Showing papers on "Earth's magnetic field published in 1984"

A survey of dayside flux transfer events observed by ISEE 1 and 2 magnetometers

Abstract: Flux transfer events (FTEs), observed on both the interior and exterior of the dayside magnetopause region by the ISEE 1 and 2 spacecraft, are noted to be a feature of the magnetopause region covered by the spacecraft when the magnetic field in the magnetosheath has a southward component, but not when it is northward. During periods of southward magnetosheath field, the average number and recurrence time of FTE signatures/magnetopause crossing are similar to those observed in the magnetopause interior, implying that the magnetosheath and magnetosphere FTEs are aspects of the same physical phenomenon. It is speculated that FTEs may provide the dominant means of flux transfer required for the driving of geomagnetic disturbances.

Time-varying magnetic fields: effect on DNA synthesis

Abstract: Human fibroblasts have exhibited enhanced DNA synthesis when exposed to sinusoidally varying magnetic fields for a wide range of frequencies (15 hertz to 4 kilohertz) and amplitudes (2.3 X 10(-6) to 5.6 X 10(-4) tesla). This effect, which is at maximum during the middle of the S phase of the cell cycle, appears to be independent of the time derivative of the magnetic field, suggesting an underlying mechanism other than Faraday's law. The threshold is estimated to be between 0.5 X 10(-5) and 2.5 X 10(-5) tesla per second. These results bring into question the allegedly specific magnetic wave shapes now used in therapeutic devices for bone nonunion. The range of magnetic field amplitudes tested encompass the geomagnetic field, suggesting the possibility of mutagenic interactions directly arising from short-term changes in the earth's field.

Large-scale Birkeland currents in the dayside polar region during strongly northward IMF: A new Birkeland current system

Abstract: Stable, well-defined patterns of transverse magnetic disturbances have been observed in the polar regions that persist during periods of strongly positive Bz (≥ 5 nT) and that increase in amplitude as Bz increases. This has been determined from an examination of magnetic field data acquired during 146 orbits of MAGSAT over the south polar regions during November 1979 to January 1980 and supplemented by four consecutive orbits of TRIAD over the north polar region in July 1977. The characteristics of these polar disturbances include the following: (1) they occur at latitudes poleward of the Region 1 Birkeland current system at daytime magnetic local times (0600 MLT through noon to 1800 MLT); (2) the spatial distribution along the dawn-dusk direction resembles the “W”-shaped distribution of electric fields observed in the polar cap during periods of positive Bz (Burke et al. [1979]); (3) these patterns show remarkable stability, showing little change from orbit to orbit, up to seven orbits of MAGSAT (equivalent to a period of 10 hours); and (4) the magnitude of the peak disturbance, ΔB, correlates with a “complementary” magnetospheric transmission function of the form: ϵ* = (By² + Bz²)½ cos θ/2, where θ is the angle between the positive z axis and the interplanetary magnetic field (IMF). If the magnetic disturbances are interpreted in terms of Birkeland currents, they flow downward on the duskside and flow away on the morningside (identical to the cusp current flow reported by Iijima and Potemra [1976b]). During periods of negative By the region of morningside (upward flowing) currents is much larger than the eveningside (downward flowing) current region in the southern hemisphere. The density of the currents in the smaller spatial region is larger than the density of the currents in the larger region. This pattern systematically reverses during periods of positive By. We interpret these observations as evidence for a large-scale, stable Birkeland current system in the polar region that is associated with merging on field lines in the geomagnetic tail. This current system intensifies and is more stable as Bz becomes more northward (reminiscent of the behavior of the Region 1 current system with increasing southward values of Bz). The new stable polar cap current system described here is referred to as the “NBZ” Birkeland current system for “northward Bz” and is important because of its relationship to a variety of other northward By phenomena such as polar cap auroral arcs (“theta aurora”) and multicell convective flow patterns. The existence of these stable NBZ currents and the correlation of their amplitudes with the IMF substantiate the fact that energy continues to flow to the earth's polar regions during periods of strongly northward IMF.

