Showing papers in "Journal of Geophysical Research in 1953"

Changes of ferromagnetic minerals and their bearing on magnetic properties of rocks

Abstract: Physical and chemical properties of samples from a group of pre-Cambrian diabase dikes characterized by an inverse magnetization have been determined. There are present two different ferromagnetic minerals intimately intermingled with one another and with non-magnetic ilmenite. Experiments to duplicate the original thermomagnetic behavior of these rocks cannot be carried out because by heating, non-reversible changes of physical structure are produced which alter the magnetic properties of the sample. Magnetization experiments show that the inverse magnetization is produced by particles having high coercive force; in natural samples a large part of the ferromagnetic material is demagnetized but can be easily magnetized. The hypothesis is advanced that the inverse magnetizations of these dikes having uniform magnetization could have been produced with the earth's magnetic field in its present-day sense, as a result of the partial low-temperature oxidation of primary magnetite forming small particles of high coercive force in the reversed induction field adjacent to large magnetite particles and the subsequent demagnetization of some of the larger particles. Various implications of the hypothesis are pointed out. The gross scattering of directions of magnetization observed in two dikes are proved not to have been caused by turbulent movements below the Curie point; the scatter is attributed to chemical changes, but no detailed mechanism for producing the scatter is proposed.

On variations of the geomagnetic field, fluid motions, and the rate of the Earth's rotation

Abstract: Estimated time-fluctuations in the rate of westward drift of the geomagnetic field, 1885–1945, are shown to correspond reasonably well with the unexplained systematic fluctuations in the earth's rate of rotation derived by Brouwer. In view of other geophysical evidence on the rigidity of the earth, it appears reasonable to conclude (1) that the source of the geomagnetic field lies within a large-scale fluid circulation inside the central core of the earth; and (2) that this fluid circulation in the core must be considered established as real, since no other adequate large source needed to conserve angular momentum is apparently available. As an independent check, a smaller time-fluctuation derived from the northerly drift of the geomagnetic field may afford, through the associated fluid motion, an explanation of the non-seasonal departures observed in the variation of latitude, yielding an estimated angular momentum transverse to the axis of rotation.

Propagation measurements in the ionosphere with the aid of rockets

Abstract: Daytime measurements of electron density, ion density, electron collision frequency, and earth's magnetic field in the ionosphere were made during V-2 rocket flights at the White Sands Proving Ground, New Mexico. Two CW harmonically related frequencies were radiated from the rocket to two ground stations to obtain measurements of the ordinary and extraordinary indices of refraction in the region around the rocket. The results for one flight show an ion layer with a maximum of 5×108 ions/cc and a small electron layer with a maximum of 7,500 el/cc just below the E1-layer. On a September day the E1-layer remained dense up to the E2-layer, while on a January day the density apparently decreased above the E1-layer maximum to much lower values. It is shown that the Lorentz polarization term should not be used in the E-layer at 4 Mc.

Radio reflections from aurora

Abstract: This paper outlines observations and experiences associated with a continuing investigation of radio echoes from aurora at Saskatoon, Canada. For the three frequencies investigated—3000, 106, and 56 Mc/s—echoes have been observed with the last two. Echoes occur when the auroral forms exhibit some ray structure and, then, only from parts of the aurora at elevations less than 15° above the horizon. Using range-amplitude and range-azimuth displays of echoes in combination with the corresponding photographs of the aurora, it is shown that the echoes originate from levels close to the lower edge of the aurora. Height distributions of the centers from which the echoes come agree closely with height distributions found from parallactic photographs of aurora in other regions. Using aerial arrays for the 106- and 56-Mc/s frequencies that are mounted on a rotatable tower so that each beam scans simultaneously the same part of the sky, the 106-Mc/s echoes are found to occur most frequently within the auroral zone and the 56-Mc/s echoes some distance to the south. Relative frequencies and diurnal variations of the two groups of echoes, as well as their heights and geographical distributions, indicate that echoes originate from a threshold process. It is suggested that the echoes arise by critical reflection from centers of high electron density (1.4× 08 and 4×107 electrons per cm3, respectively) that are formed close to but above the levels where the electron collision frequency exceeds the gyro-frequency of the electrons.

The fields of an oscillating magnetic dipole immersed in a semi‐infinite conducting medium

Abstract: The fields of an oscillating magnetic dipole immersed in a semi-infinite conducting medium are investigated. The axis of the dipole is taken to be parallel to the interface between the conducting medium and the semi-infinite insulating space above it. Various special cases are discussed in detail and are illustrated by graphs.

