Showing papers in "Journal of Geophysical Research in 1959"

Rapid gravity computations for two‐dimensional bodies with application to the Mendocino submarine fracture zone

Abstract: Expressions are derived for the vertical and horizontal components of the gravitational attraction due to a two-dimensional body of arbitrary shape by approximating it to an n-sided polygon. These expressions are put in forms suitable for solution by a high-speed digital computer. As an example of the application of this method, the crustal section across the Mendocino fracture zone is deduced from the gravity anomalies. Assuming the crust to consist of a single homogeneous layer, overlain by water and sediment, it is found to be about three km thicker to the north of the fracture zone than to the south of it.

Hydromagnetic theory of geomagnetic storms

Abstract: A hydromagnetic theory is presented which explains the average characteristics of geomagnetic storms. The magnetic storm is caused by a sudden increase in the intensity of the solar wind. Stresses are then set up in the geomagnetic field by the solar plasma impinging upon the geomagnetic field and becoming trapped in it. These stresses, which are propagated to the earth as hydromagnetic waves, account for the observed average magnetic storm variations. The sudden commencement of the magnetic storm is due to a hydromagnetic wave generated by the impact of the solar plasma on the geomagnetic field. The initial phase of the magnetic storm, during which the magnetic field is above average intensity, is due to the increased solar wind pressure. During the initial phase, instability causes small plasma clouds to become imbedded in the magnetic field. They break up and diffuse into the magnetic field to form a belt of trapped particles from the sun (principally protons and electrons). The trapped protons set up stresses, mainly due to centrifugal force, which account for the main phase of the magnetic storm. The recovery from the main phase is attributed to the relief of the stress on the geomagnetic field by the transfer of the energy of the trapped protons to neutral hydrogen by means of ion-atom charge exchange. The correct recovery time for the magnetic storm is predicted from the measured cross section of the ion-atom charge-exchange process and the hydrogen density values around the earth deduced from the scattering of solar Lyman-α radiation.

Pressure solution and the force of crystallization: a phenomenological theory

Abstract: The phenomena of pressure solution and the force of crystallization result from removal or deposition of mineral matter in the region of contact between the mineral grains. We assume that this transport takes place by diffusion in a solution film between the grains. The rate of transport depends on the grain size, the effective normal stress between the grains, the diffusion constant in the solution film, the film thickness, and the stress coefficient of solubility. Values of these parameters required to obtain the amount of pressure solution observed in the St. Peter sandstone are reasonable. The theory predicts an increase in pressure solution with decreasing grain size. The effect of clay films between the grains is also to increase the rate of pressure solution owing to the more rapid rate of diffusion in the clay layer relative to a single solution film between clean mineral grains. This explains how pre-existing clay films in the sediment may, because of the greater rate of pressure solution, develop into stylolites. If the interstitial water is supersaturated with respect to the minerals present, precipitation in the area of contact will take place as long as the supersaturation divided by the stress coefficient of solubility is greater than the average effective normal stress between the grains. If the stress is increased above this limit, pressure solution will take place.

The measurement of thermal conductivity of deep-sea sediments by a needle-probe method

Abstract: The transient heating of a needle probe is used to measure the thermal conductivity of deep-sea sediments in 10 minutes or less. An accuracy of 3 to 4 per cent compares favorably with steady-state methods, and measurements by both methods on the same sediments show good agreement. Thermal diffusivity of deep-sea sediments is shown to be proportional to thermal conductivity, in agreement with theoretical expectations.

Ice petrofabric observations from Blue Glacier, Washington, in relation to theory and experiment

