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Showing papers on "Earth's magnetic field published in 2002"


Journal ArticleDOI
TL;DR: In this paper, a new empirical magnetic field model has been developed, representing the variable configuration of the inner and near magnetosphere for different interplanetary conditions and the ground disturbance levels.
Abstract: [1] A new empirical magnetic field model has been developed, representing the variable configuration of the inner and near magnetosphere for different interplanetary conditions and the ground disturbance levels. This paper describes the mathematical structure of the model, while the results of fitting it to a new set of spacecraft data are presented in a companion paper. The general approach remains the same as in the earlier T96 model, but the mathematical description of all major sources of the magnetospheric field now applies recently developed new methods. In particular, the field deformation technique is extensively used, making it possible to realistically and flexibly represent the fields of the cross-tail current, the ring current, and the Region 1 and 2 Birkeland currents. The new model ring current includes not only the axisymmetric component but also a partial ring current with field-aligned closure currents, a feature absent in earlier data-based models. The field of the cross-tail current includes two modules whose current densities vary along the Sun-Earth line with different rates. The cross-tail current sheet warps in two dimensions in response to the geodipole tilt, its inner edge shifts along the Sun-Earth line with growing disturbance, and its thickness varies along and across the tail. Birkeland currents of Regions 1 and 2 vary in response to interplanetary conditions, so that at ionospheric altitudes they shift in latitude and change their distribution in local time. The magnetospheric boundary is specified using a most recent empirical model [Shue et al., 1998]; its size is controlled by the solar wind ram pressure, and its shape also varies in response to changes of the Earth's dipole tilt angle. The model magnetopause ensures a full confinement of the fields of all sources inside the magnetopause, regardless of its shape and size. The model also includes an interplanetary magnetic field–controlled interconnection field, allowing a finite normal Bn at the magnetopause and hence open magnetospheric configurations.

492 citations


Journal ArticleDOI
TL;DR: In this article, an empirical modeling of the Earth's inner and near magnetosphere (X ≥ −15 RE), using a new set of data and new methods, is described in a companion paper.
Abstract: [1] First results are presented of an empirical modeling of the Earth's inner and near magnetosphere (X ≥ −15 RE), using a new set of data and new methods, described in a companion paper. The modeling database included 5-min average B field data, taken in a wide range of altitudes and latitudes by the International Solar Terrestrial Physics spacecraft Polar (1996–1999) and Geotail (1994–1999), as well as by earlier missions, ISEE 2 (1984–1987), Active Magnetospheric Particle Tracer Explorers (AMPTE)/CCE (1984–1988), AMPTE/Ion Release Module (1984–1986), CRRES (1990–1991), and DE 1 (1984–1990). To take into account the delayed response of the magnetosphere to the solar wind and interplanetary magnetic field (IMF), each data record in the data set was tagged by a “trail” of 5-min averages of the IMF, solar wind, and Dst field data, covering the preceding 2-hour interval. The axisymmetric ring current (SRC) and the partial one (PRC), both parameterized by the corrected Dst* index and the solar wind pressure Pd, were found to vary in strikingly different ways. While under quiet conditions the PRC is much weaker than the SRC, it dramatically grows in magnitude and rotates to the dusk sector with rising |Dst*| and Pd, significantly exceeding the SRC even during moderate storms, in excellent agreement with recent particle simulations. The innermost part of the cross-tail current is quite sensitive to the southward IMF and yields ∼90% of the tail's contribution to the Dst index, in contrast with the more distant tail current, which responds mainly to the solar wind pressure and provides no appreciable contribution to Dst. In response to southward IMF conditions, Region 1 and 2 Birkeland currents rapidly grow in magnitude and expand to lower latitudes, while their peaks shift slightly in local time toward noon. The coefficient of the IMF penetration inside the magnetosphere was found to dramatically increase with growing IMF clock angle: while quite small (∼0.1) for northward IMF, it rises to ∼0.6 as the IMF turns southward. Priorities and challenges for future data-based modeling studies are discussed.

458 citations


Journal ArticleDOI
TL;DR: A review and comparison of the current state of research into relativistic electron dynamics, covering simple diffusion, substorm acceleration, ULF wave acceleration, recirculation by ULF waves or plasmaspheric hiss is given in this paper.

437 citations


Journal ArticleDOI
TL;DR: In this article, the authors focus on the first application of the Curlometer (direct estimation of the electric current density from curl(B), using measured spatial gradients of the magnetic field) analysis technique.
Abstract: [1] For the first time, the Cluster spacecraft have collected 3-D information on magnetic field structures at small to medium scales in the Earth's dayside magnetosphere. We focus here on the first application of the Curlometer (direct estimation of the electric current density from curl(B), using measured spatial gradients of the magnetic field) analysis technique. The applicability of this multipoint technique is tested, for selected events within the data set, in the context of various mission constraints (such as position, timing, and experimental accuracy). For the Curlometer, nonconstant spatial gradients over the spacecraft volume, time dependence, and measurement errors can degrade the quality of the estimate. The estimated divergence of the magnetic field can be used to monitor (indirectly) the effect of nonconstant gradients in the case of many magnetic field structures. For others, and at highly distorted spacecraft configurations, this test may not reflect the quality of the Curlometer well. The relative scales and relative geometry between the spacecraft array and the structures present, as well as measurement errors, all are critical to the quality of the calculation. We demonstrate that even when instrumental and other errors are known to contribute to the uncertainty in the estimate of the current, a number of current signatures within the magnetosphere can be plausibly determined in direction, if not absolute size. A number of examples show consistent currents at the magnetopause, both separate from, and nearby or in the cusp region. Field-aligned currents near the polar cap boundary are also estimated reliably. We also demonstrate one example of an anomalous current arising from the effect of a highly distorted spacecraft configuration.