Structure of the magnetotail at 220 RE and its response to geomagnetic activity

Abstract: Using plasma electron and magnetic field measurements from ISEE 3, 220 RE from earth, we find that the magnetotail at that distance is a coherent structure that evidently waves about through distances comparable to its own lateral scale size. For about one-third of the time it was inside the magnetotail, ISEE 3 was in the plasma sheet. During quiet times the plasma sheet is apparently quite thin, but in response to geomagnetic activity it expands, becoming filled with hot plasma flowing tailward at speeds sometimes exceeding 1000 km/sec, and forces the magnetotail cross-section itself to expand. The plasma sheet’s expansion is delayed typically by about 30 minutes from the onset of the associated geomagnetic activity (often a clearly identified isolated substorm). The magnetic field in the newly-expanded plasma sheet usually exhibits a few-minute steep northward excursion followed by a more prolonged (and often steep) southward excursion. We believe these to be the signatures of arrival of a plasmoid formed and released near the earth at the onset of the corresponding geomagnetic activity. The discreteness of these plasma releases through the magnetotail and their close association with onsets of geomagnetic activity at earth suggest that they are consequences of spontaneous release, probably by magnetic reconnection, of energy and plasma earlier stored in the magnetotail.

Longitudinal variability of annual changes in activity of equatorial spread F and plasma bubbles

Abstract: Global distribution maps of equatorial spread F (ESF) activity for various annual periods were derived from the topside soundings by the Ionosphere Sounding Satellite b (ISS-b) in 1978–1980. The ESF activity during the northern winter period reveals maximum enhancement at the Atlantic longitudes of large westward geomagnetic declination, and during the northern summer at the Pacific longitudes of large eastward declination. On several orbits passing over the region of the ESF activity enhancement, abrupt depletions of the electron density or equatorial plasma bubbles were observed at the satellite altitude of about 1100 km. It has also been found that the background electron density distributions tend to be symmetric with respect to the magnetic equator in the region of the enhanced ESF activity and asymmetric in the region of the suppressed ESF activity. These observations are taken account of by the influence of a transequatorial thermospheric wind upon the suppression of the Rayleigh-Taylor type plasma instability.

The Earth's Magnetic Field: Its History, Origin and Planetary Perspective

Abstract: After a historical introduction in Chapter 1, the more traditional aspects of geomagnetism relating to the present field and historical observations are presented in Chapter 2. The various methods and techniques and theoretical background of palaeomagnetism are given in Chapter 3. Chapters 4, 5 and 6 present the results of palaeomagnetic and archaeomagnetic studies in three topics. Chapter 4 relates to studies of the geomagnetic field roughly back to about 50,000 years ago. Chapter 5 is about reversals of the geomagnetic field and Chapter 6 presents studies of the field for times older than 50,000 years and on the geological time scale of millions or hundreds of millions of years. Chapters 7, 8 and 9 provide insight into dynamo theory. Chapter 7 is essentially a non-mathematical attempt to explain the physical basis of dynamo theories to palaeomagnetists. This is followed in Chapter 8 by a more advanced theoretical treatment. Chapter 9 explains theoretical aspects of secular variation and the origin of reversals of the geomagnetic field. Chapter 10 is our attempt to relate theory to experiment and vice versa. The final two chapters consider the magnetic fields of the moon, sun, planets and meteorites, in an attempt to determine the necessary and sufficient conditions for magnetic field generation in large solar system bodies.

