A method of analysis is investigated for the monochromatic aurora stereo TV observations at Antarctica. First, a camera model is obtained by using calibration data for the background stars. Second, an inverse problem is solved which reconstructs threedimensional luminosity structures of aurora photoemission from two-dimensional projection of auroral images. The method assumes prescribed spatial distributions of luminosity, parameters of which are to be determined by the nonlinear least squares method. A series of numerical tests indicates that stereo data give plausible estimation of the model parameters, but in some cases, the method is jeopardized due to unfavorable geometrical conditions. A preliminary analysis of the stereoscopic observation for the stable is carried out, and the presumed structure of luminosity at 557.7[nm] wavelength is inferred.

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A short history of studies of Birkeland currents and a summary of measurement techniques is provided. The observations made to date show that discrete auroral arcs and upward flowing Birkeland currents are usually associated. There is experimental uncertainty about whether these upward currents are strongest just above the arcs or are concentrated at the edges. A consistent pattern relating the location of the downward return current, the electrojet, and the electric field has not been found.

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Auroral observations using automatic optical instruments have been conducted during the Solar Terrestrial Energy Program (STEP) period, at several latitudes along, lower, and higher than the auroral zone, together with the chain stations along 210° magnetic meridian. This paper gives details of the instrumentation and the locations of stations. In an initial analysis, auroral data from an auroral zone station at Kotzebue, Alaska, and magnetic field data from both high- and low-latitude stations along the 210° and 250° magnetic meridians were used to demonstrate the relations between auroral substorm onsets and multiple Pi 2 magnetic pulsation. The correspondence between each Pi 2 onset and each small- or large-scale brightening and poleward expansion of auroras during an auroral substorm with multiple onsets was good.

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An instrument has been developed for observing high frequency modulations or fluctuations of auroral electron flux up to several magahertzs. The instrument utilizes on-board processing hardware for calculating auto-correlation functions (ACFs) of particle flux measured by any type of particle detector which outputs a series of pulses. The instrument is capable of measuring flux modulations in three frequency ranges, i. e., 1.5-100Hz, 0.06-8kHz and 0.03-4MHz, respectively. Two rocket observations by using the instrument were carried out as part of low energy auroral electron experiments on board the S-310JA-11 and -12 rockets which were launched into the Antarctic auroral ionosphere. Preliminary analyses of the rocket data indicate that, under relatively quiet conditions (S-310JA-11), the auroral electrons are almost uniform in the high and medium frequency ranges. Results from another rocket experiment (S-310JA-12) conducted under highly active aurora conditions show low frequency modulations (less than 40Hz) of electron flux associated with regions of a strong electron precipitation, in particular, with edges of the regions.

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In order to examine the relative locations of auroral particle fluxes and field-aligned currents and to identify the main charge carriers of the field-aligned currents at auroral latitudes, nearly simultaneous data from the vector magnetometers on MAGSAT and of precipitating electrons with energies between 300eV and 20keV observed by TIROS-N and NOAA-6 are compared. For more than fifty cases, MAGSAT and TIROS and/or NOAA orbits occurred within two minutes (mostly within one minute) of each other in the dawn/dusk sectors, during the time the IMS meridian chains of ground magnetometers were operating from November, 1979 through January, 1980. The latitudinal boundaries of precipitating electrons are found to line up within 1° with those of the field-aligned current region. Major portions of the upward field-aligned currents in the poleward half of the evening-sector auroral oval and in the equatorward half of the morining-sector auroral oval appear to be carried by the precipitating keV electrons.

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磁気圏－電離層結合系で生じるマクロ不安定性を計算する電磁流体コードと、磁気圏から電離層へ流入する電子によるミクロ不安定性を計算する粒子コードを連結したシミュレーションモデルにより、静穏時オーロラアーク形成の物理過程について調べた。その結果、フィードバック不安定性[1]により、アーク構造が形成される様子が観測された。さらに、粒子モデル計算で得られた加速電子のエネルギースペクトルから、大気分子励起による発光強度を算出し、オーロラアーク発光までいたる物理過程の再現をおこなった。

[1] T. Sato, J. Geophys. Res., 83, 1042, 1978.

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Auroral electrodynamics data from various experimental data sets are analysed and the cases are selected with steady conditions, homogeneous auroral bands and visual arcs (as seen by all-sky cameras from the ground) and with predominantly zonal flow in the middle and equatorial auroral oval away from the Harang discontinuity. Exclusion of dynamic cases with moving and bursty rayed arcs is needed to avoid the ambiguity of magnetometer measurements from a satellite due to possible contamination of the perpendicular component of the magnetic field of stable sheet currents by similar field variations due to Alfven waves. The schematics of electrodynamical patterns from these cases are described and substantiated by experimental data. It consists of hierarchy of encircled two-sheet current loops (so called Bostrom type current loop) of different scales from the largest one of the Region 1/Region 2 FAC's, to intermediate one of the inverted-V of order of 100km wide, to the smallest one of an auroral arc scale of about 10km wide, all having the same direction of the closing ionospheric current (the so called “Matreshka anodel”, named after Russian folkloric toy of several wooden dolls one inside another).
Examples of coordinated space/ground based measurements of auroral electrodynamic patterns taken from original observations and found from published data of other (mostly rocket) experiments selected with the above criteria are analysed and are shown to be generally consistent with the Matreshka schemiatics.

