgassing process, we could resolve the inconsistencies of the first model. By considering Na supply to the surface by the impact of dust streams and tuning the parameters more precisely, we could reproduce observations at all local times.
Key Points
The seasonal variability of Mercury's Na exosphere was investigated using observations by MESSENGER/MASCS and a 3‐D Monte Carlo model
Model with thermal/photon‐stimulated desorption, solar wind sputtering, and micrometeoroid impact vaporization cannot reproduce observations
The putative presence of comet dust streams could resolve this inconsistency
HISAKI (SPRINT-A) satellite is an earth-orbiting Extreme UltraViolet (EUV) spectroscopic mission and launched on 14 Sep. 2013 by the launch vehicle Epsilon-1. Extreme ultraviolet spectroscope ...(EXCEED) onboard the satellite will investigate plasma dynamics in Jupiter’s inner magnetosphere and atmospheric escape from Venus and Mars. EUV spectroscopy is useful to measure electron density and temperature and ion composition in plasma environment. EXCEED also has an advantage to measure spatial distribution of plasmas around the planets. To measure radial plasma distribution in the Jovian inner magnetosphere and plasma emissions from ionosphere, exosphere and tail separately (for Venus and Mars), the pointing accuracy of the spectroscope should be smaller than spatial structures of interest (20 arc-seconds). For satellites in the low earth orbit (LEO), the pointing displacement is generally caused by change of alignment between the satellite bus module and the telescope due to the changing thermal inputs from the Sun and Earth. The HISAKI satellite is designed to compensate the displacement by tracking the target with using a Field-Of-View (FOV) guiding camera. Initial checkout of the attitude control for the EXCEED observation shows that pointing accuracy kept within 2 arc-seconds in a case of “track mode” which is used for Jupiter observation. For observations of Mercury, Venus, Mars, and Saturn, the entire disk will be guided inside slit to observe plasma around the planets. Since the FOV camera does not capture the disk in this case, the satellite uses a star tracker (STT) to hold the attitude (“hold mode”). Pointing accuracy during this mode has been 20–25 arc-seconds. It has been confirmed that the attitude control works well as designed.
The purpose of this study was to examine the relation between cancer cellularity and the apparent diffusion coefficient (ADC) value using diffusion-weighted magnetic resonance imaging in breast ...cancer.
The subjects were 27 women who had undergone operation for breast cancer. There were 27 breast cancer lesions, 24 of which were invasive ductal carcinoma (IDC) and 3 of which were noninvasive ductal carcinoma (NIDC).
The mean ADC values of IDC, NIDC, and normal breasts were 1.07 +/- 0.19 .10(-3), 1.42 +/- 0.17 .10(-3), and 1.96 +/- 0.21 .10(-3) mm(2)/s, respectively. The mean ADC values of IDC and NIDC were significantly different from that of normal breasts (P < 0.001 each). The mean ADC values were also significantly different between IDC and NIDC (P < 0.001). There was no correlation between the ADC value and cancer cellularity.
The mean ADC values for breast cancer were significantly different from that of normal breasts. The mean ADC value for breast cancer did not significantly correlate with cancer cellularity but did correlate with histological types.
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