By -dependent convection patterns during northward interplanetary magnetic field

Abstract: Observations of Birkeland currents, electric fields, and auroral forms in the dayside polar regions during periods of northward interplanetary magnetic field (IMF) can be unified to fit into consistent convection patterns ordered principally by the IMF By component. During periods of weak IMF By, two polar convection cells are symmetrically located on either side of the noon-midnight meridian, producing sunward convection over the pole, as previously reported. As the IMF By becomes significantly positive (negative), the dawn (dusk) convection cell expands across the northern polar cap, whereas the other cell shrinks. In the southern polar cap it is the dusk (dawn) cell which expands. During strong By this cell expansion gives the appearance of a single convection cell in the polar region. At the “collapsed” cell a large convective flow gradient is developed where reversal of the northward Bz (NBZ) Birkeland current system, antisunward polar ionospheric current, and the most intense polar cap electric fields are statistically observed. We suggest that this convection gradient region is also associated with sun-aligned arcs and the transpolar arc of the theta aurora, which are observed in the polar regions during northward IMF. The convection patterns proposed here are consistent with the antiparallel merging model in which the IMF merges with the geomagnetic field lines in the tail lobe during northward IMF intervals. The resulting convection in the tail brings plasma from the plasma sheet to the tail. This process divides the open field lines of the lobes and produces an area of closed field lines in the polar cap associated with the transpolar arc of the theta aurora.

Outer-core geostrophic flow and secular variation of Earth's geomagnetic field

Abstract: In studies of the temporal variations of the main internal geomagnetic field (the secular variation or SV), it is usual to consider separately the variations of the dipolar and non-dipolar parts which appear to have different time constants. The mechanism that is generally invoked to explain the generation of SV is the advection of the lines of force of the main field by the highly conducting fluid at the top of the core. Such a mechanism involves the main field as a whole and it is not clear a priori why its two parts should behave separately. I show here that the Coriolis force will probably dominate the force budget at the top of the core and that, in such a case, the motion of the fluid involves the two parts of the field in a different way; in particular, the existing axial dipolar component is not re-engaged in the process which builds up the SV.

The nonlinear gyroresonance interaction between energetic electrons and coherent VLF waves propagating at an arbitrary angle with respect to the Earth's magnetic field

Abstract: A theory is presented of the nonlinear gyroresonance interaction that takes place in the magnetosphere between energetic electrons and coherent VLF waves propagating in the whistler mode at an arbitrary angle psi with respect to the earth's magnetic field B-sub-0. Particularly examined is the phase trapping (PT) mechanism believed to be responsible for the generation of VLF emissions. It is concluded that near the magnetic equatorial plane gradients of psi may play a very important part in the PT process for nonducted waves. Predictions of a higher threshold value for PT for nonducted waves generally agree with experimental data concerning VLF emission triggering by nonducted waves.

Evidence for slow-mode shocks in the deep geomagnetic tail

Abstract: Plasma and field data from the ISEE-3 space probe provide evidence that the lobe-plasma sheet boundary is the central part of the distant geomagnetic tail is often a slow-mode shock. Such shocks are predicted by Petchek's (1964) model of the reconnection. If this model applies, then the ISEE-3 observations place the general location of the reconnection diffusion region closer to the earth than x of about -100 earth radii.

Short-period geomagnetic secular variation

Abstract: In this paper the main problems of the theory of the short-period geomagnetic secular variation are described. An approximate analytical representation of the observational data on both the geomagnetic field variations and the associated variations in the length of the day, the kinematics of the generation of geomagnetic variations in the skin-layer at the core-mantle boundary and the influence of the electrical conductivity of the mantle on the geomagnetic variations are considered. A theory of torsional oscillations in the Earth's core is developed and an attempt is made to evaluate the associated geomagnetic variations. Magnetohydrodynamic effects in a possible thin layer of stably stratified fluid near the core-mantle boundary are investigated.

