Hiroshi Miyaoka

Coulomb lifetime of the ring current ions with time varying plasmasphere

We have developed a time-dependent model of the plasmasphere to evaluate the spatial variation of the Coulomb lifetime of ring current ions. Coulomb collision has been considered to be one of major loss processes of the ring current ions interacted with the thermal plasma in the plasmasphere. The distribution of plasmaspheric density is derived by a continuity equation under the hydrostatic assumption. The protons supplied from both conjugate ionospheres are drifted by a time-dependent convection field and a corotation electric field. Calculated profiles of the number density and the relative motion of the plasmasphere are in fairly good agreement with the observational results by EXOS-B satellite. We traced the energetic ions during a storm on June 4–8, 1991 and calculated the differential flux and the pressure to examine the loss effects on the pressure due to the both loss processes. We found that (1) the Coulomb collision loss restrictively affects at L ≤ 3 because the plasmasphere drastically shrank due to the strong convection, and that (2) there is no significant change in the ion composition ratio during the initial rapid recovery of Dst, i.e., the rapid recovery of Dst is not caused by the short charge exchange lifetime of O+ ions for this particular storm.

Auroral radio emission and absorption of medium frequency radio waves observed in Iceland

In order to study the generation and propagation processes of MF auroral radio emissions (referred to as auroral roar and MF burst) in the polar ionosphere, an Auroral Radio Spectrograph (ARS) system was installed at Husafell station in Iceland (invariant latitude: 65.3°). Data analysis of man-made transmissions also provides useful information for the ionosphere study as well as an investigation of auroral radio emissions since the propagation character of MF radio waves changes depending on electron-neutral collisions in the bottomside ionosphere. Thus, ionospheric absorption is examined in comparison with the solar zenith angle and auroral phenomena. The results indicate that the ARS data can be used to detect ionization occurring at distant regions. In late 2006, the ARS detected one auroral roar and twoMF bursts, which were identified as left-handed polarized waves. Results of data analysis, including other auroral data and particle spectra observed by the DMSP satellite, suggest that the MF bursts are generated by electrons with an average energy of several keV associated with auroral breakup. On the other hand, the auroral roar is generated as upper hybrid waves by relatively low-energy electrons over the observation site and propagates downward, being converted into L-O mode electromagnetic waves.

Temperature enhancements and vertical winds in the lower thermosphere associated with auroral heating during the DELTA campaign

[1] A coordinated observation of the atmospheric response to auroral energy input in the polar lower thermosphere was conducted during the Dynamics and Energetics of the Lower Thermosphere in Aurora (DELTA) campaign. N2 rotational temperature was measured with a rocket-borne instrument launched from the Andoya Rocket Range, neutral winds were measured from auroral emissions at 557.7 nm with a Fabry-Perot Interferometer (FPI) at Skibotn and the KEOPS, and ionospheric parameters were measured with the European Incoherent Scatter (EISCAT) UHF radar at Tromso. Altitude profiles of the passive energy deposition rate and the particle heating rate were estimated using data taken with the EISCAT radar. The local temperature enhancement derived from the difference between the observed N2 rotational temperature and the MSISE-90 model neutral temperature were 70–140 K at 110–140 km altitude. The temperature increase rate derived from the estimated heating rates, however, cannot account for the temperature enhancement below 120 km, even considering the contribution of the neutral density to the estimated heating rate. The observed upward winds up to 40 m s �1 seem to respond nearly instantaneously to changes in the heating rates. Although the wind speeds cannot be explained by the estimated heating rate and the thermal expansion hypothesis, the present study suggests that the generation mechanism of the large vertical winds must be responsible for the fast response of the vertical wind to the heating event.

First results of auroral tomography from ALIS-Japan multi-station observations in March, 1995

Auroral tomography observations have been carried out in March, 1995, as a joint international campaign between Sweden and Japan. Three unmanned Swedish ALIS stations (Kiruna, Merasjärvi, Tjautjas) and two Japanese JICCD sites (Abisko, Nikkaluokta), geographically separated by about 50 km at higher latitudes, were operated to capture multi-station monochromatic tomography images at 557.7 nm wavelength using CCD cameras. All cameras were pointing to one of the predetermined directions to secure a common field of view. Several images of auroral arcs, mostly for the core region right above Kiruna, have synchronously been taken by the multi-station imaging system. Tomographic inversion analysis for four-point images was carried out using the algebraic reconstruction technique. Reconstructions of a curved arc and of a double arc system suggest promising application of this technique to the retrieval of three-dimensional auroral luminosity.

