Akira Morioka

Main-phase creation of "seed" electrons in the outer radiation belt

During a geomagnetic storm in early November 1993, NOAA satellite observations revealed that a population of energetic electrons appeared in the center of the outer radiation belt during the main phase of the storm. At the beginning of the main phase of the magnetic storm, the number of electrons with energies from 30 keV to 100 keV increased rapidly and contributed to build up of the ring current. At the end of the main phase the flux of electrons with energies greater than 300 keV increased significantly. Akebono satellite observations showed that the flux of electrons with energies ranging from 300 keV to 950 keV increased late of the storm main phase and that the flux of electrons with energies from 950 keV to 2.5 MeV increased during the storm recovery phase. The electron flux increase observed by both NOAA and Akebono took place first in the central part of the outer radiation belt (L~4) and propagated to higher L shells with a significant time delay. We think that the ring current electrons that appeared first and near L~4 during the main phase seeded the subsequent increase in the flux of MeV electrons in the entire outer radiation belt.

Observation of short‐term variation of Jupiter's synchrotron radiation

Detailed observations of Jovian decimetric radiation (DIM) from relativistic electrons in the inner Jovian radiation belt was carried out to seek the existence of a short-term variation which has information on the electromagnetic disturbances in the inner Jovian magnetosphere. Results demonstrated the DIM flux enhancement continuing for a few days for the case of November 1996. It was shown that the relativistic electrons were highly anisotropic during the DIM flux enhancement. The correlation between the DIM flux enhancement and the solar F10.7 flux enhancement was also detected. A model simulation suggested that the observed DIM flux variation was caused by the enhanced radial diffusion due to the intense solar UV/EUV event which heated the Jupiters upper atmosphere.

Long term modulation of low altitude proton radiation belt by the Earth's atmosphere

To evaluate the atmospheric effect on low altitude proton variation, we calculated the long-term variation of the lifetimes of low altitude protons which mirror at the geomagnetic equator using empirical models of the atmosphere, the ionosphere, and the geomagnetic field. The calculated results showed that the proton lifetime variation is anti-correlated with the F10.7 solar cycle. The comparison between the estimated and the observed proton fluxes during 20 years showed good agreement at the altitude of the lower drift shell. Our results showed positively that solar cycle variations of the upper atmosphere and the ionosphere had a significant influence on the changes of the low altitude energetic proton flux.

Pitch angle distribution of relativistic electrons in the inner radiation belt and its relation to equatorial plasma wave turbulence phenomena

The pitch angle distribution of relativistic electrons in the inner radiation belt and its relation to local plasma waves were investigated using the data from the Akebono satellite. It was found that energetic electrons (>1 MeV) in the inner radiation belt showed an unexpected dumbbell distribution near the magnetic equator. This feature was commonly observed both in magnetically disturbed and undisturbed conditions. It was also found that this anomalous pitch angle distribution of energetic electrons was always accompanied with the enhancement of local UHR waves around the magnetic equator. These newly found features indicate the existence of an unknown wave-particle interaction process between the relativistic electrons and quasi-electrostatic plasma waves in the equatorial region of the plasmasphere.

Jovian electron modulations by the solar wind interaction with the magnetosphere

Variations of energetic electrons released from the Jovian magnetosphere are investigated in detail by using data from three spacecraft, Pioneer 10, 11, and Ulysses. Analyses with the three spacecraft data confirmed that the Jovian electrons are modulated by the dynamic pressure of the solar wind at the position of Jupiter. It is also found that the release rate of energetic electrons is controlled by the polarity of IMF having dawn-dusk or latitudinal dependence. The enhancements of released electron flux were detected during the toward polarity of IMF when the spacecraft were located in the upstream or dawn side of Jupiter. On the other hand, Ulysses detected enhancement in away polarity of IMF when the spacecraft was on the duskside of southern hemisphere. We made a simulation to reproduce the electron variation of Jovian electrons observed by Pioneer 11 considering the convective and diffusive transport in the interplanetary medium. According to the simulation, the observed enhancements of electron flux originating from Jupiter are well represented by the coupling of both the source modulation at the Jovian magnetosphere and the transport effect in the interplanetary medium.

Modulation of Jovian electrons by the solar wind

Abstract A cross-correlation analysis between Jovian electron flux and the solar wind shows that the flux of Jovian electrons in interplanetary space has an anti-correlation with the solar wind variation with some time delays. It is also confirmed that the release rate of electrons is controlled by the polarity of IMF By component. These results suggest that the release of Jovian electrons are modulated by the solar wind at the Jovian magnetosphere.

Observations of Jovian decimetric radiation at a frequency of 327MHz

Abstract The time variable phenomena of Jovian decimetric radiation (DIM) are expected to have relation to little known dynamic process of Jupiters inner magnetosphere. A program of regular DIM observations has been started since 1995 to reveal characteristics of DIM flux variations especially on a time scale of days to months, which must be deeply related to Jupiters global electromagnetic activities and/or solar activities. A daily monitor of DIM has been made at a frequency of 327 MHz with the 2000m 2 radio telescopes. The radio flux variations on a time scale of approximately 10 days were detected for the period from the end of August to September, 1995, around the middle of May and from the end of June to July, 1997. The flux variations were nearly 40% and 25% for the 1995 and 1997 observations, respectively.

Distribution of sodium cloud near Io and in the inner jovian magnetosphere

Abstract We made observations of sodium atoms to investigate the spatial distribution of Iogenic gas. From the observational results, the shapes of emission distribution are classified into three types. They are (i) brighter (∼kilorayleighs) cloud around Io as far as ∼ 50RIo(Io radii), (ii) cloud which is directed away from Jupiter and concentrated along the equatorial plane of Jupiter outside Ios orbit, and (iii) diffuse and faint cloud spreading outside Ios orbit. We also detected that the D-line emission distribution shows intensity asymmetry between the north and south sides of the equatorial plane. We conclude that this asymmetry results from the temperature anisotropy of the torus plasma which makes charge exchange with neutral atoms around Io. The value of the anisotropy is determined to be A = T ∥ T ∥ = 8 and T∥ = 60[eV]. From a comparison of the observational results with the results obtained from model calculation, we estimate that the sodium atom source for charge exchange is 6.0 × 1025 atoms/sec.

On the sources of Jovian hectometric radiation

Abstract Jovian hectometric radiation(HOM) emitted from the polar region of the Jovian magnetsphere reflects the information on the electro-magnetic environment of the source region. Using the Ulysses URAP and SWOOPS data, the occurrence characteristics of HOM and its relation to the solar wind were investigated. As a result, we found that HOM can be classified into two components; (1) Solar wind HOM(sw-HOM), whose amplitudes are related to the solar wind dynamic pressure, and (2) non Solar wind HOM(nsw-HOM), which is independent to the solar wind parameters and is observed with large amplitudes. These two components have different system III occurrence characteristics. Considering the relationship with Jovian aurora, L values of each HOM source are inferred to be L>30, L ⋍12 , respectively.

Variability of [SII] emission from the Io plasma torus

Io, the innnermost Galilean satellite of Jupiter, is a main source of heavy ions in the Jovian magnetosphere. Therefore variability of the plasma in the vicinity of Io produces large effects to the magnetospheric environment. We have started optical observations of the Io plasma torus in order to study the plasma environment in the Jovian magnetosphere. Observed emissions are forbidden line emissions of S+ ions (λλ 671.6 and 673.1 nm) in the plasma torus. In 1997, the observations were carried out at Alice Springs, Northern Territory of Australia. We mainly report the results of the observations at [SII] 673.1 nm.