Showing papers on "Earth's magnetic field published in 1975"

Polar cap currents for different directions of the interplanetary magnetic field in the Y‐Z plane

Abstract: During intervals of a steady state condition of the interplanetary magnetic field the geomagnetic variations at the polar cap have been examined as a function of the direction of the interplanetary field in the Y-Z plane of the geocentric solar magnetospheric coordinate system. A prominent feature of the equivalent current patterns during a positive B/sub z/ steady state is the total vanishing of the well-known two-cell current system, which is generally considered to be related to the convection across the polar cap. Another significant effect for a northward interplanetary magnetic field is a poleward shift of the B/sub y/ associated currents at the day side of the polar cap, with no decrease of the current strength. The existence of these currents even during the absence of the two-cell current system indicates that the explanation proposed by Heppner (1972) of the currents' being due to a shift toward the dawn or the dusk of the polar cap convection is not exhaustive. Rather, the magnetic perturbations for a positive B/sub z/ may correspond to merging of the northward interplanetary magnetic field with the open geomagnetic field lines at the poleward boundary of the cusp as suggested by Burch (1973). A pronounced decreasemore » of the B/sub y/ related magnetic perturbations from summer to winter compared to the seasonal variation of the B/sub z/ related disturbances at the polar cap indicates a different origin of the associated currents. (auth)« less

The magnetic field of Mercury, 1

Abstract: An updated analysis and interpretation are presented of the magnetic field observations obtained during the Mariner 10 encounter with the planet Mercury on March 29, 1974. The combination of data relating to position of the detached bow shock wave and magnetopause and the geometry and magnitude of the magnetic field within the magnetospherelike region surrounding Mercury lead to the conclusion that an internal planetary field exists with dipole moment approximately 5.1 × 1022 G cm³. The limited data set precludes quantitative determination of an intrinsic field more complex than a centered dipole. The dipole axis has a polarity sense similar to that of earth and is tilted 7° from the normal to Mercury's orbital plane. The magnetic field observations reveal a significant distortion of the modest Hermean field (350 γ at the equator) by the solar wind flow and the formation of a magnetic tail and neutral sheet which begins close to the planet on the night side. Presently, an active dynamo mechanism in the planetary interior appears to be favored in the interpretation of the field origin, although fossil remanent magnetization cannot be excluded. The composite data set is not consistent with a complex induction process driven by the solar wind flow.

Geomagnetic secular variation over the past 5 m.y.

Abstract: The study of the geomagnetic secular variation in prearcheological times is based upon investigations of the angular dispersion of paleomagnetic results and its variation with latitude. The westward drift of the geomagnetic field observed in historic times and apparently confirmed by archeomagnetic data back 1000 years or more suggests the way in which the present geomagnetic field may be analyzed for comparison with paleomagnetic results. Generalized models of paleosecular variation suppose that the angular dispersion arises from two contributions, one due to variations in the dipole field (dipole wobble) and one due to variations in the nondipole field. Attempts at distinguishing between these two contributions are inherently nonunique. To overcome the nonuniqueness problem, we have proposed a model for paleosecular variation (model M) that fits both the paleomagnetic data and the variation expected from analysis of the present field and that is also compatible with current theories of the origin of secular variation. This results in an average dipole wobble of 9$sup 0$ over the past 5 m.y. The best fit to the nondipole dispersion then arises from a combination of two mechanisms for the origin of secular variation. The major contribution arises from a nondipole field that originates frommore » some sort of interaction with the main poloidal field, probably akin to Hide's magnetohydrodynamic wave mechanism. The secondary contribution, about two-thirds the magnitude, arises from a fixed field whose intensity is independent of latitude compatible with Bullard's mechanism of fluid eddies interacting with the toroidal field near the core-mantle boundary. The two types of nondipole field are compatible with Yukutake's subdivision into drifting and standing parts, respectively, and provide a possible physical basis for his analysis. During a polarity transition the standing part of the nondipole field should predominate. (auth)« less

Substorm and interplanetary magnetic field effects on the geomagnetic tail lobes

Abstract: The average relationship of the tail lobe magnetic energy density to the north-south component of the interplanetary magnetic field (IMF) and substorm expansion phase is investigated by using a superposed epoch analysis method. An enhanced average lobe energy density is observed to be associated with a southward component of the IMF prior to substorm breakup. Coincident with the expansion onset, the lobe field rapidly recovers, and the north-south component of the IMF turns northward. The average magnitude of the substorm-associated lobe field variation is measured to be ∼14%, a value sufficiently large to enable the lobes to serve as the source of the energy dissipated during substorms.

