Abstract

In the field of geomagnetism a number of studies have been devoted to the investigation of turbulence and intermittency in the outer core fluid motions. Here, in order to obtain information on such phenomena we study the time spectral and self-similarity features of the main geomagnetic field fluctuations as measured on the Earth’s surface. The existence of a power law spectrum, characterised by an exponent a˜-11/3, and an anomalous scaling of q-th order structure functions on time scales longer than 5 years, suggests the occurrence of intermittent turbulence rather than classical Kolmogorov turbulence in the fluid core motions. These results are briefly discussed in connection with the existence of a strong magnetic field and drift-wave turbulence.

Highlights

  • The geomagnetic field is sustained within Earth’s outer core by dynamo action, which involves vigorous convective motions

  • In the field of geomagnetism, many studies are devoted to understanding the nature of the physical features of the motions inside the Earth’s core as well as to their relation with the maintenance of the main field

  • As a contribution to the knowledge of the Earth’s core processes, we have investigated the spectral and self-similarity features of the geomagnetic field recorded at 18 permanent observatories

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Summary

Introduction

Turbulent flows are generally characterized by «universal» scaling features in the so-called inertial range where fluctuations reach a stationary state due to the continuous flow of energy from large to small scales, or vice versa. Braginsky and Meytlis (1990) have suggested that this small-scale turbulence should be highly anisotropic because of the strong influence of the Earth’s rotation and magnetic field They conclude their paper by suggesting that the study of data relative to the geomagnetic secular variation may be highly relevant to understand the dynamo processes. As a matter of fact, the characteristic time interval of the local turbulence lies in the range from 10 to 100 years (of the order of decades), and data about the decade geomagnetic secular variations may be relevant Taking this suggestion into account we attempt to investigate the nature of the Earth’s fluid core turbulent motions using ground-based observations of the geomagnetic field.

The geomagnetic field equations
A brief overview of turbulence
A brief overview of intermittency
Data set and analysis
Summary and conclusions

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