The notation and classification of geomagnetic micropulsations have been discussed in two recent letters in this Journal [Matsushita, 1963; Jacobs et al., 1963]. At the 13th General Assembly of the International Union of Geodesy and Geophysics (IUGG) in Berkeley, California, August 1963, the question was considered in some detail by Committee 10 of the International Association of Geomagnetism and Aeronomy (IAGA). A small subcommittee, consisting of the four authors of this note, was set up to submit the recommendations which are presented below. From experimental knowledge, particularly that obtained since the IGY, it has been recognized that micropulsations can be divided into two main classes: those of a regular and mainly continuous character, and those with an irregular pattern. The first class covers the whole range of micropulsations with periods from about 0.2 to 600 sec. They can be divided into subgroups depending on their period, but it is extremely difficult to decide where the boundaries should be drawn. A purely mathematical division can be made, based perhaps on a logarithmic scale, or a division can be based on their physical and morphological properties. The second approach was adopted, and Table 1 gives the proposed classification and notation.

An approximate method of separating the effects of ionospheric currents from those of field‐aligned currents in ground magnetic perturbations observed in high latitudes is developed. The distribution of ionospheric electric fields can also be estimated. The procedure includes the following steps: (1) the calculation of the equivalent ionospheric current function on the basis of magnetic H and D component records on the earth's surface, (2) the computation of the electric potential distribution from the equivalent ionospheric current function by using a simple model of the ionospheric conductivity, (3) the derivation of ionospheric current vectors as well as electric fields and, (4) the derivation of the field‐aligned current intensity by taking the divergence of the ionospheric currents. Several examples for both quiet and disturbed conditions are utilized to demonstrate how our method is successful in estimating the intensities of the electric fields and the ionospheric and field‐aligned currents in the sense that the estimated values are in good agreement with those observed recently by radar and satellite methods. Significant portions of the H component in nightside auroral latitudes appear to result from the east‐west ionospheric currents, called the auroral electrojets, while both the north‐south ionospheric current and field‐aligned current are almost equally important in producing the D component excursions. It is found that the ionospheric and field‐aligned current distributions obtained are not very sensitive to the choice of the ionospheric conductivity model, unless the auroral enhancement is not given in an appropriate place. This indicates that even a simple conductivity distribution inferred from the distribution of the magnetic perturbations can make it possible to estimate the three‐dimensional current system with a reasonable accuracy.

A computer model is used to simulate the winds and temperature variations in the thermosphere which result from auroral region electric currents during a large isolated magnetic substorm. A disturbance propagates with a speed of 750 m/s poleward and equatorward, with an amplitude of about 200 m/s in the north-south velocity and about 100 K in the temperature at 400-km altitude. The amplitude decays relatively little before the disturbance reaches the equator. The time history of the disturbance is roughly that of a single sinusoid whose period increases with horizontal distance from the source and with decreasing altitude. East-west winds of over 400 m/s at 400-km altitude are created in the auroral region itself by the ion drag mechanism. The spatial distribution of these ion drag winds is significantly affected by momentum convection, so that a simple interpretation in terms of local ion drag forces is generally not sufficient. A residual electric field of about 5 mV/m remains after the substorm source is turned off, due to the dynamo effect of the ion drag winds. Vertical velocities up to about 40 m/s are produced inside the auroral region, primarily by the fact that the heated air is more buoyant than the air outside. Comparison of our simulation with numerous observations shows generally good agreement.

The geomagnetic solar quiet daily variation field, Sq, was studied separately for three longitudinal zones during each of three seasons using IGY data obtained at 69 stations. Equivalent external (overhead) and internal (induced within the earth) electric current systems responsible for Sq variations with respect to dip latitude were estimated by the method of spherical harmonic analysis in each zone for each season. Particularly interesting results are the large current intensity during equinoxes and the asymmetry between the northern and southern hemispheres for equinoctial months and even for the yearly average.

