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
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
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
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
Odprti dostop
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
Most resistance training studies of older subjects have emphasized low-intensity, short-term training programs that have concentrated on strength measurements. The purpose of this study was, in ...addition to the determination of strength, to assess intramuscular and transport factors that may be associated with strength increments. Eighteen untrained men ages 60-75 years volunteered for the study; 9 were randomly placed in the resistance-training group (RT), and the other half served as untrained (UT) or control subjects. RT subjects performed a 16-week high-intensity (85-90% 1 repetition maximum (RT) resistance training program (2 x/wk) consisting of 3 sets each to failure (6-8 repetitions based on 1 RM of 3 exercises): leg press (LP), half squat (HS), and leg extension (LE) with 1-2 minutes rest between sets. Pre- and post- training strength was measured for the 3 training exercises using a 1 RM protocol. Body fat was calculated using a 3-site skinfold method. Biopsies from the vastus lateralis m. were obtained for fiber type composition, cross-sectional area, and capillarization measurements. Exercise metabolism, electrocardiography, and arterial blood pressure were observed continuously during a progressive treadmill test, and resting echocardiographic data were recorded for all subjects. Pre- and post-training venous blood samples were analyzed for serum lipids. Resistance training caused significant changes in the following comparisons: % fat decreased in the RT group by almost 3%, strength improved for all exercises: LE = + 50.4%, LP = + 72.3%, HS = + 83.5%; type IIB fibers decreased and IIA fibers increased; cross-sectional areas of all fiber types (I, IIA, IIB) increased significantly, and capillary to fiber ratio increased but not significantly. No differences were noted for ECG and echocardiographic data. The RT group significantly improved treadmill performance and VO2max. Pre- and post-training serum lipids improved but not significantly. No significant changes occurred in any pre- to post-tests for the UT group. The results show that skeletal muscle in older, untrained men will respond with significant strength gains accompanied by considerable increases in fiber size and capillary density. Maximal working capacity, VO2max, and serum lipid profiles also benefited from high-intensity resistance training, but no changes were observed for HR max, or maximal responses of arterial blood pressure. Older men may not only tolerate very high intensity work loads but will exhibit intramuscular, cardiovascular, and metabolic changes similar to younger subjects.
In multinucleated skeletal muscle, a myonuclear domain is the region of cytoplasm governed by one nucleus, and myofibres are mosaics of overlapping myonuclear domains. Association of ageing and ...myonuclear domain is important in the understanding of sarcopenia and with prevention or combating age-related muscle declines. This study examined the effects of age, fibre type and muscle on nucleo-cytoplasmic (N/C) relationships as reflecting myonuclear domain size. The N/C was compared in fibre types of soleus and plantaris muscles from young (n = 6) and ageing (n = 8) male Fisher 344 rats. There were no significant differences in fibre type composition or cross-sectional area of the soleus across ages. The old soleus had significantly more myonuclei, resulting in a significantly smaller myonuclear domain size. The plantaris muscle showed a higher percentage of slow fibres in old compared with young fibres. There were no differences in the number of myonuclei or in myonuclear domain size between young and older animals. We found muscle-specific differences in the effects of ageing on myonuclear domain, possibly as a result of reduced efficiency of the myonuclei in the slow muscles.
An 8-wk progressive resistance training program for the lower extremity was performed twice a week to investigate the time course for skeletal muscle adaptations in men and women. Maximal dynamic ...strength was tested biweekly. Muscle biopsies were extracted at the beginning and every 2 wk of the study from resistance-trained and from nontrained (control) subjects. The muscle samples were analyzed for fiber type composition, cross-sectional area, and myosin heavy chain content. In addition, fasting blood samples were measured for resting serum levels of testosterone, cortisol, and growth hormone. With the exception of the leg press for women (after 2 wk of training) and leg extension for men (after 6 wk of training), absolute and relative maximal dynamic strength was significantly increased after 4 wk of training for all three exercises (squat, leg press, and leg extension) in both sexes. Resistance training also caused a significant decrease in the percentage of type IIb fibers after 2 wk in women and 4 wk in men, an increase in the resting levels of serum testosterone after 4 wk in men, and a decrease in cortisol after 6 wk in men. No significant changes occurred over time for any of the other measured parameters for either sex. These data suggest that skeletal muscle adaptations that may contribute to strength gains of the lower extremity are similar for men and women during the early phase of resistance training and, with the exception of changes in the fast fiber type composition, that they occur gradually.
During growth and repair of skeletal muscle fibers, satellite cells become activated, undergo mitosis, and a daughter nucleus becomes incorporated into the muscle fiber to increase myonuclear ...numbers. An increase in myonuclei appears to be required for this postnatal growth. This study examined whether muscle fibers of elderly men can hypertrophy with strength training and, if so, whether they have the capacity to incorporate nuclei into the fibers. The sarcoplasmic area associated with each myonucleus was calculated in nine elderly men before and after 16 weeks of strength training, and compared to nine elderly control men. Muscle fiber type changes and myosin heavy chain composition were also compared. All major fiber types (I, IIA, IIB) became significantly larger after training, and a transition of type IIB fibers to IIA occurred with training. The area occupied by each fiber type correlated with myosin heavy chain percentage, and both of these changed similarly with strength training. The cytoplasm-to-myonucleus ratio increased, but not significantly (p = .07), with muscle fiber hypertrophy. Number of myonuclei per fiber and myonuclei per unit length of muscle fiber increased, but not significantly. Cross-sectional areas of the muscle fibers in untrained elderly men were much smaller than in untrained young men (when compared with our earlier studies). Training increased the sizes of the elderly muscle fibers to that of the untrained young men. This hypertrophy of muscle fibers by 30% with training resulted in no change in the cytoplasm-to-myonucleus ratio. This suggests that the myonuclear population continues to adapt to growth stimuli in the elderly muscles.
The sulfur and oxygen ions in the Io plasma torus (IPT), which is located at a distance of ~6 RJ from Jupiter, emit light in various wavelength regions. In particular, radiative cooling in the ...ultraviolet wavelength range plays an important role in the energy balance of the Io torus, and it is important to explore the detailed spectral structure in the ultraviolet region. The ultraviolet spectrum of the IPT in the wavelength range 52.0–148.0 nm with a resolution of 0.3–0.4 nm was obtained by Extreme Ultraviolet Spectroscope for Exospheric Dynamics on the Hisaki satellite. Owing to its instrument performance and long integration time, 10 emission lines from the IPT were detected for the first time. By summarizing previous observations and using the results obtained by Extreme Ultraviolet Spectroscope for Exospheric Dynamics, we updated the term diagrams for S II, S III, and S IV emissions from the IPT. Four excited levels in the IPT were detected for the first time. Detection of the emission lines may improve the estimation accuracy of the fraction of hot electron density using the atomic data.
Key Points
Ten emission lines from the Io plasma torus were detected for the first time through the observation by Hisaki/EXCEED
Four excited levels in the Io plasma torus were detected for the first time through the observation by Hisaki/EXCEED
Detection of the new emission lines may improve estimation accuracy of the fraction of hot electrons in the torus
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