Showing papers on "Earth's magnetic field published in 1972"

Pitch-angle diffusion of radiation belt electrons within the plasmasphere.

Abstract: Study of the formation of the quiet-time electron slot, which divides the radiation belt electrons into an inner and an outer zone. The pitch-angle diffusion of radiation belt electrons resulting from resonant interactions with the observed plasmaspheric whistler-mode wave band is quantitatively investigated. The effects of wave propagation obliquely to the geomagnetic field direction with the resulting diffusion at all cyclotron-harmonic resonances and the Landau resonance are evaluated along with the effects of interactions occuring at all geomagnetic latitudes. The results obtained account for the long-term stability of the inner radiation zone, the location of its outer edge as a function of electron energy, and the removal of electrons to levels near zero throughout the slot. Computed pitch-angle distributions and precipitation decay rates are in good agreement with slot-region observations.

Solar Terrestrial Physics

Abstract: The work is concerned with solar influences on the earth's upper atmosphere and electromagnetic environment partly through solar ultraviolet and X rays, but particularly through corpuscular radiations. The sun and interplanetary space, the internal structure and magnetic field of the earth, the terrestrial atmosphere and photochemistry, and the dynamics of the upper atmosphere and dynamo action are considered. The formation of the magnetosphere, energetic particles, plasma, and electromagnetic waves in the magnetosphere, solar storms and their extension into interplanetary space, and magnetospheric storms are treated.

Recent satellite measurements of the morphology and dynamics of the plasmasphere

Abstract: The characteristic morphology and dynamics of the plasmasphere vary with local time and with geomagnetic conditions. On the nightside the plasmapause position changes predictably with changing magnetic activity. Once established at a specific L-shell value, the steep density gradient on the nightside corotates into the dayside, where filling from the ionosphere takes place. In the duskside bulge region the characteristic density profile inside the plasmapause displays a smooth decrease proportional to 1/R to the fourth power where R is radial distance. Plasmasphere morphology and dynamics can be understood in terms of a time-varying convection electric-field model of the magnetosphere that includes the bulge region as part of the main circulation pattern of the plasmasphere.

Changes in magnetospheric configuration during the substorm growth phase

Abstract: Investigation of various changes in the configuration of the geomagnetic tail that occur during the growth phase of magnetospheric substorms. A flaring-tail model indicates that the observed increases in geomagnetic tail field are explained if the dayside magnetopause shrinks by 1 to 2 earth radii. Increased tail flaring also requires that the tail current sheet approach near the earth during the growth phase. The motion of the inner edge of the plasma sheet, and consequently an equatorward shift of the nightside auroral oval, is consistent with the structural modifications mentioned above.

Interplanetary magnetic-sector structure, 1926--1971.

Abstract: The influence of the direction of the interplanetary magnetic field on the geomagnetic field at high latitudes is used to study the long-term behavior of the sector structure during nearly four solar cycles. It is found that the rotation period of the sector structure varies from about 28.5 days in the beginning of a solar cycle to 27.0 days in the end. Also it is shown that short-lived sectors rotate more slowly than long-lived ones.

Fluctuating magnetic fields in the magnetosphere. II - ULF waves.

Abstract: At the present time the existing satellite observations of ULF waves suggest that the level of geomagnetic activity controls the types of waves which occur within the magnetosphere. Consequently, we consider separately quiet times, times of magnetospheric substorms, and times of magnetic storms. Within each of these categories, there are distinctly different wave modes distinguished by their polarization: either transverse or parallel to the ambient field. In addition, these wave phenomena occur in distinct frequency bands. In terms of the standard nomenclature of ground micropulsation studies ULF wave types observed in the magnetosphere include quiet time transverse - Pc 1, Pc 3, Pc 4, Pc 5; quiet time compressional - Pc 1 and Pi 1; substorm compressional Pi 1 and Pi 2; storm transverse - Pc 1; storm compressional Pc 4, 5.

