Geomagnetic Dipole Field Research Articles

The Cretaceous opening of the South Atlantic Ocean

The separation of South America from Africa during the Cretaceous is poorly understood due to the long period of stable polarity of the geomagnetic field, the Cretaceous Normal Superchron (CNS, lasted between ∼121 and 83.6 Myr ago). We present a new identification of magnetic anomalies located within the southern South Atlantic magnetic quiet zones that have arisen due to past variations in the strength of the dipolar geomagnetic field. Using these anomalies, together with fracture zone locations, we calculate the first set of magnetic anomalies-based finite rotation parameters for South America and Africa during that period. The kinematic solutions are generally consistent with fracture zone traces and magnetic anomalies outside the area used to construct them. The rotations indicate that seafloor spreading rates increased steadily throughout most of the Cretaceous and decreased sharply at around 80 Myr ago. A change in plate motion took place in the middle of the superchron, roughly 100 Myr ago, around the time of the final breakup (i.e., separation of continental–oceanic boundary in the Equatorial Atlantic). Prominent misfit between the calculated synthetic flowlines (older than Anomaly Q1) and the fracture zones straddling the African Plate in the central South Atlantic could only be explained by a combination of seafloor asymmetry and internal dextral motion (<100 km) within South America, west of the Rio Grande fracture zone. This process has lasted until ∼92 Myr ago after which both Africa and South America (south of the equator) behaved rigidly. The clearing of the continental–oceanic boundaries within the Equatorial Atlantic Gateway was probably completed by ∼95 Myr ago. The clearing was followed by a progressive widening and deepening of the passageway, leading to the emergence of north–south flow of intermediate and deep-water which might have triggered the global cooling of bottom water and the end for the Cretaceous greenhouse period.

Whistler‐mode waves inside flux pileup region: Structured or unstructured?

AbstractDuring reconnection, a flux pileup region (FPR) is formed behind a dipolarization front in an outflow jet. Inside the FPR, the magnetic field magnitude and Bz component increase and the whistler‐mode waves are observed frequently. As the FPR convects toward the Earth during substorms, it is obstructed by the dipolar geomagnetic field to form a near‐Earth FPR. Unlike the structureless emissions inside the tail FPR, we find that the whistler‐mode waves inside the near‐Earth FPR can exhibit a discrete structure similar to chorus. Both upper band and lower band chorus are observed, with the upper band having a larger propagation angle (and smaller wave amplitude) than the lower band. Most chorus elements we observed are “rising‐tone” type, but some are “falling‐tone” type. We notice that the rising‐tone chorus can evolve into falling‐tone chorus within &lt;3 s. One of the factors that may explain why the waves are unstructured inside the tail FPR but become discrete inside the near‐Earth FPR is the spatial inhomogeneity of magnetic field: we find that such inhomogeneity is small inside the near‐Earth FPR but large inside the tail FPR.

Dynamics of the Electromagnetic Ion Cyclotron Nonlinear Solitary Structures in the Inner Magnetosphere

The nonlinear interaction of the electromagnetic ion cyclotron (EMIC) frequency waves with plasma particles in the inner magnetosphere is studied. The emission is considered to be circularly polarized electromagnetic waves propagating along the almost constant dipole geomagnetic field in the equatorial region of the inner magnetosphere. Under the action of the ion cyclotron ponderomotive force excitation of the magnetosonic waves through the amplitude modulation of the EMIC waves is investigated. Two dimensional nonlinear Schrodinger equation for the EMIC waves is derived. In the stationary case two solutions of the nonlinear Schrodinger equation with distinct natures are found. The generation of both vortices and of a quasistatic magnetic field across the geomagnetic field lines is discussed.

Sources for geomagnetism—a comparative study of models

The paper proposes—and argues in favor of an alternative model to account for the dipole geomagnetic field in which a hitherto unrecognized piezomagnetic effect is the actual cause. This model offers a plausible explanation for the observed field reversals, the westward drift, and the secularly decreasing field strength. It is consistent with recent satellite data, and capable of generalization to planets other than Earth. It also accounts for the fact that the dipole axis is displaced from the geographic axis, and that the center of the dipole is displaced from the geographic center.

