Characteristics of SSC amplitude at low and equatorial latitudes are studied using the data of 332 events observed simultaneously at three stations in the Indian region during the period 1958-1973. It is shown that during daytime both SSC amplitude in H and its equatorial enhancement increase with increased solar activity. During nighttime also the SSC amplitudes are found to vary with solar activity. The daytime equatorial enhancement of SSC's, which is attributed to the currents in the ionospheric E-layer, shows much sharper variation with solar activity than that shown by conductivity in that layer or the jet current intensity. SSC's which occurred during partial counter electrojet events are examined to determine the effect of change in an ambient electric field on their amplitudes. It is found that a westward ambient electric field leads to a reduction in the equatorial enhancement.

View full abstract

The altitude distribution of nitric oxide gamma band emissions in the day airglow was measured by the ultraviolet radiometer aboard a sounding rocket flown early in the morning from Thumba, India. The nitric oxide (NO) concentration could be determined for the first time in the equatorial upper atmosphere in the altitude range between 85 and 140km. The altitude distribution of NO below 105km is similar to those obtained by Meira in 1969 and by the present authors in 1973. The absolute concentration is, however, by a factor of 2 to 4 smaller than that measured in previous experiments. A maximum concentration of (2.7±0.6)×10⁷cm^-3 appears at the 105±2km altitude. The concentration drops off quickly as the altitude increases from 105 to 120km, then decreases rather slowly at higher altitudes. The small abundance of NO can be interpreted by a combined effect of the diurnal variation of odd nitrogen species and reduced solar ultraviolet fluxes producing excited atomic nitrogen N(²D). It favors again a smaller abundance of N(²D) caused by the deactivation due to atomic oxygen.

View full abstract

Geomagnetic pulsations with periods between 180 and 300sec recorded at 7 Canadian magnetic observatories during 1967 were examined to determine the relationships of occurrence, amplitude and period to latitude, time and the level of magnetic activity. Their characteristics are compared with those of long-period Pc5's (periods 300 to 600sec). Results are examined in the light of existing theoretical models. However, it has been found that these models do not explain satisfactorily the observed evening occurrence peak in the polar cap.

View full abstract

Observations of geomagnetic variations were made at fourteen temporary stations in Hokkaido, Japan, by transferring a flux-gate type variometer from one place to another every two months. From data analyses and model calculations, the following results were obtained: 1) The anomalous behaviors of geomagnetic variations for 5 minute periods are interpreted by means of the heterogeneity of electrical conductivity due to the sea and sediments. 2) It is presumed from variations of 60 minute periods that the surface of the conducting layer of the upper mantle in the southwestern part of Hokkaido is located at a depth of about 40km whereas that on the eastern side of the Ishikari Plain is at a depth of 130km. These results are not in contradiction with other geophysical data. The high heat flow area corresponds to the highly conducting region. The low P_n velocity of the seismic wave is also seen in the conducting region.

View full abstract

Variations of the past geomagnetic field in the Brunhes epoch as well as during the Matuyama-Brunhes polarity transition are investigated in detail using thin sections of a deep-sea sediment core. It is revealed that the geomagnetic field oscillates with a marked decrease in intensity for an interval of about 8, 000 years during the Matuyama-Brunhes transition. A short event with reversed polarity is found in the Brunhes epoch and is presumably correlated with the earliest Biwa event occurring at an age about 0.33 m. y. BP.

View full abstract

Forty-five oriented samples were collected from 38 different sites on northwest striking flows and gabbro sills of Lower Proterozoic (Aphebian) age in the central portion of the Labrador Trough (longitude: 67°W, latitude: 56°N). These were demagnetized in an alternating magnetic field. The decrease in NRM intensity during AF demagnetization is rapid in the range: 0.5-100Oe and relatively insignificant between 150 and 1, 100Oe. About 45% of the specimens were found to be unstable and 30% of the metalava and metagabbro specimens are characterized by the presence of a second magnetization superimposed on the first one.
The fold test was applied in all cases. After unfolding the intrusive and extrusive rocks, the main paleomagnetic pole position of the Murdoch metavolcanics and intrusives, the gabbros intrusive into the Thompson Lake formation and the gabbros intrusive into the Menihek formation combined with the metabasalts interstratified with this formation are respectively 159°E:30°N, 184°E:02°N, and 204°E:03°N. The pole position of all the different formations combined is 172°W:08°N, it is situated in central North Pacific ocean. This indicates that the Labrador Trough was near the equator at the time of extrusion and intrusion.

View full abstract

The discovery of many brief but large excursions of the paleomagnetic field in the time period 10, 000 to 30, 000 years B. P. together with the observed absence of excursions in coetaneous records from different sites led to the construction of minimum-effect models of geomagnetic excursions utilizing dipole anomaly sources close to the observations sites. We investigate here the modification of the auroral zone resulting from a minimum-effect model consisting of a main, geocentric dipole (M_e) and a radial dipole at the core-mantle boundary (M_s) representing the source of the geomagnetic anomaly. The location and size of the auroral zone is found as a function of the location, strength and direction (in or out) of the source. The auroral zone is displaced toward a positive source (M_s·M_e>0) and reduced in size; and it is displaced away from a negative source (M_s·M_e<0) and enlarged. An example based on plausible model parameters shows that the auroral zone can be displaced equatorward by 15° to 20° in certain longitudes. If excursions mark times of a reduced main dipole field, an additional equatorward displacement of perhaps 10° can be expected.

View full abstract