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Search Results: 1 - 10 of 214381 matches for " K.-I. Oyama "
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Effects of neutral wind on the electron temperature at a height of 600 km in the low latitude region
S. Watanabe,K.-I. Oyama
Annales Geophysicae (ANGEO) , 2003,
Abstract: Electron temperature observed by the Hinotori satellite with the low inclination at the height of ~600 km was studied in terms of local time, season, latitude, magnetic declination and solar flux intensity during a 16-month period from 1981 to 1982. The electron temperatures show steep rise in the early morning (well known as morning overshoot), decrease after that and again increase at ~18 hours (hereafter named as evening overshoot). Generally the morning overshoot becomes more enhanced in the winter hemisphere and for higher solar fluxes. The evening overshoot becomes more pronounced in the mid-latitude in all seasons and more enhanced in the winter hemisphere in the same way as the morning overshoot. A difference is seen between 210°–285° and 285°–360° longitudes where magnetic declination is different. The longitudinal dependence of electron temperature indicates that the neutral wind also contributes to the thermal structure in the low latitude ionosphere.
The role of vibrationally excited nitrogen and oxygen in the ionosphere over Millstone Hill during 16-23 March, 1990
A. V. Pavlov,K.-I. Oyama
Annales Geophysicae (ANGEO) , 2003,
Abstract: We present a comparison of the observed behavior of the F region ionosphere over Millstone Hill during the geomagnetically quiet and storm period on 16-23 March, 1990, with numerical model calculations from the time-dependent mathematical model of the Earth's ionosphere and plasmasphere. The effects of vibrationally excited N2(v) and O2(v) on the electron density and temperature are studied using the N2(v) and O2(v) Boltzmann and non-Boltzmann distribution assumptions. The deviations from the Boltzmann distribution for the first five vibrational levels of N2(v) and O2(v) were calculated. The present study suggests that these deviations are not significant at vibrational levels v = 1 and 2, and the calculated distributions of N2(v) and O2(v) are highly non-Boltzmann at vibrational levels v > 2. The N2(v) and O2(v) non-Boltzmann distribution assumption leads to the decrease of the calculated daytime NmF2 up to a factor of 1.44 (maximum value) in comparison with the N2(v) and O2(v) Boltzmann distribution assumption. The resulting effects of N2(v > 0) and O2(v > 0) on the NmF2 is the decrease of the calculated daytime NmF2 up to a factor of 2.8 (maximum value) for Boltzmann populations of N2(v) and O2(v) and up to a factor of 3.5 (maximum value) for non-Boltzmann populations of N2(v) and O2(v) . This decrease in electron density results in the increase of the calculated daytime electron temperature up to about 1040-1410 K (maximum value) at the F2 peak altitude giving closer agreement between the measured and modeled electron temperatures. Both the daytime and nighttime densities are not reproduced by the model without N2(v > 0) and O2(v > 0) , and inclusion of vibrationally excited N2 and O2 brings the model and data into better agreement. The effects of vibrationally excited O2 and N2 on the electron density and temperature are most pronounced during daytime. Key words: Ionosphere (ion chemistry and composition; ionosphere-atmosphere interactions; ionospheric disturbances)
Regional variations of mesospheric gravity-wave momentum flux over Antarctica
J. Kurihara, K.-I. Oyama, N. Iwagami,T. Takahashi
Annales Geophysicae (ANGEO) , 2006,
Abstract: Numerical simulations using the Direct Simulation Monte Carlo (DSMC) method are known to be useful for analyses of aerodynamic effects on in-situ rocket measurements in the lower thermosphere, but the DSMC analysis of a spin modulation caused by an asymmetric flow around the rocket spin axis has been restricted to the two-dimensional and axially symmetric simulations in actual sounding rocket experiments. This study provides a quantitative analysis of the spin modulation using a three-dimensional (3-D) simulation of the asymmetric flow with the DSMC method. Clear spin modulations in the lower thermospheric N2 density measurement by a rocket-borne instrument are simulated using the rocket attitude and velocity, the simplified payload structure, and the approximated atmospheric conditions. Comparison between the observed and simulated spin modulations show a very good agreement within 5% at around 100km. The results of the simulation are used to correct the spin modulations and derive the absolute densities in the background atmosphere.