A study of geomagnetic storms

Abstract: Twenty-one geomagnetic storm events during 1966 and 1970 were studied by using simultaneous interplanetary magnetic field and plasma parameters. Explorer 33 and 35 field and plasma data were analyzed on large-scale (hourly) and small-scale (3 min.) during the time interval coincident with initial phase of the geomagnetic storms. The solar-ecliptic Bz component turns southward at the end of the initial phase, thus triggering the main phase decrease in Dst geomagnetic field. When the Bz is already negative, its value becomes further negative. The By component also shows large fluctuations along with Bz. When there are no clear changes in the Bz component, the By shows abrupt changes at the main phase onet. On the small-scale behavior of the magnetic field and electric field (E=-VxB) studied in details for the three events, it is found that the field fluctuations in By, Bz and Ey and Ez are present in the initial phase. These fluctuations become larger just before the main phase of the storm begins. In the largescale behavior field remains quiet because the small scale variations are averaged out.

An Analysis of Near‐Bottom Magnetic Anomalies: Sea‐Floor Spreading and the Magnetized Layer

Abstract: Summary Near-bottom magnetic data over six oceanic ridge segments in the East Pacific are inverted, giving magnetization solutions with alternate positive and negative bands which correspond to geomagnetic field reversals. We estimate the average half-width of the crustal formation zone to be 2-3 km, based on the transition widths between these bands. The solutions show a narrow region of high magnetization centred directly over the centre of spreading, superimposed on a more gradual decrease of magnetization amplitudes with age. Both features are attributed to weathering of highly magnetized pillow lavas. We demonstrate that the short wavelength (<3 km) anomalies are largely due to topography. Distances to reversal boundaries give distance us age curves for each ridge which show that spreading changes occur as sudden accelerations typically separated by several million years of very constant motion. These rate changes are probably accompanied by shifts in the locations of poles of relative motion, which are necessary in a system of more than two interacting plates. Palaeomagnetic data and reversal boundary locations from near-bottom and surface data are combined to give spreading half-rates and a refined time scale for the past 6 My. Widespread spreading rate variations occurred at 2-3 MyBP and about 5 MyBP, possibly as a response to large scale changes in the plate pattern.

Can Precession Power the Geomagnetic Dynamo

Abstract: Summary The power requirement of a stationary geomagnetic dynamo driven by some agency external to the core (e. g. precession) is equal to the ohmic dissipation in the core, Q1. For a dynamo in which differential rotation is important we show that Q1ασ1−2, where σ1∼ 5 × 105 ohm−1 m−1 is the electrical conductivity of the core. Estimates of Q1 for kinematic dynamo models range from 109 to over 1012 Watts, depending on the particular regenerative scheme characterizing the dynamo. Precessional power input to the magnetic field can be estimated in terms of the electromagnetic part of the core-mantle coupling and the tiltover angle (inclination of the core angular momentum vector to that of the mantle). We correct Stacey's estimate of core-mantle coupling to take into account the diurnal frequency of precession-induced flow relative to the mantle, and show that the power available from precession to stir the core cannot exceed 108 W if the core flow remains stable. As a caution against the widespread uncritical acceptance of Malkus’ claims to have demonstrated the energetic adequacy of precession-driven hydromagnetic turbulence, we enumerate the mathematical and physical errors which cast doubt on his theoretical arguments.