The distribution of reconnection geometry in flux transfer events using energetic ion, plasma and magnetic data

Abstract: The distribution of energetic ion anisotropies in flux transfer events (FTEs) about the dayside magnetopause has been determined for ISEE 2 crossings of the boundary in 1977 and 1978. When the events are sorted according to the sign of the east-west component of the magnetic field in the magnetosphere, a clear correlation is observed on the northern morningside. When the field is eastward, particles flow antiparallel to the field, implying field line connection to the Northern Hemisphere; when the field is westward, the opposite is true. On the afternoonside, the particle anisotropies are correlated with latitude. Explanations for this pattern are discussed which involve FTE formation at low latitudes with subsequent motion at a velocity given by the vector superposition of the Alfven velocity from the release of magnetic tension and the magnetosheath bulk flow velocity. Evidence that the geomagnetic and not the geocentric solar magnetospheric equator is the source of FTEs is considered.

Plate motions and the geomagnetic field. II. Jurassic to Tertiary.

Abstract: Summary. The analysis of the time-averaged geomagnetic field is extended back to 200Ma. Palaeomagnetic poles from the major plates have been carefully selected from recent compilations of reliable results for each region. These were returned, with their corresponding sampling sites, to their locations at the estimated dates of magnetization, in a fixed-hotspots framework. The corrected results were then grouped into 20Ma windows at intervals of 10Ma representing the past 100Ma, and 40Ma windows at 30Ma intervals for the more sparse 100–200Ma data. Global means and Fisher statistics were calculated for each window having included the axial quadrupole in the calculation. The value of this coefficient which gave the maximum value for the Fisher precision parameter (tightest grouping of poles) was taken as representative of each interval. The results indicate that a small axial quadrupole of the same sign as the axial dipole may have persisted throughout the Cenozoic. This is equivalent to a northward offset axial dipole field (far-sided effect). During the late Cretaceous, this component appears to have changed sign with respect to the dipole. Negative values seem to have obtained throughout the Cretaceous long normal polarity interval, corresponding to a southward offset dipole (near-sided effect). The data distribution is inadequate for the resolution of the quadrupole at earlier times, and zero values cannot be discounted. Little relative motion is implied between the hotspots and the geomagnetic axis for the past 90 Ma, the global mean polar path curving around the predicted fixed-hotspots pole at a distance of typically 5° latitude with little sign of rapid Tertiary polar wander as implied by studies of Pacific data alone. Between 100 and 200 Ma, however, there is a clear difference between the two reference frames, amounting to 17–19° in the Jurassic. This may reflect motion of the mantle relative to the geomagnetic axis, but may also include errors due to inaccurate determination of hotspot tracks and inter-hotspot motion.

Possibility of Reconstruction of the Past Geomagnetic Field from Homogeneous Sediments

Abstract: Magnetizing process of a sediment is formulated using a linear system theory. Remanent magnetization is represented as a convolution integral of geomagnetic field variation and a moment fixing function. The formulated model predicts that a field variation is deformed by filtering to be recorded in magnetization of sediments, and that the original field is deduced from the record of magnetization by a deconvolution method. Application of the model to the magnetization data of a core of shallow water sediment reveals that the remanent intensity variation may reflect the intensity reduction due to the cancellation of magnetic moments aligned in various directions rather than decrease in a field intensity. Width of magnetizing zone is inferred to be unexpectedly small from results of measurement of magnetization and laboratory experiments. The half fixing depths, at which a half of magnetic moments in a horizon is locked in, seem to cluster around 10cm or less.

Electron energization in the geomagnetic tail current sheet

Abstract: Electron motion in the geomagnetic tail current sheet is examined analytically. An estimation of the energization of electrons trapped by the convective field is made independent of a guiding center approximation. The electrons in the energy range 100 eV - 100 keV have their pitch angle scattered and trapped in the current sheet until accelerated by the convective field. Predictions are made for the electrons that escape the current sheet, electron precipitation in diffuse auroral phenomena and current sheet interactions as a source for plasma sheet electrons and ions.

