Ionospheric currents estimated simultaneously from CHAMP satelliteand IMAGE ground-based magnetic field measurements: a statisticalstudy at auroral latitudes

摘要

One important contribution to the magnetic field measured atsatellite altitude and at ground level comes from the externalcurrents. We used the total field data sampled by the OverhauserMagnetometer on CHAMP and the horizontal magnetic fieldmeasurements of the IMAGE ground-based magnetometer network tostudy the ionospheric Hall current system in the auroral regions.For the CHAMP data a current model consisting of a series of linesand placed at a height of 110km is fitted to the magnetic fieldsignature sampled on the passage across the polar region. Thederived current distributions depend, among others, on season andon the local time of the satellite track. At dawn/dusk the auroralelectrojets can be detected most clearly in the auroralregions. Their intensity and location are evidently correlatedwith the A E activity index. For a period of almost two years theresults obtained from space and the currents determined fromground-based observations are studied. For the full IMAGE stationarray a newly-developed method of spherical elementary currentsystems (SECS) is employed to compute the 2-D equivalent currentdistribution, which gives a detailed picture of an area coveringlatitudes 60° – 80° N and 10° – 30° E in theauroral region. Generally, the current estimates from satelliteand ground are in good agreement. The results of this surveyclearly show the average dependence of the auroral electrojet on season and localtime. This is particularly true during periods of increasedauroral activity. The correlation coefficient of the results isclose to one in the region of sizeable ionospheric currentdensities.Also the ratio of the current densities, as determined from aboveand below the ionosphere, is close to unity. It is the first timethat the method of Hall current estimate from a satellite has beenvalidated quantitatively by ground-based observations. Amongothers, this result is of interest for magnetic main fieldmodelling, since it demonstrates that ground-based observationscan be used to predict electrojet signatures in satellite magneticfield scalar data.Key words. Ionosphere (auroral Ionosphere; electric fieldsand currents; ionosphere-magnetosphere interactions)