Edited by Tadahiko Ogawa, Shoichiro Fukao, and Takashi Maruyama

G1. Ionospheric Techniques

Recent development of computerized tomography techniques has provided a new tool for reconstructing two-dimensional ionospheric electron density structure in a vertical plane by using data of total electron content obtained from satellite beacon measurements. Okuzawa et al. [1995] reported preliminary results of their simulation study on the application of three kinds of reconstruction algorithm; the multiplicative algebraic reconstruction technique, a method on the principle of maximum likelihood estimation, and the spline function method. They described characteristics, merits, and disadvantages among the three algorithms. Kunitake et al. [1995] developed a technique using the modified truncated singular value decomposition (MTSVD) method to reconstruct electron density distribution from total electron content data obtained from NNSS satellite beacon experiments at four receiving stations along the Japanese meridian.

Nozaki and Kobayashi [1994] demonstrated that a 24.5 MHz FM/CW radar with a low transmitting power of 100 W at Okinawa can sufficiently detect low-latitude F region field-aligned irregularities (FAIs). Igarashi et al. [1995] developed a new 50 MHz scanning-beam auroral Doppler radar consisting of two sets of array antennas. This radar, located at Syowa Station, Antarctica, can observe auroral E region irregularities in a wide area.

Aurora stereoscopic observations at two distant stations was carried out in Iceland in 1993 to obtain stereo image data with longer baseline for the study of inverse algorithm and to clarify three-dimensional luminous structures of aurora [Aso et al., 1993]. Ejiri et al. [1994a] described the visible (557.7 nm) TV camera aboard the EXOS-D (Akebono) satellite that takes successive auroral images every 8 s (a snapshot image of a wide area with high spatial and temporal resolutions) and presented some preliminary results.

A 30 MHz imaging riometer was installed at Ny-Alesund (L=16) to observe spatial structure and motion of auroral absorption regions [Nishino et al., 1993]. The antenna consisting of a two-dimensional dipole array with 64 elements produces 64 pencil beams that scan an area of about 200 km square at 90 km altitude. The instrument can provide a two-dimensional image of enhanced absorption region with a spatial resolution of 20 km and temporal resolution of 1 s.

Since 1984, the National Institute of Polar Research and the Institute of Space and Astronautical Science have been studying the feasibility of a long-term circumpolar balloon experiment, called the Polar Patrol Balloon (PPB) project. After three test flights in 1987 and 1990 at Syowa Station, Antarctica, six PPB experiments were carried out in 1990 to 1993 as an Antarctic STEP project [Ejiri et al., 1993, 1994b, 1995; Nishimura et al., 1994]. Of six PPBs, PPB #1 accomplished a complete circumpolar flight over Antarctica [Ejiri et al., 1993] and measured the total force of the geomagnetic main field with an accuracy of 1 nT with a proton magnetometer [Tohyama et al., 1993]. PPB #2 was launched with scientific payloads for auroral X-rays, magnetic and electric fields. PPB #4 and #5 were equipped with payloads for measuring vector magnetic and electric fields and auroral X-rays in the auroral, polar cap and cleft/cusp regions.

Japan joined the EISCAT Scientific Association in April 1996. Matuura et al. [1995] discussed collaboration between the Japanese scientific community and the EISCAT Scientific Association on the new Svalbard incoherent scatter radar project. They summarized some of the important research topics to pursue in conjunction with this radar: identification of the cusp region, solar wind control of the cusp, electrodynamics in the cusp and polar cap, contracted-oval substorms, and thermospheric dynamics at high latitudes.

