Edited by Tadahiko Ogawa

G1. Ionospheric Techniques

Radio (radar, riometer and satellite), optical (Fabry-Perot interferometer) and rocket techniques have been used to observe plasma and winds in the ionosphere and the thermosphere. In addition to the ground-based instruments, instruments for radio sounding of the Martian atmosphere were developed.

Two-dimensional total electron contents (TECs) over Japan have been mapped with the Geographical Survey Institute (GSI) GPS network (GEONET) consisting of geodetic GPS receivers more than 900. The GEONET covers all Japan and records data with temporal resolution of 30 s. Its spatial resolutions are 0.15 degrees in both latitudinal longitudinal directions. The GEONET are continuously operated by the GSI now. A preliminary result indicates that this network is very useful for investigating ionospheric structures in detail [Saito et al., 1998c].

Ohta et al. [1998] discussed effective ionospheric heights by using a conversion technique of NNSS satellite differential Doppler frequency to TEC. The method was applied for possible six pairs of four receiving stations to show that there exists an optimum mean ionospheric height for which the rms sum of the difference among converted TECs from the six pairs takes a single minimum.

An in-situ rocket technique using aluminized plastic foils (foil chaff) was used to observe winds in the mesosphere and lower thermosphere [Murayama et al., 1996]. Finding the location of chaff ejected from two rockets provided in-situ wind velocities. The Yamagawa MF radar simultaneously observed winds at the same heights. The chaff winds are generally consistent with the MFR winds at 80-88 km altitudes. A qualitative agreement is found between fluctuations in the chaff falling speed and gravity wave-induced vertical winds.

A new middle and upper atmosphere observation activity started in September 1995 at Poker Flat, Alaska. A 38.2-MHz imaging riometer equipped with 16x16 antennas was constructed [Murayama et al., 1997]. Temporal and horizontal resolutions are 1 s and 11 km, respectively. Two Fabry-Perot interferometers, all-sky type and scanning type, were developed by Ishii et al. [1997] and began operation in 1998. Also MF radar measurements started in 1998.

Using the Syowa Station HF radar, Nishitani et al. [1997] have obtained one-month averaged diurnal patterns of radar echo region and Doppler velocity and found that the velocity patterns are consistent with previous plasma drift observations, thus suggesting that an HF radar is a very useful tool for the high latitude plasma convection study.

The plasma waves and sounder system onboard the Planet-B Mars orbiter was developed to observe the Martian ionosphere by using an RF sounder together with a plasma density probe and high frequency plasma wave detectors [Ono et al., 1998]. The system is equipped with two sets of long deployable dipole antennas with a tip-to-tip length of 52 m. The high power (600 W) transmitter of the sounder enables to measure electron density profiles. A new altimeter was developed to observe the land shape of a planet from a spacecraft [Oya et al., 1998]. This system utilizes an RF sounder observing electron density profile by selecting a transmission frequency f higher than the local cutoff frequency of the ionosphere by adding upper and lower side band modulations with a frequency p, i.e., at frequencies f+p and f-p.

G2. Ionospheric Structure and Modeling

G2.1. Ionospheric Structure

Many new findings in the mid-latitude ionospheric structures have been done by using the middle and upper atmosphere (MU) radar. Hinotori satellite data have been analyzed to study the low- and equatorial ionospheres. These observational results were compared with the Sheffield University plasmasphere ionosphere model (SUPIM). HF radars in Antarctica is being operated to explore electron density structures in the high-latitude ionosphere. VLF instruments on board a satellite generated global electron density distribution in the plasmasphere.

It is known that the observed similarity in ionospheric drifts between Arecibo-summer and Shigaraki-winter and again between Arecibo-winter and Shigaraki-summer are due to conjugate effects. Takami et al. [1998] theoretically showed that the Arecibo-summer Shigaraki-winter similarity is indeed due to conjugate effects, but that the Arecibo-winter Shigaraki-summer similarity has little to do with conjugacy.

Annual variations of the ionosphere and thermosphere studied with the MU radar showed that the seasonal anomaly in electron density exists only during daytime and at altitudes near the ionospheric peak and below [Balan et al., 1997a, 1998]. The observations also revealed the existence of equinoctial asymmetries. Model calculations show that the equinoctial asymmetries arise mainly from corresponding asymmetries in the thermosphere, with major contributions from neutral winds and a minor contribution from composition.

