Characteristics of F‐region dynamo currents deduced from CHAMP magnetic field measurements

Abstract

Using magnetic field observations of the CHAMP satellite we provide the first comprehensive study of F‐region dynamo currents as a function of season, local time, geographic longitude, and solar activity. From bipolar variations of the zonal magnetic field component the density of vertical current driven by the F‐region dynamo is deduced. The current strength is smallest around June solstice, which is attributed to a reduced F‐region Pedersen conductance caused by a lower electron density and neutral density at that season. During the hours around noon highest current densities are observed. They are flowing downward over the dip equator. A secondary peak of upward currents appears at dusk. The polarity switch occurs between15 and 16 (local time) independent of season. The noontime F‐region dynamo current peaks at longitudes connected to the South Atlantic Anomaly, which can be explained by the enhanced conductivity in the region of reduced B field. The F‐region current at dusk exhibits no peak in the longitude sector of the South Atlantic Anomaly. At noon, the F‐region dynamo currents exhibit a wave 4 longitudinal structure during equinoxes and June solstice. The wave 4 signature becomes weak during December solstice. At dusk the wave 4 signature of F‐region dynamo currents is much reduced in all seasons. This behavior can be explained by the DE3 tidal signature in the zonal wind at CHAMP altitude. F‐region dynamo currents increase linearly with the solar flux index, F10.7, during both noon and dusk time sectors. The increase in current strength with increasing F10.7 is slightly higher at dusk than at noon.

Climatology of the inter-hemispheric field-aligned current system in the equatorial ionosphere as observed by CHAMP

“…There are 24 overlapping GLON bins. Zonal residual field data are rejected if the amplitude is larger than 50 nT or the daily-averaged Kp index exceeds 4 (Park et al, 2010). The magnetic quasi-dipolar coordinates, as defined by Richmond (1995), are used in this work as reference frame.…”

“…1c) parts by Fourier transform. Details of the procedure are given in Park et al (2010). The antisymmetric component in Fig.…”

“…3, are quite common in ionospheric quantities (e.g., Lühr et al, 2008;England et al, 2010;Park et al, 2010, and references therein). Eastward-propagating non-migrating diurnal tides with wavenumber 3 (DE3) are a major source for wave-4 signatures in the ionospheric E-layer (e.g., Hagan et al, 2007).…”

“…A north-south asymmetry of the Sq vortices (especially during solstices) is expected to create a potential difference between the hemispheres (e.g., van Sabben, 1966;Fukushima, 1979;Onwumechili, 1997;Tomás et al, 2009), resulting in inter-hemispheric field-aligned currents (IHFACs). Asymmetries in neutral wind and conductivity (e.g., Maeda, 1974;Takeda et al, 2003) are known to contribute to the IHFAC generation.…”

“…The antisymmetric component in Fig. 1c is interpreted as caused by the equatorial F-region wind dynamo as described in Park et al (2010). The symmetric component in Fig.…”