Hermann Lühr¹ and Chandrasekharan Manoj²

1) GFZ, German Research Centre for Geosciences, Potsdam, Germany , 2) National Geophysical Data Center, NOAA, Boulder, Colorado, USA

 Based on 10 years of magnetic field measurements by the CHAMP satellite we draw a detailed picture of the EEJ tidal variations. For the first time the complete EEJ spectrum related to average solar tides has been compiled. A large fraction of the resulting spectrum is related to the switch on/off of the EEJ between day and night. This effect has carefully been considered when interpreting the results. As expected, largest amplitudes are caused by the migrating tides representing the mean diurnal variation. Higher harmonics of the daily variations show a 1/f fall-off in amplitude. Such a spectrum is required for representing the vanishing of the EEJ current at night. The migrating tidal signal exhibits a distinct annual variation with large amplitudes during December solstice and equinox seasons but a depression by a factor of 1.7 around June-July. A rich spectrum of non-migrating tidal effects is deduced. Most prominent is the four-peaked longitudinal pattern around August. Almost 90% of the structure can be attributed to the diurnal eastward propagating tide DE3. In addition the westward propagating DW5 is contributing to wave-4. Second largest non-migrating tide is the semi-diurnal SW4 around December solstice. It causes a wave-2 feature in satellite observations. The three-peaked longitudinal pattern, often quoted as typical for the December season, is significantly weaker. During the months around May-June a prominent wave-1 feature appears. To first order it represent a stationary planetary wave SPW1 which causes an intensification of the EEJ at western longitudes beyond 60°W and a weakening over Africa-India. In addition, a prominent ter-diurnal non-migrating tide TW4 causes the EEJ to peak later, at hours past 14h local time in the western sector. A particularly interesting non-migrating tide is the semi-diurnal SW3. It causes largest EEJ amplitudes from October through December. This tidal component shows a strong dependence on solar flux level with increasing amplitudes towards solar maximum. We are not aware of any earlier studies mentioning this behaviour of SW3. The main focus of this study is to present the observed EEJ spectrum and its relation to tidal driving. For several of the identified spectral components we cannot offer convincing explanations for the generation mechanisms.

Figure: Average solar tidal spectra of the electrojet for two periods, from solar maximum (left) and solar minimum (right). Colour-coded current densities are shown on a logarithmic scale.

Available EEJ solar tidal information for downloads
	Type	Format	Mbyte	Contents
	Table of EEJ tides	Excel	0.05	Estimates all EEJ tidal components for each month of a year, for seasons and all data during the years 2000 - 2010
	Table of EEJ tides	Excel	0.05	Estimates all EEJ tidal components for each month of a year, for seasons and all data during the years 2000 - 2005
	Table of EEJ tides	Excel	0.05	Estimates all EEJ tidal components for each month of a year, for seasons and all data during the years 2005 - 2010
	Article	PDF	6.9	The complete spectrum of the equatorial electrojet related to solar tides: CHAMP observations (angeo 2013)

Manoj Nair