Showing papers on "Earth's magnetic field published in 1973"

Semiannual variation of geomagnetic activity

Abstract: It is proposed that the semiannual variation in geomagnetic activity is caused by a semiannual variation in the effective southward component of the interplanetary field. The southward field arises because the interplanetary field is ordered in the solar equatorial coordinate system, whereas the interaction with the magnetosphere is controlled by a magnetospheric system. Several simple models utilizing this effective modulation of the southward component of the interplanetary field are examined. One of these closely predicts the observed phase and amplitude of the semiannual variation. This model assumes that northward interplanetary fields are noninteracting and that the interaction with southward fields is ordered in solar magnetospheric coordinates. The prediction of the diurnal variation of the strength of the interaction at the magnetopause by this model, does not, however, match the diurnal variation of geomagnetic activity as derived from ground-based data.

Solar wind interaction with the Earth's magnetic field: 1. Magnetosheath

Abstract: Properties of plasma and magnetic field within the magnetosheath in the period December 1968 to December 1969 are presented as observed by the European satellite Heos 1. Three states of the magnetosheath plasma have been distinguished on the basis of different upstream conditions and are characterized by low magnetosonic Mach number (M1 < 3) and low β, high values of M1 and β and absence of upstream waves, and presence of upstream waves. The distribution function of proton velocities and the magnetic turbulence in the magnetosheath have been studied in connection with these three upstream situations.

Polar cap magnetic variations and their relationship with the interplanetary magnetic sector structure

Abstract: The relationship between polar geomagnetic variations and the polarity of the interplanetary magnetic sectors has been studied for the quiet year 1965. It is found that during the day hours a system of ionospheric currents encircles the magnetic poles on every day. The current system may extend up to 15 deg from the pole but is strongest at 8 to 10 deg invariant colatitude. The current direction as seen from near the magnetic poles is counterclockwise during interplanetary sectors with field pointing away from the sun and clockwise during toward sectors. The current strength is dependent on season, being strongest during local summer. When the magnetic pole is on the nightside of the earth, this polar cap current is absent or very weak.

A qualitative study of the reconnection between the Earth's magnetic field and an interplanetary field of arbitrary orientation

Abstract: To the present date only the reconnection process for exactly antiparallel fields has been discussed in detail. In magnetospheric terms this restricts us to the consideration only of southward interplanetary fields. The qualitative study presented here shows how reconnection takes place between arbitrarily oriented fields in infinite and finite geometries. The process may best be thought of as a continuous exchange of field-line partners during the time a field line maps into the diffusion region. In a finite geometry the diffusion regions lie on field lines which connect neutral points of the field configuration, and along which a potential drop is imposed. Although the discussion here centers principally on the magnetosphere, the described reconnection process and field topology should also be applicable to other astrophysical problems.

Partial ring current models for worldwide geomagnetic disturbances.

Abstract: In recent years the longitudinal asymmetry of the ring current in the magnetosphere encircling the earth has been advocated to explain the geomagnetic field variation in middle and low latitudes. Direct observations of particles and magnetic field by means of satellites in the magnetosphere have given some supporting evidence for the existence of partial ring current in the magnetosphere. In this article a review is made of the analysis of low-latitude geomagnetic variation during magnetic storms or substorms, and the contribution of the partial ring current system to low-latitude geomagnetic variations is discussed. After examining the magnetic effect of each part of the partial ring current system, i.e., (1) the equatorial ring current in a limited longitude range, (2) the Birkeland (field aligned) current in the magnetosphere connecting the equatorial plane and the ionosphere, and (3) the current in the ionosphere, including an electrojet along the auroral zone or oval, we conclude that the main contribution to the longitudinal ΔH asymmetry in middle and low latitudes (H is the horizontal component of the geomagnetic field) comes primarily from the Birkeland current and neither from the partial ring current in the equatorial plane nor from the auroral electrojet. Therefore the models of partial ring current need to be modified so as not to overemphasize the ring-form current closure in the equatorial plane. This article explains the reason for the above conclusions and gives some necessary considerations for interpreting the geomagnetic variations in low latitudes during magnetic storms and substorms and on quiet days.