Abstract: Ice observed on the surface of Blue Glacier is classified texturally into three types: coarse bubbly ice, coarse clear ice, and fine ice. The three types occur intercalated to form the observed foliated structure of the bulk glacier ice. Petrofabric study of fine ice reveals consistently a broad maximum in the density of c-axis orientations, centered about the pole of the foliation plane. This single-maximum fabric is in some respects similar to fabrics of stressed ice from polar glaciers, and the textures of fine ice and polar ice are similar. The fine-ice layers, also resemble layers that have recently been produced by rapid shearing deformation of glacier ice in the laboratory. It is inferred that the fine-ice layers in the glacier constitute zones that are undergoing (or have recently undergone) rapid mechanical plastic flow, and that the adjacent coarse-ice layers originate by recrystallization from fine ice and are not now deforming rapidly by mechanical plastic flow (basal glide). Whether the fine-ice layers have predominately a tectonic origin or whether they originate predominately as in-fillings of snow in crevasses in the icefall is not known for certain. Coarse bubbly ice fabrics generally show more than one maximum in the density of c-axis orientations. The statistical significance of multiple-maximum fabrics is tested by a comparison of several independent fabrics from given stress situations, and it is shown that the basic four-maximum pattern is reproducible, though subject to unexplained fluctuations in orientation. The ‘diamond-shaped’ four-maximum pattern is characteristic of ice subjected to long-continued shear stress of persistent orientation, and the long axis of the ‘diamond’ is (approximately) parallel to the direction of the stress vector that acts across the persistent plane of maximum shear stress. It is inferred that the basic features of the pattern develop at some depth within the glacier, and that subsequent deformation has affected the pattern to some extent. The results of recent experimental studies of the origin of ice fabrics are in moderately good agreement with the Blue Glacier observations. Recent theoretical treatments are in sufficient disagreement to be ruled out. A new method of presenting orientation data allows statistical inferences to be drawn directly from the fabric diagrams.

A general theory of the unit hydrograph

Abstract: By the single assumption that the reservoir action in a catchment can be separated from translation, the general equation of the unit hydrograph is shown to be This is simplified by two further simple assumptions to give which can be conveniently calculated.

Turbulent spectra in a stably stratified atmosphere

Abstract: After noting the discrepancy between the predictions of turbulence theory and the empirical evidence from radio experiments, the author suggests that this may be the result of modification of the turbulent spectra by the effects of buoyancy in stably stratified layers. He points out that in such situations kinetic energy of turbulence is converted, over a wide range of scales, to potential energy of the resulting density deviations, that this potential energy is subsequently destroyed by the action of further turbulent mixing and molecular diffusion, and, finally, that the primary effect is to reduce the viscous dissipation rate significantly below that which normally would be estimated on the basis of large-scale turbulent motions. Universal forms are predicted for the kinetic energy and density fluctuation spectra, and in the buoyancy subrange (the part of the equilibrium range that reflects the anisotropy induced by the density gradient) the energy spectrum is found to be proportional to k−11/5, the density spectrum to k−7/5.

The pattern of fresh‐water flow in a coastal aquifer

Abstract: Formulas are developed for the flow pattern followed by the seaward-moving fresh ground water as it nears a beach. It is found that, under steady flow conditions, a sharply defined interface is formed between the fresh and salt water. Along the interface the pressure of the static salt water, owing to its greater density, is counterbalanced by the pressures which drive the fresh water seaward. The fresh water escapes through a gap between this interface and the shore line. An increase in the flow of fresh water widens the gap. Tidal action causes a diffusion of salt water across the interface. This salt is carried back to sea with the fresh-water flow.

Motions in the magnetosphere of the Earth

Abstract: The conditions determining the dynamical behavior of the ionized gas in the outer atmosphere of the earth are discussed. It is proposed to call this region in which the magnetic field of the earth dominates the ‘magnetosphere.’ Observations by Van Allen and others indicate that this zone reaches out to between 5 and 10 earth radii, depending on the degree of magnetic disturbance. It is shown that the existence of an insulating layer at the base of this region, namely the non-ionized atmosphere, completely changes the type of control exerted by the magnetic field, allowing a class of motions to occur freely without the need to overcome any magnetic forces. The extent to which such motions may occur is discussed, and some of the indications from airglow and magnetic observations are mentioned. The theory predicts that, at the level of the F2 layer and above, most motions will show strict symmetry between the two base points of a magnetic line of force.

Formulas for Computing the Tidal Accelerations Due to the Moon and the Sun

Abstract: A summary of formulas with which the tidal accelerations due to the moon and the sun can be computed at any given time for any point on the earth's surface, without reference to tables, is presented in this paper. These formulas are convenient for computer use.