To determine the effects of the interplanetary magnetic field (IMF) on the electric potential as well as on ionospheric and field‐aligned currents, a recently available numerical algorithm is applied to an empirical model of high‐latitude magnetic perturbations, parameterized in terms of the By and Bz components of the IMF. The empirical model is derived from 20‐min average magnetometer data observed during summer at the chain on the west coast of Greenland and the corresponding IMF information from the HEOS 2 satellite. The calculated results reproduce fairly well overall features of the influence of the IMF on high‐latitude electric fields which have been reported on the basis of more direct measurements. This confirms the validity of the numerical method and the conductivity distribution models. In addition, our results indicate that the system of ionospheric and Birkeland currents near the polar cusp, which has been shown to depend strongly on By, exists independently of the system of region 1 and region 2 field‐aligned currents, which, on the other hand, depends strongly on Bz. The direction of the field‐aligned currents in the dayside polar cap is uniquely controlled by the sign of the By component of the IMF, namely upward currents for By > 0 in the northern polar cap and oppositely directed for By < 0. At the dayside boundary of the polar cap the DPY ionospheric Hall current is sandwiched between the polar cap field‐aligned currents and an oppositely directed field‐aligned current sheet on the equatorward side. For Bz >0 and By small the ionospheric and field‐aligned currents are localized near the dayside polar cusp, and the electric field has a dusk‐dawn component in a narrow region near magnetic local noon in agreement with reported satellite measurements. The associated distribution of field‐aligned currents consists of the region 1 current system and an additional pair of oppositely directed currents located poleward of the region 1 currents.

The dynamo theory of Sq variations is reexamined to see if it can be reconciled with recent observations of ionospheric winds and electric fields. Dynamo simulations are performed by using steady winds and both diurnal and semidiurnal tidal winds, observational and theoretical evidence being used to specify the amplitude and phase variations of the winds with altitude. We find that the first negative diurnal tidal mode, present in the upper E and lower F regions of the ionosphere, is capable of accounting for most of the Sq currents. The structure of this tide, which we call the (1, −2)* mode, is significantly influenced by the ion drag force. Lower E region winds, most notably the semidiurnal (2, 4) tidal mode, also contribute somewhat to the Sq currents. Our simulated electric fields using the combined (1, −2)* and (2, 4) modes are in fair agreement with observations; uncertainties in the ionospheric winds as well as errors in obtaining quiet day mean electric fields from the observations can easily account for the remaining discrepancies. Day‐to‐day variations in the daytime electric fields are probably related to variability of the E region winds. A simulation of electric currents and fields caused by hypothesized magnetospheric sources at quiet times shows that such sources alone cannot account for most of the observed middle‐ and low‐latitude currents and fields, at least during daytime. Clarification of remaining problems requires further observational and theoretical work.

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A study was made of the variations of the maximum electron number density in the ionospheric F2 layer during magnetic storms. Fifty‐one strong storms and 58 weak storms were studied. The data were collected during the ten‐year period 1946–1955, at 38 ionospheric stations between 60.4°N and 60.4° geomagnetic latitudes. The ionospheric stations were put into eight zones according to their geomagnetic latitudes. Storm‐time variations in the maximum electron number density (Dst) and disturbance daily variations during each six‐hour period (DS) were obtained for each of the eight zones. The Dst variation in higher middle‐latitudes was characterized by an initial short increase followed by a much larger decrease, the amplitude of the decrease being accentuated in summer. In the equatorial region, however, the phase of the variation was the opposite of that in higher latitudes. There was generally an increase after an initial short decrease, with no seasonal effect. The Dst variation at intermediate latitudes resembled that at higher latitudes in summer and that at the equatorial region in winter, with the average over all seasons being relatively flat. The diurnal component of the DS variation for each six‐hour period indicated, on the harmonic dial, a change in the clockwise sense except in the equatorial region. The maximum amplitude of the diurnal component of the mean of the DS variations showed a gradual decrease from higher toward lower latitudes, with a subsequent increase in the equatorial region. A remarkable change of the phase of the diurnal component also occurred from higher toward lower latitudes.