Critical component of the interplanetary magnetic field responsible for large geomagnetic effects in the polar cap

Abstract: .An observed influence of the interplanetary magnetic sectorstructure on the geomagnetic variations in the polar cap appearsto be due to the component of the interplanetary magnetic fieldnear the ecliptic perpendicular to the earth-sun direction. Thissuggests that the observed effect on the ground originates in thefront of the magnetosphere.A clear correspondence between the sector polarity of the inter­planetary magnetic field and the geomagnetic variations in the polarcap has been found to exist by Svalgaard (1968) and by Mansurov (1969).For a positive polarity of the interplanetary magnetic field (a direc­tion away from the sun) the vertical component, Z, of the magneticfield at Thule (INVL, invariant latitude, = 86.8°N) is depressedduring several hours around magnetic noon and the horizontal compo­nent,H, at Godhavn (INVL = 77.5 0 N) 1S increased. Correspondingly anegative sector polarity gives r1se to an increase of the Z-componentat Thule and a decrease of the H-component at Godhavn. This correlationbetween the interplanetary magnetic field and the geomagnetic field 1nthe polar cap is so distinct that it has been possible to infer thesector polarity from the geomagnetic field variations at the polar capobservatories Thule and Godhavn (Friis-Christensenet al., 1971),(Svalgaard, 1972).But a more detailed examination of the observed relationship (Friis­Christensen, 1971) revealed a few cases in which the geomagnetic var1a­tions at Thule and Godhavn apparently disagreed with the sector polarity.Especially May 29th and June 21st in 1968 showed evidence of beingsituated in a sector of opposite polarity to the one observed by space­craft (Wilcox and Colburn, 1970). As these days occurred around shiftsin the sector polarity a possibility of a time delay was at firstlooked for. The fact, however, that the shift in the type of dailyvariation of the geomagnetic field in the polar cap did occur nearly

Local-time survey of plasma at low altitudes over the auroral zones.

Abstract: A local-time survey of the low energy proton and electron intensities precipitated into the earth's atmosphere during periods of magnetic quiescence was constructed. A typical, individual satellite crossing of this region in each of eight local-time sectors was selected from a library of similar observations with the polar-orbiting satellite Injun 5. The trapping boundary for more energetic electron intensities, E 45 keV, is a natural coordinate for delineating the boundary between the two major types of lower energy, 50 or = E or = 15,000 eV. The main contributors to electron energy influx are inverted V precipitation poleward of the trapping boundary and the plasma sheet intensities equatorward. These are interpreted in terms of a magnetospheric model.

Evidence for short geomagnetic polarity intervals in the Early Cenozoic

Abstract: To test the validity of the theory that geomagnetic reversals are produced by a stochastic process in the earth's core, a search was made for heretofore undetected short polarity intervals in marine magnetic anomalies over segments of ocean floor with ages between 55 and 63 m.y. The original Heirtzler time scale shows almost no short events during this time, and it is thus indicated that reversals are not a Poisson-distributed stochastic process. In our analysis we reduced profiles to the pole to eliminate asymmetry, stretched them to bring them to a common spreading rate, and stacked them to attenuate incoherent noise. Six new short events were determined by modeling the profiles with a single crustal layer. To simulate the process by which new oceanic crust is created by dike injection and extrusion of lava flows, the magnetization of the layer was determined by convolving model time scales with a Gaussian function. A standard deviation of 3 km gave the best fit to the anomalies. All the new events were found to be <60,000 years. When the geomagnetic time scale is modified by the addition of these six short polarity intervals, it has a frequency distribution close to that predicted by the stochastic model.

Three‐dimensional polarization characteristics of high‐latitude Pc 5 geomagnetic micropulsations

Abstract: In 1969 and 1970, a line of magnetometer stations was operated in western Canada between 58.5° and 77.7°N corrected geomagnetic latitude and within 2° of 302°E corrected geomagnetic longitude. This paper deals with the three-dimensional polarization characteristics of low-frequency (f 250 sec) Pc 5 geomagnetic micropulsations recorded at these stations. The center of activity of these pulsations follows the auroral oval during the whole day. The early morning (0200–1000 local geomagnetic time) has the greatest number of pulsations, with almost circular polarization in the horizontal plane, whereas the afternoon normally has few sinusoidal pulsations and almost linear polarization in the direction of magnetic north. A reversal in the sense of polarization in the horizontal plane, occurring between 1130 and 1230 local geomagnetic time, suggests that the Pc 5 events derive their energy from a Kelvin-Helmholtz instability at the magnetopause. A latitudinal change in the sense of polarization, occurring near the latitude of the peak intensity, indicates that the energy of the pulsations may be distributed in a resonant hydromagnetic mode within the earth's inner dipole field or that field lines at high latitudes (λ > 70°) may pass into the magnetosheath. The vertical components of the Pc events are likely caused by spatial gradients in the ionospheric currents associated with the pulsations. During part of the pulsation cycle, strong shear currents flow to the east and the west in the ionosphere, giving a large vertical magnetic field near the latitude of the peak in the intensity of the horizontal components.