336 citations


Journal ArticleDOI
TL;DR: In this article, Kageyama et al. reviewed the results of numerical integrations of fully three-dimensional, fully nonlinear dynamos and compared them with the observed geomagnetic field.
Abstract: In 1995, two groups [Kageyama et al., 1995; Glatzmaier and Roberts, 1995a, 1995b] reported results of numerical integrations of fully three-dimensional, fully nonlinear dynamos. Their papers were precursors of a stream ofsuch models that have focused particularly on the geodynamo. They provide us, in unprecedented detail, with spectacular realizations of interesting geomagnetic field behaviors, such as secular variation and even polarity reversals. The proliferation of models has, however, created some confusion and apparently conflicting results. This can be partly attributed to the different ways in which different groups have modeled the core, normalized their equations, defined their dimensionless parameters, chosen their boundary conditions, and selected their energy sources. This has made it difficult to compare the results of different simulations directly. In this paper, we first try, as far as possible, to overcome this difficulty, so that all reported results can be compared on common ground. We then review the results, emphasizing three major topics: (1) onset and evolution of convection, (2) character of the magnetic field generated, and (3) comparison with the observed geomagnetic field. Although there are large differences in the way that the simulations are defined, the magnetic fields that they generate have some surprising similarities. The fields are dominated by the axial dipole. In some models they are most strongly generated in shear layers near the upper and lower boundaries and near the tangent cylinder, an imaginary surface touching the inner core on its equator. Convection rolls occur within which a type of the a effect distorts the toroidal field lines to create poloidal magnetic field. Some features of the models are found to strongly affect the fields that they produce. In particular, the boundary conditions defining the energy flow (e.g., an inhomogeneous heat flux or distribution of buoyancy sources) are very influential and have been extensively studied. They change the frequency and the mode of magnetic polarity reversals as well as the ratio in strengths of the dipole and nondipole moments. As the ultimate goal of geodynamo simulations is to explain the features of the real geomagnetic field, it is essential that proper comparisons be made between simulation results and observations. It is remarkable that polarity reversals reminiscent of the paleomagnetically observed field reversals have already been simulated by some of the models. Other features such as drift of the field, its secular variation, and statistical properties of Gauss coefficients are discussed in this paper and are compared with observations. These comparisons are rather primitive, not only because self-consistent dynamo models are still too new and too few but also because many of the observations (and especially the paleomagnetic data) are themselves not yet reliable or decisive enough. The aim of the third part of this paper is therefore more to demonstrate the potential use of simulations than to elucidate the nature of geomagnetic field generation.

234 citations


Journal ArticleDOI
TL;DR: In this article, the authors derived geomagnetic paleointensity proxies from the slope of the NRM versus anhysteretic remanent magnetization plot for alternating field demagnetization in the 30-60 mT peak field range.
Abstract: [1] We report natural remanent magnetization (NRM) directions and geomagnetic paleointensity proxies for part of the Matuyama Chron (0.9–2.2 Ma interval) from two sites located on sediment drifts in the Iceland Basin. At Ocean Drilling Program Sites 983 and 984, mean sedimentation rates in the late Matuyama Chron are 15.9 and 11.5 cm kyr−1, respectively. For the older part of the record (>1.2 Ma), oxygen isotope data are too sparse to provide the sole basis for age model construction. The resemblance of the volume susceptibility record and a reference δ18O record led us to match the two records to derive the age models. This match, based on Site 983/984 susceptibility, is consistent with available Site 983/984 benthic δ18O data. Paleointensity proxies were derived from the slope of the NRM versus anhysteretic remanent magnetization plot for alternating field demagnetization in the 30–60 mT peak field range. Paleointensity lows correspond to polarity reversals at the limits of the Jaramillo, Olduvai, Cobb Mountain, and Reunion Subchrons and to seven excursions in NRM component directions. Magnetic excursions (defined here by virtual geomagnetic polar latitudes crossing the virtual geomagnetic equator) are observed at 932, 1048, 1115, 1190–1215 (Cobb Mountain Subchron), 1255, 1472–1480, 1567–1575 (Gilsa Subchron), and 1977 ka. The results indicate that geomagnetic directional excursions, associated with paleointensity minima, are a characteristic of the Matuyama Chron and probably of polarity chrons in general.

