Instantaneous, not statistical, convection patterns in the polar ionosphere are examined for periods of the northward interplanetary magnetic field (IMF) on the basis of the equivalent ionospheric current systems obtained from data of the IMS ground magnetometer. The equivalent current system is sufficient for this purpose, as long as one knows the relationship between the equivalent current system and the electrostatic potential distribution for such relatively quiet periods when no great spatial gradients in the ionospheric conductivity are expected to be present in the polar cap. In spite of the complexity of individual patterns, the following characteristic features are found. (1) During the late recovery phase of substorms, the clockwise current vortex in the postmidnight sector decays more rapidly than the counterclockwise one in the dusk sector. (2) As the IMF B_z component increases, a current system consisting of two current vortices with a sunward convection flow between them appears in the polar cap region. (3) As the IMF B_z component decreases, the clockwise current vortex in the afternoon sector decays rapidly while the counterclockwise vortex in the noon sector is shifted toward the afternoon sector. In the meantime, a new clockwise vortex emerges in the nightside and becomes enhanced.
Thus the whole current system evolves gradually into the familiar two-cell type with an antisunward convection flow over the polar cap. In particular, it is important to note that all these changes occur before the IMF turns southward. (4) The effect of the IMF B_y dependence on the equivalent current system is also confirmed on our instantaneous data base during a prolonged B_z positive condition.

View full abstract

Fourier spectra of X, Y, and Z components for both latitudinal and longitudinal circular profiles are deduced analytically from a n*=64 spherical harmonic model of the internal geomagnetic field. Expressions and numerical results demonstrate that the spectra behave differently for different field components and different profiles. Simplified anomaly model studies indicate that, despite being much less in amplitude with respect to the spectra of the core field, the spectra with long wavelengths of crustal field are generally more significant than those with short wavelengths of the same source. Wavelength should not be used as a criterion to discuss the depth of the field sources. The crustal field may be seriously distorted by high-pass filtering, whereas truncation at the commonly accepted level of spherical harmonic model of the core field has little effect on localized patterns of the anomalies.

View full abstract

In 1984, a regional Magnetotelluric (MT) survey was carried out in the northern Tohoku district in the frequency range from 17.4kHz to 1/256Hz. MT stations were distributed along the latitude of 40°N from the Japan Sea coast to the Pacific coast. The shallow lateral inhomogeneity was evaluated from ELF-MT (8 to 20Hz) and VLF-MT (17.4kHz) survey at 35 sites, and the deep structure was investigated using ULF-MT (1/4 to 1/256Hz) survey at 7 sites.
Two-dimensional modelings with north-south strike are carried out. Analyses are done with special reference to the anisotropic behavior of phase of impedance. From the modeling with north-south strike, the following features are found. (1) For TM mode, where electric field is directed eastward, frequency characteristics and spatial dependence of impedance are primarily due to effect of oceans and shallow resistivity distribution. For 1/16Hz, coastal effect alone cannot account for the observed impedance and conductor (300ohm-m) is required beneath the eastern part of the profile whose top is located at 10km depth in the crust. (2) On the other hand, for TE mode, where electric field is directed northward, the spatial and frequency dependence of impedance requires the existence of a conductor (10ohm-m) in lower crust beneath the volcanic region whose depth ranges from 20km to 30km. This conductor is in harmony with low seismicity, low velocity, and shallow Curie isotherm depth.

View full abstract

The spatial spectrum of secular variation (SV) from 1955 to 1985 was studied using recent spherical harmonic models. The drifting part of the spectrum was decomposed into longitudinal (LD) and meridional (MD) parts. In the IGRF 1980 the mean relative magnitudes of the LD, MD and non-drifting (ND) parts were 51%, 29% and 20%, respectively. During the last 20 years there has been a slight increase in the LD and ND parts. The ND part dominates (63% in 1985) the SV of the dipole field. An interesting feature in the recent dipole SV is a rapid decrease of the LD part and an increase in the MD part. In 1965 the relative magnitudes were 2% (MD) and 11% (LD) in 1985 they were 37% and 0.2%, respectively.
A correlation was found between the jerk signal in 1970 and a change in the curvature of the LD and MD curves. The ND part did not show any change around 1970.

View full abstract