Annual and solar-magnetic-cycle variations in the interplanetary magnetic field, 1926-1971.

Abstract: : The polarity of the interplanetary magnetic field has been inferred by Svalgaard from observations of the polar geomagnetic field during the interval 1926-1971. On the basis of a few years of spacecraft observations Rosenberg and Coleman have suggested that there may be an annual variation in the predominant polarity of the interplanetary field. The present analysis of 45 years of inferred field polarity shows clearly an annual variation and also a variation of about twenty years, which may be associated with the solar magnetic cycle. The annual variation changes its predominant polarity within a few days of the times when the heliographic latitude of the earth is zero. (Author)

Identification of short polarity events by transforming marine magnetic profiles to the pole

Abstract: A method is presented that permits marine magnetic anomaly profiles from all parts of the world to be stacked for the purpose of enhancing coherent small anomalies due to short geomagnetic polarity events. In general, the shape of a profile depends not only on the sequence of normally and reversely magnetized magnetic prisms that constitute the source, but also on the following five angles: the declination and inclination of the regional field; the declination and inclination of the magnetization; and the azimuth of the magnetic stripes. Our method transforms a measured anomaly to a second anomaly which would have been produced by an identical source with vertical magnetization and vertical regional field. The method uses Fourier transform and linear systems theory and employs filters with a flat spectral response, so that no information is lost at short wavelengths. No assumptions are made about the true depth or shape of the source: this may be any two-dimensional body or bodies in which the direction of magnetization is constant and the intensity varies arbitrarily. A preliminary application of the method to marine profiles from the East Pacific rise has yielded encouraging results.

Geomagnetic effect on the neutral temperature of the F region during the magnetic storm of September 1969

Abstract: A spherical Fabry-Perot interferometer on Ogo 6 (launched June 5, 1969) gives the profile of the 6300-A oxygen line in the dayglow at different altitudes between 200 and 320 km. The temperature of the neutral components of the atmosphere is deduced from these measurements with an accuracy of ±65°K. The results presented in this paper correspond to the storm period September 25 to October 5, 1969. During this period a severe geomagnetic storm occurred. The temperature measurements show a very strong effect in the two polar regions, while the equatorial regions present a smooth temperature variation.

Hydromagnetic oscillations of the Earth's core

Abstract: The dispersion relationship for hydromagnetic waves in a rotating system indicates that, under plausible assumptions concerning the strength of the toroidal magnetic field in the earth's liquid core, the periods associated with slow planetary-scale waves in the core would be comparable with the characteristic time scales found in the geomagnetic secular variation, decades to centuries, and it now seems likely that such waves could also play a key role in the production of the geomagnetic field by the ‘dynamo’ process. Attempts to interpret specific features of the field in terms of hydromagnetic oscillations of the core (or of any other type of fluid motion, for that matter) have not yet proved entirely successful, for they encounter serious and in some cases fundamental mathematical difficulties. The present paper reviews theoretical work on free hydromagnetic oscillations of a bounded rotating fluid in the context of the geomagnetic problem.