202 citations


Journal ArticleDOI
TL;DR: In this article, the authors presented new data obtained from fired archeological structures found in two French sites (Loupiac and Aspiran) dated within the first millennium b.c.

190 citations


Journal ArticleDOI
TL;DR: In this article, the authors presented a comprehensive model for estimating the near-Earth magnetic field caused by sources in the Earth's core, ionosphere, magnetosphere, and the lithosphere.
Abstract: SUMMARY The near-Earth magnetic field is caused by sources in the Earth’s core, ionosphere, magnetosphere, lithosphere and from coupling currents between the ionosphere and the magnetosphere, and between hemispheres. Traditionally, the main field (low degree internal field) and magnetospheric field have been modelled simultaneously, with fields from other sources being modelled separately. Such a scheme, however, can introduce spurious features, especially when the spatial and temporal scales of the fields overlap. A new model, designated CM3 (Comprehensive Model: phase 3), is the third in a series of efforts to coestimate fields from all of these sources. This model has been derived from quiet-time Magsat and POGO satellite and observatory hourly means measurements for the period 1960‐1985. It represents a significant advance in the treatment of the aforementioned field sources over previous attempts, and includes an accounting for main field influences on the magnetosphere, main field and solar activity influences on the ionosphere, seasonal influences on the coupling currents, a priori characterization of the influence of the ionosphere and the magnetosphere on Earth-induced fields, and an explicit parametrization and estimation of the lithospheric field. The result is a model that describes well the 591 432 data with 16 594 parameters, implying a data-to-parameter ratio of 36, which is larger than several popular field models.

184 citations


Journal ArticleDOI
TL;DR: In this paper, the seasonal dependence of the dynamic properties of the dayside and nightside field-aligned current systems over the Northern and Southern Hemispheres was investigated using three-component geomagnetic field observations made on board the Danish satellite Orsted over polar regions.
Abstract: [1] In this paper we report on field-aligned currents inferred from high-precision three-component geomagnetic field observations made on board the Danish satellite Orsted over polar regions. Because of a slow drift in local time of the satellite orbit through the “noon-midnight” sector, we were able to study the seasonal dependence of the dynamic properties of the dayside and nightside field-aligned current systems over the Northern and Southern Hemispheres. We find an average over-the-pole distance between dayside and nightside currents of 32° during summer but 37° during winter and 36° during equinox. The decrease in the size of the summer polar cap is caused by a shift of both daytime and nighttime current systems to higher magnetic latitudes. For comparison, the dawn-dusk cross-polar distance of the Region 1/Region 2 field-aligned currents has been determined from high-precision data observed by Magsat, a satellite flown in 1979–1980 in a “dawn-dusk” orbit. The latter results show that the dawn-dusk distance between R1/R2 currents exhibits little seasonal dependence and amounts to ∼34° for all seasons in both polar caps. The seasonal dependence is confirmed for the high-latitude field-aligned intensities; they are larger by a factor of 1.5–1.8 in the sunlit (summer) polar cap in comparison with the winter hemisphere. Our results suggest that the R1/R2 and dayside field-aligned currents are well balanced between the pairs of downward/upward currents for all seasons as well as between hemispheres during equinox. We were not able to confirm results reported in earlier studies that the net currents tend to increase with an enhancement of ionospheric conductivity caused by the solar illumination or substorm activity.

180 citations


Journal ArticleDOI
TL;DR: In this article, the time-averaged geomagnetic field on the core-mantle boundary is interpreted using numerical models of fluid dynamos driven by non-uniform heat flow.
Abstract: SUMMARY The time-averaged geomagnetic field on the core‐mantle boundary is interpreted using numerical models of fluid dynamos driven by non-uniform heat flow. Dynamo calculations are made at Prandtl number Pr = 1, magnetic Prandtl numbers Pm= 1‐2, Ekman numbers E = 3×10 −4 ‐ 3 × 10 −5 and Rayleigh numbers 10‐30 times the critical value for different patterns of heat flow on the outer boundary of a rotating, electrically conducting spherical shell. The results are averaged over several magnetic diffusion times to delineate the steady-state magnetic field and fluid motion. When the boundary heat flow is uniform the time-averaged flow approaches axisymmetry and the magnetic field is mostly a geocentric axial dipole (GAD). The largest departure from GAD in this case is the octupole field component. When the amplitude of the boundary heat flow heterogeneity exceeds the average heat flow, the dynamos usually fail. Lesser amounts of boundary heterogeneity produce stable dynamos with time-averaged magnetic fields that depend on the form of the boundary heterogeneity. Elevated heat flow in the northern hemisphere produces a time-averaged axial quadrupole magnetic field comparable to the inferred paleomagnetic quadrupole. Azimuthally periodic boundary heat flow produces a time-averaged magnetic field component with the same azimuthal wavenumber, shifted in longitude relative to the heat flow pattern. Anomalously high and anomalously low magnetic flux density correlate with downwellings and upwellings, respectively, in the time-averaged fluid motion. A dynamo with boundary heat flow derived from lower-mantle seismic tomography produces anomalous magnetic flux patches at high latitudes and westward fluid velocity in one hemisphere, generally consistent with the present-day structure of the geodynamo.

