Journal of geomagnetism and geoelectricity

Study of the Return Current of the Equatorial Electrojet

C. Agodi ONWUMECHILI

1992 Volume 44 Issue 1 Pages 1-42
Published: 1992
Released on J-STAGE: September 14, 2010

DOIhttps://doi.org/10.5636/jgg.44.1

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A review of indications of the return currents of the equatorial electrojet (EEJ) and associated phenomena has been presented. The need for good fitting of the observed dip equatorial magnetic variation profiles had compelled the inclusion of westward currents on the flanks of the dip equator in practically all the cases. The three sources of the return currents: the curvature of the field lines through the ionospheric dynamo region, the divergence of east-west currents close to the magnetic dip equator, and the local neutral winds varying with height combine constructively, each peaking around 5° dip latitude, to provide for the return currents of practically all the eastward electrojet current. The return currents flow on the flanks of the dip equator from about 3° and terminate at a dip distance L₁ in the interval 7°≤L₁≤17° with a mean of L₁=10±3°, sometimes extending further but not beyond the Sq focus. At a location about 5.2±0.8° dip latitude, they reach a peak of about -27±3 per cent of the peak of the eastward equatorial electrojet. The negative correlation between the width and the intensity of the equatorial electrojet has been confirmed with data derived from physical model, indicating its origin in the westward return currents. The ionospheric current system so far detected by rockets is essentially in two layers. It is suggested that the intense lower layer including the return currents around 5° dip latitude, be associated with the equatorial electrojet; and that the weak upper layer that maintains more steady altitude extent everywhere be associated with the worldwide part of the Sq currents.

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Conductivity Structure in the South-East Pacific Inferred from the Island Effect on Tahiti Island

S. YAMAGUCHI, N. ISEZAKI, K. YASKAWA

1992 Volume 44 Issue 1 Pages 43-54
Published: 1992
Released on J-STAGE: September 14, 2010

DOIhttps://doi.org/10.5636/jgg.44.43

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The geomagnetic field variation was measured for about a month at two points on Tahiti Island in the south-east Pacific. Measurements were made with ocean bottom magnetometers which were set 1 meter under the ground. This method reduced the magnetic field noise originating from the diurnal temperature change by 1/20 of the surface value. The data indicated a typical anomaly of the geomagnetic field variation characteristic of the island effect.
The conductivity structure beneath Tahiti Island is estimated through comparison between observed response functions and expected ones from a number of numerical models. A reasonably well-constrained, four-layer model is summarized as follows; (1) The first, shallowest, layer is highly conductive at the depth of 5-10km with a conductivity of 1S/m. (2) The second layer is less conductive with a conductivity of 0.05S/m and is 40-50km thick. (3) The third layer has a characteristic high conductivity of 0.1 or 0.2S/m and is 9-10km thick at the depth of 50-60km. (4) The bottom layer is assumed to be uniform with a conductivity of 0.06S/m. We interpret that the lithosphere is composed of the first and second layers and that the asthenosphere corresponds to third layer beneath Tahiti Island. The thickness (50-60km) of the lithosphere is consistent with that postulated for a thickened plate under Tahiti Island at 70 m. y.

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Lava Block Rotation around Vertical Axis Inferred from Remanence Directions of Miocene Yoka Formation, Southwest Japan

Masanori SAKAMOTO

1992 Volume 44 Issue 1 Pages 55-63
Published: 1992
Released on J-STAGE: September 14, 2010

DOIhttps://doi.org/10.5636/jgg.44.55

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Paleomagnetic study was made on the lava flows of the Miocene Yoka Formation in the Hokutan Group, Southwest Japan. Although the stable remanence directions are consistent within the same sampling site, the site-mean directions are widely scattered and are distributed along a small circle on an equal area projection. The radius of the small circle is 45.4° and its center locates at the point of D=187.5°, I=81.0°. The shifting of the center by 9.0 degrees is concordant with northward tilting of the Yoka Formation. Average inclination value 44.6° of the Yoka Formation derived from the radius of the small circle is consistent with inclination values in Southwest Japan during 20-30Ma. These results lead to the conclusion that the distribution of remanence directions along a small circle is caused by rotations of lava blocks around a vertical axis after cooling below blocking temperature.

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