Annual and seasonal variations in the low-latitude topside ionosphere
Y. Z. Su,G. J. Bailey,K.-I. Oyama
Annales Geophysicae (ANGEO) , 2003,
Abstract: Annual and seasonal variations in the low-latitude topside ionosphere are investigated using observations made by the Hinotori satellite and the Sheffield University Plasmasphere Ionosphere Model (SUPIM). The observed electron densities at 600 km altitude show a strong annual anomaly at all longitudes. The average electron densities of conjugate latitudes within the latitude range ±25° are higher at the December solstice than at the June solstice by about 100 during daytime and 30 during night-time. Model calculations show that the annual variations in the neutral gas densities play important roles. The model values obtained from calculations with inputs for the neutral densities obtained from MSIS86 reproduce the general behaviour of the observed annual anomaly. However, the differences in the modelled electron densities at the two solstices are only about 30 of that seen in the observed values. The model calculations suggest that while the differences between the solstice values of neutral wind, resulting from the coupling of the neutral gas and plasma, may also make a significant contribution to the daytime annual anomaly, the E×B drift velocity may slightly weaken the annual anomaly during daytime and strengthen the anomaly during the post-sunset period. It is suggested that energy sources, other than those arising from the 6 difference in the solar EUV fluxes at the two solstices due to the change in the Sun-Earth distance, may contribute to the annual anomaly. Observations show strong seasonal variations at the solstices, with the electron density at 600 km altitude being higher in the summer hemisphere than in the winter hemisphere, contrary to the behaviour in NmF2. Model calculations confirm that the seasonal behaviour results from effects caused by transequatorial component of the neutral wind in the direction summer hemisphere to winter hemisphere.
Comparison of the measured and modelled electron densities and temperatures in the ionosphere and plasmasphere during 20-30 January, 1993
A. V. Pavlov,T. Abe,K.-I. Oyama
Annales Geophysicae (ANGEO) , 2003,
Abstract: We present a comparison of the electron density and temperature behaviour in the ionosphere and plasmasphere measured by the Millstone Hill incoherent-scatter radar and the instruments on board of the EXOS-D satellite with numerical model calculations from a time-dependent mathematical model of the Earth's ionosphere and plasmasphere during the geomagnetically quiet and storm period on 20–30 January, 1993. We have evaluated the value of the additional heating rate that should be added to the normal photoelectron heating in the electron energy equation in the daytime plasmasphere region above 5000 km along the magnetic field line to explain the high electron temperature measured by the instruments on board of the EXOS-D satellite within the Millstone Hill magnetic field flux tube in the Northern Hemisphere. The additional heating brings the measured and modelled electron temperatures into agreement in the plasmasphere and into very large disagreement in the ionosphere if the classical electron heat flux along magnetic field line is used in the model. A new approach, based on a new effective electron thermal conductivity coefficient along the magnetic field line, is presented to model the electron temperature in the ionosphere and plasmasphere. This new approach leads to a heat flux which is less than that given by the classical Spitzer-Harm theory. The evaluated additional heating of electrons in the plasmasphere and the decrease of the thermal conductivity in the topside ionosphere and the greater part of the plasmasphere found for the first time here allow the model to accurately reproduce the electron temperatures observed by the instruments on board the EXOS-D satellite in the plasmasphere and the Millstone Hill incoherent-scatter radar in the ionosphere. The effects of the daytime additional plasmaspheric heating of electrons on the electron temperature and density are small at the F-region altitudes if the modified electron heat flux is used. The deviations from the Boltzmann distribution for the first five vibrational levels of N2(v) and O2(v) were calculated. The present study suggests that these deviations are not significant at the first vibrational levels of N2 and O2 and the second level of O2, and the calculated distributions of N2(v) and O2(v) are highly non-Boltzmann at vibrational levels v > 2. The resulting effect of N2(v > 0) and O2(v > 0) on NmF2 is the decrease of the calculated daytime NmF2 up to a factor of 1.5. The modelled electron temperature is very sensitive to the electron density, and this decrease in electron density results
The neutral temperature in the ionospheric dynamo region and the ionospheric F region density during Wenchuan and Pingtung Doublet earthquakes
Y. Y. Sun, K.-I. Oyama, J. Y. Liu, H. K. Jhuang,C. Z. Cheng
Natural Hazards and Earth System Sciences (NHESS) & Discussions (NHESSD) , 2011,
Abstract: One of the possible candidates which modifies the ionosphere before large earthquake is electric field. We presume that the electric field associated with large earthquakes is generated in the ionosphere dynamo region (100–120 km). This paper tries to identify the evidence of the contribution of the neutral atmosphere in the dynamo region. The relationship between the critical frequency at the F2 peak (foF2) and the height profile of the neutral atmosphere temperature was studied for two large earthquakes: Wenchuan, 2008 and Pingtung Doublet, 2006. It is found that the wave amplitude of the vertical wavelength 20–30 km which is usually superposed on the height profile of the neutral atmosphere temperature enhances when the foF2 increases. The correlation between the wave amplitude and foF2 is found better along a longitudinal direction than along latitude direction.