Penetration of thundercloud electric fields into the ionosphere and magnetosphere: 1. Middle and subauroral latitudes

Abstract: The mapping of thundercloud electric fields at middle and subauroral latitudes is investigated analytically as a three-dimensional boundary value problem. The electrical conductivity is represented by several piecewise exponential functions of altitude, and the anisotropy of the medium is taken into account above 70-km altitude. The geomagnetic field lines are assumed to be straight and vertical below 150-km altitude. Electric field strength at great heights depends sensitively on conductivity and thundercloud models used in the calculations. Sample calculations using representative nighttime profiles show that ‘giant’ thunderclouds can produce transverse electric fields of tens of microvolts per meter in the equatorial plane of the midlatitude magnetosphere. In the daytime, corresponding electric fields are about an order of magnitude less. These results suggest that giant thunderclouds may be an important source of localized electric fields that can form field-aligned electron density irregularities in the ionosphere and the magnetosphere.

Plasma and potential at the lunar surface

Abstract: The various plasma environments of the Moon are described for the lunar orbit through the solar wind and geomagnetic tail. The sources of lunar surface charge, including plasma, photo, and secondary currents, are compraed for their expected range of values. The electric potential is calculated from probe theory as a function of local position on the lunar surface; the theory includes plasma environments which are both stationary and flowing. In agreement with several other calculations, the potential at the subsolar point is likely to be a few volts positive. However, for the case when the Moon is in the solar wind, the potential is calculated to be a few tens of volts negative at the terminator, and is expected to go to larger negative values on the dark side. If the Moon traverses a significant plasma sheet in the geomagnetic tail, then the dark side potential tends toward several kilovolts negative but may be strongly limited by the secondary electron current. Associated surface electric fields are estimated.

Energy and momentum theorems in magnetospheric processes

Abstract: This review deals with the several energy and momentum theorems that relate to magnetospheric processes that have been developed. The region of primary consideration in this paper is the magnetospheric domain that extends between the ionosphere and the interplanetary medium, although, for studying certain phenomena, ionospheric and solar wind properties are of central importance and must be included. Both energy theorems and momentum theorems with their applications are presented. Since energy is an integral property of the system variables, analytical results can be found without knowledge of detailed dynamical processes. Thus, relations are derived between particle and magnetic system energies, and application is made to the shape of the magnetopause and various phases of a magnetic storm. Particular attention is given to symmetric and asymmetric ring currents, including energy and momentum equilibrium conditions; a review of nonlinear self-consistent models and a discussion of how charge exchange and energy diffusion participate in the recovery phase are presented. Comprehensive expressions for the storm time disturbance field are given in terms of both ring current and boundary current energies, and changes that occur during magnetospheric compressions are discussed. The momentum theorems center around the requirement of static force balance during geomagnetically quiet intervals. Whereas the energy theorems give expressions for the average disturbance field over the earth, the momentum theorems give the gradient in the disturbance field across the earth. The forces between earth and the boundary current, the ring current, and the tail current are derived for various models. It is noted that existing vacuum models of the geomagnetic tail are deficient in meeting the combined requirements of energetics and dynamics of the quiet time tail. Introducing the plasma sheet removes the difficulty by allowing an extra degree of freedom in adjusting the force between the earth and the tail. The role of the plasma sheet in making force adjustments is shown to be consistent with the observed thinning of the plasma sheet before sub storms.

The reconnection rate of magnetic fields.

Abstract: Rapid reconnection of magnetic lines of force occurs whenever two regions of opposite field are pressed together without sufficient fluid pressure between to keep them apart. It has been suggested that rapid reconnection is the basis for solar flares, and that in the magnetopause and the neutral sheet of the geomagnetic tail it plays an essential role in geomagnetic activity. It is pointed out that the escape of fluid from between the fields is the result of magnetohydrodynamic exchange instabilities, leading to reconnection rates which are generally some fraction of the Alfven speed.