G2. Ionospheric Structure and Modeling

G2.1. Ionospheric Structure

There have been some attempts to improve the IRI model for altitudes above about 1000 km by using satellite data. Sawada et al. [1993] proposed a new method to construct global plasma distribution in the plasmasphere using wave normal directions of Omega signals observed on the Akebono satellite and electron density measured along every trajectory of this satellite. Kimura et al. [1995] further developed this method to include data of delay times of Omega signals from transmitters to Akebono. Electron temperatures observed with two satellites (Hinotori and Akebono satellites) were compared with the IRI model [Oyama, 1994]. The Hinotori data show that the nighttime temperatures except for temperatures relating to a few peculiar phenomena (in particular, a morning overshoot at 600 km altitude and anomalous features occurring near the nighttime equatorial ionization anomaly) are quite well consistent with the IRI90. The temperature profiles up to 8000 km revealed by Akebono depend on local time at all latitudes and on solar activity.

A time-dependent three-dimensional computer simulation of equatorial F region was carried out to understand electron temperature structures observed by the Hinotori satellite near 600 km altitude in the low- and mid-latitude ionosphere [Watanabe et al., 1995; Watanabe and Oyama, 1995]. This model provides three-dimensional distributions of ion densities, electron temperature, and ion temperatures and is consistent with the equatorial electron density and temperature data from Hinotori. Su et al. [1995] studied Hinotori observations of electron density and temperature by comparison with values from the Scheffield University plasma-ionosphere model to discuss similarities and dissimilarities between the observations and the model.

Oyama et al. [1993] discussed energy budget in an equatorial plasma bubble to explain electron temperatures observed by Hinotori at 600 km altitude and showed from calculations that electron temperature profile inside bubble can basically be explained as a function of energy input to the bubble, ion temperature, and electron density. The calculated results are consistent with the satellite observations.

Based on in-situ observations of electron density by the Hinotori satellite, Takahashi and Oya [1994] presented a new concept of three dimensional large scale structure of the low-latitude ionosphere. The structures are classified into two types, crest type (cusp type enhancement) and canyon type (cusp type depletion), both of which are often accompanied by plasma bubbles in their central portion. Takahashi et al. [1994] used the same satellite data to clarify two types of equatorial plasma bubbles: one is large scale array type appearing between the sunset and local midnight, and the other type is related to local density enhancement due to substorm activity.

Much of the thermospheric research in Japan concerned the 46.5 MHz MU (Middle and Upper atmosphere) radar of the Radio Atmospheric Science Center of Kyoto University. Rishbeth and Fukao [1995] reviewed much of this work.

G2.2. Ionospheric Disturbances

Studies of thermospheric gravity waves with a multiple-beam capability of the MU radar have been ongoing. Waves of a magnitude detectable by the radar, produced by wind perturbations of typically 7 m/s and giving rise to electron density perturbations of typically 3 %, are always observed during radar experiments. Spatial gradient of the ionospheric density between radar beams was used to distinguish between propagating and non-propagating waves and, for the propagating waves, their speed, wavelength, direction of travel, and rate of decay transverse to their direction of travel [Fukao et al., 1993]. Wave fronts are slanted to give downward phase propagation below 300 km altitude but gradually change above that altitude owing to the dissipative effects of viscosity in such a way that they are nearly vertical above 400 km altitude. Individual wave packets are identified in time-height plots of wave vector velocity, with one or two cycles per wave packet forming the rule and apparently longer wave trains being the result of a juxtaposition of shorter trains [Oliver et al., 1994]. Strong waves are seen only during the daytime during magnetically quiet periods but occur during both the daytime and nighttime during disturbed periods. By filtering according to wave period, the wave dispersion (horizontal wave trace speed versus wave period) was obtained [Oliver et al., 1995]. The speed of about 200 m/s is found to be largely independent of period for 60 to 130 minutes period at 200-300 km altitude, indicating largely dissipationless propagation. The dispersion relation shows no change with either solar or magnetic activity and only small change with season. These results was summarized by Fukao et al. [1994b]. Ogawa et al. [1995] conducted a comparison observation of medium- and large-scale traveling ionospheric disturbances (TIDs) between the MU radar and the NNSS satellite method to determine the wave parameters (propagation direction, period, wavelength, and wave front tilt). They also showed that the NNSS method is difficult to detect large-scale TIDs due to the limited spatial coverage of this method.