Energetics and structure of the lower E region associated with sporadic E layer
K.-I. Oyama, K. Hibino, T. Abe, R. Pfaff, T. Yokoyama,J. Y. Liu
Annales Geophysicae (ANGEO) , 2008,
Abstract: The electron temperature (Te), electron density (Ne), and two components of the electric field were measured from the height of 90 km to 150 km by one of the sounding rockets launched during the SEEK-2 campaign. The rocket went through sporadic E layer (Es) at the height of 102 km–109 km during ascent and 99 km–108 km during decent, respectively. The energy density of thermal electrons calculated from Ne and Te shows the broad maximum in the height range of 100–110 km, and it decreases towards the lower and higher altitudes, which implies that a heat source exists in the height region of 100 km–110 km. A 3-D picture of Es, that was drawn by using Te, Ne, and the electric field data, corresponded to the computer simulation; the main structure of Es is projected to a higher altitude along the magnetic line of force, thus producing irregular structures of Te, Ne and electric field in higher altitude.
Electron temperature in nighttime sporadic E layer at mid-latitude
K.-I. Oyama, T. Abe, H. Mori,J. Y. Liu
Annales Geophysicae (ANGEO) , 2008,
Abstract: Electron temperature in the sporadic E layer was measured with a glass-sealed Langmuir probe at a mid-latitude station in Japan in the framework of the SEEK (Sporadic E Experiment over Kyushu)-2 campaign which was conducted in August 2002. Important findings are two fold: (1) electron temperature and electron density vary in the opposite sense in the height range of 100–108 km, and electron temperature in the Es layer is lower than that of ambient plasma, (2) electron temperature in these height ranges is higher than the possible range of neutral temperature. These findings strongly suggest that the heat source that elevates electron temperature much higher than possible neutral temperature exists at around 100 km, and/or that the physical parameter values, which are used in the present theory to calculate electron temperature, are not proper.
Possible interaction between thermal electrons and vibrationally excited N2 in the lower E-region
K.-I. Oyama, M. Shimoyama, J. Y. Liu,C. Z. Cheng
Annales Geophysicae (ANGEO) , 2011,
Abstract: As one of the tasks to find the energy source(s) of thermal electrons, which elevate(s) electron temperature higher than neutral temperature in the lower ionosphere E-region, energy distribution function of thermal electron was measured with a sounding rocket at the heights of 93–131 km by the applying second harmonic method. The energy distribution function showed a clear hump at the energy of ~0.4 eV. In order to find the reason of the hump, we conducted laboratory experiment. We studied difference of the energy distribution functions of electrons in thermal energy range, which were measured with and without EUV radiation to plasma of N2/Ar and N2/O2 gas mixture respectively. For N2/Ar gas mixture plasma, the hump is not clearly identified in the energy distribution of thermal electrons. On the other hand for N2/O2 gas mixture, which contains vibrationally excited N2, a clear hump is found when irradiated by EUV. The laboratory experiment seems to suggest that the hump is produced as a result of interaction between vibrationally excited N2 and thermal electrons, and this interaction is the most probable heating source for the electrons of thermal energy range in the lower E-region. It is also suggested that energy distribution of the electrons in high energy part may not be Maxwellian, and DC probe measures the electrons which are non Maxwellian, and therefore "electron temperature" is calculated higher.
Yearly variations in the low-latitude topside ionosphere
G.J. Bailey,Y. Z. Su,K.-I. Oyama
Annales Geophysicae (ANGEO) , 2003,
Abstract: Observations made by the Hinotori satellite have been analysed to determine the yearly variations of the electron density and electron temperature in the low-latitude topside ionosphere. The observations reveal the existence of an equinoctial asymmetry in the topside electron density at low latitudes, i.e. the density is higher at one equinox than at the other. The asymmetry is hemisphere-dependent with the higher electron density occurring at the March equinox in the Northern Hemisphere and at the September equinox in the Southern Hemisphere. The asymmetry becomes stronger with increasing latitude in both hemispheres. The behaviour of the asymmetry has no significant longitudinal and magnetic activity variations. A mechanism for the equinoctial asymmetry has been investigated using CTIP (coupled thermosphere ionosphere plasmasphere model). The model results reproduce the observed equinoctial asymmetry and suggest that the asymmetry is caused by the north-south imbalance of the thermosphere and ionosphere at the equinoxes due to the slow response of the thermosphere arising from the effects of the global thermospheric circulation. The observations also show that the relationship between the electron density and electron temperature is different for daytime and nighttime. During daytime the yearly variation of the electron temperature has negative correlation with the electron density, except at magnetic latitudes lower than 10°. At night, the correlation is positive. Key words: Ionosphere (equatorial ionosphere; ionosphere-atmosphere interactions; plasma temperature and density)
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