The equatorial electrojet strengths are computed by a simple technique at Indian and American sectors utilising the hourly mean values of the horizontal component (H) of the geomagnetic field at pairs of appropriately selected observatories. After testing the validity of the index to its faithful representation of the strengths, detailed comparative study of the monthly mean and seasonal behaviours of the diurnal variations at the two sectors for low solar activities are made.
The diurnal variations would permit themselves in identifying the intervals of “Counter electrojet” events separately during the “morning” and “afternoon” hours. Besides the morphological features of this special phenomenon at the two sectors, their dependence on the degree of magnetic activity and lunar control are studied and the results are reported. In the seasonal progression of the occurrence of counter electrojet, during low solar activity period, the afternoon/early evening events are confined to J-season at Indian sector whereas they are observed more frequently in the morning hours in the D-season at American sector. During high solar activity years, morning counter electrojet events are larger in number in both the sectors. Lunar influence on the occurrence frequency of the counter electrojet appears to be confined to the morning events only.

抄録全体を表示

Strengths of the equatorial electrojets are derived at Indian and American sectors employing the same procedure as in Part I, for high solar activity years. On examining the morphological features of the strengths at this activity including the counter electrojet events, a comparative study is made with these results and those obtained for low solar activity years. It is noticed that the strengths in the American sector are uniformly larger during high solar activity when compared with low activity years. Similar increase at high over low is within the theoretically derived ratio at the Indian sector. Probable reasons for this enhanced ratio at American sector are discussed.
Morning counter electrojet events are found to be larger in number in both the sectors during high solar activity years. However, afternoon counter electrojet phenomenon is confined to low solar activity years and is more prominent in the Indian sector. The simultaneous occurrence of counter electrojet intervals at both the sectors in the two phases of solar activity show that the morning simultaneous events are more during high and afternoon events more in low solar activity years. Lunar influence appears to exist in the occurrence of morning counter electrojet which maximises at the lunar ages 05 and 17 at both phases of solar activity.

抄録全体を表示

Observations of the signal strength of MF broadcasting signals (774/770kHz) transmitted from Akita, Japan, on board the Japanese antarctic ice breaker Fuji, bound from Japan to Syowa station, Antarctica, have revealed an interesting positive correlation between strengths of long distance signals propagating at night and solar activity. It is already known that MF propagation characteristics in North America show a negative correlation with solar activity. In the present paper, we have tried to interpret the results of the measurements on board Fuji by using the multi-hop method with the full wave analysis. The difference in correlation with solar activity between the results of Fuji and those in North America can be elucidated if we assume that there is a ledge in the electron density profile around an altitude range of 85 to 90km and that the density of the ledge is smaller in the North American region than in the equatorial region.

抄録全体を表示

Paleomagnetic data were obtained from the Lower-Middle Devonian Torbrook Formation (7 sites) which consists of fossiliferous shale, siltstone and quartzite and mafic intrusions (10 sites) composed mainly of gabbro sills. Some but not all the sills intrude the Torbrook Formation. Thus, some of these intrusives are of Silurian age while others are Late Devonian. The natural remanent magnetization site-mean directions are scattered but discordant from the present Earth's field for both the sedimentary rocks and the sills. Stepwise alternating field demagnetization up to 100mT and thermal treatment up to 625°C for the Torbrook Formation revealed the presence of a high unblocking temperature (400°-550°C) and an intermediate coercive force (20-50mT) direction (tilt corrected) at 342°, -10° with a corresponding paleopole at 38°N, 138°E. An additional direction (in situ) is observed at 26°, -72° with lower or overlapping unblocking temperature and coercivity spectra. This appears to be a post-folding magnetization and the corresponding ill-defined pole is 14°S, 101°E. The mafic sills yielded the following directions of magnetization with corresponding poles: a) 6 of the 10 sites gave 323°, -65°[139°E, 7°S], b) 4 sites with a component at 96°, -25°[150°W, 13°N], c) 9 sites with a component at 322°, -37°[152°E, 16°N] and with closely overlapping unblocking temperature and coercivity spectra. The age of intrusion of six sills is probably Early Silurian; their site-mean tilt corrected direction and paleopole are 320°, +17°[175°E, 38°N]. The age of intrusion of the other sills ranges from Silurian to Late Devonian. The pole positions are compared with those of the same age from Laurentia, Gondwana and the Avalon Terrane. During the Silurian period, the Meguma and Avalon zones were apparently not part of the same landmass. By Early-Middle Devonian time, both the Meguma Terrane and Avalon Zone had docked unto the North American craton. The dissimilarity in the apparent polar wander paths and paleolatitudes of Africa and the Meguma Terrane does not support a North African provenance for the Meguma Zone.

抄録全体を表示

Since the first time geomagneticists tried to derive the expression of the fluid flow at the top of the core, various assumptions have been made in order to reduce first, the non-uniqueness and second, the number of free parameters of the problem. We show that a geostrophic flow with a plane reflexion symmetry about the equatorial plane and a simple symmetry about the Earth's centre for the non-zonal component accounts for the 1980 secular variation (S.V.) although the number of parameters of the flow is reduced by a factor eight (from the number necessary when no assumption is made); the relative misfit between the predicted and the observed S.V. is of the same order as those existing between various 1980 S.V. models. The so-obtained flow exhibits a rather simple configuration. We suggest a means of downward continuing the flow into the body of the core which leads to some integral constraints on the density heterogeneity in the core.

抄録全体を表示