Ionospheric and atmospheric disturbances due to the eruptions of Mount Pinatubo were detected by the Japanese ionospheric observation network [Igarashi et al., 1994]. Wave fronts of the disturbances traveled northward with a velocity of about 290 m/s, being comparable to a horizontal velocity of the pressure fluctuations detected by the microbarograph chain in Japan.

Low-latitude penetration of magnetospheric electric field was corroborated from low-latitude ionospheric disturbances which accompanied severe ionospheric scintillations [Tanaka, 1994]. An important feature associated with this event is the enhancement of equatorial ionization anomaly that correlates with the asymmetric ring current in the evening.

The winter D region anomalies (anomalous f-min enhancements) detected during the DYANA campaign in 1992 were analyzed to find their temporal and geographical distributions [Ohtani et al., 1993]. The f-min anomaly that might be related to planetary wave activity was most intensified around the north latitudes of 40-45 degrees, became weaker toward higher and lower latitudes, and occasionally extended down to 26 degrees.

Using data from the EISCAT CP1 observation mode, Shibata and Schlegel [1993] studied vertical structure of atmospheric gravity waves (AGWs) associated fluctuations of ionospheric plasma parameters at altitudes of 100-240 km in daytime. They found that the essential features of AGWs can be explained in terms of the energy conservation in a dissipative thermosphere. They also derived intrinsic propagation parameters of the dominant AGW and clearly identified a downcoming AGW.

G2.3. Ionospheric Irregularities

Small-scale (3.2 m) field-aligned irregularities (FAIs) in the midlatitude nighttime E region have been observed with the MU radar. Yamamoto et al. [1994] (see also Fukao et al. [1994a]) made multibeam observations in 12 directions to find that quasi periodic (QP) radar echoes have a monochromatic wave structure propagating toward SSW, and also used simultaneously an interferometry technique to investigate much smaller scale structure of the irregularities. The results are consistent with a model in which a southward propagating gravity wave modulates the sporadic E layer to generate the QP echoes [Tsunoda et al., 1994a, 1994b]. Ogawa et al. [1995] summarized the characteristics of both QP echoes appearing at altitudes of 100-125 km only in the nighttime and continuous echoes appearing at altitudes of 90-100 km in the nighttime and in the morning. They also discussed the turbulent state of the QP and continuous echoes. Bowman et al. [1994] examined FAIs in the F region on one particular night with the MU radar and compared the FAIs with ionogram data from ionosondes in Japan to find that medium-scale traveling ionospheric disturbances passed over the MU radar when the radar detected the FAIs.

Irregularities that produce quasi-periodic scintillations of the field strength of radio waves transmitted from a satellite were investigated by numerical calculations for various shapes of plasma enhancement in the sporadic E layer, and the results were compared with observations [Maruyama, 1995].

Saito et al. [1995] studied electric field fluctuations in the nighttime topside ionosphere at midlatitude observed by the DE-2 satellite. The fluctuations with wavelengths of several tens of kilometers and amplitudes of a few mV/m extended over several hundred kilometers along the latitude and were often detected at magnetically conjugate points in the northern and southern hemispheres.

To explain the seasonal-longitudinal variation of equatorial spread F irregularities, the Rayleigh-Taylor instability growth rates were calculated with a non-local treatment and compared with the calculations with local ionospheric parameters at the equator [Maruyama, 1994].

Ohtaka and Tanaka [1993] presented long term variations of VHF auroral radar echo occurrences from 1978 to 1990. The diurnal variation exhibits a maximum after local midnight and a minimum around noon, and the monthly-averaged occurrences show semi-annual variations with minima in summer and winter. For the solar cycle variation, there exist two occurrence peaks: major peak in the declining phase of the sunspot number and secondary peak in the increasing phase. By analyzing data from 50 MHz auroral radar, Kunitake [1993] showed a statistical result of the radar echoes having very narrow spectral peaks with low Doppler shifts less than 20 Hz. A mechanism responsible for this type of echoes is unknown.