A Hypothesis Concerning the Dynamic Balance of Fresh Water and Salt Water in a Coastal Aquifer

Abstract: The dispersion of salts produced by reciprocative motion of the salt-water front in a coastal aquifer induces a flow of salt water from the floor of the sea into the zone of diffu- sion and back to the sea. The head losses that accompany the landward flow tend to lesson the extent to which the salt water occupies the aquifer. Introduction--Published explanations of the steady-state balance between salt water and fresh water in a coastal aquifer, beginning with Badon Ghyben (1889) and Herzberg (1901), commonly assume that the salt water is static. Under this assumption the balance would be as shown in Figure 1. The depth below sea level to a point on the interface would be (Hubbert, 1940, p. 872)

A crustal section across the Puerto Rico trench

Abstract: A crustal section across the Puerto Rico Trench, from 450 km north to 250 km south of San Juan, was deduced from seismic refraction and gravity data. The result, a refinement of previous work, was made possible through more complete seismic refraction coverage and a program for high-speed electronic computation of two-dimensional gravity problems. On the basis of refraction data, the crust was divided into five layers having compressional wave velocities of 1.54, 2.1, 3.8, 5.6, and 7.0 km/sec. Densities taken from a density-velocity curve compiled by Nafe and Drake are 1.03, 2.0, 2.4, 2.7, and 3.0 gm/cc, respectively. Depths to the Mohorovicic discontinuity were computed from the gravity data using sub-crustal density of 3.4 gm/cc, which corresponds to 8.2 km/sec on the density-velocity curve. (A similar calculation was made using a sub-crustal density of 3.3 gm/cc). Depth to M under the trench is about 20 km decreasing sharply on both sides. Northwards, it reaches a minimum of about 10 km under the Outer Ridge and then deepens gradually to about 13 km beneath the southern margin of the Nares Basin. South of the trench M rises under the Puerto Rico Shelf to about 17 km and then deepens sharply to about 30 km beneath Puerto Rico. South of Puerto Rico the depth decreases again to about 14 km under the Venezuelan Basin. Depths to M were also obtained by using Airy isostatic anomalies and assuming constant crustal density of 2.67 gm/cc over a mantle of density 3.27 gm/cc. The crustal section thus deduced differs significantly from that obtained when the density structure within the crust was considered.

Systematic determination of unit hydrograph parameters

Abstract: In unit hydrograph correlations the number of degrees of freedom which it is useful to maintain in the form of the instantaneous unit hydrograph is shown to be limited by the number of significant independent correlations with the catchment characteristics. The moments of the instantaneous unit hydrograph are suggested as a series of parameters of the response, for which correlations should be sought. A simple method of obtaining these moments is evolved, and a method of choosing, in any given case, between several two-parameter forms is demonstrated.

Calculations on the thermal history of the Earth

Abstract: The possible thermal history of a spherically symmetric earth is studied by comparing numerical calculations of the development of temperature within a number of model earths with observations on the present thermal state of the earth. The pertinent observations are the following: (1) The average surface heat flow is 50 ergs/cm2 sec. (2) The mantle of the earth is solid. (3) The electrical conductivity increases rapidly in the outer few hundred kilometers of the earth. It is shown that models which incorporate a wide range of initial conditions and distributions of radioactivity reproduce in a qualitative fashion the electrical conductivity of the earth. Models which have a radioactive content equal to an earth composed of chondritic meteorites reproduce the present surface heat flow to within a factor of 2. This is a consequence of the near coincidence of the present rate of heat production in a chondritic earth and the heat flux of the actual earth. The variability in heat flow among models is due to varying contributions of ‘initial heat,’ higher rate of radioactive heat production in the past, and the depth of burial of heat sources. The solid nature of the mantle provides a further qualitative restraint on possible earth models. However, a lack of experimental data on melting relations at high pressures precludes detailed conclusions. The general features of temperature-depth curves are similar for many models in which energy is transmitted by radiation as well as by ordinary lattice conduction. The gradient of temperature is high near the surface but decreases within the earth as the effective conductivity increases with increasing temperature. The melting temperature is most closely approached or exceeded in the outer few hundred kilometers of the earth. The variation of surface heat flow with time is examined. It is shown that the surface heat flow is constant in time for a wide range of models, provided that heat is transmitted solely by radiation and conduction.