177 citations


Journal ArticleDOI
TL;DR: Significant anomalous changes in the ultra low frequency range were observed in both geoelectric and geomagnetic fields before the major volcano-seismic activity in the Izu Island region, Japan.
Abstract: Significant anomalous changes in the ultra low frequency range (≈0.01 Hz) were observed in both geoelectric and geomagnetic fields before the major volcano-seismic activity in the Izu Island region, Japan. The spectral intensity of the geoelectric potential difference between some electrodes on Niijima Island and the third principal component of geomagnetic field variations at an array network in Izu Peninsula started to increase from a few months before the onset of the volcano-seismic activity, culminating immediately before nearby magnitude 6 class earthquakes. Appearance of similar changes in two different measurements conducted at two far apart sites seems to provide information supporting the reality of preseismic electromagnetic signals.

Journal ArticleDOI
07 Nov 2002-Nature
TL;DR: In this article, it was shown that geomagnetic jerks can be explained by the combination of a steady flow and a simple time-varying, axisymmetric, equatorially symmetric, toroidal zonal flow.
Abstract: Geomagnetic jerks, which in the second half of the twentieth century occurred in 1969 (refs 1, 2), 1978 (refs 3, 4), 1991 (ref. 5) and 1999 (ref. 6), are abrupt changes in the second time-derivative (secular acceleration) of the Earth's magnetic field. Jerks separate periods of almost steady secular acceleration, so that the first time-derivative (secular variation) appears as a series of straight-line segments separated by geomagnetic jerks. The fact that they represent a reorganization of the secular variation implies that they are of internal origin (as has been established through spherical harmonic analysis7), and their short timescale implies that they are due to a change in the fluid flow at the surface of the Earth's core (as has also been established through mapping the time-varying flow at the core surface8). However, little is understood of their physical origin. Here we show that geomagnetic jerks can be explained by the combination of a steady flow and a simple time-varying, axisymmetric, equatorially symmetric, toroidal zonal flow. Such a flow is consistent with torsional oscillations in the Earth's core, which are simple oscillatory flows in the core that are expected on theoretical grounds9, and observed in both core flow models10 and numerical dynamo models11.

Journal ArticleDOI
TL;DR: In this paper, the authors derived a spherical harmonic model of the main field (up to degree/order 29) and the secular variation of the magnetic field from Orsted data spanning more than two years (1999 March-2001 September).
Abstract: Summary The availability of high-precision geomagnetic measurements from satellites such as Orsted and CHAMP opens a new era in geomagnetic field research. However, in order to take full advantage of the improved data accuracy it is necessary to refine the usual way of deriving field models from satellite data. This paper describes the derivation of a spherical harmonic model of the main field (up to degree/order 29) and of the secular variation (up to degree/order 13) using Orsted data spanning more than 2 yr (1999 March–2001 September) and applying new modelling approaches for a correct statistical treatment of the data errors and for considering external field contributions. Magnetospheric contributions are modelled up to degree/order two; the zonal terms vary with annual and semi-annual periodicity, and terms with degree n= 1 are modulated with the strength of the magnetospheric ring current as measured simultaneously by globally distributed geomagnetic observatories. In addition, the observatory data are used to constrain secular variation. The model is estimated using an iteratively reweighted least-squares method with Huber weights to account for the non-Gaussian data error distribution. The rms misfit achieved at non-polar latitudes is 3 nT for the scalar intensity and for one of the vector components perpendicular to the magnetic field; the third vector component (rms misfit of 6.4 nT owing to attitude noise) is downweighted when estimating the model. Comparing model predictions with actual scalar magnetic field observations from the CHAMP satellite yields an rms misfit of 3.4 nT at non-polar latitudes and 5.4 nT at polar latitudes.

Journal ArticleDOI
TL;DR: In this article, a taxonomy of echo types is proposed to distinguish bottom-type, bottom-side, topside, and post-midnight irregularities in the Peruvian ionospheric electric field.

Journal ArticleDOI
TL;DR: In this paper, a model of field-aligned currents is derived from high-precision magnetic field measurements from the Orsted and Magsat satellites, being parameterized by the interplanetary magnetic field strength and direction for summer, winter and equinox.
Abstract: [1] A model of field-aligned currents is derived from high-precision magnetic field measurements from the Orsted and Magsat satellites, being parameterized by the interplanetary magnetic field strength and direction for summer, winter and equinox. The high-precision data allow the model to be determined directly by a simple 2-D curl technique combined with fitting of spherical harmonic functions. New elements of the model are: (a) the FAC patterns are determined separately for both polar regions, resolving the seasonal dependence of interhemispheric asymmetries, (b) the IMF ∼ 0, ground-state patterns are also resolved; these elements are obtained for the first time. From the model, the total upward/downward currents have been determined for various IMF conditions. The ratio of the summer/winter currents is ∼1.35 and the equinox currents ∼1. The model allows FAC mapping for IMF |B| ≤ 12 nT, except during magnetic storms and substorms.