Satellite studies of magnetospheric substorms on August 15, 1968: 1. State of the magnetosphere

Abstract: The sequence of events occurring throughout the magnetosphere during a substorm has not been precisely determined. This paper introduces a collection of papers that attempts to establish this sequence for two substorms on August 15, 1968. Data from a wide variety of sources are used, the major emphasis being changes in the magnetic field. In this paper we use ground magnetograms to determine the onset times of two substorms that occurred while the Ogo 5 satellite was inbound on the midnight meridian through the cusp region of the geomagnetic tail (the region of rapid change from taillike to dipolar field). We conclude that at least two worldwide substorm expansions were preceded by growth phases. Probable beginnings of these phases were at 0330 and 0640 UT. However, the onset of the former growth phase was partially obscured by the effects of a preceding expansion phase around 0220 and a possible localized event in the auroral zone near 0320 UT. The onsets of the corresponding expansion phases were 0430 and 0714 UT. Further support for these determinations is provided by data discussed in the subsequent notes.

Geomagnetic anomaly pattern of the Japan Sea

Abstract: Using all available geomagnetic data, including those obtained in a detailed survey conducted by the authors in 1970, the geomagnetic anomaly pattern if the Japan Sea has been studied. It has been established that sublinear magnetic anomalies run subparallel to the general trend of the Japanese Islands. The peak to peak amplitudes of most of these anomalies are less than 300y, their wavelengths 20 to 40 km. The anomalies are much less distinct in linearity than those found in the northwestern corner of the Pacific basin off northeastern Honshu. The linear trend is better developed in the deeper basin areas and less recognizable in the Yamato- and Kitayamato-areas. The anomaly pattern appears to support the view that the Japan Sea floor evolved through a spreading process from numerous spreading centers. A definite conclusion about the genesis of the Japan Sea, however, must await further investigation.

Solar wind and substorm‐related changes in the lobes of the geomagnetic tail

Abstract: Variations in the magnetic energy density in the lobes of the geomagnetic tail appear to be fundamentally associated with magnetospheric substorms. Thus in this paper the relationship of the tail lobe energy density to solar wind dynamic pressure, the north-south component of the interplanetary magnetic field, and substorm expansion onsets is investigated. Enhanced lobe energy densities are observed to follow both southward turnings of the interplanetary magnetic field and solar wind dynamic pressure increases. An empirical relationship is developed relating the tail lobe magnetic energy density to the solar wind dynamic pressure by using data obtained when the interplanetary field was northward. Localized, off-center, or multiple-onset substorm expansion phases are shown to have little effect on the lobe magnetic field. A positive correlation is found between magnetospherewide substorm expansions and decreases in the lobe energy density. The positive correlation of the energy density with both the solar wind dynamic pressure and a southward interplanetary magnetic field and the positive correlation between magnetospheric substorm activity and lobe energy density decreases are demonstrated to be consistent with our phenomenological model of substorm behavior.

Worldwide Distribution of Geomagnetic Tides

Abstract: Geomagnetic lunar-daily variations result from the Moon's tidal action on the ionosphere and oceans. They provide information that bears on the general large-scale dynamics of the atmosphere, and can also add to our understanding of ionospheric processes. For these reasons the variations, though small, are worth determining from geomagnetic data. The data for this study are hourly mean values of the geomagnetic elements from 100 observatories for the interval 1957.5 to 1960.0. These are analysed by the Chapman-Miller method to obtain parameters of the principal lunar harmonics. For each observatory, the contribution of the sea tidal dynamo to these parameters is empirically determined; this contribution has been ignored in previous worldwide studies, but is shown here to be an appreciable proportion of the total lunar effect, and worthy of study in its own right. After removal of the ocean dynamo effect, the remaining geomagnetic lunar daily variation, ascribed to the ionospheric dynamo and its associated induced currents, is represented by a series of spherical harmonic coefficients, and separated into parts of internal and external origin. (In this context, the relative merits of spherical harmonic analysis and the Price-Wilkins method of analysis are discussed.) Internal and external current systems associated with the ionospheric dynamo effect are deduced. Similar analyses, for the solar daily geomagnetic variation and for the lunar semidiurnal tide in the atmospheric pressure, are appended.