Polar patrol balloon experiment in Antarctica

Abstract Since 1984 the National Institute of Polar Research, the Institute of Space and Astronautical Science and collaborative scientists have searched for feasibility of the long-term circumpolar balloon experiment called Polar Patrol Balloon (PPB) project. This project aims at establishing a station network in the stratosphere over the Antarctic region for geophysical and astrophysical observations. Three test flights in 1987 and 1990 at Syowa Station have convinced us that PPB would come back to the launching site with a good possibility. During 1991–1993 the PPB experiments will consequently be made for scientific researches at Syowa Station. This paper reports unique advantages of this PPB experiment and briefly reviews the past PPB experiments.

On the simultaneity of substorm onset between two hemispheres

Simultaneous observations of auroral kilometric radiation from the Northern and Southern Hemispheres showed some cases in which the buildup of field-aligned acceleration occurred only in one hemisphere at the substorm onset. This indicates that a substorm does not always complete the current system by connecting the cross-tail current with both northern and southern ionospheric currents. Conjugate auroral observations showed that in one case, the auroral breakup in the Northern and Southern Hemispheres was not simultaneous; rather, they occurred a few minutes apart. This time difference in the breakup between two hemispheres suggests that the local auroral ionosphere controls auroral breakup in each hemisphere independently. The evidence in this study may indicate that the buildup of the field-aligned acceleration region at the auroral breakup does not result only from the magnetospheric process and that the auroral ionosphere finally controls and/or ignites the substorm onset, that is, the auroral breakup.

Rocket observation of electron fluxes over a pulsating aurora

Abstract A sounding rocket S-520-12 was launched from Andoya, Norway at 02:06:00 U.T. on 26 February 1990, into pulsating aurora. Electron energy spectra were observed with a quadrispherical electrostatic analyzer (QESA). The rocket flew from one pulsating patch to another, and we observed the spectral variation of precipitating electron flux following this transition. Pulsation of particle flux was observed in the precipitating electrons above 4 keV and the spectrum was fitted with a power-law distribution, although the electrons with energy less than 4 keV did not show significant pulsation. We found that the pulsation periods obtained through Fourier analysis for the auroral emission recorded by ground TV camera and for the in-situ energy flux data of the precipitating electrons agreed well. However, the one-to-one correlation between the electron energy flux and the auroral intensity was relatively poor. We attributed this to the spatial nonuniformity of the boundary region between two pulsating patches, and to the unstable phase relationship between the dominant Fourier components of the auroral emission and the electron energy flux. This might be caused by the propagating and streaming nature of the pulsating aurora during the time of the observation. We also found low-energy electron precipitation at the boundary region between the two pulsating patches, which can be attributed to the acceleration of the electrons at an altitude of several thousand kilometers by upward-propagating kinetic Alfven waves. That wave might be generated in association with the ionospheric conductivity change caused by the precipitation of the auroral electrons and the resultant enhancement of ionization of the upper atmosphere.

Tidal waves in the polar lower thermosphere observed using the EISCAT long run data set obtained in September 2005

[1] Characteristics are presented of the lower thermospheric wind from a long run data set obtained by the EISCAT UHF radar at Tromso (69.6°N, 19.2°E) over ~23 days, from September 6 to 29, 2005. The derived semidiurnal amplitude exhibited day-to-day variations (~5-30 ms -1 ) at and above 109 km, while the phase varied little with the day. We have found a mode change of the semidiurnal tide occurring during September 17-22, 2005. Between September 6 and 16, the vertical wavelengths were estimated to be ~58 km and ~76 km for the meridional and zonal components, respectively, while between September 23 and 29, they became less than -24 km. The day to day variability of the diurnal tide was less obvious than that of the semidiurnal tide. The diurnal amplitude of the meridional component increased with height except for 8 days between September 13 and 20, when the diurnal amplitudes were smaller values (<40 ms -1 ) at and above 111 km than those for the other intervals. Furthermore, the shapes of the altitude profiles of the meridional phase differ from those for the other intervals. We have evaluated contributions due to the electric field and the ion drag acceleration and showed that they were not the causes. From the analysis of 22.5 days of wind data, we found about 5-6 day oscillations in the lower thermosphere, probably where there were planetary wave activities in the lower thermosphere.