Showing papers on "Earth's magnetic field published in 1986"

Magnetopause stability threshold for patchy reconnection

Abstract: This review is devoted to the problem of the internal fine structure of the Earth's magnetopause. A number of theoretical and experimental papers dealing with this subject is discussed from a unified viewpoint. The Vlasov kinetic approach is used to study the stability of magnetopause magnetic surfaces that can be destructed by the growth and overlapping of magnetic islands. The stochastic wandering of magnetic field lines between the destructed surfaces can result in magnetic percolation, i.e. the appearance of a topological connection of interplanetary and geomagnetic field lines. Such a process may be considered as a mechanism of the macroscopic (but spatially localized) reconnection. We discuss this in relation with the phenomena of spontaneous ‘patchy’ reconnection, recently observed at ISEE satellites and now known as flux transfer events. Drift tearing mode, which is responsible for the growth of magnetic islands can be stabilized due to its coupling with ion sound waves, and the process of percolation will be interrupted if even a thin region with smooth stable magnetic surfaces exists within the magnetopause. Accordingly, we obtain a magnetopause stability threshold for localized reconnection. It is represented in the form of dependence of marginal dimensionless thickness of the magnetopause on the angle of magnetic field rotation within it. Further, we discuss the possible role of lower hybrid turbulence permanently observed within the. magnetopause and speeding up the process of reconnection. Nonlinear calculations supporting the developed model are given in the appendices. We consider briefly the motion of reconnecting flux tubes and evaluate the time necessary for the accomplishment of percolation. The calculations show that the appearance of reconnection ‘patchies’ at the dayside magnetopause cannot occur too far from the stagnation region. The latter agrees with experimental indications on the most probable site of the formation of flux transfer events. In the concluding part of the review we discuss the necessary limitations on the theory, possible lines of its future advance and comparison with the experimental data.

A statistical study of the central plasma sheet: Implications for substorm models

Abstract: The University of Iowa Lepedea on board ISEE 1 is used to investigate the characteristics of the central plasma sheet under all levels of geomagnetic activity. Positive ion responses from 1 eV to 45 keV are used in this study. All the periods during 1978 when the central plasma sheet is encountered are included. This study excludes all boundary layer samples. The results of this study show that the central plasma sheet consists of plasma with high thermal energy (several keV) but low bulk speeds. This remains true even during high geomagnetic activity. The main effect of increasing activity is heating of the plasma sheet, preferentially at the high-latitude boundaries.

Possible evidence of flux transfer events in the polar ionosphere

Abstract: We present magnetic field data from the cusp-latitude South Pole station that exhibit, under appropriate local time and interplanetary magnetic field conditions, the signature expected in the ionosphere from a flux transfer event (FTE) at the magnetopause. In particular, the model of multiple X-line reconnection at the magnetopause predicts field-aligned currents in helical flux tubes, with transverse magnetic fields propagating as Alfven waves toward the ionosphere. The distinctive magnetic signature at a polar cap magnetic station, particularly in the vertical component, can be used to infer the signs of the By and Bz components of the interplanetary magnetic field.

Reduction to the pole as an inverse problem and its application to low-latitude anomalies

Abstract: Traditionally, reduction to the pole has been accomplished either by space- or wavenumber-domain filtering. In the two-dimensional case, this procedure is stable regardless of the latitude, as long as the source strike is not parallel to the horizontal projection of the geomagnetic field. In the three-dimensional case, however, reduction-to-the-pole filtering is stable only at high magnetic latitudes. At latitudes lower than 15 degrees, it is of no practical use due to a sharply increasing instability toward the magnetic equator.The three-dimensional instability of this filtering technique is demonstrated, and the reduction-to-the-pole problem is formulated in the context of a general linear inverse problem. As a result, stable solutions are found by using well-known stabilizing procedures developed for the inverse linear problem. The distribution of magnetization of an equivalent layer of doublets that reproduces the observed data is computed. The magnetic doublets are parallel to the magnetization direction which is assumed constant throughout the sources. The magnetic field reduced to the pole is then obtained by changing the inclinations of the geomagnetic field and the doublets to 90 degrees and recalculating the total field.The usefulness and limitations of the method at low magnetic latitudes are assessed using theoretical data. The effects of noise and anomaly truncation are also investigated for both high and low latitudes. In all cases, application of the proposed method produced meaningful results regardless of the latitude. The method is applied to field data from two different low-latitude anomalies. The first anomaly is due to a seamount in the Gulf of Guinea with reversed magnetization. The geomagnetic field at this location is about -23 degrees. The second anomaly is an intrabasement anomaly from Parnaiba Basin, Brazil, where the magnetization is assumed to be induced by a geomagnetic field with -1.4 degree inclination. The results obtained confirm that the proposed method produces stable, meaningful, reduced-to-the-pole maps.