Earth’s habitable loop: Water, atmospheric structure, the geomagnetic field and plate tectonics

Complex and intelligent life developed on Earth because it has retained sufficient water, primarily in liquid form, for several billion years. The loss of Earth’s water to outer space is limited by a cold trap at the top of its thick troposphere. Earth maintains a thick troposphere because the dipolar geomagnetic field deflects the solar wind, preventing ionized particles from heating the upper atmosphere and lowering the tropopause. Earth’s magnetic field is sustained by the dynamo action of convective motions within the outer core, which require a large flux of heat from core to mantle. Plate-tectonics, which provides the required rate of planetary cooling over billions of years, requires the presence of liquid water to make the plates ductile and to lubricate the plate boundaries. The sustained, coupled interaction of liquid water, atmospheric structure, the geomagnetic field and plate tectonics forms a feedback-loop which maintains Earth’s long-term habitability.

Hybrid simulation of foreshock waves and ion spectra and their linkage to cusp energetic ions

A three‐dimensional global hybrid simulation is carried out to investigate energetic ions and electromagnetic waves in the quasi‐parallel (Q‐∥) bow shock and cusp for a typical interplanetary magnetic field configuration during the cusp energetic particle events. The bow shock, magnetosheath, and dayside magnetosphere form by interaction between the solar wind and geomagnetic dipole field. Ions of solar wind characteristics injected into the system evolve along with electromagnetic waves in a self‐consistent manner. Several important features are yielded from the simulation. Solar wind ions are accelerated by the waves and turbulence at the bow shock and foreshock as indicated by the ion distribution. The shock‐accelerated ions possess an exponential energy spectrum with their differential flux scaled by the field‐aligned distance to the bow shock, consistent with satellite observations. Second, the compressive and transversal waves in the foreshock are strongly correlated with the diffuse ion dynamics. Third, energetic ions in the magnetosheath and cusp downstream from the Q‐∥ shock also exhibit a spectrum similar to those at the shock. By tracing trajectories of cusp energetic ions in the simulation, the origin of these ions is revealed. Their source is predominantly associated with the Fermi acceleration at the Q‐∥ shock and foreshock. Waves in the magnetosheath and cusp, as well as magnetic reconnection at the high‐latitude magnetopause, are relatively insignificant in accelerating cusp energetic particles.

Paleomagnetism of the Fentale-magmatic segment, main Ethiopian Rift: New evidence for counterclockwise block rotation linked to transtensional deformation

We carried out the first ever paleomagnetic investigations in the area around the Fentale volcanoes, Main Ethiopian Rift. A total of 28 paleomagnetic sites were sampled around the two Fentale (Tinish and Tilik) volcanic centers. Lava and pyroclastic flows ranging in age from 2 Ma to 0.1 Ma was sampled for this study. After routine paleomagnetic collection and sample preparation procedures specimens were subjected to progressive alternating fields and thermal demagnetizations. The directional analysis shows either one or two components of Natural Remanent Magnetization to characterize the entire measurements. The low stability components are removed either by alternating fields up to 15 mT or by heating up to 250 °C. The high stability and primary components are isolated above those steps and vector is directed towards the origin, and it represents the Characteristic Remanent Magnetization. The principal component analysis and site means directions calculations yielded normal and reversed polarities together with two anomalous directions. When an overall mean direction is calculated from the individual site mean directions for the Fentale region, D s = 354.3°, I s = 19.3°, N = 27, K = 33, α 95 = 4.9° are obtained. When this result is compared with the mean expected reference dipole geomagnetic field direction ( D = 1.0°, I = 16.4°, α 95 = 2.3°, K = 105.6, N = 32) for an average age of 1.5 Ma, the following differences of Δ D = −6.7° ± 4.3°, Δ I = −2.9° ± 4.3° can be obtained. These results from the region indicate ∼7° counterclockwise block rotations about vertical axis consistent with the transtentional deformations that characterize the Fentale magmatic segment.