Otsuka et al [1998] presented the diurnal, seasonal and solar activity variations of ion (Ti) and electron (Te) temperatures at altitudes of 200-550 km by using MU radar data from 1986 to 1995. Ti and Te have similar diurnal and altitude variations and different seasonal and solar activity dependence. Analyzing MU radar data, Kawamura et al [1998] reported that the H+ profiles in the topside ionosphere for solar-minimum summer and equinox closely resemble. No change was detected in H+ between magnetically quiet and disturbed periods. The H+ proportions above the MU radar are much smaller than those above Arecibo, and much more comparable to those above Saint Santin and Millstone Hill.

Balan et al. [1997b] found that the daytime plasma fountain and its effects in the regions outside the fountain lead to the formation of an additional layer, the F3 layer, at latitudes within about plus or minus 10 degrees of the magnetic equator in each ionosphere. The maximum plasma concentration of the F3 layer, which occurs at about 550 km altitude, becomes greater than that of the F2 layer for a short period before noon when the vertical ExB drift is large. Within the F3 layer the plasma temperature decreases by as much as 100K.

Su et al. [1996] investigated the latitudinal, altitude and diurnal variations of electron temperature at equatorial anomaly latitudes in December using SUPIM and data from the Hinotori satellite. It is found that the modeled altitude variations of the temperature are in good agreement with incoherent scatter radar observations. Annual and seasonal variations in the low-latitude topside ionosphere were investigated by using Hinotori and SUPIM [Su et al., 1998]. The observations show strong seasonal variations at the solstices with the electron density at 600 km altitude being higher in the summer hemisphere, contrary to the behavior in NmF2. Model calculations confirm that the seasonal behavior is caused by transequatorial component of neutral wind from summer hemisphere to winter one.

The morning overshoot in electron temperature in the equatorial topside F-region was studied using Hinotori [Oyama et al., 1996]. The enhancement depends on season and solar activity; it is strong during the northern summer months and grows with the increase in solar activity. Model calculations show that the enhancement is caused by a reduction in electron density in the topside due to a downward drift of plasma. High electron temperatures in the equatorial anomaly detected by the Kyokko and Hinotori satellites were found to be closely associated with the ionization crests of the anomaly [Oyama et al., 1997]. The mechanism is concluded to be based on the plasma transport in the evening equatorial ionosphere resulting from the sunset electrodynamical processes [Balan et al., 1997c].

Large-scale, enhanced electron density structures (i.e., polar cap patch and auroral blob) in the high latitude F region were studied by using the Antarctic HF radars [Ogawa et al., 1998b, 1998c]. The results indicate that the polar patches drift antisunward in the polar cap with a convection speed. The electron density within the patch is not uniform but highly structured. The blobs, that have been transported from the polar cap, move sunward at the auroral latitudes and their spatial shapes are controlled by convection pattern.

Kimura et al. [1997] deduced flexible, general global distributions of the plasmaspheric electron density by using propagation characteristics of Omega signals obtained by the VLF instruments on board the Akebono satellite. The validity of the model was checked by comparing it with SUPIM. The density profile thus obtained can contribute to improve the topside electron density model of the International Reference Ionosphere (IRI).

G2.2. Ionospheric Disturbances

Research activities for ionospheric disturbances have been continued in the mid- and high-latitude regions. In the mid-latitudes MU radar observations have contributed to the atmospheric gravity wave study. An interesting topic is the start of using total electron content (TEC) data from the GPS satellite receiver network in Japan. Researches of ionospheric disturbances in the high-latitudes have been continued in the Arctic and Antarctica using radars, satellites and riometers.

By observing the ionospheric F-region simultaneously in multiple beams with the MU radar, propagation characteristics of gravity waves in the thermosphere were measured. Oliver et al. [1997] analyzed data of 58 daytime experiments during 1986-1994 and showed that the waves have a moderate preference of southward propagation. The propagation direction is shifted to southeastward during disturbed periods. On average the horizontal phase trace speeds remain near 240 m/s for all wave periods inspected (40-130 minutes). Waves on disturbed days seem to travel moderately faster on solar minimum mornings.

Saito et al. [1998a] studied electric field fluctuations with ten kilometer scale size by using data from the Freja satellite and the MU radar conjugate observations in the period from 1993 to 1995. They found that the fluctuations were embedded in the medium-scale traveling ionospheric disturbances (MSTID). Large scale modulation of the ionosphere, such as TIDs, may cause small scale modulations which grow up through ionospheric instabilities such as the Perkins instability. The evolutional process of the ionospheric structures with the Perkins instability was discussed by Saito et al. [1998b].

