The mechanism for omega bands and torch structures as observed on the poleward boundary of the postmidnight diffuse aurora is studied by a two-dimensional electrostatic particle simulation for the motion of magnetospheric plasma perpendicular to the geomagnetic field. This type of diffuse aurora is caused by precipitation of energetic electrons with energies in excess of a few keV. In the model, the diffuse aurora (which may be identified with the central plasmasheet) is embedded within a less energetic but hot plasma which extends to higher latitudes. The temporal evolution of the diffuse auroral pattern can be visualized by following the cluster of the energetic electrons. Electric dipoles can grow on the poleward boundary of the hot plasma, due to charge separation induced by magnetic drifts. As a result of these dipoles, there is a spatial modulation of the energetic electrons, which leads to the formation of omega bands and the subsequent evolution into torch structures. The magnetospheric (or ionospheric) potential distribution obtained in the simulation is consistent with observed distribution of ionospheric electric fields and field-aligned currents in events of torches/omega bands.

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Significant features of a rare sequence of noon-time counter electrojet events in the Indian sector are analysed. They are compared with records from nearby electrojet stations in the east and west to show the limited longitudinal extent of the noon-time CEJ. It is also shown that quiet-time CEJ is not altogether controlled by lunar tidal oscillations.

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Concurrent with the midday maximum of horizontal geomagnetic field, H, the eastward field Y at the equatorial electrojet station Kodaikanal shows a distinct minimum around noon during any of the years of solar cycle. The sunspot cycle variations of the daily variations of ΔY are sympathetical with those of ΔH. Even the day to day fluctuations of the daily range Y are correlated with corresponding changes of H. Abnormal seasonal changes are seen in the daily variations of Y field at Kodaikanal as well as at other. observations in India. These significant daily variations of eastward field at Kodaikanal are suggested to be due to a systematic meridional current associated with the electrojet current.

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A record of the secular variation of the geomagnetic field for the last 11000 years has been obtained from the remanent magnetization of seven wide-diameter (20 cm) cores of marine and lacustrine sediment from central to south-west Japan. Magnetization directions of five cores, possessing high-amplitude variations, exhibit good correlation. Two cores show low- amplitude variations, but they can be correlated well with other records after deconvolution to remove filtering effects of the post-depositional magnetization process. A composite secular variation curve was constructed by stacking the field direction records. Time constraints were obtained from radiocarbon ages of shell and wood fractions or tephrochronology. The secular variation curve agrees well with the archeomagnetic record after 1400 yrBP, with a slight difference in the time range 1400-2000 yrBP. The secular variation for the last 11000 years shows an elongated distribution of VGP's. The azimuth of the elongation, about 40°E, is consistent with that of VGP's from Japanese volcanic and sedimentary rocks during Brunhes epoch. The angular standard deviation is 14.5° (upper limit = 15.4°, lower limit = 13.7°). This estimate is slightly larger than that observed globally during Brunhes epoch and the position of the average VGP deviates 8.5° from the geographic north pole. These VGP analyses suggest a stationary nondipole source. The secular variation records from five northern hemisphere sites between 135°E and 95°W in longitude possess a single prominent feature: an extreme easterly swing in declination. A plot of age versus longitude of the swing at each site shows a clear westward drift, at a rate of about 0.13° /yr. The field vector around the swing represents clockwise looping at all the sites. This suggests that the swing is caused by a large non-dipole source which drifts at least from 135°E to 95°W in longitude.

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The f-min data obtained at eight ionosonde stations in the Far East during the DYANA campaign in January 1990 is analyzed to investigate the latitudinal extent of the ionospheric D-region winter anomaly region. The f-min anomaly is most intensified around the geographic latitudes of 40-45°N, becomes weaker toward higher and lower latitudes, and on occasion ex- tends down to 26°N. Anomalous VLF and LF radio wave propagations strongly suggest the ex- istence of D-region electron density enhancements during the winter anomaly. We hypothesize that this anomaly is triggered by planetary wave activity in the stratosphere and mesosphere.

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