Crustal structure from gravity and seismic measurements

Abstract: Gravity data indicate that there is a regular relationship between crustal structure, crustal density (composition), and surface elevation. Earthquake and surface seismic refraction and reflection evidence as to the composition and structure of the earth's crust have not yielded a simple, unambiguous relationship to the surface elevation. The velocity dispersion of earthquake surface waves, on the other hand, indicates variations in the thickness and composition of the crust that are in general accord with the variations in surface elevation and the Bouguer gravity anomalies. Why seismic refraction measurements have not agreed everywhere with gravity and surface wave indications of crustal structure appears to be a result of masking of crustal layering. On the basis of the slope of the curve that describes the relationship between the seismic depth to the Mohorovicic discontinuity and Bouguer gravity anomalies, the density difference between the crust and the mantle appears to decrease as the thickness of the crust increases. On the assumption that the mantle has a constant mean density of 3.32 gm/cc, the mean crustal density would appear to increase from a minimum value of 2.86 gm/cc in the oceans to about 3.08 gm/cc beneath the high plateaus and mountains. If the mean crustal density is essentially constant, the effective density of the mantle must decrease by a comparable amount. The existence of a low-density zone in the upper part of the mantle, as suggested by the velocity dispersion of very long period Rayleigh waves, would explain the relationships observed. Isostatic relationships suggest that the mean density of the continental crust is essentially constant (2.85 gm/cc to 2.88 gm/cc). These values imply that a basaltic layer is present everywhere. That there is possibly an increase in mean crustal density as the crust thickens is suggested by U.S.S.R. seismic studies in Central Asia. These show that the intermediate (basaltic) layer is usually thicker beneath areas of uplift. Although the origin of the basaltic layer can only be surmised, its general inhomogeneity, as indicated by variations of seismic velocity from 6.4 to 7.3 km/sec, and its varying thickness suggest that it may be a zone of phase transformation within the underlying mantle rock. Despite the lack of homogencity in the crust, it appears possible that empirical relationships may be used to predict approximate crustal thickness from the regional Bouguer gravity anomalies or from surface elevations with a reliability approaching that for seismic measurements.

A theory of electrostatic fields in a horizontally stratified ionosphere subject to a vertical magnetic field

Abstract: A theory is developed to describe quantitatively the idea that in an ionized gas subject to an imposed magnetic field, such as the ionosphere, the lines of magnetic flux are approximately equipotential lines. The ionosphere is assumed to be horizontally stratified, and the case in which the earth's magnetic field is vertical is considered. Small-scale electrostatic fields are studied with a view towards elucidating the phenomena of spread F and radio star scintillation. The analysis indicates that in the ionosphere the results are strongly affected by the variation of conductivity with height, as well as by the anisotropy. For a reasonable model of the ionosphere it is shown that it is possible, under certain conditions, for a horizontal field three kilometers or larger in extent, at a height of about 120 or more kilometers, to produce a similar, localized electric field in the F region, not appreciably reduced in strength. The height of the source is the most important factor, but the temperature and ionization-density profiles are also significant. The fact that the strength of the small-scale fields in the F region could vary by one or two powers of 10 for plausible diurnal variations of the ionospheric parameters suggests that these fields could perhaps be responsible for the puzzling diurnal behavior of spread F and radio star scintillation.

Gamma‐ray burst from a solar flare

Abstract: A burst of high-energy radiation coincident with a solar flare has been detected during a balloon flight at 10 gm/cm2 atmosphere depth and 30° geomagnetic latitude over Cuba. The flare occurred at 1305 UT on March 20, 1958 and was associated with solar radio bursts on 1,500 and 10,000 Mc/s. Terrestrial effects included a SID, earth-current disturbances, and a magnetic crotchet. The 18-second burst was detected with an integrating ionization chamber and a single Geiger counter. From these two instruments and their ratio, it is inferred that the radiation is due to a gamma-ray flux of about 2×10−5 ergs/sec cm2 peaked in the 200 to 500-Kv region. This radiation can be interpreted as bremsstrahlung produced in the solar photosphere from electrons of 0.5 to 1 Mev energy. These same electrons, spiraling in a 1000-gauss field in the flare region, can produce the observed radio burst by betatron radiation. The high-energy electrons represent about 1 per cent of the flare energy. Only about 0.01 per cent of the emitted betatron radiation escaped from the flare region toward the earth.