Interplanetary streams and their interaction with the earth

Abstract: Plasma and magnetic field observations of interplanetary streams near 1 AU are summarized. Two types of streams have been identified corotating streams and flare-associated, and other flow patterns are present due to interactions among streams. The theory of corotating streams, which attributes them to a high temperature region near the sun, satisfactorily explains many of the effects observed at 1 AU. A correspondingly complete theory of flare-associated streams does not exist. Streams are a key link in the chain that connects solar and geomagnetic activity. The factors that most influence geomagnetic activity are probably related to streams and determined by the dynamics of streams. The evolution of streams on scales of 27 days and 11 years probably determines the corresponding variations of geomagnetic activity.

A three-dimensional kinematic dynamo

Abstract: A number of steady (marginal) solutions of the induction equation governing the magnetic field created by a particular class of threedimensional flows in a sphere of conducting fluid surrounded by an insulator are derived numerically. These motions possess a high degree of symmetry which can be varied to confirm numerically that the corresponding asymptotic limit of Braginsky is attained. The effect of altering the spatial scale of the motions without varying their vigour can also be examined, and it is found that dynamo action is at first eased by decreasing their characteristic size. There are, however, suggestions that the regenerative efficiency does not persistently increase to very small length scales, but ultimately decreases. It is further shown that time varying motions, in which the asymmetric components of flow travel as a wave round lines of latitude, can sustain fields having co-rotating asymmetric parts. It is demonstrated that, depending on their common angular velocity, these may exist at slightly smaller magnetic Reynolds numbers than the corresponding models having steady flows and fields. The possible bearing of the integrations on the production of the magnetic field of the Earth is considered, and the implied ohmic dissipation of heat in the core of the Earth is estimated for different values of the parameters defining the model.

A Model of the Geomagnetic Field at Epoch 1975

Abstract: Summary The main magnetic field at epoch 1975 is defined by 168 spherical harmonic coefficients (those representing the internal part to degree and order 12). A further 80 coefficients (degree and order 8) define the secular variation and, in addition, 26 secular acceleration coefficients (those most significantly determined) are included. Details are given of the data used and the methods of reduction. This paper presents the latest in a series of models of the geomagnetic field that have been derived in association with the World Magnetic Charts published by the British Hydrographic Office. The models are listed in Table 1 together with the epochs to which they refer. The next three columns give the number of spherical harmonic coefficients derived for the main field (MF), its first time derivative (secular variation: SV) and its second time derivative (secular acceleration: SA). An S in the final column indicates that the Earth has been assumed to be spherical; an E, that its ellipticity has been taken into account. Although responsibility for the preparation of both charts and models passed from the Royal Greenwich Observatory to the Institute of Geological Sciences in 1967, there has been continuity of experience and personnel throughout the period. The increasing number of coefficients is partly a reflection of the increase in computing capability with time, but also indicates the improvement of our knowledge of the geomagnetic field due to the improved distribution, accuracy and quantity of data available for analysis. Significant advances have also been made in the methods of analysis. For the present model we have determined all the coefficients up to twelfth order and degree for the main field. The data do not justify so many coefficients for the time derivatives, so the secular variation model has been truncated after eighth order and degree, and for secular acceleration only the 26 most significant coefficients are included. For any temporal extrapolation of the main field coefficients, it is appropriate to treat those coefficients not present in the SV and SA models as though they were zero, We have deliberately omitted standard deviations since these are of limited value and can be misleading. The British World Charts for 1975 are derived from a composite spherical harmonic model that has been recommended to the International Hydrographic Organisation as the source of declination values for the International Series of Navigation Charts. By

Ionosphere-magnetosphere coupling

Abstract: The large-scale electrical coupling between the ionosphere and magnetosphere is reviewed, particularly with respect to behavior on time scales of hours or more. The following circuit elements are included: (1) the magnetopause boundary layer, which serves as the generator for the magnetospheric-convection circuit; (2) magnetic field lines, usually good conductors but sometimes subject to anomalous resistivity; (3) the ionosphere, which can conduct current across magnetic field lines; (4) the magnetospheric particle distributions, including tail current and partial-ring currents. Magnetic merging and a viscous interaction are considered as possible generating mechanisms, but merging seems the most likely alternative. Several mechanisms have been proposed for causing large potential drops along magnetic field lines in the upper ionosphere, and many isolated measurements of parallel electric fields have been reported, but the global pattern and significance of these electric fields are unknown. Ionospheric conductivities are now thoroughly measured, but are highly variable. Simple self-consistent theoretical models of the magnetospheric-convection system imply that the magnetospheric particles should shield the inner magnetosphere and low-latitude ionosphere from most of the time-average convection electric field.

