Abstract
Abstract. One important contribution to the magnetic field measured at satellite altitude and at ground level comes from the external currents. We used the total field data sampled by the Overhauser Magnetometer on CHAMP and the horizontal magnetic field measurements of the IMAGE ground-based magnetometer network to study the ionospheric Hall current system in the auroral regions. For the CHAMP data a current model consisting of a series of lines and placed at a height of 110km is fitted to the magnetic field signature sampled on the passage across the polar region. The derived current distributions depend, among others, on season and on the local time of the satellite track. At dawn/dusk the auroral electrojets can be detected most clearly in the auroral regions. Their intensity and location are evidently correlated with the A E activity index. For a period of almost two years the results obtained from space and the currents determined from ground-based observations are studied. For the full IMAGE station array a newly-developed method of spherical elementary current systems (SECS) is employed to compute the 2-D equivalent current distribution, which gives a detailed picture of an area covering latitudes 60° – 80° N and 10° – 30° E in the auroral region. Generally, the current estimates from satellite and ground are in good agreement. The results of this survey clearly show the average dependence of the auroral electrojet on season and local time. This is particularly true during periods of increased auroral activity. The correlation coefficient of the results is close to one in the region of sizeable ionospheric current densities. Also the ratio of the current densities, as determined from above and below the ionosphere, is close to unity. It is the first time that the method of Hall current estimate from a satellite has been validated quantitatively by ground-based observations. Among others, this result is of interest for magnetic main field modelling, since it demonstrates that ground-based observations can be used to predict electrojet signatures in satellite magnetic field scalar data. Key words. Ionosphere (auroral Ionosphere; electric fields and currents; ionosphere-magnetosphere interactions)
Highlights
The most intense current system in the ionosphere is that of the auroral electrojets in the auroral oval
The strength and latitudinal position of these current flows depend on many factors, for example, on the solar zenith angle, solar wind activity and magnetospheric convection and substorm processes
In the same diagram we show the field-aligned currents (FACs) estimated from CHAMP magnetic field vector data
Summary
The most intense current system in the ionosphere is that of the auroral electrojets in the auroral oval. The strength and latitudinal position of these current flows depend on many factors, for example, on the solar zenith angle, solar wind activity and magnetospheric convection and substorm processes. The characteristics of the auroral electrojet have been of interest, since they reflect the dynamics and the processes at the magnetopause and in the outer magnetosphere. Ground-based observations at auroral latitudes have been interpreted in terms of equivalent ionospheric currents. In case of additional information the different current components could be determined independently Kamide and Richmond, 1982; Untiedt and Baumjohann, 1993, and references therein). A recent refinement of equivalent ionospheric current estimates from ground-based data has been presented by Amm and Viljanen (1999)
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