Plasma and Magnetic Fields in the Solar System

Abstract: I am planning to talk about the interplanetary gas in the inner part of the solar system, in which we live. I should like to discuss the circumstances that we shall find when we send up suitable instruments to investigate that gas. The subject is the counterpart to meteorology on the earth; we shall be concerned with the permanent and the variable features in the gaseous content of the inner part of the solar system. We first ask, what will be the experiments equivalent to the meteorological measurements of temperature, pressure, and wind? What are the quantities we should be interested in, and what are the orders of magnitude that we now expect we shall find?

A study of the morphology of ionospheric storms

Abstract: 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.

Radiation observations with satellite 1958 ε

Abstract: A preliminary account is given of the radiation observations made with Satellite 1958e. The earlier discovery of the great radiation belt around the earth with Satellites 1958α and 1958γ has been confirmed and greatly extended with an apparatus of much greater dynamic range and discrimination. It appears likely that many important geophysical phenomena are intimately related to the reservoir of charged particles found to be trapped in the outer reaches of the earth's magnetic field.

The geomagnetically trapped corpuscular radiation

Abstract: Introduction—One of the most interesting geophysical discoveries of recent years was that made with the early United States satellites Explorer I (satellite 1958α) and Explorer III (satellite 1958γ). It was found [Van Allen, 1958; Van Allen and others, 1958] that an immense region around the earth is occupied by a very high intensity of charged particles (protons and electrons), temporarily trapped in the geomagnetic field. The detailed study of this radiation has been a major endeavor of the past year and a half by a group at the State University of Iowa in the United States, and by IGY workers in the Soviet Union. Important additional information at relatively low altitudes has been obtained in rocket experiments flown by other workers.

Particle coatings affecting the wettability of soils

Abstract: The fundamental principles underlying the process of wetting show that a reduction in the surface tension of a solid substance to be wetted reduces the wettability. Conversely, a reduction in the surface tension of the applied liquid increases the wettability. The wetting of a soil, which exhibits water-repellency in an air-dry state, can sometimes be achieved by leaving water in contact with this soil. There is some evidence that this wetting is achieved by an interaction of the solid and the liquid phase, leading to a reduction in the surface tension of the liquid. The phenomenon of advancing and receding angle of contact had led to an hypothesis that the angle of contact changes as the soil wets up or dries out. Some evidence is presented which does not support such an assumption, as the angle of contact seems fixed at a moisture content just above air-dry. There is some evidence that under certain conditions an improvement in the base status of the soil may lead to improved wettability. Particle coatings by hydrophobic films are in some cases responsible for the observed water-repellency. There is some evidence that this film is strongly adsorbed. No quantitative relationship could be detected between the characteristics of substances extracted by chloroform from a problem soil and the degree of unwettability of such a soil.

Salt intrusion into fresh-water aquifers

Abstract: In a coastal aquifer a steady flow of fresh water toward the sea can limit the encroachment of the salt water into the aquifer. This action is treated on the assumptions that the flow is steady and two-dimensional, that the salt and fresh water are immiscible, and that there is no fingering. Theoretical equations for the shape and location of the interface and for the boundary velocities are derived for several sets of boundary conditions. The uncertainty of the location of the interface is circumvented by use of a hodograph plane. In addition, a complex potential is employed and related to the hodograph by conformal mapping. Certain boundary conditions represent inversions of gravity seepage through dams for which solutions already exist. Numerical computations are also presented for a semi-infinite aquifer having a vertical seepage face and one having a horizontal seepage face.

Trend surface analysis of contour-type maps with irregular control-point spacing

Abstract: Trend surface analysis is a procedure for separating the relatively large-scale systematic changes in mapped data from essentially non-systematic small-scale variations due to local effects. The method can be applied to any contour-type map, and has been used for analysis of gravity maps, isopach maps, facies maps, and maps of igneous and sedimentary rock attributes. When observations can be collected on a rectangular grid, orthogonal polynomial analysis permits convenient separation of the trend from the residuals. When observations are limited in number or are irregularly distributed over the map, non-orthogonal polynomial analysis can be used to determine at least the linear and quadratic components of the trend. These surfaces and deviations from them have value in geological interpretation, and they suggest that even when the complete trend is known, maps of selected trend components may be useful for examining special problems. Polynomial analysis of maps is facilitated by use of high speed computers. Organization of an IBM 650 program is outlined in this paper.