Wave polarizations of geomagnetic pulsations observed in high latitudes on the Earth's surface

Abstract: A new theory is put forward, which accounts for the general characteristics of wave polarizations of low-frequency geomagnetic pulsations observed in the auroral and subauroral zones on the earth's surface. Because the displacement current can be neglected in the neutral atmosphere, the character of the polarization ellipses observed on the ground surface depends on the functional form of the localization of the pulsation disturbance. Whenever a pulsation has an elliptical polarization in a horizontal plane on the ground surface, the disturbance should be bidirectionally localized; in one direction the perturbation is propagative and in the direction orthogonal to the propagation direction it is specified by a stationary function having a sharp peak at the center line of activity. As a result, a train of magnetic potential eddies travels along the auroral zone in the neutral atmosphere near the earth's surface. The observed polarization reversals between morning and afternoon hours, between geomagnetically conjugate stations, and across the auroral zone are consistently explained by the proposed mechanism. To test this mechanism, a series of computer simulations has been performed. A new way of mapping the magnetospheric convection pattern is also proposed by utilizing the global distribution of the wave polarization of geomagnetic pulsations observed on the earth's surface.

Kinematic Dynamos and the Earth's Magnetic Field

Abstract: The Bullard—Gellman formalism is applied to investigate the existence of convergent solutions for steady kinematic dynamos. It is found that the solutions for the Bullard—Gellman dynamo, as well as for Lilley’s modification of it, do not converge. Convergent solutions have been found for a class of spherical convective cells which would be stationary in a perfect fluid in the absence of rotation and of the magnetic field. By calibrating the theoretical magnetic dipole so as to fit the observed value at the Earth’s surface, one can find a dynamo in the above class which also matches the observed equatorial magnetic dipoles. There is a dynamo which has a rate of total ohmic dissipation of only 1.8 x 1016 erg s-1 for an assumed electrical conductivity of 3 x 10~6 e.m.u.9f This is one thousandth the rate of tidal dissipation, and one hundred thousandth the rate of heat outflow from the surface of the Earth. The required velocities are of the order of 10~3 cm s_1, and the average magnetic energy density is 4 erg cm-3. The internal structure of the magnetic field in this model shows a dynamo mechanism situated in the outer part of the liquid core and is thus insensitive to possible rigidity of the material in the * inner core.

Recent studies of magnetospheric electric field emissions above the electron gyrofrequency.

Abstract: In a previous paper Kennel et al [1970] presented a survey of VLF electric field observations in the magnetosphere made with the short electric dipoles aboard Ogo 5. Particular emphasis was placed on the geomagnetic equatorial region between L = 4 ,and L = 10. The preliminary findings in that paper suggested that the predominant emissions were those at odd half-integral multiples of the local electron gyrofrequency ƒce and that, among these multiples, emissions at ƒ ≈ 3ƒce/2 were by far the most common, although other multiples, including ƒce/2, were also observed at times. Kennel et al. [1970] noted that the waves have large electric field amplitudes (1–10 mv/m), and they suggested that 3ƒce/2 emissions could be important sources of turbulent energization and pitch angle diffusion for auroral zone electrons.

Geomagnetic line spectra-2 to 70 years

Abstract: Application of a new data adaptive approach to power spectrum estimation has yielded greatly improved knowledge of the geomagnetic spectrum in the range 2 to 70 yr. The first successful line spectrum detection of the solar and double solar cycle variations in absolute geomagnetic element data are presented; also detected are the first four harmonics of the solar cycle and, excepting one, the first nine harmonics of the double solar cycle. Finally, evidence is found for a ∼ 60 yr line. The implications of these results for a variety of problems in planetary and space physics are discussed.