Reversals of the Earth's Magnetic Field

Abstract: By measuring the direction and intensity of magnetism in rocks of different ages, a record of the Earth's magnetic field in the past can be obtained. This 1994 book deals with the particular case of reversals of the Earth's magnetic field. These have played a major role in the development of plate tectonics and in establishing a geological time scale. The magnetism of rocks is discussed in some detail with a warning of possible misinterpretations of the record. Observational results and theories are reviewed with special attention to the structure and geometry of the transition field. Changing conditions at the core-mantle boundary, their effect on reversals, the generation of plumes and the possible correlation of reversals with tectonic changes, ice ages or mass extinctions are thoroughly discussed, including suggested periodicities in the reversal record and in other geophysical data.

Slow mode shocks in the Earth' magnetotail: ISEE-3

Abstract: High time-resolution magnetic field measurements adjacent to the boundary of the distant magnetotail plasma sheet have been analyzed. The nature of the changes in the field between the lobe and the plasma sheet is consistent with the boundary being a slow mode shock. The crossing times are relatively long (about 30 sec), implying a shock thickness estimated to be approximately 2000 km. An increase in the entropy of the electrons is consistent with dissipation at the shock. Reasonable shock-normal directions are derived, and may be used to determine the location of the spacecraft relative to the reconnection or merging region. It is suggested that a foreshock exists in which upstream wave and particle phenomena are occurring.

A physical model for palaeosecular variation.

Abstract: Summary. A new model to describe the latitude dependence of the angular dispersion of the palaeomagnetic field (palaeosecular variation) is developed following previous models, but with crucial differences. It is shown that if the probability distribution of virtual geomagnetic poles (VGPs) is circularly symmetric about the rotation axis then the geometry of the distribution of field directions is latitude dependent. This has a significant effect on the latitude dependence of dispersion and is accounted for in the model. The dipole and non-dipole parts of the field are not artificially separated but are intimately linked through an observationally determined relation that the time averaged intensity of the non-dipole field is dependent upon the intensity of the dipole field. It is shown that a consequence of this relation is that no knowledge of the probability distribution of the geomagnetic dipole moment is required. This is a fundamental improvement over previous models. The model provides excellent fits to the palaeodata and, unlike previous models, is not inconsistent with the latitude variation of the non-dipole field dispersion determined from the present field. For the past 5 Ma the point estimate of the VGP dispersion due to dipole wobble is 7.2° and of the VGP dispersion at the equator due to variation in the non-dipole field is 10.6°. This estimate of the dispersion due to variation in the non-dipole field is in excellent agreement with the value predicted from an analysis of the variation in field intensities over the same period. Fits of the model to data from earlier periods indicate that dispersion due to variation in the non-dipole field is essentially independent of the geomagnetic reversal rate while dipole wobble is positively correlated with reversal rate.

Associations of geomagnetic activity with plasma sheet thinning and expansion: A statistical study

Abstract: Associations of geomagnetic activity in the auroral zone with thinnings and expansions of the magnetotail plasma sheet are examined statistically in this paper. We first identified many plasma sheet thinnings and expansions in plasma and particle data from VELA satellites and from OGO 5 without reference to ground magnetic data. These events were grouped according to the location of the detecting satellite in the magnetotail. For each such group the times of thinning or expansion were then used as fiducial times in a superposed-epoch analysis of the geomagnetic AL index values that were recorded in 8-hour intervals centered on the event times. The results show that many plasma sheet thinnings and expansions are related to discrete negative bay structures that are the classical signature of substorms. Furthermore, they support earlier findings that plasma sheet thinning and expansion at the VELA orbit (r ≈ 18 RE) tend to be associated with the onset of the auroral zone negative bay and the beginning of its subsidence, respectively. Earthward of r ≈ 13–15 RE, plasma sheet expansion occurs near the time of the onset of the negative bay, again in agreement with earlier findings. A large fraction of plasma sheet expansions to half thicknesses of ≳ 6 RE at the VELA orbit are associated not with a baylike geomagnetic disturbance but with subsidence of a prolonged interval of disturbance. The study also shows that many plasma sheet expansions are related simply to generally enhanced geomagnetic activity showing no baylike or other distinctive features.