Steady flows at the top of Earth's core derived from geomagnetic field models

Abstract: Select models of the main geomagnetic field and its secular variation are extrapolated to the base of an insulating mantle and used to estimate the adjacent fluid motion of a perfectly conducting outer core. The assumption of steady motion provides formally unique solutions and is tested along with that of no upwelling. The hypothesis of no upwelling is found to be substantially worse than that of steady motion. Although the actual motion is not thought to be steady, the large-scale secular variation at the top of the core can be adequately described by a large-scale, combined toroidal-poloidal circulation which is steady for intervals of at least a decade or two. The derived flows include a bulk westward drift but are complicated by superimposed jets, gyres, and surface divergence indicative of vigorous vertical motion at depth. The circulation pattern and key global properties including rms speed, upwelling, and westward drift are found to be fairly insensitive to variations in modeling parameters.

International Geomagnetic Reference Field Revision 1985

Abstract: The International Association of Geomagnetism and Aeronomy (IAGA) Division I, Working Group 1, deals with the topic “Analysis of the Main Field and Secular Variations.” One of the more important functions of the working group is the periodic revision of the International Geomagnetic Reference Field (IGRF). The 13 members of the working group have professional interests covering a broad spectrum of geomagnetic science, including the theory and practice of geomagnetic analysis and modeling, the theory of the origin of the magnetic fields of the earth and other bodies, the theory of geomagnetic secular variation, the application of field models in magnetic survey data processing, and geomagnetic charting.

Chaotic particle dynamics near a two-dimensional magnetic neutral point with application to the geomagnetic tail

Abstract: The dynamical system composed of a charged particle moving in a two-dimensional magnetic field containing a neutral point, with and without a perpendicular electric field, is studied. The system is found to be nonintegrable and chaotic. Lyapunov characteristic exponents are calculated for the various types of chaotic orbits. When parameters relevant to the magnetotail are used it is found that the stochastic nature of the motion is more important for a thick current sheet (20,000 km) than for a thin sheet (1000 km). Some consequences of this stochasticity are discussed, including the suggestion of a collisionless, “chaotic conductivity”; this represents the collisionlike randomizing effect of the chaotic dynamics.

Geomagnetic evidence for fluid upwelling at the core-mantle boundary

Abstract: Summary. Previous studies, both geomagnetic and seismic, have been unable to show conclusively whether or not there is fluid upwelling at the core-mantle boundary. Here a new method is developed, in which an attempt is made to invert geomagnetic secular variation data measured at the Earth's surface for a frozen-flux purely toroidal core-mantle boundary (CMB) velocity field, under the assumption that the mantle is electrically insulating and flux is frozen in at the CMB. These data have previously been inverted for the core-mantle boundary radial secular variation, from which the appropriate fit between model and data is known. Two different main field models were used to assess the effect of uncertainty in its radial component at the CMB. The conclusions were the same in both cases: frozen-flux purely toroidal motions provide a poor fit. A statistical test allows very firm rejection of the hypothesis that the residuals are not significantly larger, whereas there is no statistical difference between the residuals of inversions for radial secular variation and frozen-flux velocity fields at the CMB if upwelling and down-welling is included. The inherent non-uniqueness in the velocity field obtained is not of concern, since only their statistical properties are utilized and no physical significance is attached to the flows obtained.