Variations of the occurrence rate of discontinuities in the interplanetary magnetic field

Abstract: A statistical study of magnetic field discontinuities for the interval from February to October 1968 using data from the NASA Goddard Space Flight Center/University of Rome experiment on Pioneer 8 has been conducted. High-resolution data (10-sec averages) have been employed, stringent criteria being utilized to select more than 16,000 events, about 50 per cent being identified as disturbances that are probably tangential discontinuities (TD). Near 1 AU the average occurrence rate of discontinuities is about 3.6/hr, whereas that of tangential-like discontinuities is about 1.6/hr. The rate of occurrence of discontinuities is found to be correlated with the directional change of the field across the discontinuity surface. The normal to the TD discontinuity plane is most often perpendicular to the Archimedean spiral field configuration. A decrease in the rate of occurrence of TD is found with increasing heliocentric distance and/or with decreasing heliocentric latitude. Possible interpretations of the effect are discussed.

Solar wind interaction with the earth's magnetic field. II - Magnetohydrodynamic bow shock.

Abstract: The earth's bow shock has been investigated as a magnetohydrodynamic discontinuity using the plasma and magnetic data supplied by the European satellite Heos 1. The jumps of the fluid parameters through the shock have been studied as a function of the Mach number and of the geometry of the shock surface. The solar wind specific heat ratio has been found to be equal to 1.75 or 2.25, depending on the presence or absence of upstream waves. Computations of the shock velocity performed for 23 crossings gave an average speed of about 85 km/sec.

Magnetospheric Mapping with Quantitative Geomagnetic Field Models

Abstract: The Mead-Fairfield geomagnetic field models were used to trace field lines between the outer magnetosphere and the earth's surface. The results are presented in terms of ground latitude and local time contours projected to the equatorial plane and into the geomagnetic tail. With these contours various observations can be mapped along field lines between high and low altitudes. Low altitudes observations of the polar cap boundary, the polar cusp, the energetic electron trapping boundary and the sunward convection region are projected to the equatorial plane and compared with the results of the model and with each other. The results provide quantitative support to the earlier suggestions that the trapping boundary is associated with the last closed field line in the sunward hemisphere, the polar cusp is associated with the region of the last closed field line, and the polar cap projects to the geomagnetic tail and has a low latitude boundary corresponding to the last closed field line.

The direction and spectral variability of a cosmic gamma-ray burst

Abstract: Description of the simultaneous observations of a gamma-ray event by five different satellites on May 14, 1972 in widely differing geomagnetic environments. The direction of the source was unambiguously determined, and it is concluded that the event was real and not caused by some unexplained spurious or instrumental effects. Since the obtained direction is far from the earth and the sun, the conclusion is that this gamma-ray event was almost certainly of cosmic origin. The total energy at the earth in this event was about .0005 ergs/sq cm from 11 to 1500 keV.

Aeromagnetics and the Transition Between the Colorado Plateau and Basin Range Provinces

Abstract: An aeromagnetic survey of central Utah has given improved resolution of a regional anomaly previously recognized from satellite magnetometer data. The anomaly consists of a gradient from a high over the Colorado Plateau to a low over the Basin Range province. Magnetic profiles are fit by a model in which the Curie isotherm deepens eastward in a belt up to 80 km wide, whose western side is about 50 km east of the Basin Range physiographic margin. The magnetic profiles could also be fit be a change of average crustal susceptibility instead of Curie depth. In either case we conclude that the major lateral change between Basin Range and Colorado Plateau crustal geophysical parameters occurs not at the Fenneman physiographic boundary but more than 50 km eastward of it. This suggestion is found to be consistent with data from seismic refraction, geomagnetic variation, geochemistry, and geologic structures.