Meridional transport and deposition of atmospheric &amp;lt;sup&amp;gt;10&amp;lt;/sup&amp;gt;Be

Abstract. 10Be concentrations measured in ice cores exhibit larger temporal variability than expected based on theoretical production calculations. To investigate whether this is due to atmospheric transport a general circulation model study is performed with the 10Be production divided into stratospheric, tropospheric tropical, tropospheric subtropical and tropospheric polar sources. A control run with present day 10Be production rate is compared with a run during a geomagnetic minimum. The present 10Be production rate is 4–5 times higher at high latitudes than in the tropics whereas during a period of no geomagnetic dipole field it is constant at all latitudes. The 10Be deposition fluxes, however, show a very similar latitudinal distribution in both the present day and the geomagnetic minimum run indicating that 10Be is well mixed in the atmosphere before its deposition. This is also confirmed by the fact that the contribution of 10Be produced in the stratosphere is dominant (55%–70%) and relatively constant at all latitudes. The contribution of stratospheric 10Be is approximately 70% in Greenland and 60% in Antarctica reflecting the weaker stratosphere-troposphere air exchange in the Southern Hemisphere.

Modeling the partial ring current effect in a disturbed magnetosphere

The mathematical model of the magnetic field of the partial ring current has been proposed. This current is considered as a pair of spatial current circuits in the Northern and Southern hemispheres, either of which includes two ring zones, in the geomagnetic equator plane and ionosphere, and two zones of the field-aligned current, flowing along the geomagnetic dipole field lines and joining ring fragments of the circuit. The model parameters are: colatitude of the eastward electrojet, longitudinal shift relative to the Sun-Earth axis, circuit half-angle, and the total current flowing in the circuit. The Biot-Savart-Laplace law has been used to calculate the magnetic field of the current circuit. The magnetic field of the partial ring current has been calculated under the conditions typical of a strong magnetic storm. The technique for calculating the partial ring current intensity, using the Asym-H geomagnetic index, has been proposed.

Age calibrated relative paleointensity for the last 1.5 Myr at IODP Site U1308 (North Atlantic)

Integrated Ocean Drilling Program (IODP) Site U1308 (central North Atlantic) records paleomagnetic directional and relative paleointensity (RPI) variations for the last 1.5 Myr, in 110 m of the sediment sequence at a mean sedimentation rate of 7.3 cm/kyr. A detailed benthic oxygen isotope record was combined with RPI to produce an integrated, high-resolution magneto-isotopic stratigraphy for Site U1308. Apart from the well-known polarity reversals in this interval, the Punaruu excursion is recorded at 1092 ka and the Cobb Mountain Subchron in the 1182–1208 ka interval. The paleointensity proxies are determined as slopes of NRM versus ARM and NRM versus ARMAQ (ARM acquisition) with linear correlation coefficients to monitor the quality of the linear fit. The RPI record for Site U1308 is compared with the three other paleointensity records (one from the Western Equatorial Pacific and two from the North Atlantic) that cover the same time interval and have accompanying oxygen isotope records. The Match protocol of Lisiecki and Lisiecki (2002) is used to optimize the correlation of paleointensity records. Beginning with the original (published) age models for each record, the Match routine is used to optimize the RPI correlations to Site U1308, with checks to ensure compatibility with oxygen isotope records. Squared wavelet coherence (WTC) indicates significant improvement in RPI (and oxygen isotope) correlations after matching each RPI record to Site U1308, particularly for periods > 10 kyr. The level of coherence for the Atlantic RPI records and the lower resolution Pacific record implies synchronous global variability (at scales > 10 kyr) that can be attributed to the axial dipole geomagnetic field.