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Short-period disturbances of the ionosphere were observed by a network of HF Doppler receivers in central Japan at the time of typhoons 18 and 19 which crossed the Islands of Japan successively in September 1982.
Both dynamic and static spectral analyses on sample records revealed that (1) these waves have the periods ranging between 1.4 and 9.7min that correspond to infrasound at F-layer heights (and partly to gravity wave at E-layer heights), (2) the spectral content varies from hour to hour, from station to station for a given typhoon, and further one typhoon to the other, and (3) the spectral fine structure does not accord with existing theoretical prediction for thunderstorms. It is, therefore, plausible that the observed spectral peaks arose from the characteristics of the radiation sources in the typhoon air mass.
Horizontal and vertical phase velocities, and corresponding wavelengths of the ionospheric waves were also measured using cross-correlation method. It is shown that (1) the horizontal phase velocities take similar values to those observed during the severe thunderstorms and tornadoes so far reported, with scatter range of the values being slightly larger than the latter two, and (2) the mean values of the upward vertical phase velocities are several to ten times larger than those of the horizontal ones.

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Recent observations of electric fields in the ionosphere using various measurement techniques have led to important advances in the understanding of magnetosphere-ionosphere coupling. In particular the discovery of evidence for parallel electric fields in the altitude range 2, 000-10, 000km has an important impact on the question of auroral acceleration processes. More detailed mapping of large scale field-aligned current configurations on field lines penetrating the auroral oval are leading to more complete models for the generator processes in the outer magnetosphere which are associated with energy dissipation in the near-earth environment. Recent observations of significant electric fields equatorward of the auroral oval indicate that significant interaction between the magnetosphere and ionosphere takes place outside of the auroral oval. New techniques are presently being developed for more quantitative studies of magnetosphere-ionosphere coupling.

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An aurora-TV camera for studies of aurora dynamics and global monitoring of magnetospheric activities with simultaneous in-situ experiments is installed on the satellite Akebono (EXOS-D). The camera is a two-channel, visible and VUV, snapshot imager with the maximum rating of one shot every 8 seconds. Initial results of VUV imaging are providing promising data for imposed missions.

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Bright auroral forms several hundreds of km in spatial extent were observed repeatedly (a few to about 15min, periodicity) in the premidnight hours, following auroral breakups. The auroral forms drifted towards the northwest or the southwest, and were related to the magnetic field fluctuations on the ground in a similar period range. The leading front of the drifting auroral form has a discrete auroral (vortexchain) feature. After the passage of discrete forms pulsating aurora appears. From the comparison with magnetic field perturbations at the geosynchronous orbit satellites GOES 5 and GOES 6, these auroral forms are likely to correspond to the depression of the magnetic total intensity. A possible source of these bright auroral forms is hot plasmas injected from the plasma sheet. The concurrent magnetic pulsations, here called Ps5, would be produced by the fluctuating electric field and conductivity in the ionosphere due to the electron precipitation into the auroral forms. Hot plasma clouds, quasi-periodically injected during the course of a substorm, are related to the auroral and magnetic field pulsations. A steep injection front of hot plasma in the magnetosphere is likely related to discrete auroras at the ionosphere.

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Relationships between average characteristics of particle precipitation and substorm activity are investigated in the low latitude region as well as in the high latitude region within the dusk-to-midnight sector. “Low latitude” is defined here as the region below the equatorward boundary of the most equatorward electron BPS, while “high latitude” is defined as the region higher than that boundary. DMSP particle data are divided according to the phase of the substorm and according to the longitude with respect to the onset region by referring to ground-based magnetograms and DMSP auroral images. At high latitude, not only electron precipitation but also ion precipitation significantly intensifies in accordance with substorm activity. The energization of these particles presumably takes place in the near-tail region where an unstable tail current develops and locally diverges associated with substorm onset. At low latitude, temporal and spatial changes of energetic precipitation would be caused by injected, drifting particles from the substorm onset regions, particularly in the case of electrons.

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Three sounding rockets were launched in 1984 from Syowa Station in Antarctica into different types of aurora, designated as AURORA I (an active auroral arc at the substorm expansion phase), II (a stable arc prior to the substorm onset) and III (a diffuse aurora during the recovery phase). A new television camera was used to take an auroral picture, together with the electron spectrometers to measure the energy spectrum of auroral particles. AURORA I showed greater values in both auroral emission and electron density than for AURORA II. In AURORA III, although the auroral emission intensity was much less in comparison with AURORA I, the electron density showed a pronounced enhancement in the D-region and was comparable with the case of AURORA I above the E-region. As for an energy spectrum of auroral electrons, AURORA I and II showed an inverted-V type monoenergetic peak with much higher energy for AURORA I than for AURORA II, whereas AURORA III exhibits a power-law spectrum with a small superposing monoenergetic peak around 0.5keV above 200km in altitude. The television camera showed a dependence of auroral color spectrum on the auroral activity, i. e., the ratio of an apparent emission rate at 427.8nm and an illuminance in a visible range (400 to 800nm) becomes greater for brighter aurora.

