Sunlight effects on the 3D polar current system determined from low Earth orbit measurements

• Main Authors: Laundal, Karl M, Finlay, Christopher C, Olsen, Nils
• Format: Journal Article
• Language: English
• Published: 21-11-2016
• Subjects: Physics - Space Physics
• DOI: 10.48550/arxiv.1611.06770

Interaction between the solar wind and the Earth's magnetosphere is
associated with large-scale currents in the ionosphere at polar latitudes that
flow along magnetic field lines (Birkeland currents) and horizontally. These
current systems are tightly linked, but their global behaviors are rarely
analyzed together. In this paper, we present estimates of the average global
Birkeland currents and horizontal ionospheric currents from the same set of
magnetic field measurements. The magnetic field measurements, from the low
Earth orbiting $\textit{Swarm}$ and CHAMP satellites, are used to co-estimate
poloidal and toroidal parts of the magnetic disturbance field, represented in
magnetic apex coordinates. The use of apex coordinates reduces effects of
longitudinal and hemispheric variations in the Earth's main field. We present
global currents from both hemispheres during different sunlight conditions. The
results show that the Birkeland currents vary with the conductivity, which
depends most strongly on solar EUV emissions on the dayside and on particle
precipitation at pre-midnight magnetic local times. In sunlight, the horizontal
equivalent current flows in two cells, resembling an opposite ionospheric
convection pattern, which implies that it is dominated by Hall currents. By
combining the Birkeland current maps and the equivalent current, we are able to
calculate the total horizontal current, without any assumptions about the
conductivity. We show that the total horizontal current is close to zero in the
polar cap when it is dark. That implies that the equivalent current, which is
sensed by ground magnetometers, is largely canceled by the horizontal closure
of the Birkeland currents.