A detector telescope’s pitch angle sampling of magnetospheric particles

Abstract A geometrical method developed for a particle detector can be extended for application to the determination of the pitch angle sampling capability of the particle telescope of any orientation and for the parent satellite of any orbital orientation. A unit vector tangent to a dipole geomagnetic field line has been calculated from the dipole magnetic field equations in the geomagnetic coordinate system. Rotation matrices have been used, step-by-step, to align the geomagnetic coordinate frame with the detector coordinate frame. The detector selected in this work has its axis tilted with the local zenith direction. The component of the unit vector in the detector coordinate system is found at an observation point characterized by longitude, latitude, and the geocentric radial distance. Pitch angles are measured with respect to the guiding field direction. The pitch angle sampling factor is used in the calculation of detector response functions to particles of different pitch angles. The sampling efficiency functions are useful to make an absolute comparison of magnetospheric particle fluxes measured by particle telescopes under different solar conditions pertaining to different epochs.

Resonant diffusion of radiation belt energetic electrons by field-aligned propagation whistler-mode chorus waves

Adopting the dipole geomagnetic field， Gaussian spectral density for the waves， and semi-empirical latitudinal electron density models obtained from available in situ data， this paper has calculated the local and bounce-averaged quasi-linear resonant electron diffusion coefficients due to chorus and then determined the timescales for electron precipitation loss and stochastic acceleration， in the range of 4≤L≤7 outside the plasmapause. The results indicate that the spatial extent where gyroresonance occurs depends on electron energy， equatorial pitch angle， wave spectrum， and the local electron number density and magnetic field. Besides these five parameters， the actual values of resonant frequency rely on magnetic latitude where resonance occurs. The acceleration of radiation belt energetic electrons occurs predominantly due to equatorial chorus， and the mid-latitude chorus preferably contributes to the precipitation loss of relativistic electrons. The timescales for both electron loss and acceleration due to chorus-driven diffusion have been evaluated to be of hours for lower-energy electrons (about 200keV) and of days for higher-energy electrons (about 1MeV). It is also found that variation of latitudinal density distributions contributes importantly to chorus-driven electron resonant diffusion. In general， an increasing latitudinal electron density increases the loss timescales for untrapped electrons with small equatorial pitch angles， but has negligible effect on the acceleration of trapped electrons with large equatorial pitch angles. The variations of chorus wave amplitude and wave spectrum with magnetic latitude and L-shell also make important contributions to the lifetime and acceleration of radiation belt electrons， which are generally greater than the effects of varying latitudinal distribution of cold plasma density.

The Secular Variation Characteristics and Mechanisms of Geomagnetic Field

AbstractThe variations of geomagnetic field are divided into three parts: the spontaneous variations of dipole and non‐dipole geomagnetic fields and the dipole geomagnetic field variation resulting from geomagnetic anomalies of non‐dipole field through modulating the ring current in core‐mantle boundary. The secular variation characteristics and distributions of geomagnetic field at different depths are calculated and analyzed in this paper by using the model of International Geomagnetic Reference Field (1900~2000). Possible mechanisms are discussed also. It can be concluded that the westward driftage and reversal of geomagnetic field are not only resulted from non‐dipole field, but also dipole.

Palaeomagnetic intensity data: an Achilles heel of solar activity reconstructions

Reconstructions of solar activity prior to AD 1600 rely on estimates of the past production rates of cosmogenic radionuclides in the Earth's atmosphere. These rates are modulated by the strength of the solar magnetic field and the intensity of the Earth's geomagnetic dipole. One of the largest uncertainties associated with these reconstructions originates from palaeointensity data and to show how errors in palaeomagnetic data influence reconstructions of solar activity we present different radiocarbon based reconstructions of the solar magnetic modulation parameter (Φ) during the Holocene. We considered geomagnetic field changes according to (i) a global archaeomagnetic compilation, (ii) a geomagnetic field model covering the last 7000 years and (iii) a regional sediment relative palaeointensity record. Comparison between alternative solar modulation reconstructions shows that the uncertainties in palaeomagnetic data prevent us from making robust conclusions about solar variability at timescales longer than a few hundred years. Our analyses indicate that the geomagnetic dipole field dominates radiocarbon production at timescales longer than 500 years although a combination of solar and geomagnetic modulation could apply to all timescales. Long-term trends depend on which palaeomagnetic data set is used for correction and there are additional unexplained differences between 10Be and 14C records that question results about long-term changes in solar activity. The higher-resolution geomagnetic field records, which are usually very much smoothed in stacked records, also indicate that part of the shorter-term changes (~500 yr) in the radiocarbon production rate could be attributed to the geomagnetic dipole field. To test this hypothesis and improve our knowledge of past solar activity a higher resolution palaeomagnetic reconstruction of geomagnetic dipole moment is required.