Saito et al. [1998c] derived two-dimensional distribution of TEC perturbations by using the GEONET TEC data (see G1 section). A case study revealed spatial structures of nighttime MSTIDs propagating southwestward with temporal evolution over Japan. Before the GEONET TEC experiment, Taylor et al. [1998a] conducted imaging of the 630 nm thermospheric nightglow in 1996 to find clear wave-like structures that propagated southwestward, which was consistent with the waves detected by the GEONET, and provided new evidence for the coupling between medium-scale gravity waves and the ionospheric F-region.

Ogawa [1998a] reviewed the atmospheric gravity wave observations in the upper mesosphere and thermosphere over Syowa Station, Antarctica. Gravity waves detected with a meteor radar were well manifested in the sodium abundance and density perturbations simultaneously measured with a sodium lidar. Statistical analysis of about 400 MSTIDs detected by the NNSS satellite beacon wave reception indicated that MSTIDs appeared quite often during geomagnetically quiet and moderately disturbed conditions and that most of them propagated from south toward the equator.

Nishino et al. [1997] showed examples of daytime cosmic radio noise absorption events in the cusp-latitude ionosphere. One case observed near local magnetic noon at Ny-Alesund displayed a small-scale absorption event of 100-200 km in extent superposed upon large-scale (at least 700 km) absorption features extending in longitude. Many of the small-scale absorption events showed quasi-periodic variations with repetition periods of 1-3 minutes. Nishino et al. [1998] also showed peculiar daytime absorption events at Ny-Alesund in which the polar cap F-region plasma patches were responsible for the absorption.

G2.3. Ionospheric Irregularities

The topic in the research of ionospheric irregularities in Japan was the SEEK (Sporadic-E Experiment over Kyushu) campaign in 1996. It was the observation campaign to investigate the mechanism for the generation of quasi-periodic (QP) radar backscatter from field-aligned irregularities (FAI) imbedded in nighttime sporadic-E (Es) layers. SEEK was designed to determine in-situ small-scale electrodynamical properties by using two sounding rockets and large-scale dynamics and electrodynamics by using ground-based radar and optical sensors. The results from the SEEK campaign were published as 14 papers in Geophysical Research Letters in 1998. Other researches of E-region FAI have been continued based on the MU radar and other radar system in Antarctica.

In the SEEK campaign two sounding rockets were successfully launched at different nights, both while intense FAI echoes were observed by a transportable radar on the ground [Fukao et al., 1998; Tsunoda et al., 1998]. Primary Es layers existed at almost the same altitude near 100 km on their upleg and downleg [Yamamoto Mas. et al., 1998]. The results suggested that the layers were horizontally stratified. Irregularities in the electron density were measured by a fixed-bias Langmuir probe and showed the existence of multiple-Es layers which were separated by 10-12 km within a wide altitude range of 90-130 km [Mori and Oyama, 1998]. Meter-scale irregularities generated in association with the multiple-Es layers were found to exist in a similar altitude range as QP echoes. The chemical release experiment revealed that the primary Es was collocated in a region of an intense vertical shear of zonal wind. Although this was consistent with the classical wind-shear theory for the formation of Es layers, an important finding was that the wind shear was intense enough to produce Kelvin-Helmholtz instability which might provide an in-situ source of km-scale atmospheric gravity waves [Larsen et al., 1998]. From two types of electric-field measurements conducted with the rockets, large DC electric fields were detected [Pfaff et al., 1998; Nakamura M. et al., 1998]. Especially the double-probe experiment found a localized polarization electric field of as large as 20 mV/m associated with narrow (few km) plasma density depletions near 125 km altitude. The electric fields were predominantly eastward, corresponding to upward motion of the plasma, and were consistent with Doppler shift of FAI echoes observed by the portable radar [Yamamoto et al., 1998]. Ground-based optical CCD imagers revealed ubiquitous atmospheric gravity waves in the mesopause region. Their characteristics were quite consistent with those of QP echoes. However, the gravity waves almost always propagated with a northward component, the propagation direction which was opposite to that expected for the QP echoes [Nakamura T. et al., 1998; Taylor et al., 1998b].

Yamamoto et al. [1997] conducted simultaneous observations of E-region FAI with the MU radar and the Frequency Agile Radar with the horizontal separation of 50 km. From comparisons between echo appearance between the radars southwestward propagation of QP structures were confirmed. More recently Ogawa et al. [1998d] made the first detailed comparison of heights of the FAI echoes with that of Es layers by using a high resolution FM-CW ionospheric sounder. They found that the production QP echoes after sunset was closely related to the oscillation of Es height, supporting a recent theory of the QP echo generation.