Journal ArticleDOI
TL;DR: In this paper, large-scale equatorial F-region airglow depletions extending to low-midlatitudes in both hemispheres were observed with two all-sky imagers at Sata, Japan and Darwin, Australia.
Abstract: [1] We report for the first time large-scale equatorial F-region airglow depletions extending to low-midlatitudes in both hemispheres. The observational sites were located at low-midlatitude geomagnetic conjugate points. Clear depletions of 630.0-nm airglow intensity due to equatorial plasma bubbles were simultaneously observed with two all-sky imagers at Sata, Japan (magnetic latitude 24°N), and its geomagnetic conjugate point, Darwin, Australia (magnetic latitude 22°S), on the night of November 12, 2001. Airglow depletion regions with east-west scale sizes of 40–100 km extend poleward. The maximum apex altitude of the plasma bubbles is about 1,700 km over the geomagnetic equator. The depletions move eastward at about 100 m/s, without changing their structures. The Darwin depletion structures mapped onto the northern hemisphere along the geomagnetic field coincide closely with structures in the Sata images, even for the 40-km structure. These observations indicate that plasma depletions in the equatorial ionosphere elongate along the geomagnetic field lines.

Journal ArticleDOI
TL;DR: In this paper, it was shown that the initial flux dropout was due to the development of local taillike magnetic field stretching, rather than due to more global processes such as ring current buildup or large-scale radial diffusion.
Abstract: [1] The radiation belt electrons in Earth's magnetosphere exhibit substantial variability driven by changing solar wind conditions. The electron dynamics are due to a number of different adiabatic and nonadiabatic processes that can result in rapid increases and decreases in the particle flux levels. In this paper we present observations of abrupt flux decreases driven by a moderate geomagnetic storm. The particle dynamics are found to have significant local time and energy dependence that developed over roughly a 10-hour period beginning with the onset of the storm. The electrons with energies greater than 2 MeV dropped fairly abruptly at various local times, but not simultaneously at different local times. It is shown that the initial flux dropout was due to the development of local taillike magnetic field stretching, rather than due to more global processes such as ring current buildup or large-scale radial diffusion. It is also found that while the lower energy electrons (E 2 MeV electrons were lost from the magnetosphere and did not recover. These results indicate that the initial dropout of the radiation belt electrons at geostationary orbit was controlled by the adiabatic response to localized changes in the geomagnetic field that develop over many hours, but that eventually nonadiabatic processes acted to cause the loss of electrons from the magnetosphere. It is also shown that during geomagnetically quiet conditions, the energetic electron flux can remain at nearly constant levels for as long as 1 week, suggesting that in the absence of geomagnetic activity either the outer radiation belt electron loss rate becomes quite small or the loss and growth rates are balanced.

Journal ArticleDOI
TL;DR: In this paper, the authors discuss some of the subtle compromises that have been made in current models and propose a grand challenge for the future, requiring significant improvements in numerical methods and spatial resolution.
Abstract: ▪ Abstract The past seven years have seen significant advances in computational simulations of convection and magnetic field generation in the Earth's core. Although dynamically self-consistent models of the geodynamo have simulated magnetic fields that appear in some ways quite similar to the geomagnetic field, none are able to run in an Earth-like parameter regime because of the considerable spatial resolution that is required. Here we discuss some of the subtle compromises that have been made in current models and propose a grand challenge for the future, requiring significant improvements in numerical methods and spatial resolution.

Journal ArticleDOI
TL;DR: In this article, the authors simulate the propagation of an Alfven wave driven by an oscillating potential in the outer magnetosphere and show that more than 50% of the downgoing wave Poynting flux is dissipated through electron acceleration parallel to the geomagnetic field.
Abstract: [1] Observations of electric and magnetic field oscillations and accelerated electron distributions within an inverted-V region suggest the propagation of an Alfven wave from the outer magnetosphere into the auroral acceleration region. This hypothesis is tested for a case study event by simulating the propagation of an Alfven wave driven by an oscillating potential in the outer magnetosphere. At the spacecraft altitude the waveform and the associated electron distributions and spectra formed due to acceleration in the Alfven wave field are similar to those observed. The results show that more than 50% of the downgoing wave Poynting flux is dissipated through electron acceleration parallel to the geomagnetic field.