The fields of an electric dipole in a semi‐infinite conducting medium

Abstract: The behaviour of an electric dipole situated above a semi-infinite conductor is investigated. Expressions for the electric and magnetic fields within the conductor are derived for the case that significant distances are much less than a free space wave-length. Curves are plotted which show the dependence on conductivity, frequency, and position of the observer. The application of the results to propagation in sea-water is indicated.

Correlation of magnetic, auroral, and ionospheric variations at Saskatoon

Abstract: An analysis has been made for the five-month period from December 1951 to April 1952 of the variations at Saskatoon of the horizontal component (H) of the earth's magnetic field, the position in the sky and intensity of auroral light, and of critical frequencies and heights of the ionospheric reflecting regions. There is a relationship between the maximum elevation above the northern horizon of auroral light and the maximum amplitude of variation of H. Some types of sporadic E reflecting layers appear more frequently during disturbances. Detailed analysis of magnetically disturbed nights shows that magnetic bays and certain other phenomena are correlated. An increase in the intensity of aurora is related to the rate of decrease of H in the bay. Radio wave absorption or weak reflections at levels below 100 km correspond to the periods when H is of the order of 500 gammas or more from its normal value.

A series of stratospheric temperature profiles obtained with the searchlight technique

Abstract: Measurements of atmospheric densities have been made with the searchlight technique, using improved instrumentation. The density distributions are sufficiently accurate so that corresponding temperature profiles are readily obtained. The density-temperature equation requires the evaluation of an integration constant, which is accomplished by using radiosonde information and, in addition, considering physically acceptable temperature lapse rates. Five temperature profiles are obtained which represent stratospheric conditions over New Mexico during October 1952. The average height of the temperature maximum is 53 km and the mean value of the temperature maximum is 313°K. The average temperatures are calculated to 67.6 km.

Dynamic probe measurements in the ionosphere

Abstract: Preliminary rather successful attempts to determine the ionization in the E-layer by means of a probe technique are described. The probe current showed an extremely rapid rise between 90 and 105 km altitude. The result indicates a positive-ion density about ten times larger than the electron density. Further measurements with improved equipment are recommended.

Radio measurements and auroral electron densities

Abstract: Radar echoes from aurora have been recorded simultaneously at frequencies of 56 and 106.5 Mc/sec. The ratio of the echo amplitudes at the two frequencies varies between wide limits. An attempt is made to reconcile the experimental evidence with (i) partial reflections from the surfaces of large ionized regions in the aurora, (ii) scattering from inhomogeneities in the auroral ionization, and (iii) critical reflections from small volumes of intense ionization. It is concluded that (iii) is responsible for the typical auroral echoes which are observed at Saskatoon, although (ii) may be responsible for echoes of small amplitude which are often observed in the early stages of an auroral display. If these conclusions are correct, the occasional existence is indicated of electron densities of 108 per cm3 in small volumes of the aurora.

Extended‐range radio transmission by oblique reflection from meteoric ionization

Abstract: It has been found that radio communication between relatively low-power stations operating at 14 Mc and separated by distances of roughly 1,200 km may be maintained at times when no layer transmission to any point on the earth's surface can be demonstrated to be present. The signal obtained is subject to considerable fading, but some signal is nearly always detectable. The contribution of overlapping oblique-incidence meteor reflections to the observed signal is considered in the light of some preliminary theoretical and experimental findings. It is clearly important to assess the meteoric contribution with care, since the possibility that meteoric reflections alone could account for the signal does not seem unreasonable. Suggestions for further investigation are given.

Elasticity of olivine and constitution of the Earth's mantle

Abstract: The object of this paper is to show that Birch's recent suggestions on the occurrence of phase changes in the upper mantle are based on a pressure-density relation that is not likely to be applicable to silicate minerals, and are inconsistent with the assumption that the depth-velocity curve is continuous. It is suggested that the pressure coefficient of incompressibility (dK/dP) of a homogeneous phase may increase at low pressures and go through a maximum. Bridgman's experimental data on quartz and olivine suggest that this is so; yet the accuracy of his measurements is not sufficient to warrant any definite conclusion. There is still no way of extrapolating to zero pressure the elastic properties of the lower mantle.