In Antarctica Igarashi et al. [1998] have operated a 50-MHz scanning-beam auroral radar with two sets of array antennas at Syowa Station. The system is called STARS (Syowa Station Auroral Radar System), and has been used to study E-region irregularities associated with aurora. Ogawa [1996a] briefly overviewed the auroral radar observations that had been made for 30 years at Syowa Station to study electron density irregularities in the southern high-latitude E-region. Some observational results from VHF radars were presented to discuss long-term variation of radio aurora, Doppler spectra and spatial relationship between radio and optical auroras.

G2.4. Ionospheric Dynamics

To interpret the ionospheric dynamics, ionosphere-thermosphere coupling is important. Ionospheric observations by the MU radar were analyzed in this context and thermospheric neutral winds were discussed. Modeling studies were done to see how the ionosphere is affected by the variation in thermospheric parameters including neutral winds and electrodynamics. Several theoretical simulations of the thermosphere were also made to investigate responses to magnetic storms.

Fukao et al. [1996] reviewed dynamical features of the mid-latitude ionosphere and thermosphere in the Asian sector as quantified by the MU radar and compared these with dynamics at other locations. The thermospheric wind and temperature in the Asian sector differ from those at other mid-latitude locations. They gave an evidence that global features cannot be described by a simple zonal mean.

Oliver et al. [1998] developed an improved method, the "layer wind" technique, to determine meridional neutral wind from MU radar measurements. This method integrates all wind information in the F region profile into its result. The advantages of the layer wind method is to avoid selecting a particular altitude for computing diffusion velocity.

Modeling studies were made of the mid-latitude and equatorial ionosphere. Su et al. [1997] made model calculations using the ExB drift and neutral wind velocities derived from the MU radar observations. These showed that the poleward neutral wind during the daytime and eastward electric field in the postsunset hours are important for the diurnal variations of the electron density profile. Maruyama [1996] found that the transequatorial thermospheric wind significantly modifies the equatorial ionospheric height during evening hours, which affects the accuracy of zonal electric fields derived from the time derivative of F layer virtual heights on the ionograms.

A two-dimensional, time-dependent model of the thermosphere was used to investigate thermospheric response to high-latitude energy inputs during magnetic storms. Maeda [1996] showed that the primary cause of composition changes is the horizontal and vertical advection of circulation for storms lasting longer than nine hours while it is the vertical advection of atmospheric gravity waves (AGWs) for three-hour storms. Fujiwara et al. [1996] showed that short-duration energy injection preferentially generates AGWs, while long-duration energy injection is more effective in generating a meridional circulation.

Using numerical simulations for various geomagnetic field strengths, Takeda [1996] examined how the geomagnetic main field strength affects the electrodynamics of the ionosphere. He showed that altitudes of Pedersen conductivity maxima shift upward, and that height-integrated Pedersen and Hall conductivities are enhanced when the field strength decreases.

Iyemori et al. [1996] statistically detected solar and IMF effects in mid-latitude ionospheric electric field data obtained by the MU radar. They found that the IMF-Bx component and the solar radio flux (SRF) correlate with the north-south component of the electric field at night. A strong mutual correlation between IMF-Bx and SRF suggests that the night side correlation between electric fields and IMF-Bx comes from the effect of solar activity rather than the direct influence of IMF-Bx.

G2.5. Particles and Auroras

Extensive studies have been made on the high latitude auroral ionosphere from satellite and ground-based observations. Based on analyses of data from the Akebono satellite and visible auroral images at Qaanaaq, Obara [1997] and Obara et al. [1996a, 1996b, 1996c, 1998a, 1998b] investigated dynamical features of polar cap arc and patch and their response to interplanetary magnetic field (IMF) polarity, which demonstrates an importance of the polar arc studies in understanding basic magnetospheric processes during northward IMF. Shiokawa et al. [1996a, 1997b] found that morningside sun-aligned arcs tended to move poleward repeatedly with a period of several minutes. Makita et al. [1998] also showed that postnoon auroras sometimes had a periodic latitudinal movement with periods of 120-160 s. Ayukawa et al. [1996] studied relationships between polar auroras and electron precipitation.

At lower latitudes, Shiokawa et al. [1996b, 1997a, 1998] showed that low latitude red auroras observed in Japan during magnetic storms were caused by intense electron precipitation termed "broadband electrons" around the equatorial edge of the auroral oval.

