The results of statistical analysis of a new phenomenon, the Sudden Change in the Integrated Intensity of Atmospherics (SCIIA) to a high or low value occurring within a few seconds followed by a slow exponential recovery to the normal existing value within a relatively long time reckoned in minutes are reported. A number of physical parameters were defined to quantitatively describe the phenomenon and the results of measurements on the records of three selected VLF bands were statistically analysed to bring out the characteristics of the phenomenon. The results are discussed and a plausible explanation of the phenomenon of SCIIA based upon the sudden increase in ionisation at the “control points” in the lower ionosphere due to the incidence of a sufficiently large single meteor or a shower of small meteors penetrating to different depths in the ionosphere is put forward.

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Experimental conditions under which the OI line λ5577 is emitted in a low-pressure glow discharge tube containing a mixture of oxygen and xenon gas is studied. It is clear from the experimental data that the excited xenon atom acts only upon the oxygen molecule and not upon the oxygen atom. On the basis of this study, it is proposed that the emission mechanism of OI line λ5577 should be classified into two processes: A. initial process,
Xe″+O₂(x³Σ^-_g)→Xe(¹S₀)+O(³P)+O(¹S₀)
(Xe″ indicates a higher excited state of xenon) and B. main process,
Xe*(³P₂)+O₂(a¹Δ_g)→Xe(¹S₀)+O(³P)+O(¹S₀).
This emission mechanism is different from the mechanism which uses the reaction of argon with a small amount of oxygen in a discharge tube.

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The spatial, diurnal and seasonal distribution of the modulation of a particular type of Sq(H) in the equatorial regions is examined. It appears most clearly as an abnormal depression of Sq(H) in the afternoon hours, sometimes resulting in the reduction of the H component well below its nighttime values. These abnormal depressions of Sq(H) occur at about the same local time in a very narrow zone (at least ±4° dip latitude) flanking the dip equator. They occur more frequently from noon to dusk in the months around the December solstice. General considerations of their characteristics are consistent with some associations with the lunar tide. However, detailed comparison of the time of maximum depression with the time of minimum geomagnetic lunar tide is not consistent with its association with the lunar tide. Some evidence from f₀F₂ suggests that the depressions arise from modulation or reversal of the electric field at the equatorial electrojet regions of the ionosphere.

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The growth and decay of the main phase of the September 21-23, 1963 magnetic storm are studied in detail on the basis of iso-intensity contour maps of the main phase field, which were generated by a computer.
It is shown that in contrast to the rather smooth variation of the Dst index during the storm period, the development of the main phase is quite dynamic. In particular, the deepest main-phase depression in the afternoon-evening sector evolves in a complex way, waxing and waning as magnetospheric substorms occur. Eventually, a fairly uniform main-phase depression results, as the deepest depression subsides rapidly and also spreads toward the least depressed sector.
In order to interpret these changes properly in terms of the growth and decay of distant (beyond the ionosphere) currents, similar analyses should be performed for future storms, in conjunction with space observations by several geostationary satellites.

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A study of the daily variations of earth-current and geomagnetic fields at stations lying north and south of the magnetic equator in Nigeria is presented. The experimental results show that, during the period of measurements, the east-west component of earth-current variation is predominant for stations lying within about 2° from the magnetic equator. Results also show that the latitudinal variation of earth current is controlled partly by the equatorial electrojet and partly by the differences in the surface geological structures. Modified magnetotelluric relations that can explain, to some extent, the magnetotelluric variations in the equatorial region are derived and, using these relations together with experimental results, a good conducting layer is found at about a depth of 160km at Ibadan and about 60km at Zaria.

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Time-dependent behaviour of a dynamo model proposed by Lilley is numerically investigated. In order to avoid the mathematical complexity, only four types of magnetic fields, S₁, T₂, T^2c₂ and T^2s₂ say, are taken into account ignoring fields expressed by spherical harmonics of higher order. Under an initial condition that a uniform magnetic field is given to the system, growth of the magnetic fields is studied. It then turns out that a steady state is reached within a period of several thousand years provided the velocity takes on a value as determined by Lilley as an eigenvalue. When the velocity is larger or smaller than the eigen-value, growth or decay of the fields respectively takes place.
In the next place, time-dependent behaviour of the system is examined assuming that a disturbance is given to one of the magnetic fields keeping other fields at their steady state values. It seems likely that the system is most disturbed after all when a new steady state is reached by the disturbance in the S₁-type field, although the intensity of the field is weaker than that of the T₂-type field by one order of magnitude.

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Yukutake and Tachinaka have demonstrated that the geomagnetic nondipole field can be separated into two parts, standing and drifting. Assuming that these two fields are created by an interaction between a strong toroidal field prevailing in the core as a result of the dynamo action and convective motions, near-surface core motions are obtained.
It is striking that the drifting field is produced mostly by a P¹₁-type motion which represents a simple large-scale convective motion. On the other hand, two marked downflows beneath the Pacific and Indian Oceans are found for the standing field. No physical interpretation of these flows has been put forward yet.

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The time-scale of the variation in the length of the day is comparable to, or shorter than, the electromagnetic decay time, τ_η, of the mantle. This suggests that an electromagnetic theory for the origin of these fluctuations should not only depend on the electromechanical coupling time, τ_I, introduced by ROCHESTER (1960), but also on τ_η. It is shown that, when τ_I is small compared with τ_η, the fluctuation time for perturbations in the angular velocity of the mantle is of order τ_c=τ_I^2/3τ_η^1/3, and not τ_η. The possibility that τ_I is not large compared with τ_η for the case of the Earth is considered. Also, by means of a simple shellular model of the core, the rôle of Alfvén waves in modifying the coupling of core to mantle is discussed. The relationship of this theory with that of BRAGINSKII (1970) is briefly noted. A new account is given of mantle induction which, although only valid at small magnetic Reynolds numbers (based on the conductivity of the mantle), is sufficiently general to permit the coupling of the mantle to an arbitrary slow motion on the core surface to be treated.

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The possibility that VLF hiss observed at low latitudes is a consequence of waveguide mode propagation of energy from sources located in the auroral zone is examined in some detail. From the locii of the hypothetical point sources which would yield the observed power ratios for any given pair of stations, it is found that sources located in a region around the auroral zone can explain the observations at all stations. It is also shown that the accepted values of the dominant mode attenuation in the terrestrial waveguide can explain the observed hiss spectral densities at low latitudes only if we assume the presence of an extended source along the auroral zone.

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