Journal ArticleDOI
TL;DR: A detailed study of 43 lava flows comprising two stratigraphic sequences exposed along the north and south walls of Barranco de los Tilos on the island of La Palma, Canary Islands, reveals a complex, temporally segmented record of geodynamo behavior that contains no less than three distinct geomagnetic events as discussed by the authors.
Abstract: [1] A detailed study of 43 lava flows comprising two stratigraphic sequences exposed along the north and south walls of Barranco de los Tilos on the island of La Palma, Canary Islands, reveals a complex, temporally segmented record of geodynamo behavior that contains no less than three distinct geomagnetic events. The Matuyama-Brunhes (M-B) reversal is recorded in five transitionally magnetized lava flows from the north (TN) section. The isochrons obtained from three of the lower four M-B lavas are defined by 14 incremental heating experiments that, together with a previous age determination, yielded a weighted mean of 798.4 ± 6.2 ka (all uncertainties ±2s). In addition, a 780.3 ± 10.3 ka isochron was determined for the overlying transitionally magnetized flow, indicating that it was erupted during a distinctly younger portion of the transition. Near the base of the south (TS) section one finds a sequence of weakly magnetized flows associated with virtual geomagnetic pole (VGP) positions in the southwest Indian Ocean between latitudes 56� S and 65� S, suggesting instability of the geomagnetic field beyond that of typical secular variation. 40 Ar/ 39 Ar isochrons from three of these flows, defined by 11 separate incremental heating experiments, gave a weighted mean of 822.2 ± 8.7 ka. This anomalous field behavior recorded 24 ± 11 kyr prior to the M-B reversal may coincide with an event featured in several marine sediment records. Directly above two normal polarity flows ( 40 Ar/ 39 Ar isochrons of 751.9 ± 18.1 ka and 675.0 ± 15.7 ka) are nine transitionally magnetized lavas having magnetization directions associated with low to midlatitude VGPs spanning 23� –60� N. These flows are then capped by a single flow possessing normal polarity. Based on 12 incremental heating experiments, 40 Ar/ 39 Ar isochrons of five of these nine lavas, along with the uppermost flow, gave a weighted mean age of 580.2 ± 7.8 ka for this period of transitional to normal field behavior. From these same transitional lavas, Quidelleur et al. [1999] reported three unspiked K-Ar ages with a weighted mean of 602 ± 24 ka and proposed a new event called the ‘‘La Palma’’ excursion. However, the 40 Ar/ 39 Ar age presented here is three times more precise than the K-Ar age and is indistinguishable at the 95% confidence level from the 40 Ar/ 39 Ar age of a lava from the Snake River Plain, Idaho, that originally defined the Big Lost event. Transitional field behavior of similar age observed in astronomically dated marine cores further establishes that the Big Lost event recorded at La Palma was indeed global in extent. Rigorous temporal and geomagnetic constraints for several additional periods of geomagnetic field instability during the last several million years will comprise a geomagnetic instability timescale that can be factored confidently into models of the dynamo process. INDEX TERMS: 1560 Geomagnetism and Paleomagnetism: Time variations—secular and long term; 1520 Geomagnetism and Paleomagnetism: Magnetostratigraphy; 1035 Geochemistry: Geochronology; 1513 Geomagnetism and Paleomagnetism: Geomagnetic excursions; KEYWORDS: argonargon, dating, lavas, excursion, paleomagnetism, reversal

Journal ArticleDOI
TL;DR: In this paper, the authors investigate the relationship between solar irradiance and cosmogenic isotope variations by simulating with a flux transport model the effect of solar activity on the Sun's total and open magnetic flux.
Abstract: [1] We investigate the relationship between solar irradiance and cosmogenic isotope variations by simulating with a flux transport model the effect of solar activity on the Sun's total and open magnetic flux. As the total amount of magnetic flux deposited in successive cycles increases, the polar fields build up, producing a secular increase in the open flux that controls the interplanetary magnetic field which modulates the cosmic ray flux that produces cosmogenic isotopes. Non-axisymmetric fields at lower latitudes decay on time scales of less than a year; as a result the total magnetic flux at the solar surface, which controls the Sun's irradiance, lacks an upward trend during cycle minima. This suggests that secular increases in cosmogenic and geomagnetic proxies of solar activity may not necessarily imply equivalent secular trends in solar irradiance. Questions therefore arise about the interpretation of Sun-climate relationships, which typically assume that the proxies imply radiative forcing.

Journal ArticleDOI
TL;DR: P paleomagnetic and paleointensity data from lavas of the Cretaceous Normal Polarity Superchron that formed at high latitudes near the tangent cylinder that surrounds the solid inner core suggest that the basic features of the geomagnetic field are intrinsically related.
Abstract: If relationships exist between the frequency of geomagnetic reversals and the morphology, secular variation, and intensity of Earth's magnetic field, they should be best expressed during superchrons, intervals tens of millions of years long lacking reversals. Here we report paleomagnetic and paleointensity data from lavas of the Cretaceous Normal Polarity Superchron that formed at high latitudes near the tangent cylinder that surrounds the solid inner core. The time-averaged field recorded by these lavas is remarkably strong and stable. When combined with global results available from lower latitudes, these data define a time-averaged field that is overwhelmingly dominated by the axial dipole (octupole components are insignificant). These observations suggest that the basic features of the geomagnetic field are intrinsically related. Superchrons may reflect times when the nature of core–mantle boundary heat flux allows the geodynamo to operate at peak efficiency.

