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Ann. Geophys., 25, 721-736, 2007
www.ann-geophys.net/25/721/2007/
© European Geosciences Union 2007
A model for estimating the relation between the Hall to Pedersen conductance ratio and ground magnetic data derived from CHAMP satellite statistics
L. Juusola, O. Amm, K. Kauristie, and A. Viljanen
Finnish Meteorological Institute, Space Research Unit, P.O. Box 503, 00101, Finland
Abstract. The goal of this study is to find a way to statistically estimate the Hall to
Pedersen conductance ratio α from ground magnetic data. We use vector
magnetic data from the CHAMP satellite to derive this relation. α is
attained from magnetic satellite data using the 1-D Spherical Elementary
Current Systems (SECS). The ionospheric equivalent current density can either
be computed from ground or satellite magnetic data. Under the required 1-D
assumption, these two approaches are shown to be equal, which leads to the
advantage that the statistics are not restricted to areas covered by ground
data. Unlike other methods, using magnetic satellite measurements to
determine α ensures reliable data over long time sequences. The
statistical study, comprising over 6000 passes between 55° and
76.5° northern geomagnetic latitude during 2001 and 2002, is carried out
employing data from the CHAMP satellite. The data are binned according to
activity and season. In agreement with earlier studies, values between 1 and
3 are typically found for α. Good compatibility is found, when
α attained from CHAMP data is compared with EISCAT radar measurements.
The results make it possible to estimate α from the east-west
equivalent current density Jφ; [A/km]: α=2.07/(36.54/|Jφ|+1) for
Jφ<0 (westward) and α=1.73/(14.79/|Jφ+1) for Jφ0
(eastward). Using the same data,
statistics of ionospheric and field-aligned current densities as a function
of geomagnetic latitude and MLT are included. These are binned with respect to
activity, season and IMF BZ and BY. For the first time, all three
current density components are simultaneously studied this way on a
comparable spatial scale. With increasing activity, the enhancement and the
equatorward expansion of the electrojets and the R1 and R2 currents is
observed, and in the nightside, possible indications of a Cowling channel
appear. During southward IMF BZ, the electrojets and the R1 and R2
currents are stronger and clearer than during northward BZ. IMF BY
affects the orientation of the pattern.
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