Recent seasonal interactions between north Pacific waters and the overlying atmospheric circulation

Abstract: The recent pronounced anomalous warming of surface waters of the eastern North Pacific is related to prevailing abnormalities in the overlying atmospheric circulation. The gross features of the abnormalities in water temperature patterns during the cold season appear to be caused by abnormal wind components. The data suggest that a preponderant surface drift of water masses from warmer (or colder) sources is induced, and this may be augmented by divergence and upwelling factors. The summer surface-water temperature patterns suggest, in addition to surface advection, the possibility of heating through enhanced insolation combined with light wind. An attempt is made to ascribe long-period continuity to the atmospheric mean seasonal circulations from summer 1957 through spring 1958 and to hypothesize that this continuity is the result of a certain interplay (involving feedback) between ocean and atmosphere against the slowly changing climatological background.

The origin of thermoremanent magnetization

Abstract: Thermoremanent magnetization (trm) generally has several components characterized by a range of coercive force. The component of trm which has the highest stability with respect to ac-demagnetization is believed to reside in stressed regions surrounding dislocations. Because of their size and shape, these regions behave much as single-domain particles.

Primary and secondary objects

Abstract: In considering our space program, it is well to study objects that arrive from extraterrestrial sources quite without any effort or expense on our part. It is my purpose to discuss recent and older observations of meteorites and to draw some tentative conclusions from them. Meteorites are objects of variable structure and chemical composition, and a complete review of their properties is impossible in a brief time; therefore, this discussion will be limited to some specific features from which certain conclusions will be drawn. I believe that these conclusions will not be contrary to evidence that is not reviewed.

Statistical and harmonic analysis of gravity

Abstract: Markov theory is developed in terms of two correlated functions, the free-air gravity anomaly and the elevation of the topography. The Markov methods are applied to the mean anomalies of 1°×1° blocks to extrapolate from all available observations to obtain estimates of mean anomalies of 10°×10° blocks world-wide. These estimates are adjusted so that the even-degree zonal harmonics are consistent with the precession of the node of satellite 1958β and so that the inadmissible first-and second-degree harmonics are absent. Spherical harmonic coefficients up to the eighth degree (P8,8 terms) for free-air gravity are computed. An independent autocorrelation analysis is made in order to estimate the variance of mean anomalies of blocks and the variance of each degree of the spherical harmonics. This analysis is utilized as a control on the error variances and covariances of the mean anomaly estimates made by the Markov method. The results are used in conjunction with the zonal harmonics derived from satellite motions to obtain a best estimate of the exterior potential in spherical harmonics from terrestrial gravimetry up to June 1958 and satellite data up to December 1958. It is planned to revise this estimate periodically as new observational data become available.

Low-energy cosmic-ray events associated with solar flares

Abstract: As a result of the IGY riometer program, it has been found that the measurement of ionospheric absorption in arctic regions is a sensitive method of detecting low-energy cosmic rays associated with solar flares. The normal morphology of these events is described, and details are given of the 24 such events that have been detected in the period from May 1957 through July 1959. Two features have been noted: an apparent asymmetry in the distribution of cosmic-ray-producing flares across the solar disk; a pronounced degree of uniformity in the distribution of the radio-wave absorption over the terrestrial polar cap. These features are discussed, and tentative explanations are suggested.

The diurnal development of the anomalous equatorial belt in the F2 region of the ionosphere

Abstract: The latitudinal variation in the critical frequency of the F2 layer was studied for each hour of the day during the equinoctial months of a year at sunspot minimum. The middle latitude maxima first develop at low latitudes and shift poleward with the progress of the day, the course being reversed in the evening hours. The double maxima in the diurnal variation of ƒoF2 are less separated with increasing latitude and finally converge to a single maximum at a dip of about 25°. These two anomalies in ƒoF2 are suggested as being due to the vertical drift of ionization, together with its motion towards the poles in the morning and towards the equator in the afternoon. Other anomalies of F2 can also be explained by a meridional transport of ionization.