Journal ArticleDOI
TL;DR: In this article, a case study of plasma, energetic particle, and magnetic field perturbations seen by Wind in the dawn foreshock on 19 April 1996 is presented, and the results of hybrid simulations confirm model predictions for a diamagnetic cavity flanked by regions of enhanced density and magnetic fields strength but filled with a suprathermal ion population.
Abstract: [1] We present a case study of plasma, energetic particle, and magnetic field perturbations seen by Wind in the dawn foreshock on 19 April 1996 A comparison with the results of hybrid simulations confirms model predictions for a diamagnetic cavity flanked by regions of enhanced density and magnetic field strength but filled with a suprathermal ion population In contrast to previously reported hot flow anomalies, flow velocities within the foreshock cavities are nearly identical to those in the ambient solar wind, ion temperatures do not rise greatly, thermal pressures are only slightly greater than those in the ambient solar wind, and the events do not lie centered upon interplanetary magnetic field (IMF) tangential discontinuities intersecting the bow shock We attribute the cavities to the diamagnetic effects of ions Fermi accelerated within the foreshock A review suggests that foreshock cavities are far more common than hot flow anomalies

Journal ArticleDOI
TL;DR: In this article, a series of new high-latitude ionospheric convection models have been constructed using Defense Meteorological Satellite Program (DMSP) thermal ion drift measurements, which are obtained by sorting cross polar cap electrostatic potentials into magnetic latitude/magnetic local time bins.
Abstract: [1] A series of new high-latitude ionospheric convection models have been constructed using Defense Meteorological Satellite Program (DMSP) thermal ion drift measurements. The models are obtained by sorting cross polar cap electrostatic potentials into magnetic latitude/magnetic local time bins. A regression analysis of the potentials in each bin is then implemented for establishing the relationships to the interplanetary magnetic field (IMF) for three seasons: summer, winter, and equinox. A linear modeling formula for the ionospheric electrodynamics (LIMIE) yields a convection response to the average solar wind (i.e., the “quasi-viscous” interaction) and to changes in the IMF By, Bz ≤ 0, and Bz > 0 components. The modeled convection is a superposition of the first two parameters with either the IMF Bz ≤ 0 or the Bz > 0 component. A global model is created by fitting the regression analysis results to a spherical harmonic function. The resulting DMSP-based ionospheric convection model (DICM) is fully parameterized by the IMF strength and direction. With this model, ionospheric convection patterns can be generated for any IMF configuration during quiet to moderate geomagnetic conditions. We compare the DICM model with other available high-latitude convection patterns organized by the IMF. The new elements in DICM are its quasi-viscous and separate IMF-dependent terms for both the northern and southern polar regions, which are not explicitly found in other ionospheric convection studies. The DICM's seasonal dependence and interhemispheric symmetry/asymmetry features show that the summer cross-polar potentials are 10–15% smaller than the winter potentials. The latter is in agreement with the seasonal dependence of field-aligned currents and with the voltage-current relationship required for the proper magnetosphere-ionosphere coupling.

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TL;DR: In this paper, the effect of electromagnetic torques on nutation was calculated by combining a solution for the full hydromagnetic response of the fluid core with the nutation theory of Mathews et al.
Abstract: [1] The presence of an internal magnetic field influences of the Earth's nutation through the effects of electromagnetic torques at the boundaries of the fluid core. We calculate the effect of electromagnetic torques on nutation by combining a solution for the full hydromagnetic response of the fluid core with the nutation theory of Mathews et al. [2002]. The coupling of the fluid outer core to the mantle and solid inner core is described by two complex constants, KCMB and KICB, that characterize the electromagnetic torques at the core-mantle boundary (CMB) and the inner core boundary (ICB). Predictions for KCMB and KICB are compared with estimates inferred from observations of the Earth's nutation. The estimate of KCMB can be explained by the presence of a thin conducting layer at the base of the mantle with a total conductance of 108 S. The overall root-mean-square (RMS) radial field at the CMB is 0.69 mT, which is partitioned into a dipole component (0.264 mT) and a nondipole component (0.64 mT). (The latter is represented using a uniform radial field.) The estimate of KICB can be explained with a mixture of dipole and nondipole components. The overall RMS field at the ICB is 7.17 mT, though smaller values are inferred when small adjustments are made to the dynamic ellipticity of the inner core and/or the fluid density at the boundary. The minimum RMS radial field required to explain the nutation observations is 4.6 mT.

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TL;DR: In this paper, a linear one-dimensional simulation of the ionospheric Alfven resonator including parallel electric fields due to electron inertial effects and statistically determined altitude dependent density and composition profiles in a dipole geomagnetic field yield waveforms and electron energy spectra qualitatively similar to observations.
Abstract: [1] FAST wave and particle observations on the nightside polar cap boundary indicate the operation of the ionospheric Alfven resonator (IAR). Large impulsive electric and magnetic field deviations on the boundary between the auroral oval and the polar cap close to magnetic midnight are correlated with accelerated electrons and excite semi periodic oscillations with a frequency of ∼0.5 Hz. Linear one-dimensional simulations of the Alfven resonator including parallel electric fields due to electron inertial effects, the ionospheric feedback instability and statistically determined altitude dependent density and composition profiles in a dipole geomagnetic field yield waveforms and electron energy spectra qualitatively similar to observations. However, from comparison with a case study example observed above a sunlit ionosphere, the observed electron energies (which exceed 10 keV) suggest that the observed wave carries a parallel electric field larger than possible from electron inertial effects in the linear approximation particularly if this acceleration occurs at altitudes within the ionospheric Alfven resonator.