Holocene secular variation in western North America: Paleomagnetic record from Fish Lake, Harney County, Oregon

Abstract: An 8.9-m composite section of lake sediment from Fish Lake, Steens Mountain, southeastern Oregon, has provided a high-resolution record of Holocene geomagnetic secular variation in western North America. The section is composed of overlapping core segments with stratigraphic control based on six tephra layers and other thin, distinct bands and with age control from 37 radiocarbon dates. The 455 paleomagnetic samples possess a strong and stable magnetic remanence which is carried by pseudo-single-domain magnetite. Paleomagnetic records for individual core segments show low scatter and high serial correlation, and where the segments overlap, there is excellent agreement between records. The azimuthal orientation of the composite section was recovered from the known direction of the geomagnetic field at the time of the climactic eruption of Mount Mazama tephra which is present in Fish Lake. Both internal and external evidence, including the correspondence with an archaeomagnetic record from the southwestern United States, support the conclusion that the Fish Lake record represents the history of geomagnetic field behavior during at least the last 10,000 years B.P. Analysis of that record reveals that the average geomagnetic direction can not be distinguished from that of a geocentric axial dipole field and that the overall circularity of the field was clockwise but the detailed behavior was quite complex. Angular standard deviations of both field directions and virtual geomagnetic poles differ significantly from the values predicted from theoretical models of secular variation but are consistent with those of other Holocene records. When the record from Fish Lake is compared to the proposed type curve for east central North America, there is excellent agreement for the past 6000 years, provided the curves are offset by about 400 years. This offset corresponds to a rate of westward drift of about 0.062°/yr. Beyond about 6000 radiocarbon years ago, good agreement between the two curves occurs when there is no offset, suggesting that the field was standing rather than drifting during this time.

Geomagnetic field analysis—IV. Testing the frozen-flux hypothesis

Abstract: Summary. We present a model of the magnetic field at the core–mantle boundary, for epoch 1959.5, based on a large set of observatory and survey measurements. Formal error estimates for the radial field at the core are 50 μT, compared with 30 and 40 μT for our previous MAGSAT (1980) and POGO (1970) models. Current work on the determination of the velocity of the core fluid relies on the assumption that the core behaves as a perfect conductor, so that the field lines remain frozen to the fluid at the core surface. This frozen-flux condition requires that the integrated flux over patches of the core surface bounded by contours of zero radial field remain constant in time. A new method is presented for constructing core fields that satisfy these frozen-flux constraints. The constraints are non-linear when applied to main field data, unlike the case of secular variation which was considered in an earlier paper. The method is applied to datasets from epochs 1969.5 and 1959.5 to produce fields with the same flux integrals as the 1980 model. The frozen-flux hypothesis is tested by comparing the changes in the flux integrals between 1980/1969.5, 1969.5/1959.5 and 1980/1959.5 with their errors. We find that the hypothesis can be rejected with 95 per cent confidence. The main evidence for flux diffusion is in the South Atlantic region, where a new null flux curve appears between 1960 and 1970, and continues to grow at a rapid rate from 1970 to 1980. However, the statistical result depends critically on our error estimates for the field at the core surface, which are difficult to assess with any certainty; indeed, doubling the error estimates negates the statistical argument. The conclusion is therefore, at this stage, tentative, and requires further evidence, either from older data, if good enough, or from future satellite measurements.

A new shipboard three-component magnetometer

Abstract: The three components of the geomagnetic field were measured at sea by STCM (Shipboard Three Component Magnetometer) with a relative accuracy of 50 ± 25 nT. The STCM was controlled by a microcomputer, and the three geomagnetic components were calculated by reducing the magnetic field due to the ship’s remanent and induced magnetic moments, the only assumption being that the magnetic field is linearly related to the magnetic moments. Although STCM has no particular magnetic compensation device, it can be set at any place on board of any kind of ship. In the KH82-5 cruise, the three component measurements were conducted along latitude 42 °N north of the Mendocino fracture zone where there are well‐defined magnetic anomaly lineations trending north and south. STCM measured the north component anomalies with very small amplitudes compared to the east and downward component anomalies, which shows that lineations trend north and south. The three measured component anomalies are almost identical to the anomalies ...