Ueda et al. [1994] studied mechanism of stimulated electromagnetic emissions, called Broad Upshifted Maximum, observed in high-frequency ionospheric heating experiments with the aid of two-dimensional electromagnetic particle computer simulation. The results show that the irregularities are important for the excitation of the emissions.

G2.4. Ionosphere Dynamics

The multiple-beam Doppler capability of the MU radar has been used to derive the first ionospheric electric field climatology, including time-of-day, seasonal, and solar-cycle dependences, for the midlatitude Asian sector [Oliver et al., 1993]. The perpendicular-northward component of the field resembles that measured at other longitudes in basic diurnal form but shows additional seasonally varying fluctuations tied to the time of sunset, perhaps indicating a sunset shift in dynamo control between the E and F regions. The eastward component has a strong evening excursion, similar to those shown at other longitudes, that strengthens with solar activity, but additionally exhibits a secondary peak in the morning that is completely missing at American and European longitudes. The multiple-beam Doppler capability of the MU radar has also been used to study the meridional wind in the upper thermosphere. A comparative study with the meridional winds deduced from ionogram readings shows good agreement between techniques and verifies the MU radar finding that the winds in the Asian sector are a factor of 1.5-2 smaller in amplitude than those reported at other longitudes [Igi et al., 1995]. Verification of the ionosonde wind method opens the way to comprehensive studies of the thermospheric wind over Japan from the long and continuous ionogram libraries from the Japanese ionosonde chain.

Using a two-dimensional, time-dependent model of coupled dynamics and composition, Maeda and Fujiwara [1994] simulated dynamical response of the thermosphere to energy input associated with geomagnetic storms. The response to model storms of various durations are interpreted in terms of meridional circulation and atmospheric gravity waves.

G2.5. Particles and Auroras

Imaging riometer observations at three cusp-latitude observatories and one auroral zone observatory were reported by Stauning et al. [1995]. The high-latitude nightside observations support the model of substorms being a series of enhancements distributed in latitude and longitude; the individual enhancements could be explained by an augmented current wedge mechanism. The dayside results indicate that disturbances associated with a strong convection shear may cause enhanced and strongly variable precipitation of the eastward drifting cloud of high energy electrons.

Makita et al. [1995] studied characteristics of extremely high latitude auroras (polar arc and polar corona), electron energies for producing these auroras, and origins of the energetic electrons. Ono [1993] operated a high speed multi-channel photometer at Syowa Station, Antarctica to derive energy spectrum of precipitating electrons through an intensity comparison of auroral emission lines and bands. They showed that the intensity ratio of 844.6 and 427.8 nm band is a useful indicator for an average energy of precipitating electrons. This method was applied for estimation of an average energy and a total flux for active discrete auroras associated with auroral break-up [Ono and Morishima, 1994]. Using a multicolor all-sky imaging system (MAIS) and a tilting-filter photometer at Syowa Station, Takahashi et al. [1995] investigated the dynamics of proton auroras and its relation to electron auroras that were monitored with an all-sky SIT camera. Simultaneous TV observations of pulsating auroras at three, nearly geomagnetically conjugate stations (one in Iceland and two in Antarctica) showed a distinct lack of correlation between pulsating auroras in both hemispheres for a certain period [Minatoya et al., 1995].

Motions of auroral arcs in the high-latitude morning sector (0300-0900 magnetic local time) during magnetically quiet periods was studied, which suggests that the observed duskward auroral motion is related to poleward shrinkage of the morningside oval on closed field lines [Shiokawa et al., 1995a].