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TL;DR: In this paper, the Imager for Magnetopause to Aurora Global Exploration (IMAGE) Spectrographic Imager (SI12) detects Doppler-shifted Lyman alpha emissions created by charge exchange and de-excitation of precipitating protons in the atmosphere.
Abstract: [1] The Imager for Magnetopause to Aurora Global Exploration (IMAGE) Spectrographic Imager (SI12) detects Doppler-shifted Lyman alpha emissions created by charge exchange and de-excitation of precipitating protons in the atmosphere. At high latitudes near local noon, emissions consistent with the location of the cusp foot point have distinct interplanetary magnetic field (IMF) dependence. For northward IMF a spot of emissions is observed poleward of the dayside auroral oval. By tracing the magnetic field lines from this spot using a model magnetosphere it is shown that the cusp foot point maps to a narrow region on the high-latitude magnetopause where antiparallel magnetic reconnection may be occurring. As the IMF turns southward, the spot merges into the auroral oval, producing a broad region of intense emissions centered near local noon. By tracing the magnetic field lines from this broad region using a model magnetosphere it is shown that the cusp foot point maps to a relatively broad region on the dayside magnetopause where component magnetic reconnection may be occurring.

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TL;DR: In this article, the authors focus on the first application of the discontinuity analyzer analysis technique (determination of the geometry and motion of magnetic discontinuities, using interspacecraft timing and variance information) for the case of boundaries that are planar over the four-spacecraft array.
Abstract: [1] The Cluster spacecraft have collected 3-D information on magnetic field structures at small to medium scales (100–1000 km) in the Earth's dayside magnetosphere, magnetosheath, and near solar wind. We focus here on the first application of the discontinuity analyzer analysis technique (determination of the geometry and motion of magnetic discontinuities, using interspacecraft timing and variance information) for the case of boundaries that are planar over the four-spacecraft array. This multipoint technique uses minimum variance of the magnetic field to determine the boundary normals at each spacecraft. Identification of the four spacecraft crossings requires accurate timing of encounters at each spacecraft, which depends on key features in each time series being stationary in the frame of the boundary. The technique is tested for selected boundary crossing events (magnetopause and bow shock, and one TD in the magnetosheath), for which the independently determined normals are closely (∼1–2°) colinear. Closely aligned normals, on spatial scales of ∼600–1000 km, were found for most crossings studied. For such a planar geometry, the motion can be determined unambiguously and shows that significant acceleration (up to ∼10 km s−2) of the boundary over the spacecraft is nearly always present. This has the consequence that previous estimates of boundary scales, based on constant motion between two spacecraft, may be in error by factors of two or three and that implied variations in boundary thickness could therefore be predominantly due to variations in speed. We comment on the error introduced through assumptions of constant motion.

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29 Mar 2002-Science
TL;DR: A model in which the strength of the geocentric axial dipole field varies with 100,000-year periodicity, whereas persistent nondipole components do not is proposed, which suggests that the magnetic field is modulated by orbital eccentricity.
Abstract: A continuous record of the inclination and intensity of Earth's magnetic field, during the past 2.25 million years, was obtained from a marine sediment core of 42 meters in length. This record reveals the presence of 100,000-year periodicity in inclination and intensity, which suggests that the magnetic field is modulated by orbital eccentricity. The correlation between inclination and intensity shifted from antiphase to in-phase, corresponding to a magnetic polarity change from reversed to normal. To explain the observation, we propose a model in which the strength of the geocentric axial dipole field varies with 100,000-year periodicity, whereas persistent nondipole components do not.

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TL;DR: Palaeomagnetic analyses were conducted on two varved lake-sediment sequences in northern Sweden as discussed by the authors, where the magnetic properties of the sediment sequences are dominated by stable single-domain magnetite with characteristics typical of bacterial magnetosomes.
Abstract: Palaeomagnetic analyses were conducted on two varved lake-sediment sequences in northern Sweden. The magnetic properties of the sediment sequences are dominated by stable single-domain magnetite with characteristics typical of bacterial magnetosomes. Alternating field demagnetization measurements indicate that the single-domain magnetite is the dominant carrier of a stable natural remanent magnetization. Temporal variations in inclination and declination were obtained from a total of four cores and the data points were stacked according to their independent calendar-year (varve) ages. Statistically significant patterns in inclination and declination form a regional palaeomagnetic secular variation (PSV) curve, which possesses features that are identical in form to the UK Holocene PSV master curve. However, the calibrated radiocarbon ages of UK features identified prior to 1500 BC are approximately 500 years older than their Swedish varve-dated equivalents, which points to dating errors and/or drifting of the geomagnetic field. The sediments meet the uniformity criteria proposed for palaeointensity reconstruction and estimates of relative geomagnetic field intensity are calibrated against global dipole-moment compilations. A calculated nuclide production curve is derived from the reconstructed geomagnetic field intensity, which empirically demonstrates the dominant modulation of cosmogenic nuclide production by dipole-moment between 5000 BC and AD 1500.