| The geomagnetic field variations measured at the surface of the Earth are
composed of both internal and external parts. The external field arises
from the sources in the magnetosphere and ionosphere, whereas the internal
field is generated by the currents induced within the Earth. The internal
part may in some situations comprise a notable part of the measured total
field and thus a blind usage of geomagnetic field recordings potentially
produces significant errors to estimated ionospheric currents. In this paper
the role of geomagnetic induction in auroral ionospheric studies is
investigated by modeling the induction using simultaneously the realistic
ionospheric source and a realistic three-dimensional Earth conductivity
structure.
The modeling results imply that the effects of the lateral ground
conductivity anomalies on ionospheric equivalent current patterns are, though
clearly detected, less severe than anticipated for fields varying with
periods from 5 to 120min. However, the amplification of the determined
currents caused by induction is significant, leading to an overestimation of
up to 30% of the main current flow intensities, with the overestimation
increasing sharply when moving away from the region of the main flow.
In addition to the 3-D modeling, a simple method is introduced to help
estimate the internal contribution to the measured variations of the IL
index (local variant of the AL index). A test with the 26 June 1998
substorm event indicates that the method can help to extract the internal
contribution from the IL index. |