A new plasma regime in the distant geomagnetic tail

Abstract: Observations are reported of an extensive region of low-energy plasma particles (LEP) flowing antisunward along the ordered field lines in the lobes of the geomagnetic tail at lunar distances. The flow was detected by three suprathermal ion detectors deployed on the lunar surface during the Apollo 12, 14, and 15 missions. This particle regime is found to be similar to the 'boundary layer' and 'plasma mantle' observed at smaller geocentric distances and to an interior flow region parallel to the magnetopause in the dayside magnetosphere. It is located exterior to the plasma sheet across essentially the entire tail and adjacent to the magnetopause on both the dawn and dusk sides of the magnetosphere. Variations in the integral flux, temperature, and number density are described. It is suggested that this flow and the three similar regimes are simply connected along the inner surface of the magnetopause and are, in fact, the same phenomenon.

An ancient lunar magnetic dipole field

Abstract: SINCE the landing of the first Luna spacecraft it has been known that the present magnetic moment of the Moon is negligible. Later, the Explorer 35 data1 gave an upper limit of 4 γ (= 4 nT) to the dipole field at the surface. Now, Russell et al.2 by analysing the Apollo 15 subsatellite magnetometer data obtained during passages of the Moon through the geomagnetic tail (where the magnetic field is reasonably steady) have calculated an upper limit of 0.05 γ. They have shown that both the induced and the permanent magnetic dipole moments can be determined by separately analysing data obtained during passages of the Moon through the north and south lobes of the tail, in which the field is directed towards and away from the Sun, respectively. Only the permanent dipole component in the plane of the subsatellite orbit is determinable and is less than 1.3 × 1018 gauss cm3: it is unlikely that the component perpendicular to it is appreciably greater. Russell et al.2 estimated the error to be 1.33 × 1018 gauss cm3, and therefore concluded that the lunar magnetic moment is not significantly different from zero. Paradoxical as it may seem, it follows from this observation that the Moon possessed a magnetic field of internal origin in its early history.

Polarization changes of geomagnetic Pi 2 pulsations associated with the plasmapause

Abstract: The polarization characteristics of substorm-associated Pi 2 pulsations are studied in detail on statistical and nonstatistical bases by using magnetic data from a network of stations (L = 3.2–4.4) along a geomagnetic meridian and a conjugate station (L ∼ 4) in the southern hemisphere. The predominant periods in the Pi 2 wave trains are usually constant at all latitudes, whereas the polarization characteristics of the predominant oscillations change drastically among the stations. That is, reversals of the sense of polarization between two stations were often observed. Such polarization reversals result from the latitude dependence of the wave phase: the D (azimuthal) pulsation components oscillate in phase at all stations, whereas the oscillations in the H (north-south) components have large phase shifts among the stations. The H component phase shift is often 180° between Durham (L = 3.2) and Lac Rebours (L = 4.0). A satellite measurement of the plasmapause location during one Pi 2 event showed that the large H component phase shift occurred near the plasmapause. Therefore the Pi 2 oscillations, which are originally excited at high latitudes in the magnetosphere, apparently couple to the shear Alfven waves of the resonant local field lines at the plasmapause because of the sharp decrease in the wave phase velocity just inside the plasmapause. This result suggests the possibility of determining the location of the night side plasmapause by monitoring the polarization changes of Pi 2 pulsations along a geomagnetic meridian. The Pi 2 data from the conjugate pair stations showed that the H components oscillate in phase, whereas the D components oscillate out of phase. Therefore it is concluded that the Pi 2 waves observed near L = 4 can be characterized as odd mode standing Alfven waves.

