Induction studies with satellite data

The natural variations of the Earth's magnetic field of periods spanning from milliseconds to decades can be used to infer the conductivity-depth profile of the Earth's interior. Satellites provide a good spatial coverage of magnetic measurements, and forthcoming missions will probably allow for observations lasting several years, which helps to reduce the statistical error of the estimated response functions.Two methods are used to study the electrical conductivity of the Earth's mantle in the period range from hours to months. In the first, known as the potential method, a spherical harmonic analysis of the geomagnetic field is performed, and the Q-response, which is the transfer function between the internal (induced) and the external (inducing) expansion coefficients is determined for a specific frequency. In the second approach, known as the geomagnetic depth sounding method, the C-response, which is the transfer function between the magnetic vertical component and the horizontal derivative of the horizontal components, is determined. If one of these transfer functions is known for several frequencies, models of the electrical conductivity in the Earth's interior can be constructed.This paper reviews and discusses the possibilities for induction studies using high-precision magnetic measurements from low-altitude satellites. The different methods and various transfer functions are presented, with special emphasis on the differences in analysing data from ground stations and from satellites. The results of several induction studies with scalar satellite data (from the POGO satellites) and with vector data (from the Magsat mission) demonstrate the ability to probe the Earth's conductivity from space. However, compared to the results obtained with ground data the satellite results are much noisier, which presumably is due to the shorter time series of the satellite studies.The results of a new analysis of data from the Magsat satellite indicate higher resistivity in oceanic areas than in continental areas. However, since this holds for the whole range of periods between 2 and 20 days, this difference probably is not caused purely by differences in mantle conductivity (for which one would expect less difference for the longer periods). Further studies with data from recently launched and future satellites are needed.