Ensemble Alfvén Wave Model for CHAMP Small-Scale Field-Aligned Currents
Abstract
Intense, small-scale field-aligned currents (SSFACs) deduced from CHAMP measurements of magnetic field fluctuations at 400 km altitude in Earth's ionosphere are correlated with cusp-region, thermospheric density anomalies and are thought to underlie anomaly formation in both empirical and theoretical studies. The analysis of SSFACs from magnetic time series in the CHAMP studies derives from a calculation of the curl of the magnetic field under two basic assumptions: (1) SSFACs are sheet currents and (2) the spatial derivative across the sheet current can be approximated as the Doppler derivative ∂/∂(vst) where vs is the satellite velocity normal to the sheet. Assumption (2) requires the time dependence of SSFACs to be sufficiently slow that the variations in the magnetic time series may be considered spatial on the transit time of the satellite across them. We examine this assumption in the context of an Alfvén wave series expansion, first using the model to create an ensemble of SSFACs "events" with statistically similar properties for model and observed events. In contrast with an analysis of time series recorded on a single satellite, the Alfvén expansion disambiguates the spatial and temporal dependence of the magnetic variability and allows for comparison of SSFACs derived from the Doppler derivative versus the curl of the magnetic field. SSFACs derived from both the curl and Doppler derivative are compared with observational SSFACs reported by Rother et al. (2007). We have found that the ensemble Alfvén wave model is capable of simulating SSFACs with spectral and intermittency characteristics similar to Rother et al. It is shown that the FAC derived from the curl of the magnetic field contains more power at low frequencies relative to the FAC derived from the Doppler derivative. The model may be further used to calculate time series and spectral properties of electric and magnetic fields at all altitudes in the ionosphere-thermosphere, along with ensemble-averaged altitude profiles of thermospheric heating rates (see paper by Lotko et al., this meeting).
Rother, M., K. Schlegel, and H. Lühr (2007), CHAMP observation of intense kilometer-scale field-aligned currents, evidence for an ionospheric Alfvén resonator, Ann. Geophys. 25- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMSM51F2802H
- Keywords:
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- 2431 Ionosphere/magnetosphere interactions;
- IONOSPHEREDE: 2475 Polar cap ionosphere;
- IONOSPHEREDE: 2721 Field-aligned currents and current systems;
- MAGNETOSPHERIC PHYSICSDE: 2736 Magnetosphere/ionosphere interactions;
- MAGNETOSPHERIC PHYSICS