Shiokawa et al. [1994] observed low-latitude auroras in Japan with a meridian scanning photometer and all-sky TV camera. These auroras were found to occur in the region of L=2 even during moderate magnetic storms. The low-latitude aurora event on September 13, 1993 showed that this aurora occurred in association with the expansion onset of an intense magnetospheric substorm [Shiokawa et al., 1995b]. Electron auroras was theoretically studied by using the Monte Carlo method, the results of which may be applicable to low latitude electron auroras [Onda and Itikawa, 1995]. Low-latitude auroras associated with magnetic storms are very rare phenomena and therefore are not well understood.

Global features of particle precipitation and field-aligned electron acceleration during isolated substorms were investigated using energetic particle data from the DMSP F6 and F7 satellites. Some dynamical features of the precipitation region of central plasma sheet type electrons were revealed [Shiokawa and Yumoto, 1993].

A comparison study was made between the electron energy spectra calculated from electron density profiles at the 90-160 km altitudes obtained from the EISCAT radar and in-situ observations by the DMSP satellites at the top of the ionosphere [Fujii et al., 1995].

G2.6. Polar Electrodynamics

Ion composition in the lower F region for the daytime in summer was obtained by correcting the EISCAT composition model with the ratio between the measured and modeled ion temperatures [Shibata et al., 1995]. The results reveal that the transition altitude between molecular and oxygen ions has a clear diurnal variation with a minimum altitude of about 187 km around 13 LT.

The characteristics of the large-scale electrodynamic parameters, field-aligned currents, electric fields, and electron precipitation, which are associated with auroral substorm events in the nighttime sector, was obtained through a unique analysis which places the ionospheric measurements of these parameters into the context of a generic substorm determined from global auroral images [Fujii et al., 1994; Hoffman et al., 1994].

Questions about a basic concept of the dayside field line merging are raised, and alternative new mechanisms were proposed on the IMF control of the magnetosphere and for the magnetosheath plasma injection into the magnetosphere [Matuura, 1995]. The EISCAT Svalbard radar is important to pursue mechanisms on the solar wind-magnetosphere interactions [Matuura, 1995].

G2.7. Modeling of Planetary Ionospheres

The Pioneer Venus observations have been thought to suggest that all the ionospheric magnetic fields of Venus are originated from the solar wind. Shinagawa et al. [1993], however, suggested that a very small but non-negligible magnetic field (1-10 nT) could be generated due to the polarization electric field in the region where ionospheric plasma is not homogeneous. The magnetic field of this origin may be detectable, when the Venus ionosphere is practically unmagnetized, i.e., when the solar wind dynamic pressure is very low for a long time. The dayside ionosphere of Venus at solar minimum was studied using a one-dimensional MHD model [Shinagawa, 1993a]. The observed electron density profiles were reproduced reasonably well, assuming that the ionosphere of Venus at solar minimum is magnetized by the solar wind. A bump or a ledge structure of the electron density profile at altitudes of 170-200 km observed by the Pioneer Venus radio occultation measurements was also reproduced under magnetized conditions. A time-dependent, two-dimensional MHD model of the solar wind interaction with the Venus ionosphere was developed [Shinagawa, 1993b]. It was found that the day-to-night horizontal plasma flow necessary for maintaining the nightside ionosphere reaches several kilometers per second and that the horizontal pressure gradient forces are sufficient to produce the observed acceleration of ionospheric plasma. In the magnetic barrier region, the calculated horizontal velocities were of the order of 10 km/s. Horizontal and vertical variations of the electron density, and the magnetic field were also reproduced reasonably well.

