Analysis Of Geomagnetic Variations Research Articles

State and evolution of the geodynamo from numerical models reaching the physical conditions of Earth’s core

SUMMARY Our understanding of the geodynamo has recently progressed thanks to geomagnetic data of improved quality, and analyses resting on numerical simulations of increasing realism. Here, these two advances are combined in order to diagnose the state and present dynamics of Earth’s core in physically realistic conditions. A sequential, ensemble-based framework assimilates the output of geomagnetic field models covering the past 180 yr into a numerical geodynamo simulation, the physical realism of which is also advanced as data is assimilated. The internal dynamical structure estimated for the geodynamo at present reproduces previously widely documented features such as a planetary-scale, eccentric westwards gyre and localization of buoyancy release beneath the Eastern (0°E−180°E) hemisphere. Relating the typical magnetic variation timescale of the assimilated states to the power at which they operate, the present convective power of the geodynamo is estimated at 2.95 ± 0.2 TW, corresponding to an adiabatic heat flow out of the core of 14.8 ± 1 TW if the top of the core is convectively neutrally stratified at present. For the first time, morphologically and dynamically relevant trajectories are obtained by integrating the estimated states forward for a few decades of physical time using a model reaching the physical conditions of Earth’s core. Such simulations accurately account for the spatio-temporal content of high-resolution satellite geomagnetic field models and confirm earlier interpretations in terms of rapid core dynamics. The enforcement of a realistic force balance approaching a Taylor state allows for propagation of weak (velocity perturbation of about 0.6 $\mathrm{km\,yr^{-1}}$) axisymmetric torsional waves with period about 5 yr, supported by a magnetic field of root-mean-squared amplitude of 5.6 mT inside the core. Quasi-geostrophic magneto-Coriolis waves of interannual periods and significantly stronger velocity perturbation (about $7 \mathrm{km\,yr^{-1}}$) are also reproduced, with properties that converge towards those recently retrieved from the analysis of geomagnetic variations before fully achieving Earth’s core conditions. The power spectral density of magnetic variations falls off rapidly at frequencies exceeding the inverse Alfvén time (about $0.6\mathrm{yr^{-1}}$), which indicates that the excitation of hydromagnetic waves occurs preferentially at large spatial scales. The possibility to account for geomagnetic variations from years to centuries in physically realistic models opens the perspective of better constraining properties of the deep Earth through geomagnetic data assimilation.

Tidal effects in geomagnetic variations

This paper reports the results of processing and analysis of geomagnetic variations, estimates of their spectra, and the first identified waves of lunar–solar tides on the geomagnetic variation envelopes in the circadian range and in the periods of 13.66 and 27.55 days. The specific features of the geomagnetic variation spectrum open up new possibilities for evaluation of the viscosity of the Earth’s outer core, description of the geomagnetic dynamo, development of new models of movement of the Earth’s inner core and the current system dynamics in the outer (liquid) core, and establishment of the general regularities controlling the energy exchange conditions in the geosphere.

Impulse Bursts of Geomagnetic Pulsations as the Ground Signature of Contact of Interplanetary Irregularity with the Dayside Magnetopause

The aim of the present article was to analyze features of interactions between an interplanetary shock wave and the magnetosphere on the basis of comparative analysis of geomagnetic variations observed during a sudden commencement (SC) by satellites and gr...

Морфологические признаки в структуре геомагнитных вариаций в период подготовки сильнейшего землетрясения 25 марта 1998 г. в Антарктиде

Морфологические признаки в структуре геомагнитных вариаций в период подготовки сильнейшего землетрясения 25 марта 1998 г. в Антарктиде

Analysis of geomagnetic variations in south-western Nigeria

Geomagnetic variations, observed at 11 sites in south-western Nigeria, have been analysed to derive interstation transfer functions with the site at Ile-Ife as reference station. The study involves frequencies from 1 to 6 c.p.h. The reference station Ile-Ife is 160 km northward of the continental slope off the Nigerian coastline and 400 km southward of the dip equator. The analysis has been carried out separately with selected data sections of a few hours length during daytime and during the night. Thus expressed linear relations between field components are in the case of day events of dual implication: (1) for the source field structure of the equatorial electrojet, (2) for internal conductivity conditions, including the coast effect from the Bight of Benin. Conductivity anomalies are the sole cause for an observed spatial variability of night events. A 2-D thin-sheet conductivity model has been derived taking both the source and the coast effect into consideration. This model provides a reasonably good fit between observed and computed transfer functions during day and night.

The Solar Cycle Contribution to the Secular Change in the Geomagnetic Field

Analyses of geomagnetic variations, having a period of about 11 years, have been made using the data from 27 observatories for the last 55 years. Using a particular differentiation process together with a numerical high-pass filter, in-phase variations of about 11-year period have been extracted for all the observatories against a background of the gradually changing large scale secular variation. Fourier analyses of these modified mean annual values revealed that the amplitude of the 11-year variation in the north component varies from about 2γ to 17γ according to the observatory. Approximating the 11-year variation field by a P1 field, the coefficient of the first spherical harmonic was determined by a least square calculation as 8.5γ for the north component and 9.1γ for the vertical component. The ratio of the coefficient of internal origin i to that of external origin e was obtained as 0.16±0.08. From this, the electrical conductivity in the lower mantle has been estimated as 6×10-10 e. m. u. at about 1600km depth.It has also been shown that the effect of the 11-year variations on analyses of the rates of change in the field are negligible, and that the variation is likely to be caused by the growth or decay of the equatorial ring current.