The Syowa and Iceland conjugate observations have been one of the Japanese unique observations since early 80s, which can give insight of auroral physical processes. Minatoya et al. [1996], based on TV observations, found a large-scale (500~800 km) longitudinal displacement of conjugate auroras during recovery phase and discussed its cause. Sato et al. [1998a] found clear cases of both conjugate and non-conjugate auroral arcs and studied similarity and differences between the two hemispheres in terms of spatial scale, temporal and spatial variations. They found for one case that the onset of auroral breakup was about one minute earlier at Syowa than at Husafell, suggesting that the triggering process did not occur near the equatorial plane in the magnetosphere but rather in a localized region near the ionosphere. Sato et al. [1998b] also showed a case of non-conjugate pulsating auroras, which implies that an asymmetry in the generation or precipitation mechanism is possible for which active ionospheric processes probably play an important role. The conjugacy of auroral related phenomena was also investigated by another means, multiple imaging riometer observations. The riometer observation is capable of observing spatial and temporal auroral precipitation even in the daylight, thereby providing excellent data base to cover the whole seasons in dayside high latitudes. Fujita et al. [1998] and Yamagishi et al. [1998] showed from the whole seasons imaging riometer observation that the latitude of the conjugate point showed seasonal variations by moving to higher latitude in winter and lower latitude in summer, which agree with the seasonal drift of conjugate points calculated from the Tsyganenko 1987 magnetosphere model.

Kasahara et al.[1997] studied characteristics of broad and low frequency noise observed with Akebono satellite mainly in the polar region in the frequency range below a few kHz. The noise was closely correlated with transversely accelerated ions and ion conics, and was electromagnetic below local oxygen cyclotron frequency but electrostatic at higher frequencies. It was suggested that the low frequency noise is the energy source of heating/acceleration of ions.

Onda et al. [1997, 1998] calculated a photoemission rate with the Monte Carlo method from differential electron precipitation flux and compared with observed auroral emission. They made it clear that the collision process of 1st positive band of N2 is more efficient to excite oxygen atoms from O(3P) to O(1S) than the electron impact in a wide range of electron energy observed by a sounding rocket.

Some technical progresses for auroral research have also been made. Aso et al. [1998a, 1998b] conducted auroral tomography observations from the unmanned Swedish ALIS stations and Japanese CCD TV sites. 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. Matsui and Hayashi [1997] applied an inversion method based on the entropy maximization method and the Bayesian information criterion to all-sky TV camera data in order to determine the altitude and location of auroral arcs. A low-light TV camera with an extremely bright fish-eye lens was developed for monochromatic and panchromatic optical measurements and was installed at the Amundsen-Scott South Pole Station [Okada et al., 1997; Ejiri et al., 1998].

G2.6. Polar Electrodynamics

Considerable progresses on the solar wind-magnetosphere-ionosphere-thermosphere coupling have been achieved. Although thorough researches taking account of all these regions have not been made as yet, effort has extensively been directed to researches as comprehensive as possible.

Miura [1996] simulated a sheared flow equilibrium in the magnetosphere-ionosphere coupling system and evaluated its stability against the Kelvin-Helmholtz instability within an ideal MHD. Without forcing, the unperturbed convection electric field declines exponentially with time due to the ionospheric Joule dissipation and the decay time is larger than one-half of the Alfven bounce period. Araki et al. [1997] and Yamada et al. [1997] investigated geomagnetic sudden commencements (SC) and showed that ionospheric currents play important roles in interpretation of the global distribution of amplitude of wave form of SC.

Watanabe et al. [1998], based on HF radar observations, investigated quantitatively for the first time the ionospheric signatures of distant tail neutral line just prior to substorm onsets and suggested from the relative motion of the plasma and the polar cap boundary that flow burst phenomena at the nightside polar cap boundary were attributed to reconnection processes.

By analyzing data from DE-2 satellite, Taguchi et al. [1996a, 1996b, 1998] found some controlled parameters for polar ionospheric convection during northward interplanetary magnetic field.

Based on Akebono visible-imager data, Kadokura et al. [1998] studied time evolution of and interrelationship between the auroral luminosity and ionospheric equivalent current system associated with a small pseudo-breakup like auroral brightening event, and showed that the substorm current wedge field-aligned current system, ionospheric conductivity, auroral luminosity variation and ionospheric closure pass of the substorm current should be closely related with each other.