Magnetospheric mapping with a quantitative geomagnetic field model

Abstract: Mapping the magnetosphere on a dipole geomagnetic field model by projecting field and particle observations onto the model is described. High-latitude field lines are traced between the earth's surface and their intersection with either the equatorial plane or a cross section of the geomagnetic tail, and data from low-altitude orbiting satellites are projected along field lines to the outer magnetosphere. This procedure is analyzed, and the resultant mappings are illustrated. Extension of field lines into the geomagnetic tail and low-altitude determination of the polar cap and cusp are presented. It is noted that while there is good agreement among the various data, more particle measurements are necessary to clear up statistical uncertainties and to facilitate comparison of statistical models.

The Terrestrial Magnetosphere: A Half-Wave Rectifier of the Interplanetary Electric Field

Abstract: A study of geomagnetic disturbances during 1967 and 1968, for which in situ solar wind observations are available, reveals that the magnetosphere acts as a half-wave rectifier of the interplanetary electric field. The rate of injection of energy into the magnetosphere, as inferred from the strength of the disturbance, is approximately linearly proportional to the component of the electric field from dawn to dusk but is effectively zero if the electricfield has a component from dusk to dawn.

Can the Earth's magnetic field be sustained by core oscillations?

Abstract: Some recent suggestions for generating the earth's magnetic field via short period oscillations of the liquid core are critically examined. Seismic energy seems inadequate for the purpose. Limits on the minimum amplitude of such oscillations are derived and these are shown to be so large that they would be accompanied by observable gravity anomalies. It is concluded that waves with periods much less than years are unimportant for the dynamo problem.

Auroral electrojet current density deduced from the Chatanika radar and from the Alaska meridian chain of magnetic observatories

Abstract: The use of the equivalent overhead current approximation for geomagnetic perturbations is critically examined by using the height-integrated ionospheric current density deduced by the Chatanika incoherent scatter radar and the simultaneous ground magnetic variations from the Alaska meridian It is found that time variations of H on the earth's surface are in good agreement with those of the observed east-west ionospheric current density if variations in width and location of the electrojet with respect to a ground-based observatory are carefully examined However, by using the conventional infinite overhead current approximation the height-integrated current density in the ionosphere is, on the average, underestimated by a factor of 2 or more

On the possibility of deducing interplanetary and solar parameters from geomagnetic records

Abstract: While at present we are able to deduce from ground records only qualitative properties of the solar wind, in the future quantitative deductions may be possible, in a statistical sense, from an examination of polar cap magnetograms together with records of geomagnetic activity. The qualitative inferences that are possible now indicate several important features of the behavior of the solar wind over the last 100 years. First, there appear to be significant long term changes in either the solar wind velocity, the magnetic field strength, the variability of the field or some combination of all three. Second, a heliographic latitude dependence of these parameters exists, whose amplitude depends on sunspot number. Third, with the exception of the most recent solar cycle, there is little north-south asymmetry in these solar parameters. Finally, there is a double sunspot cycle modulation of geomagnetic activity, the most likely cause of which is a modulation of the interplanetary magnetic polarity with latitude, and which in turn implies the presence of a solar polar magnetic dipole. The amplitude of this modulation has undergone significant changes since 1868, being large then and at the present, but effectively disappearing from 1908 to 1948.

Strong Geomagnetic Fields during a single Icelandic Polarity Transition

Abstract: Summary Both the magnitude and direction of the palaeomagnetic field have been determined during a field reversal. The results indicate that the geomagnetic field was large and stable when the magnetic pole was close to the equator.

The Sun's Magnetic Sector Structure

Abstract: The synoptic appearance of solar magnetic sectors is studied using 454 sector boundaries observed at Earth during 1959–1973. The sectors are clearly visible in the photospheric magnetic field. Sector boundaries can be clearly identified as north-south running demarcation lines between regions of persistent magnetic polarity imbalances. These regions extend up to about 35 ° of latitude on both sides of the equator. They generally do not extend into the polar caps. The polar cap boundary can be identified as an east-west demarcation line marking the poleward limit of the sectors. The typical flux imbalance for a magnetic sector is about 4 × 1021 Mx.