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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.

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The damped-type long-period (T=10-20min) magnetic variation associating with a substorm is called the Ps6-type Pi3 pulsation, or briefly Ps6. The two-snake model including meandering ionospheric currents during a substorm which has been proposed in previous papers to explain the morphological characteristics of Ps6 is examined in the present paper to interpret the observed polarization of the ground-based Ps6. A model calculation suggests that from the viewpoint of the polarization the area near a snake-type model electrojet is expected to be divided into five domains: pole-side pre-midnight domain (right-handed), pole-side post-midnight domain (L), central domain (R & L), equator-side pre-midnight domain (L), and equator-side post-midnight domain (R). This theoretically expected polarization rule is examined by two ways: statistical study and case study of the observed Ps6.
The polarization of 214 Ps6 events observed at Point Barrow and College is statistically found to be governed by the theoretical polarization rule. A case study on Ps6 waves sorted out through a digital filter from five substorm events corresponding to the five domains indicates that the polarization rule obtained observationally in this way also agrees with the theoretical polarization rule.

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The visible (Oxygen green line: 557.7 nm) TV camera aboard EXOS-D (Akebono) takes successive auroral images every 8 sec, which are characterized as a snapshot image of a wide area with high spatial and temporal resolutions. The followings are some preliminary results, when the satellite was located at altitudes of 3000 to 4700 km in the southern hemisphere. (1) At the magnetospheric disturbances, the faint enhanced aurora (MLT width less than about 0.2 hours ≈3°) along the meridian at the pre-midnight was formed, extending from 74° CGL down to 65° CGL. The brightening of the spot was first activated at the higher latitude with subsequent propagation/elongation to the lower latitude. (2) During the storm recovery phase the brightening was confined to a small region (MLT width being less than 0.2 hours or about 330 km across in the ionosphere) of discrete aurora in the high latitude side of the midnight auroral oval. Newly intensified auroral arcs along the pre-existing faint discrete aurora were successively coming out to the westwards. (3) The discrete auroral arc showed a clockwise rotation viewed in the direction of the geomagnetic field. (4) Interestingly, a 557.7 nm airglow layer was observed whenever the field of view covered the earth's limb.

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An attempt is made to construct a road map for translating various plasma domains in the magnetosphere to their counterparts in the auroral ionosphere. Results of previous work have allowed much to be inferred about where in the magnetosphere auroral processes are taking place. The main auroral oval appears to be associated with magnetospheric processes well separated from boundary layer processes with the exception of the dayside sector between about 8 and 16 MLT. This sector is apparently dominated by sources in the dayside boundary layer and by the region where the nightside cross-tail current intersects the boundary layer regions. The division between the stably trapped particles and isotropic distributions probably coincides with the most equatorward discrete arc system involved with the substorm onset. Thus the diffuse aurora equatorward of the discrete aurora would be a ring current related phenomena and the nightside discrete aurora would originate dominantly from the central plasma sheet. During magnetically active conditions the outer edge of the central plasma sheet plays a more important role in the production of the discrete aurora. Diffuse aurora poleward of the discrete system is probably linked to velocity dispersed ion signatures seen in the low altitude ionosphere.

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A new TV camera system was developed for monochromatic digital imaging of auroras. On the basis of observation in 1984 at Syowa Station, Antarctica by the TV system, three types of auroral activities were analysed, i. e., i) stable auroras in the dusk sector, ii) active auroras and iii) pulsating auroras in the dawn sector. The results are; 1) diffuse auroral belt which appears at the lower latitude border of auroral activity coincides with the distribution of H_β emission, 2) enhancement of auroral 5577Å emission is much larger than that of 6300Å emission at the westward travelling surge and 3) period of auroral pulsation in the dawn sector shows regional variations. The observation suggests; 1) that the diffuse aurora in the dusk sector at the lower latitude side is excited by auroral protons, 2) that a hardening of energy spectrum of precipitating electron is associated with the westward travelling surge.

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Irregular magnetic pulsations and IPDP plasma wave events related to the low latitude aurora were observed at Yonezawa observatory on October 20 and 21, 1989. Three positive bays appeared in the magnetograms on October 20. Concurrently, IPDP, Pi 2, Pi B and Pi C are registered during the respective positive bays. The power spectrum estimations of irregular magnetic pulsations are carried out by the maximum entropy method indicating that the spectral peaks are mainly distributed in the frequency band between 0.1 Hz and 0.2 Hz. When the magnetopause is compressed by a sudden increase of the solar wind pressure inside the geosynchronous orbit, transient magnetic field oscillations are observed on the ground. Two distinct positive bays at the time of the minimum H during the main phase of the geomagnetic storm occurred in the magnetograms on October 21. The irregular magnetic pulsations accompanying the IPDP plasma wave events and the low latitude optical aurora were observed during each positive bays.

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