On the Production of Glow Discharges in the Ionosphere by Winds

Abstract: It is suggested that a source of excitation of the airglow and aurora is to be found in potential differences generated by zonal ionospheric winds cutting the earth's magnetic field. A return path of current flow (glow discharge) in the higher ionosphere appears plausible. The principal effect in this process arises from the cutting of the vertical component of the field by broad horizontal winds. An auroral, zone results as the region where the product of wind velocity and vertical field intensity has a maximum. From this point of view, the aurora and the airglow (at least a portion of it) are manifestations of the same thing. The aurora occurs under high excitation when the winds are strong in high latitudes, while this portion of the airglow is always present in some measure in most latitudes, arising from lower velocity winds. Patterns in the large-scale air circulation should lead to patterns in the intensity of the emission, and the regular diurnal disturbance of these upper atmosphere zonal winds should lead to diurnal features in the aurora and airglow.

Orientations of auroral displays in west‐central Canada

Abstract: Orientations of auroral arcs relative to the geomagnetic meridians of the centered dipole have been determined from 628 single-station photographs taken during 1949, 1950, and 1951 at Saskatoon, Canada. A statistical analysis of the orientations brings out the reality of a seasonal variation in their direction. After this seasonal variation is taken into account, latitudinal and longitudinal variations of direction disappear and the existence of a diurnal variation of direction becomes at least doubtful. The angle (measured from south around by west) between the westward extension of an auroral arc and the geomagnetic meridian is at a minimum during October and November, and again during April and May.

Some regularities of the ionospheric F region

Abstract: It has been shown recently by Ratcliffe that, although the behaviour of the critical frequency (and hence the maximum ionization density) of the F2 region is very irregular, that of its total ion content (in a column of unit cross-section) is much less so. However, the regularities observed by Ratcliffe are confined only to a few stations and disappear when the F1-F2 bifurcation is large. It is shown in the present paper that if, instead of the total ion content of the F2 region, that of the F1 cum F2 region (nT) is considered as a whole, then the regularities become much more marked. Further, the regularities are found to persist for all the three stations considered—Slough (northern hemisphere), Falkland Islands (southern hemisphere), and Singapore (equatorial region)—even when the bifurcation is large. Certain peaks, however, are found to occur on the otherwise smooth variations of the monthly mean values of nT. These are explained as due to tidal effects. The paper also shows that the observed regularities of the composite F region are in conformity with the recent hypothesis that F1 and F2 belong really to one ionized region, being produced by a common ionizing radiation from the sun.

Irregularities in the ionosphere

Abstract: Electron clouds, both in the E and F regions of the ionosphere, have been detected at vertical incidence by means of a high-precision ionosphere sounding equipment. The clouds give rise to scattered echoes, which are delineated on an expanded sweep as individual pips, resolved out from the normal echoes. An estimate of the received power by back-scattering, made on the basis of the theory of tropospheric radio scattering by Booker and Gordon, shows that it is far in excess of the receiver noise power. Some of the records indicate that sometimes at night the normal layer structure of the ionosphere ceases to hold; the ionospheric regions then contain only electron clouds at different heights. The scattered echoes generally appear for a very short duration. The short persistence (1 to 2 minutes) of these echoes has been explained by considering the drift of the clouds by the upper air winds.

Magneto-Ionic Multiple Splitting Determined with the Method of Phase Integration

Abstract: The method used is a first-order W.K.B. approximation with an integration path in the complex height plane. The refractive index versus height is represented on a Riemann surface with four corresponding to upgoing and downcoming waves and ordinary and extraordinary types of polarization. The branch points connecting the streets are characterized as reflection-points or ascoupling points. Suitable paths of integration in the Riemann surface determine the reflection coefficient for the various magneto-ionic components. Five possible fundamental components have been found, and an infinite number of additional components due mainly to multiple reflections between the different reflection points. Numerical computations have been carried out for ionospheric model with a Chapman distribution of electrons and an exponential decrease of collision frequency located in a moderate magnetic latitude. The results are represented in a sweep frequency picture of virtual height and absorption for five fundamental modes of propagation.