Some early results from the ATS‐6 Radio Beacon Experiment

Abstract: The multifrequency satellite radio beacon enables the measurement of the columnar electron content of the ionosphere and plasmasphere along the ray path and its spatial and temporal structure. Measurements include modulation phase, Faraday rotation, and amplitude. The characteristics of the beacon transmitter and its design are presented together with the design of the Boulder receiver and antennas and the calibration procedures. A shape factor F is defined which depends on the electron density and geomagnetic field distributions. It is found that F varies by about 30% from day to night. It is shown that the ratio of the plasmaspheric content to total content varies from about 0.08 during the day to about 0.35 at night. Other examples which are presented to illustrate the uses of the radio beacon include sunrise effects, solar flare enhancements of total content, and the ionospheric storms of early July 1974.

Pioneer 7 observations of plasma flow and field reversal regions in the distant geomagnetic tail

Abstract: The present paper gives the results of an extensive analysis of plasma and magnetic-field data from Pioneer 7 taken in the geomagnetic tail approximately 1000 earth radii downstream from earth. The principal observations are: (1) measurable fluxes of protons in the tail, flowing away from earth, sometimes with a double-peaked velocity distribution; (2) field reversal regions in which the field changes from radial to antiradial by a vector rotation in the north-south plane; and (3) general characteristics of the tail similar to those observed near earth with good correlation between taillike magnetic fields and plasma.

Latitude dependence of ionosphere total electron content: Observations during sudden commencement storms

Abstract: The latitude dependence of the changes in the ionosphere total electron content (TEC) has been studied during 12 magnetic storms. TEC observations were obtained at Hamilton, Massachusetts, and Arecibo, Puerto Rico. Definite latitude differences are observed in the TEC responses during the magnetic storms: both TEC enhancements and TEC depletions are observed at Hamilton, while only enhancements are measured at Arecibo. A pre-local midnight TEC ‘ledge’ is frequently observed in the Arecibo storm data but is seldom observed in the higher-latitude Hamilton data. These enhanced ledges in the Arecibo TEC, together with the nighttime TEC depletions measured at Hamilton, produce large south-to-north differences in the ionosphere TEC. The ledges at Arecibo may arise from electrodynamic drift effects associated with westward electric fields; the role of the motion term in the continuity equation for the TEC is assessed under these conditions. The local evening enhancements in the TEC during certain large storms are discussed in the context of the latitude dependence of the storm time magnetic field variations measured from Great Whale River to San Juan. It is shown that the large TEC enhancements appear to be associated with large positive geomagnetic bays at middle to high latitudes.

HEOS Observations of the Configuration of the Magnetosphere

Abstract: Summary The preliminary results of an analysis of magnetic field measurements made over a five-year period in the Earth‘s magnetosphere are presented. These measurements were obtained from Imperial College experiments in the European Space Research Organization’s two HEOS satellites which provided data in the near-Earth region of the tail sheet and in the relatively unexplored polar regions of the magnetosphere. Using a new co-ordinate system which allows for the variable orientation of the geomagnetic dipole, the data provide a graphic illustration of the field topology in the polar and near-tail regions: the polar cusp or cleft and the average width of the tail sheet are clearly revealed. The results of a comparison between the measured field and the predictions of magnetospheric models are outlined. The paper concludes by using data taken during two transits through the Earthward edge of the tail sheet to indicate the extreme variability of the field configuration in that region.

Comments on Field Line Resonances and Micropulsations

Abstract: Summary Features and limitations of a simple plasma model exhibiting hydro-magnetic transverse mode field line resonances are discussed in relation to geomagnetic micropulsations.

Error enhancement in geomagnetic models derived from scalar data

Abstract: An investigation conducted by Backus (1970) regarding the possible existence of two harmonic functions of certain characteristics in three-dimensional space is considered. The derivation of a model of the main geomagnetic field from scalar data is discussed along with a numerical simulation study. It is found that experimental discrepancies between vector field observations and the predictions of the model may have a mathematical origin, related to the work of Backus.