Eigenmode analysis of ballooning perturbations in the inner magnetosphere of the Earth

Abstract. We analyze coupled Alfvén and slow magnetosonic eigenmodes in a dipole geomagnetic field with different ionospheric conductivities in the framework of ideal magnetic hydrodynamics (MHD) with finite pressure. We use numerical and, if possible, analytical methods to describe eigenmode frequencies, growth rates and eigenfunctions. The spectrum of Alfvén and slow magnetosonic modes is discrete and equidistant. The frequencies of the first Alfvén and slow magnetosonic eigenmodes are estimated as ~1 Hz and ~1 mHz, respectively. In the case of finite conductivity, periodic and aperiodic modes are separated and their interaction analyzed. It was shown that periodic and aperiodic perturbations can mutually transform into each other. A new flute stability criterion is derived (α~4.25), which is stricter than the Gold criterion (α=20/3). Here, as usual, α=−L/p dp/dL. For flute perturbations, the deviations of transversal displacement from a constant are calculated. An approximation for longitudinal displacement is derived. We determined the position of the main longitudinal peak, which can be responsible for nonlinear structures observed by Freja. An influence of nonlinear terms in pressure is estimated as well.

Symplectic tracing of high‐energy charged particles in the inner magnetosphere

To investigate the radiation belt, the Gauss Runge‐Kutta method (RKG), one of the symplectic numerical integration methods, is introduced and tested by tracing protons and electrons in geomagnetic dipole field. The tracings of protons having the energies from 10 keV to 10 MeV with pitch angles from 90° to 30° at L ∼ 5 are practiced by fourth‐ and sixth‐order RKG with time step 0.016 of the Larmor period on the equatorial plane. The accuracy of calculations are examined with Cartesian, cylindrical, and polar coordinates. With all coordinates, we confirmed that the calculations are stable for 10,000,000 steps with RKG. On the other hand, the calculations for protons in the same conditions with standard Runge‐Kutta methods fail within 10,000 steps. The tracings of relativistic electrons having energies from 10 to 150 MeV with pitch angles from 90° to 30° at L ∼ 5 are also practiced. For the electrons above 50 MeV, the calculations are successful for 10,000,000 steps with the time step 0.1 × RE/c by fourth and sixth RKG with cylindrical coordinates. The calculations by standard Runge‐Kutta method are failed in any coordinate system, and the calculations by fourth or sixth RKG with Cartesian or polar coordinates are also failed for all energy range above. The calculations for the electrons below 10 MeV with this time step size are all failed by any methods and with any coordinate systems mentioned above. For the effective calculations, for example, for 10 MeV electron with pitch angle 90° by fourth order RKG, we must use shorter timescale size than 0.2 of the above size.