Méthode de détermination des hauteurs vraies des couches de l'ionosphère, compte‐tenu de l'action du champ magnétique terrestre: I. Utilisation d'une expression approchée de l'indice de réfraction (cas du rayon ordinaire

Abstract: Using an approximate formula for the refractive index of an ionized medium influenced by the earth's magnetic field, we reach new relations for the characteristic parameters of an ionospheric region when the ionization follows the parabolic law. Then we compare our results with those that can be reached according to Appleton's method. So doing, we obtain an important corrective term for the value of the layer's thickness. That corrective value shows good agreement with the correction indicated by Shinn and Whale. The mathematical terms representing action of the earth's magnetic field always act to make smaller the value of the thickness evaluated according to classical methods (Appleton, Pekeris, Rydbeck). These calculations show that results of interest are found using approximate relations other than the ones that are in general use (quasi-transverse, quasi-longitudinal).

On the ionization produced by gamma radiation from the ground and from the atmosphere

Abstract: Two new methods for quick determination of the ionization produced by the gamma rays from the radioactive substances in the ground are discussed. The first (“absorption method”) utilizes partial screening of a portable ionization meter with a lead shield of one centimeter from the bottom and from the sides, and empirical determination of its absorbing power. This method was tried out in field experiments and its results agree very well with the results obtained with the conventional method (alternative measurements over soil and over water). The second method (“well method”) consists in placing a cylindrical ionization chamber inside an iron housing with a wall 10 cm thick, but open at the top and on the bottom. The chamber is used in two positions inside this “iron well” and from the difference of ionization observed at these two positions the total value of the terrestrial gamma radiation can be derived. Both methods can also be used for determining the gamma radiation coming from the atmosphere (“air radiation”); however, the smallness of this effect makes it difficult to get reliable results. Discrepancies arising in this case are discussed.

On a self‐exciting process in magneto‐hydrodynamics

Abstract: In connection with the convection-current model proposed by E. C. Bullard, the problem here considered is whether the magneto-hydrodynamical equations have a solution representing the self-exciting process which is appropriate to the model. In §2, the general expressions for the magneto-hydrodynamical couplings are obtained. In §3, using the results in §2, the problem is reduced to solve (3.6) under the boundary conditions (3.3). This is the eiger-value problem for 4πκaV (κ is the electric conductivity, α the radius of the earth's core, and V the fluid velocity). This eigen-value problem is solved in an approximate way. In §4 and §5, this approximate solution is shown to be good enough for studying the present problem. The results obtained in this paper show that such a self-exciting dynamo is possible by which the earth's main magnetic field is produced and maintained.

Atomic and molecular transitions in auroral spectra

Abstract: The identifications of lines and bands in auroral spectra, as reported by several observers over the past few years, are subjected to a critical analysis in order to determine which atomic transition arrays and molecular progressions are most important. The various transitions are analyzed from the standpoint of expected relative intensities of lines or bands originating from common upper levels, and the probable validity of the identifications is discussed. Tables are presented listing the emissions that seem to be well established in at least some auroral displays. Variations in auroral spectra with latitude, type of display, etc., undoubtedly account for many of the discrepancies among the wavelength lists of different authors. Hence it is not always possible to differentiate between real and fortuitous identifications. A large number of fainter lines and bands still remain unidentified.

Radio noise from aurora

Abstract: A search for radio noise of 10-cm wavelength from aurora during 1951 and 1952 with improved equipment was unsuccessful. The failure to detect the auroral radio noise, observed previously in 1949, is attributed to the decrease of the intensity of auroral displays and of sunspot-activity.

Continental maps of four ionospheric disturbances

Abstract: Measurements of ƒ°F2 at 13 North American ionospheric stations are used to study the geographical distribution of ƒ°F2 deviations from normal and the changes with time during four ionospheric disturbances In two of these, storm centers can be identified which appear to move across the maps during the disturbance In the other two, the contours of percentage deviation are roughly directed east and west, and show relatively little change with time

Dissociative recombination in the E‐layer

Abstract: The present paper discusses the consequences of the assumption that dissociative recombination determines electron removal in the E-layer. The observation that the apparent recombination coefficient, α, does not depend markedly on the altitude of the E-layer, electron density, or time of day imposes restrictions on the rates of the various atomic collision processes. It is shown that α must be very nearly equal to the actual dissociative recombination coefficient of ions and electrons. The conversion of O+ to by charge transfer is consistent with ionospheric observations provided that the transfer cross-section is ≳10−17 cm2. In addition, the negative ion concentration is required to remain smaller than the electron concentration at all times. It is concluded that dissociative recombination is far more satisfactory than ion-ion recombination in explaining the E-layer observations, and that most of the atomic processes proposed for this mechanism are subject to accurate laboratory measurement.