The complete spectrum of the equatorial electrojet related to solar tides: CHAMP observations

Abstract

Abstract. Based on 10 yr of magnetic field measurements by the CHAMP satellite we draw a detailed picture of the equatorial electrojet (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 to represent 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. The 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 represents 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 14:00 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 previous 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.

Nonmigrating tidal signatures in the magnitude and the inter-hemispheric asymmetry of the equatorial ionization anomaly

“…This asymmetry can well be described by the planetary waves, SPW1 and SPW2. Although we cannot offer a conclusive explanation for the observed EIA displacement, it is worth noting that Lühr and Manoj (2013) report about an enhanced EEJ intensity at all daytime hours in the same western longitude sector during June solstice months (see their Fig. 18).…”

“…The amplitude of the SW3 tide in EEJ is found to be quite large during certain seasons. In particular during fall and winter the SW3 amplitude depends on solar activity (Lühr and Manoj, 2013). In order to deduce the SW3 tidal dependence on solar flux we sorted the CHAMP EIA observations into high (P10.7 ≥ 120 sfu) and low (P10.7 ≤ 120 sfu) solar flux bins.…”

Tidal signatures of the thermospheric mass density and zonal wind at midlatitude: CHAMP and GRACE observations

“…Hagan et al (2009) reported the existence of stationary planetary wave-4 (SPW4) oscillation in the upper mesosphere and lower thermosphere (MLT) region, caused by the nonlinear interaction between DE3 and the migrating tide DW1. Since then, the importance of stationary planetary waves, SPW4/SPW3, contributing to the longitudinal WN4/WN3 patterns in the upper atmosphere, has also been supported by both model simulations (e.g., Oberheide et al, 2011a;Pancheva et al, 2012) and in situ observations (e.g., Kil et al, 2010;Lühr and Manoj, 2013;Xiong and Lühr, 2013;Chang et al, 2013).…”

On the day‐to‐day variation of the equatorial electrojet during quiet periods

“…Using the zonal wind for May and June 2009, the tidal components were extracted on the basis of least squares fitting [ Wu et al , ]. Satellite‐borne magnetometer measurements have shown that those tidal components are dominant in the electrojet spectra [ Lühr and Manoj , ].…”