Abstract
Abstract. Three geomagnetic satellite missions currently provide high precision magnetic field measurements from low altitude polar orbiting spacecraft. We demonstrate how these data can be used to determine the intensity and location of the horizontal currents that flow in the ionosphere, predominantly in the auroral electrojets. First, we examine the results during a recent geomagnetic storm. The currents derived from two satellites at different altitudes are in very good agreement, which verifies good stability of the method. Further, a very high degree of correlation (correlation coefficients of 0.8–0.9) is observed between the amplitudes of the derived currents and the commonly used auroral electrojet indices based on magnetic measurements at ground. This points to the potential of defining an auroral activity index based on the satellite observations, which could be useful for space weather monitoring. A specific advantage of the satellite observations over the ground-based magnetic measurements is their coverage of the Southern Hemisphere, as well as the Northern. We utilize this in an investigation of the ionospheric currents observed in both polar regions during a period of unusually steady interplanetary magnetic field with a large negative Y-component. A pronounced asymmetry is found between the currents in the two hemispheres, which indicates real inter-hemispheric differences beyond the mirror-asymmetry between hemispheres that earlier studies have revealed. The method is also applied to another event for which the combined measurements of the three satellites provide a comprehensive view of the current systems. The analysis hereof reveals some surprising results concerning the connection between solar wind driver and the resulting ionospheric currents. Specifically, preconditioning of the magnetosphere (history of the interplanetary magnetic field) is seen to play an important role, and in the winther hemisphere, it seems to be harder to drive currents on the nightside than on the dayside.Key words. Ionosphere (electric fields and currents) – Magnetospheric physics (current systems; magnetosphere-ionosphere interactions)
Highlights
The currents flowing in the high latitude ionospheres constitute an important space weather parameter
Associated with the differences in altitude, the orbit periods are slightly different, that of CHAMP being roughly 93 min and that of Ørsted and SAC-C being roughly 100 min. This implies that even during times when CHAMP and Ørsted cover nearly the same local time regions, they will be at opposite locations in their orbits roughly once per day, i.e. part of each day they will be over the same polar region at roughly the same time and we can use this to make consistency checks of their measurements of the ionospheric currents
They continue to play a large role as a quantitative parameter describing various aspects of the solar-terrestrial environment (e.g. Weimer et al, 1990; Lu et al, 1998), and the auroral electrojets (AE) index is recognized as a metric for space weather modeling and forecasting (e.g. Vassiliadis et al, 1993; Gleisner and Lundstedt, 1997; NSWP, 2000)
Summary
The currents flowing in the high latitude ionospheres constitute an important space weather parameter. The effect of the horizontal ionospheric currents in the auroral electrojets are felt in the magnetic measurements made at low altitude, near-polar orbiting satellites. Such measurements offer an alternative means for determining and monitoring these currents. The orbits of the three satellites are widely separated in local time and they all cross the same pole nearly simultaneously Analyzing these results puts to the test our ability to relate observed ionospheric current systems in the two hemispheres to the value of the interplanetary magnetic field. We describe briefly the main characteristics of the satellite missions, their measurements, and the modeling technique that we apply to the data
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