Journal of geomagnetism and geoelectricity

Morning Overshoot of T_e Enhanced by Downward Plasma Drift in the Equatorial Topside Ionosphere

K.-I. Oyama, N. Balan, S. Watanabe, T. Takahashi, F. Isoda, G. J. Bail ...

1996 Volume 48 Issue 7 Pages 959-966
Published: July 20, 1996
Released on J-STAGE: May 25, 2007

DOIhttps://doi.org/10.5636/jgg.48.959

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The morning overshoot in electron temperature, T_e, which occurs in the sunlit atmosphere of low electron density during early morning hours, is a well known phenomenon. Studies of the phenomenon carried out using the Hinotori satellite observations reveal that the overshoot enhances in the topside equatorial F-region for a short period of time in the morning. The enhancement in the overshoot is found to depend on season and solar activity; it is strong during the northern summer months and grows with the increase in solar activity. Theoretical model calculations show that the enhancement in T_e is caused by a reduction in electron density in the topside ionosphere due to a downward drift of plasma.

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Large Displacements of Conjugate Auroras in the Midnight Sector

Hirokazu Minatoya, Natsuo Sato, Thorsteinn Saemundsson, Takeo Yoshino

1996 Volume 48 Issue 7 Pages 967-975
Published: July 20, 1996
Released on J-STAGE: May 25, 2007

DOIhttps://doi.org/10.5636/jgg.48.967

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Simultaneous TV camera data were obtained at three nearly geomagnetic-conjugate stations: Husafell in Iceland, and Syowa and Asuka in Antarctica, on September 9-10 in 1991. Of the two southern stations, Syowa is more nearly conjugate to Husafell. With two stations in the Antarctica, the field of view in the southern conjugate area extends longitudinally to ∼ 1600 km at auroral heights, making it possible to investigate the spatial characteristics of auroral conjugacy. When the correlation of auroral shape and intensity between conjugate areas was examined, an interesting event was discovered in the geomagnetic recovery phase in the midnight sector. During this event, good correlation was maintained between Husafell and Asuka while no correlation existed between Husafell and Syowa, suggesting a large (500-800 km) longitudinal displacement of conjugate auroras. The cause of the longitudinal displacement is discussed.

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Palaeomagnetism of Dyke Swarms from the Deccan Volcanic Province of India

J. N. Prasad, S. K. Patil, P. D. Saraf, M. Venkateshwarlu, D. R. K. Ra ...

1996 Volume 48 Issue 7 Pages 977-991
Published: July 20, 1996
Released on J-STAGE: May 25, 2007

DOIhttps://doi.org/10.5636/jgg.48.977

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Palaeomagnetic results of 19 dykes situated south of Narmada river (21°30'N, 74°15'E) in Dhule district of the Maharashtra state, India are reported. Of the 19 dykes, 11 dykes exhibit a normal magnetic polarity, and 4 dykes a reverse polarity. The remaining 4 dykes yielded mostly scattered and unstable sample directions. The normal and reverse directions are almost antipodal. The characteristic remanence is indicated to reside in magnetite, and is most probably primary. The N-Pole position corresponding to the mean directions of 11 dykes (7 normal and 4 reversed) based on a minimum of 4 sample characteristic directions per dyke is at 37.2°N, 80.5°W (A₉₅=9.7°). This pole is concordant with the Deccan Superpole, indicating a similar age of magnetization for the Deccan basalt flows and the dykes intruding them. A joint consideration of similarity of palaeopoles of the dykes and the lava flows, magnetic polarity of dykes, and their stratigraphic positions of intrusions in the lava flow sequence support the view that the volcanic activity in the Deccan area spanned a short duration. The post-trappean tectonic activity resulting in the dyke swarms may possibly have coincided with the opening of the Arabian Sea and the rifting of the Seychelles-Mascarene oceanic plateau.

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Palaeomagnetism of the Rajmahal Traps of India: Implication to the Reversal in the Cretaceous Normal Superchron

G. V. S. POORNACHANDRA Rao, J. MALLIKHARJUNA Rao

1996 Volume 48 Issue 7 Pages 993-1000
Published: July 20, 1996
Released on J-STAGE: May 25, 2007

DOIhttps://doi.org/10.5636/jgg.48.993

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The Upper Gondwana Rajmahal Traps in northeast India were studied over the last four decades and these are referred to as normally magnetized rocks. Our study of these rocks from thirty-five sites has brought into light a palaeomagnetic record revealing the existence of a reverse magnetization event also. In the light of the ⁴⁰Ar/³⁹Ar age data of the Rajmahal Traps and the geomagnetic polarity time scale during the middle Cretaceous period, the observed magnetic reversal in the Rajmahal Traps has been interpreted to constitute a geomagnetic reversal in the Cretaceous Normal quiet magnetic interval (Cretaceous Normal Superchron, K-N). Similar short magnetic reversals have also been reported at other periods in this superchron from some other places.

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Source Effect on Geomagnetic Induction Vectors in the Fennoscandian Auroral Region

Ari Viljanen

1996 Volume 48 Issue 7 Pages 1001-1009
Published: July 20, 1996
Released on J-STAGE: May 25, 2007

DOIhttps://doi.org/10.5636/jgg.48.1001

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Geomagnetic induction vectors were calculated at the IMAGE magnetometer stations in Fennoscandia using hundreds of four-hour events of quiet time variations, and of magnetic storms due to eastward and westward electrojets. Quiet time vectors were found to be very similar to those calculated previously using a set of carefully selected plane wave events. Due to the source effect during electrojet events, the average direction of the induction vectors may rotate tens of degrees compared to the quiet time. The length generally increases tens of percent, but at the stations MAS and SOR near the electrojet and highly-conducting anomalies, a decrease is observed. If the quiet time induction vector is longer than about 0.50 then the source effect does not affect the direction of the vector remarkably even if the length may change considerably. If the conductivity anomaly is weaker (quiet time vector shorter than about 0.30) then the source effect dominates near the electrojet (PEL, MUO, KEV, KIL), but is less important farther away (NUR, OUJ). If a large number of magnetically quiet events are available of local morning hours, no special reduction of the source effect is needed in the research area.

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On the Occurrence of Geomagnetic Storms with Sudden Commencements

V. M. Silrergleit

1996 Volume 48 Issue 7 Pages 1011-1016
Published: July 20, 1996
Released on J-STAGE: May 25, 2007

DOIhttps://doi.org/10.5636/jgg.48.1011

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Geomagnetic storms with sudden commencement happened from June 1957 to December 1980 are considered. The characteristic size of the storm intensity is assumed to be represented by the absolute value of D_st index in a time scale of seven months. As this magnitude is bounded in maximum size, Gumbel's third extreme value distribution is applied to predict the occurrence of storms. According to that, modal extreme values and return periods are estimated. Events with modal extremes values of 400 nT are predicted to happen within the next (17 ± 3) years since 1980.

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