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.

Evidence of transmission of polar electric fields to the low latitude at times of geomagnetic sudden commencements

Abstract: The HF Doppler frequency deviation associated with the geomagnetic sudden commencement (sc), called SCF, consists of the negative preliminary frequency deviation (PFD) and the succeeding positive main frequency deviation (MFD) when observed during the night at geomagnetic low latitude (geomagnetic latitude = 25°) This type of HF Doppler frequency deviation, SCF(∓), corresponds to the sc(+) at low latitude (Kakioka, Japan) and the sc(∓) (=sc*) at high latitude (Husafell, Iceland) The SCF(∓) starts earlier than the low latitude sc by 50 s, while it occurs simultaneously with the high latitude sc* within a time accuracy of 10 s This result implies that the negative PFD of the nighttime SCF(∓) is caused by a dusk-to-dawn polar electric field responsible for the preliminary reverse impulse (PRI) of sc*, the occurrence of which is earlier than the occurrence of the world-wide geomagnetic increase Consequently, the magnetospheric electric field generated at the onset of sc is transmitted to the low-latitude ionosphere by way of the polar ionosphere, earlier than the direct transmission through the magnetosphere by compressional hydromagnetic waves It is also shown that the daytime SCF(∓) starts simultaneously with the low-latitude sc(+), suggesting that the positive PFD of the SCF(±) is caused by the westward electric field transmitted directly from the magnetosphere by the compressional hydromagnetic wave

Theory and simulations of broadband electrostatic noise in the geomagnetic tail

Abstract: The excitation mechanism for broadband electrostatic noise (BEN) and the effects of BEN particles in the geomagnetic tail are examined using the linear analysis theory and particle simulations. The linear theory for electrostatic instabilities is discussed. The plasma sheet particle population is simulated using counter-streaming cold ion beams, and warm ions and electrons. The ion-ion instability, ion-acoustic mode, and the electrostatic ion cyclotron harmonic waves are studied. The velocity distributions, electric field intensity, and electron plasma waves for the plasma sheet boundary layer are evaluated. The frequency wave spectrum and particle distributions are computed and analyzed. The conditions for the two simulations, which differ only in beam ion drift speed, are described; it is observed that in the first simulation the dominate modes propagate parallel to the magnetic field and in the second simulation the propagation modes are oblique. The simulation data reveal that when beam temperature is smaller than plasma sheet temperature ion-acoustic and ion-ion instabilities grow to large amplitudes heating both electrons and ions. The data are compared to ISEE-1 observations and good correlation is obtained.

Auroral zone thermospheric dynamics: 1. Averages

Abstract: Forty-four nights of thermospheric neutral wind and temperature measurements were obtained with a ground-based Fabry-Perot spectrometer at College, Alaska (65° invariant latitude), mostly during high to moderate levels of solar activity. The spectrometer scanned the O I 15,867 K (6300 A) emission line yielding line profile measurements that originate from an altitude range between 170 and 280 km. The observed average wind direction is to the northwest in the early evening, changing to the south near midnight and then to the southeast in the morning. The overall wind pattern generally persists, but the wind speed increases with increasing geomagnetic activity. A change in the zonal wind direction occurs earlier in magnetic local time with increasing magnetic activity. The meridional wind pattern also shifts equatorward with the auroral oval during increasing magnetic activity. Consequently, the average wind pattern in the north during low geomagnetic activity is rather similar to the moderate activity average in the south, and the moderate geomagnetic activity average wind pattern in the north is similar to the high geomagnetic activity average in the south. The observed average thermospheric temperature is found to be governed by geomagnetic activity and the previous day's 10.7-cm solar flux.