Dynamics of high‐energy electrons interacting with whistler mode chorus emissions in the magnetosphere

Close examination of whistler mode chorus emissions reveals that a chorus emission is a coherent monochromatic wave typically with a fast rising tone. The frequency of the emission increases rapidly along with growth of the wave amplitude. We first consider the generation mechanism of whistler mode chorus emissions. The essential mechanism of the frequency change is critically related to the inhomogeneity of the geomagnetic field in the equatorial region. The rising tone emission is only possible when the coherent wave propagates away from the equator interacting with a sufficient flux of counterstreaming resonant electrons. Depletion of the resonant trapped electrons from the wave phase space results in formation of an electromagnetic electron hole, which gives rise to a transverse resonant current causing both wave growth and frequency increase. The wave growth of a rising tone can elongate the nonlinear trapping zone, which works as an effective wave train that guides a fraction of the resonant electrons moving toward the equator. We perform test particle simulations where we solve the relativistic equations of motion for high‐energy electrons under the action of the electromagnetic field of a coherent whistler mode wave and the assumed dipole geomagnetic field. We find that a fraction of weakly relativistic electrons are trapped by a coherent wave and that the trapped electrons are effectively accelerated by the chorus wave with a rising tone.

The influence of a southward and northward turning of the interplanetary magnetic field on the geomagnetic cut-off of cosmic rays, on the mirror points positions of geomagnetically trapped particles, and on their rate of precipitations in the atmosphere

A number of geomagnetic observations indicate that the orientation of the interplanetary magnetic field (IMF) plays a definite role in triggering several magnetospheric processes. In this theoretical study based on the superposition of a dipole geomagnetic field model and an uniform IMF, it is shown that when the northward component of the IMF decreases ( d B z / d t < 0 ), the geomagnetic cut-off surface moves to larger equatorial distances, the altitude of the mirror points of charged particles trapped in the geomagnetic field increases, and the value of their equatorial pitch angle decreases. Furthermore, we show that the loss cone angle of trapped particles decreases when d B z / d t is negative. The southward component of the magnetic field generated by an enhancement of the ring current produces similar effects everywhere inside this ring current loop.

The Geomagnetic Dipole Field in the 20th Century

The study of variations of the geomagnetic field in the 20th century becomes possible after the IGRF (International Geomagnetic Reference Field) data of 1900~2000 are available. This study shows that during the 20th century, even since the 19th century, the magnetic dipole moments have changed much. And the all three components of the Earth's center dipole kept a decline trend in the last century. The variation of the positions of the geomagnetic south pole around 1930 turned over. It began to drift westward and southward since 1960. The direction of the axis of the dipole only changed by about 1° in latitude and by about 3° in longitude in the 20th century. That is far away from the reverse of the geomagnetic poles.

Do superchrons occur without any palaeomagnetic warning?

The mainly dipolar geomagnetic field generated by the geodynamo within the Earth’s liquid core has reversed its polarity many times in the past. This succession of intervals of alternate polarity defines the geomagnetic polarity time scale (GPTS), usually interpreted as resulting from a Gamma renewal process, the rate of which would be controlled by the boundary conditions imposed by the mantle on the core. In this interpretation, boundary conditions would have occasionally evolved towards being unfavourable to reversals, leading the (reversal) rate of the process to progressively decrease and reach zero at least twice, causing the onset of the so-called Kiaman and Cretaceous very long polarity intervals (superchrons). Here we reconsider this causal link. Analysing the latest and best constrained GPTS (thanks to the continuous record provided by marine magnetic anomalies) describing the past 160 Myr, we found that contrary to earlier claims, no long-term behaviour over the ∼40 Myr preceding the Cretaceous superchron can be seen in the reversal rate that could explain its onset at ∼120 Ma. More generally, it turns out that hardly any special behaviour can be identified in the GPTS, which could have announced this superchron. Only the occurrence of the longest of all pre-superchron intervals (CM1n), ∼3 Myr before the onset of the superchron, could be identified as a precursor. Such a behaviour could possibly be the consequence of medium-term (on the 10 Myr time scale) changes in the boundary conditions imposed by the mantle on the core. But we note that the sole analysis of the GPTS does not allow this to be tested. In fact, it appears that a single stationary process could also explain the entire pre-superchron GPTS, with the only possible exception of CM1n. This suggests that the occurrence of superchrons does not necessarily require changes in boundary conditions and could simply attest for a sudden non-linear transition between a reversing and a non-reversing state of the geodynamo.