Freddy Christiansen

On the Modelling of Field-Aligned Currents from Magnetic Observations by Polar Orbiting Satellites

The comprehensive data from Orsted and CHAMP adding to the data base from the earlier Magsat magnetometry mission have made it possible to develop sophisticated models for the high-latitude field-aligned currents (FAC) relating to solar wind and magnetospheric parameters. However, there are still large uncertainties involved in using the new models primarily due to the lack of precise knowledge of the temporal and spatial development of the currents in relation to the highly variable solar wind parameters and the still unpredictable effects of magnetospheric substorms. A further problem is the lack of proper account of fine-structure currents in the statistical models. By considering separately the upward and downward FAC intensities the paper presents some first attempts to examine the possible diminishing effects on modelled FAC magnitudes from averaging oppositely directed currents.

Mapping of Field-Aligned Current Patterns during Northward IMF

Magsat and Orsted high-precision magnetic measurements have been combined to provide detailed and consistent mapping of ionospheric field-aligned current (FAC) patterns during intervals of northward interplanetary magnetic field (NBZ) conditions. The satellite observations indicate that the NBZ FAC currents extend from two rounded ionospheric regions centered at 85° geomagnetic latitude and magnetic local times of approximately 0900 for the upward and 1500 for the downward FAC. The analysis has revealed that the locations of the NBZ FAC regions are almost invariant to seasonal variations and to variations in the magnitude of IMF B z . The NBZ FAC currents are sizeable for summer conditions only; they almost vanish during equinox and winter seasons. The NBZ FAC intensity is a strongly non-linear function of IMF B z rising strongly above a marked threshold at around +5 nT. The observations support experiences from polar magnetometer observations, among other from Greenland, on the gradual development of high-latitude convection changes following IMF changes.

Field-aligned Currents Inferred from Low-Altitude Earth-Orbiting Satellites and Ionospheric Currents Inferred from Ground-Based Magnetometers — Do They Render Consistent Results?

Field-aligned currents (FACs) and ionospheric Hall currents were inferred from spatially and temporally coincident vector magnetometer measurements made along the Greenland west coast onboard the Orsted satellite and on the ground, respectively. We selected satellite passes which were closely aligned with the ground-based magnetometer chain, and investigate whether the trajectoryintegrated FAC (TIFAC) is consistent with the ionospheric Hall current across the line of magnetometers. Our analysis builds on the assumption that FACs are closed in the ionosphere via Pedersen currents driven by an electric field which also drives ionospheric Hall currents. Solar radiation conductance models suggest that the Hall/Pedersen conductance ratio, and consequently the Hall/Pedersen current ratio, and eventually the Hall/TIFAC ratio, should be slightly greater than 1. We examined 236 cases covering the midnight-morning-noon sector and found at times small and at other times large deviations from this ratio. We suggest that the latter cases represent conditions where the infinite current sheet assumption is violated or the ionosphere exhibits strong conductivity gradients or conductances are largely caused by charged particle precipitation. Deviations from a ratio of 1 appear to occur predominantly equatorward of ≈74° corrected geomagnetic latitude (CGML) in the midnight sector and poleward of ≈72° in the noon sector.

The Low-Altitude Cusp: Multi-Point Observations During the February 2002 SIRCUS Campaign

The first SIRCUS campaign covered the days February 16–22, 2002. It involved the CHAMP and Orsted LEO satellites and the EISCAT (Tromso, Svalbard) and Sondrestrom incoherent scatter radars. The primary data set was supplemented by DMSP-F13 and -F14 particle flux and ACE solar wind measurements. On February 21 all instruments were operated in campaign mode and delivered usable data. We identify the low-altitude cusp based on signatures in our multi-instrument data set and derive a consistent time-dependent mapping of the cusp in terms of magnetic local time and latitude. We demonstrate that small-scale variations of the magnetic field resp. field-aligned currents (spatial scales of several hundred meters) can be used to identify the cusp. However, their general ability as cusp signature has not yet been proven. An extended analysis of several SIRCUS campaigns is expected to give a qualified answer.

Detection of Intense Fine-Scale Field-Aligned Current Structures in the Cusp Region

The Orsted high-precision magnetic measurements made at high temporal resolution of 25 and occasionally 100 samples/sec corresponding to spatial resolutions down to less than 100 m have demonstrated the occurrences of highly structured field-aligned currents (FAC) in the low-altitude Cusp region. The observed magnetic perturbations indicate structures of very intense but thin sheets or narrow filaments of mixed up- and downward currents up to several hundreds of µA/m2 embedded in large-scale FAC structures of only up to a few µA/m2. The intensities and locations of fine-scale FAC structures are closely related to solar wind conditions.