A magnetic anomaly map of the Japan Sea

Abstract: A magnetic anomaly contour map (contour interval is 50nT) is compiled with all the available data of the total intensity of geomagnetic field Data are very dense in the coastal areas of the Japanese islands so that contour control is good while in the coastal areas of U S S R and the Korean Peninsula, data are not so dense The amplitudes of anomalies are generally small compared with that in the northwestern Pacific Ocean The magnetic anomaly lineations exist in the southern half of the Japan Basin From identification of the lineations, the Japan Basin stopped opening 15Ma BP on the assumption of two-limbed opening and the spreading rate of 34cm/yr There are many seamounts and uplifts which have anomaly patterns suggesting their reversed magnetization This seems to be a crucial point towards understanding the tectonics of the Japan Sea

Direction finding of ELF hiss emissions in a detached plasma region of the magnetosphere

Abstract: Wave normal directions and wave distribution functions of ELF hiss emissions in a detached plasma region of the magnetosphere have been determined, using data from the geostationary satellite GEOS 2 located in the equatorial plane at L = 6.6. Three different methods of direction finding have been utilized: (1) Means's method based on the hypothesis of a single plane wave, (2) the maximum likelihood method, assuming a few plane waves, and (3) the maximum entropy method of determining the wave distribution function. An intercomparison of the results from those methods has proven that they provide very reliable and definite information on the wave normals. From analyses of two equatorial ELF hiss events it is found that the wave distribution function is composed of a single peak (i.e., the waves have a single propagation direction) and that the wave normals of the ELF hiss originating in the detached plasma region make very small angles with the Earth's magnetic field; also, we wore able to make an experimental estimate of the angular width of the unstable cone of the emissions. Both our initial direction finding results in the equatorial plane and the corresponding previous results at higher geomagnetic latitudes are compared with theoretical predictions, assuming that the waves are generated by the electron cyclotron instability due to medium-energy (3–30 keV) electrons. It is concluded that all of the characteristics (morphology and direction-finding results) are consistent with the electron cyclotron instability.

The warped neutral sheet and plasma sheet in the near‐Earth geomagnetic tail

Abstract: An analysis of ISEE 2 plasma and magnetic field data indicates that the plasma sheet and neutral sheet in the near-earth magnetotail are warped in such a manner that in summer (winter) the neutral sheet rises above (dips below) the solar magnetospheric equatorial plane near the center of the tail, but dips below (rises above) the equatorial plane along the tail flanks. In the near tail, the neutral sheet crosses the equatorial plane at about 12 earth radii from the aberrated X axis, considerably closer to the center of the tail than has been inferred from data obtained farther downstream. This increase in the warp with decreasing distance from the earth is consistent with theoretical predictions. In the near tail, the warp is sufficiently strong when the dipole tilt angle is large that even in quiet times the upper or lower edge of the plasma sheet can be found close to the solar magnetospheric equatorial plane along the tail flanks. The seasonal dependence of the warp can produce certain dawn-dusk asymmetries in satellite data which are more apparent than real.

Standing wave pattern of HF radio waves in the ionospheric reflection region: 2. Applications

Abstract: General analytical formulas derived within a uniform approximation in a previous paper by the present authors are utilized for determining the wave pattern of a vertically propagating HF wave that is totally or partially reflected from the ionosphere. The full three-dimensional wave is calculated accurately, also at the reflection point. Parameter values typical of ionospheric modification experiments at Arecibo and Tromso are chosen. The influence of the geomagnetic field and collisions are properly taken into account but nonlinear and coupling effects are excluded. It is shown that the geomagnetic field affects the wave pattern strongly, especially in the Tromso case, and leads to a very high swelling in the first wave maximum below the reflection point.

Geomagnetic temporal change: 1903-1982 - A spline representation

Abstract: The secular variation of the earth's magnetic field is itself subject to temporal variations. These are investigated with the aid of the coefficients of a series of spherical harmonic models of secular variation deduced from data for the interval 1903-1982 from the worldwide network of magnetic observatories. For some studies it is convenient to approximate the time variation of the spherical harmonic coefficients with a smooth, continuous, function; for this a spline fitting is used. The phenomena that are investigated include periodicities, discontinuities, and correlation with the length of day. The numerical data presented will be of use for further investigations and for the synthesis of secular variation at any place and at any time within the interval of the data - they are not appropriate for temporal extrapolations.