Energetic ion regimes in the deep geomagnetic tail: ISEE‐3

Abstract: Bame et al. (1983) have given plasma measurements for two 12-hour intervals and have identified the plasma regions observed during the January 1-March 30, 1983 traversal by the ISEE-3 spacecraft of a previously unexplored part of the distant geomagnetic tail. Attention is presently given to the 35 keV-1.6 MeV energetic ion and magnetic field measurements made by ISEE-3, relating them to the plasma measurements. The plasma sheet is found to be a region of energetic ion enhanced fluxes, with unidirectionally tailored flow. A characterization accounting for these characteristics involves occurrence of reconnection at a neutral line that lies earthward of the spacecraft.

Plasma entry into the distant tail lobes: ISEE‐3

Abstract: ISEE-3 measurements indicate that a broad mantle-like boundary layer plasma often exists within the distant geomagnetic tail lobes at all latitudes, directly adjacent to the tail magnetopause. The presence of this boundary layer at large tail distances indicates that plasma from the magnetosheath often crosses the magnetopause locally along much of the length of the tail, and is evidence that the tail is 'open'.

Intense fluxes of low‐energy electrons at geomagnetic latitudes above 85°

Abstract: A systematic survey was performed to determine the frequency of occurrence of intense fluxes of electrons above the level of the polar rain at geomagnetic latitudes ≥85° and their relationship to the orientation of the interplanetary magnetic field, the density and velocity of the solar wind, and geomagnetic activity. These intense fluxes we identify as the “polar showers” and “polar squalls” reported by Winningham and Heikkila (1974). For the study, 452 orbits of data from the SSJ/3 sensor on the DMSP F2 satellite were used. The orbits selected satisfied the criteria that the satellite attained a geomagnetic latitude ≥85° during the orbit and that interplanetary magnetic field (IMF) and solar wind data were available both for the hour in which the satellite passed over the northern hemisphere and for the previous hour. The key result of the study is that in 50% of the cases studied an electron flux exceeding the level of the polar rain was found in both hemispheres at geomagnetic latitudes ≥85° and that such occurrences are dependent predominantly, if not exclusively, on the Bz component of the IMF being positive. Unlike visible polar cap arcs, a positive value of the IMF Bz component appears to be both a necessary and a sufficient condition for the occurrence of such fluxes above 85° geomagnetic latitude. Occurrence is independent of the IMF Bx and By components. The occurrence is strongly skewed toward Kp = 0, but an occurrence exceeding 30% is found up to a Kp = 4. The occurrence is independent of the number density or velocity of the solar wind. The values of the peak energy flux and number flux observed above 85° are roughly logarithmically distributed over the ranges from 3×106 to 3×109 keV/cm² s sr and 107 to 1010 el/cm² s sr, respectively. Average energies of the peak fluxes are typically between 50 and 300 eV. The maximum energy flux and number flux above 85° appear to lie above a threshold dependent on the IMF Bz component magnitude but are independent of the solar wind flux nV. The maxima in integral number flux and energy flux above 85° fall in a continuum of values consistent with visible polar cap arcs being cases near the maximum of the continuum.

Thermospheric circulation, temperature, and compositional structure of the southern hemisphere polar cap during October–November 1981

Abstract: Results of NCAR thermospheric general circulation model (TGCM) simulations were compared with Dynamics Explorer (DE2) data on the neutral wind, temperature and O and N2 number variations over the south polar cap in the F region in October-November 1981. Attention was focused on variations in the thermospheric characteristics. All variations reached a maximum over the magnetic polar cap, with the largest O and N2 changes being present in the dawn sector of the auroral oval, where Joule heating was maximized. The TGCM simulations for conditions similar to the DE2 passes agreed well with the satellite data, indicating that the variations in the wind, temperature and compositional characteristics are dependent on the magnitude of magnetospheric convection and influenced by the separation between the geomagnetic and geographic poles.