| In addition to field observations and
numerical models, geomagnetic induction effects can be studied by scaled
analogue model experiments. We present here results of analogue model studies of
the auroral electrojet with an Earth model simulating the Arctic Ocean and
inland conductivity structures in northern Fennoscandia. The main elements of
the analogue model used were salt water simulating the host rock, an aluminium
plate corresponding to the ocean and graphite pieces producing the inland highly
conducting anomalies. The electrojet was a time-harmonic line current flowing at
a (simulated) height of 100 km above northern Fennoscandia. The period simulated
was 9 min.
The analogue model results confirmed the well-known rapid
increase of the vertical field when the coast is approached from the continent.
The increase of the horizontal field due to induced ocean currents was
demonstrated above the ocean, as well as the essentially negligible effect of
these currents on the horizontal field on the continent.
The behaviour of the magnetic field is explained with a simple
two-dimensional thin-sheet model. The range, or the adjustment distance, of the
ocean effect inland was found to be some hundreds of kilometers, which also
agrees with earlier results of the Siebert-Kertz separation of IMAGE
magnetometer data. The modelled inland anomalies evidently had too large
conductivities, but on the other hand, their influence decayed on scales of only
some tens of kilometers.
Analogue model results, thin-sheet calculations, and field
observations show that the induction effect on the horizontal magnetic field Bx
near the electrojet is negligible. On the other hand, the vertical component Bz
is clearly affected by induced currents in the ocean. Evidence of this is the
shift of the zero point of Bz 0-1° southwards from the
maximum of Bx. The importance of these results are discussed,
emphasizing the determination of ionospheric currents. |