G3. Ionospheric Radio Propagation


Joint experiments using the Akebono satellite, the ionospheric heater and the EISCAT radar near Tromso were carried out to measure simultaneously signals from a polar electrojet antenna in the ionosphere heated at 2.5 and 4.0 kHz waves, electron density profiles and dc electric field [Kimura et al., 1994]. The results were compared with those obtained by theoretical approaches, that is, ray tracing and full wave analyses. Yagitani et al. [1994] developed a full wave technique to calculate field intensities both in the upper ionosphere and on the ground when a dipole source immersed in the lower ionosphere radiates ELF/VLF waves. The radiated wave was divided into a large number of elementary plane waves, for each of which the propagation in the horizontally stratified model consisting of the ionosphere, the free space and the ground was calculated by the full wave technique. This technique was applied to investigate the propagation characteristics of radiation from a polar electrojet antenna modulated at an ELF/VLF frequency [Kimura et al., 1995]. A new method to determine k-vector direction of a whistler mode signal observed onboard spacecraft was proposed, which only uses information of amplitudes of three wave magnetic components and two wave electric components [Sakamoto et al., 1995]. This method brings about a tremendous reduction of telemetry bandwidth but cannot determine a sense of k-vector.

Nishino et al. [1994] reported characteristics of the low-latitude VLF hiss observed at dawn at Moshiri (L=1.6) and Kagoshima (L=1.2), Japan during a severe geomagnetic storm and discussed a generation mechanism of the band-limited hiss spectra due to hot and cold plasmas in the magnetosphere. Iwata et al. [1995] compared storm-related energetic electrons detected by a satellite with both LF whistler-mode signals from a Decca navigation station in Japan measured at Birdsville, Australia and VLF/ELF emissions measured at Moshiri and Kagoshima. They found that the energetic electrons penetrating from the magnetosphere to low L-shell regions below L=2 generated the waves.

On the basis of application of both digital spectral analysis and field-analysis direction finding techniques, Ohta et al. [1994] investigated subionospheric propagation characteristics of the magnetospheric whistlers observed in South China and found that very low latitude whistlers exhibit very clear additional dispersion effects near the cutoff frequencies of the subionospheric 1st- and higher-order modes. These wave properties were explained in terms of a full wave theory in the earth-ionosphere waveguide in which realistic electron density profiles of the lower ionosphere are assumed [Hayakawa et al., 1994]. Hayakawa et al. [1995] reviewed recent activities on VLF/ELF sferics. The paper describes a new method for estimating both propagation distance and ionospheric reflection height by using a signal processing for the dispersive tails of tweek sferics near cutoff frequencies and the first attempt to apply the field-analysis direction finding to tweek sferics.

A new direction finding procedure, based on simple processing of averaged Poynting vector in time domain, for groups of nearby and distant atmospherics was developed to estimate the azimuth of a wideband ELF-VLF source, and was compared with the corresponding azimuths acquired by some other single-station techniques [Nickolaenko et al., 1994]. Baba and Hayakawa [1995] formulated a finite element method for investigating the effect of localized perturbations in the lower ionosphere on subionospheric VLF propagation and presented a few computational results showing the effectiveness of this method.

Molchanov et al. [1994] considered some propagation effects occurring when the ionosphere consists of several layers and found that a multi-layer structure can yield a new set of ELF resonance phenomena. The resonances appear in the space between the earth and layer interfaces or inside the layers and are liable to give rise to peaks or minima in the frequency dependencies of ELF electromagnetic fields for both cosmic and terrestrial sources.

G3.2. HF

Fading phenomena of standard HF waves near the sunrise are caused by an interference of two or more waves separated from original wave due to the Doppler effect in the ionosphere. Yano et al. [1993, 1994] identified experimentally four types of frequency variation of the waves in a frequency-time domain and modeled an electron density profile to produce each type.


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Baba, K., and M. Hayakawa, The effect of localized ionospheric perturbations on subionospheric VLF propagation on the basis of finite element method, Radio Sci., 30, 1511-1517 (1995)

Bowman, G. G., S. Fukao, M. Yamamoto, and K. Igarashi, MU radar recorded field-aligned irregularities in the F2 region and associated sporadic-E disturbances, J. Geomag. Geoelectr., 46, 873-889 (1994)

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