Internal Structure of the Geomagnetic Neutral Sheet

Abstract: Models for internal structure of geomagnetic tail neutral sheet deduced from Explorer 34 magnetic data

Electric field and plasma observations in the magnetosphere

Abstract: Satellite-borne electric field measurements using the double probe technique have now provided a comprehensive survey of convection electric fields at low altitudes in the magnetosphere. The most prominent features of the convection electric fields are reversals located at high magnetic latitudes, with generally anti-sunward convection poleward and sunward convection equatorward of the electric field reversal location. On the day side of the magnetosphere the electric field reversal is observed to coincide with the equatorward boundary of the polar cusp. In the local afternoon and evening regions inverted V electron precipitation bands occur at or near the electric field reversal and in regions usually characterized by large fluctuations in the electric field. In the local midnight region strong convection electric fields have also been observed deep within the magnetosphere, near the equatorward boundary of the plasma sheet. Recent measurements of electric fields near the inverted V electron precipitation bands suggests that these events are associated with large electrostatic potential gradients along the geomagnetic field.

Palaeomagnetic Evidence about the Source of the Geomagnetic Field

Abstract: Summary The availability of a large body of palaeomagnetic data from Eastern and Western Iceland has made possible several deductions about the nature of the source of the geomagnetic field. We conclude that the assumption of a centred dipole source leads to a simple and reasonable time-average dependence of absolute dipole strength on dipole orientation (dipole moment on pole position). The relationship is nearly axially symmetrical, and provides natural definitions of ‘normal’ and ‘reversed’ fields. The normal state has been more stable over the past 12 My than the reversed state. The dependence of time-average dipole strength on dipole orientation provides a considerable constraint on any postulated mechanism for field generation, although it does not necessarily give direct insight into the behaviour in the core.

IMP 5 magnetic field measurements in the high latitude outer magnetosphere near the noon meridian

Abstract: IMP 5 magnetic field measurements at high geomagnetic latitudes to observe broad depressed field region centered on polar or dayside cusp

Geomagnetic Attitude Control of an Axisymmetric Spinning Satellite

Abstract: The results of this study yield a general control law for geomagnetic attitude stabilization of a satellite. The proposed law, called the switching function, is derived from the asymptotic stability condition. Use of this law enables us to select the pattern of the onboard dipole moment satisfying the criteria for achieving the maximum effective torque and minimum transverse torque at every instant of activation. Analysis and formulation have been made for a simplified case and then extended to the general case including the canted geomagnetic dipole and the elliptical orbit. The derived switching point at which the polarity of a dipole is reversed is found to be different from those presented by the averaging method. This difference makes the spin axis attain the desired direction faster. The narrow pulse pattern activated halfway between the switching points is found advisable, in a practical application, since a considerably effective utilization of the applied torque is made possible. For spin rate control, operation around the selected point on the orbit is concluded desirable, satisfying the above criteria. Simulation runs have verified the feasibility of the law thus derived and showed a notable improvement in performance.

Response time of the magnetosphere to the interplanetary electric field.

Abstract: Data from the IMP-1 satellite are used to determine the lag time between the response of the index of geomagnetic activity AE to fluctuations in the azimuthal component of the interplanetary electric field as measured in the solar ecliptic coordinate system. The lag time is found to lie in the range 30–50 min. The magnitude of the lag time is related to the average energy content of the magnetosphere.

Palaeomagnetic Differences Between Normal and Reversed Field Sources, and the Problem of Far-sided and Right-handed Pole Positions

Abstract: Summary Analysis of palaeomagnetic data from the USSR, which Khramov and Sholpo have separated into normal and reversed mean data, reveals that there is considerable and obvious dissimilarity between normal and reversed regimes of the geomagnetic field source. These new data also strengthen the case for a world-wide eastward declination of the geomagnetic field during normal regimes, and westward (west of south) declination during reversed regimes. The question of how such fields could be maintained is discussed.

Kinematic Reversal Schemes for the Geomagnetic Dipole

Abstract: Fluctuations in the distribution of cyclonic convective cells, in the earth's core, can reverse the sign of the geomagnetic field. Two kinematic reversal schemes are discussed. In the first scheme, a field maintained by cyclones concentrated at low latitude is reversed by a burst of cyclones at high latitude. Conversely, in the second scheme, a field maintained predominantly by cyclones in high latitudes is reversed by a fluctuation consisting of a burst of cyclonic convection at low latitude. The precise fluid motions which produce the geomagnetic field are not known. However, it appears that, whatever the details are, a fluctuation in the distribution of cyclonic cells over latitude can cause a geomagnetic reversal.