Energy and momentum coupling among the magnetosphere, ionosphere and thermosphere has extensively been studied. Asamura and Iyemori [1995] analyzed the ionospheric equivalent current system derived from ground-based geomagnetic observations, and tried to ascertain the existence of the flywheel effect and to obtain its global pattern in the polar region. They found several characteristics in favor of the flywheel effect taking place after IMF northward turning, but the results also showed some differences with the prediction by computer simulations in the global current pattern. Fujii et al. [1998a] showed, based on EISCAT radar data, that electromagnetic energy input from the magnetosphere is provided into both Joule heating and neutral wind mechanical energy. They showed that these quantities are highly height dependent. The electromagnetic energy was usually positive but occasionally negative. Fujii et al. [1998b] studied one of the very fundamental problems; that is, how the ionospheric ion is affected by electric fields and neutral wind drag. They found that the ion motion relative to the local magnetic field line and the horizontal plane depends greatly on altitude and the magnetospheric electric field strength; in particular, under weak electric fields the ions tend to move horizontally throughout the E region and even in the F region, which must be due to neutral winds. Statistical characteristics of the E region neutral wind was extensively studied by Nozawa et al. [1997].

G2.7. Modeling of Planetary Ionospheres

Thermosphere, ionosphere, and solar wind-ionosphere interaction of planets were studied using various simulation techniques. Dynamics and structure of the ionosphere-thermosphere system, bow shock, and magnetosheath were calculated. The results were compared with observations of Venus and Mars by space probes.

Shinagawa [1996a, b] constructed a time-dependent two-dimensional model of the Venus ionosphere. Horizontal and vertical variations of the electron density and the magnetic field were reproduced reasonably well. He demonstrated that above 240 km both vertical and horizontal flows play an important role in transporting magnetic flux as well as in determining electron density profiles.

Tanaka and Murawski [1997] and Tanaka [1998] did three-dimensional MHD simulations for the ionospheres of the non-magnetized planets to study the solar wind-ionosphere interaction and obtained the global structure of the bow shock, the magnetosheath, and the ionosphere. The results were generally consistent with observations of the Venus ionosphere.

Bougher and Shinagawa [1998] studied the structure of the Mars thermosphere-ionosphere using the NCAR Mars Thermosphere General Circulation Model. They predicted significant dust-driven impacts in the lower thermosphere (100-120 km) and showed that the ionospheric peak height changes greatly with the passage of a Mars global dust storm event.

Choi et al. [1998] solved the coupled, one-dimensional electron and ion energy equations, with a combination of small steady and fluctuating horizontal magnetic fields imposed for the Mars ionosphere. They showed that solar EUV heating alone does not lead to the observed temperature profiles and that assuming reasonable heat fluxes at the top results in good agreement.

G3. Ionospheric Radio Propagation


The most important recent topics in ELF/VLF range are (1) the local perturbations in the lower ionosphere (classical Trimpi or direct heating effect by lightning discharges) detected by ELF/VLF wave scattering, and (2) ELF transient waves (Q bursts, slow tails) seeming to be associated with the cloud-to-ionosphere discharges.

An analysis was performed of the parameters of nearby lightning discharges, based on the measurement of three field components (two horizontal magnetic and one vertical electric). Spectra of the current moments of the strokes were calculated from the records, after the source distance was estimated [Shvets et al., 1997]. Detailed analysis of Trimpi phenomena revealed two different regimes in their temporal behavior, and these features were studied by using a rather simple theoretical model of VLF wave scattering from about ion and electron density perturbations [Molchanov et al., 1998].

Three field component measurements of tweeks were performed, and multi-mode analysis of the frequency-time structure of tweeks allowed us to study the polarization effects of tweeks [Shvets and Hayakawa, 1998]. ELF propagation in the Earth-ionosphere waveguide was studied, and the ELF transients (Q bursts and slow tails) were treated in a unified form [Nickolaenko and Hayakawa, 1998a, 1998b].

The modeling of ionospheric perturbations associated with particle precipitation (and/or direct lightning effect) was carried out by the finite element method, and it is suggested that the mode coupling is important in this modeling [Baba and Hayakawa, 1996].

G3.2. VHF

Ogawa et al. [1996b] observed for the first time sea surface echoes with the MU radar to show that the radar wave propagated toward the target by reflection due to intense sporadic E layer. Fundamental characteristics of the Doppler spectra obtained are well explained by a first-order theory of Bragg scatter from sea surface.


The editor thanks K. Oyama, R. Fujii, T. Maruyama and M. Yamamoto for their collaboration in preparing this review article.


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