Prediction of geomagnetic activity on time scales of one to ten years

Abstract: The long-term prediction of geomagnetic indices that characterize the state of the magnetosphere is discussed. While a prediction of the yearly average sunspot number is simultaneously a prediction of the yearly number of sudden-commencement storms, it is not a prediction of the number of disturbed or quiet half days. Knowledge of the sunspot cycle phase leads to a good estimate of the correlation expected between activity during one 27-day solar rotation period and the next.

Electron acoustic instabilities in the geomagnetic tail

Abstract: Electron acoustic waves present in a two temperature electron plasma can be driven unstable when ion beams propagate along the magnetic field. Both linear theory and numerical simulations indicate that this instability contributes to the generation of broadband electrostatic noise (BEN) in the geomagnetic tail.

Solar Wind Control of Electric Fields and Currents in the Ionosphere

Abstract: KERN (1966) substantiated theoretically the feasibility of using geomagnetic field variations on the earth's surface to calculate the spatial structure of the current system. Algorithms of the electric field and current calculation being used at present are examined. The methods for discriminating the geomagnetic variations controlled by various solar wind parameters are discussed. The calculation results obtained for the assumption of the uniform ionospheric conductivity are presented. This approximation made it possible to account for the existing regularities in the distribution of field-aligned currents underdefinite solar wind conditions and to predict the character of the three-dimensional current system in the intervals with the positive Bz IMF. The methods for and the results of calculating the electrostatic potential, the electric field, the ionospheric and field-aligned currents in the highlatitude ionosphere for nonuniform conductivity are presented in detail. The calculation results are compared with the direct measurements of the respective parameters.

Major flattening of the distant geomagnetic tail

Abstract: The development of a magnetotail cross section model is examined. The characteristics of the model are described. The flux content and magnetic field component are evaluated. The model predictions for the magnetotail are compared with observations. The model predicts insignificant flattening in the near-earth magnetotail, and sizable flattening in the deep and the distant magnetotail caused by the anisotropic magnetic pressure of magnetosheath field lines. Correlation between the model and observations for the distant magnetotail, magnetopause occurrence, and the magnetopause attitude is observed.

High-resolution sedimentary record of a geomagnetic reversal

Abstract: A detailed study of a geomagnetic reversal shows that during the transition the geomagnetic field undergoes both short-and long-term fluctuations. The first are thought to be linked to the reversal process; the second are present with the same time constants both before and during the reversal. This suggests that the secular variation, at least in its longer-period terms, is not affected by the reversal of the main field. This feature has important consequences for the validity of the frozen-flux approximation.

Geomagnetic-heliomagnetic modulation of atmospheric radiocarbon production.

Abstract: New Arizona high precision L\\14C data back to 6500 BC plot close to an 11,300-yr period sinusoid extrapolated from the post 5300 BC data (offset = +32%o, half amplitude = 51%o and phase lag = 2.29 radians). The trend curve is modulated by high latitude components of the non-dipole field with a fundamental period of 2400 yr. Based upon a model of Lund and Banerjee (1985), the non-dipole field rotates and every 1200 yr the high latitude maxima pass over the north magnetic pole and near the south magnetic pole in reversed polarity. This modulates cosmic ray production producing extended maxima ca AD 1700, 700 BC, 3100 BC, and 5500 BC. The 2400 period appears to be stationary. The magnetic field also modulates the amplitude of the solar activity induced cycles of periods 200, 80, and 11 yr as can be seen in the Zurich-Bern Camp Century ice core data as well as in the &4C fluctuation data. Reinterpretation of the Camp Century 10Be data indicates that it is in agreement with magnetic field as well as solar activity modulation of terrestrial radioisotope production.