Zonal winds in the equatorial upper thermosphere: Decomposing the solar flux, geomagnetic activity, and seasonal dependencies

Abstract

[1] Using 3 years (2002)(2003)(2004), over 16,400 orbits of measurements from the accelerometer on board the CHAMP satellite, we have studied the climatology of the equatorial zonal wind in the upper thermosphere. Several main features are noticed. The most prominent one is that the solar flux significantly influences both the daytime and nighttime winds. It overrides the geomagnetic activity effect, which is found to be rather limited to the nightside. An elevation of the solar flux level from F10.7 % 100 Â 10 À22 W m À2 Hz À1 to F10.7 % 190 Â 10 À22 W m À2 Hz À1 produces an eastward disturbance wind up to $110 m s À1 . This consequently enhances the nighttime eastward wind but suppresses the daytime westward wind. A seasonal variation with weaker wind (by over 50 m s À1 at night) around June solstice than in other seasons has been observed regardless of solar flux and geomagnetic activity levels. The zonal wind is eastward throughout the night except around June solstice, where it ebbs to almost zero or turns even westward in the postmidnight sector at low solar flux level. The daytime wind is found to be generally more stable than the nighttime wind, particularly unresponsive to geomagnetic activities. Predictions from the Horizontal Wind Model find good agreement with the CHAMP-observed wind at high solar flux levels during nighttime. At low solar flux levels, however, the model strongly underestimates the westward wind during morning hours by 50-120 m s À1 depending on season. The major difference between the HWM-predicted and the CHAMP-observed wind is seen in the phase of its diurnal variation. The CHAMPobserved wind turns eastward around 1200-1300 MLT instead of 1600-1700 MLT predicted by the model. Comparisons with ground FPI observations and the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM) predictions show that the solar flux effect obtained from CHAMP is consistent with that modeled by TIEGCM. The solar flux dependence of zonal wind found here together with that of the zonal ion drift found in previous studies reflect the relative importance of the E-and F-region wind dynamo in the thermosphere-ionosphere coupling process. Furthermore, these wind measurements indicate that the Earth's atmosphere superrotates. The average superrotation speed amounts to about 22 m s À1 for a solar flux level of F10.7 % 100 Â 10 À22 W m À2 Hz À1 but increases to 63 m s À1 for F10.7 % 190 Â 10 À22 W m À2 Hz À1 . Finally, the wind behavior presented in this study is longitudinally averaged and may differ from wind measurements at a certain longitude.

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

“…As the background thermospheric mass density is largely controlled by solar activity and magnetic storms, data from solar minimum years (from 2007 to 2009, mean P10.7 = 71 sfu) have been selected for the mass density to avoid their influence on our tidal signatures analysis. In contrast, the wind shows much less dependence on solar activity, as shown by Liu et al (2006). Unfortunately, the wind signals at CHAMP altitude are too weak and unreliable during solar minimum conditions.…”

Thermospheric zonal mean winds and tides revealed by CHAMP

“…All aspects of the mission and the accelerometer experiment are described by Bruinsma et al [2004]. Cross-track winds have been derived from acceleration values by Liu et al [2006] and Doornbos et al [2010]. Liu et al [2006] compared 2002-2004 ı of the magnetic equator with the Horizontal Wind Model (HWM-93) and documented the sensitivity of longitudinally averaged equatorial zonal winds to solar and geomagnetic activity.…”

“…Cross-track winds have been derived from acceleration values by Liu et al [2006] and Doornbos et al [2010]. Liu et al [2006] compared 2002-2004 ı of the magnetic equator with the Horizontal Wind Model (HWM-93) and documented the sensitivity of longitudinally averaged equatorial zonal winds to solar and geomagnetic activity. More recently, Doornbos et al [2010] developed an iterative scheme to derive winds and densities from accelerometer measurements that operates independently of the instrument orientation in space.…”

Thermospheric vorticity at high geomagnetic latitudes from CHAMP data and its IMF dependence

“…It is located at the spacecraft's center of mass and effectively samples the in situ acceleration with an accuracy of ∼ 3 × 10 −9 m s −2 (Doornbos et al, 2010). From the air drag observations, thermospheric mass density and cross-track neutral wind data have been obtained using a simplified methodology as described by Liu et al (2006). This method neglects lift and sideways forces on the spacecraft or requires that these forces are modeled and removed from the acceleration beforehand, as it was done later by Sutton et al (2007), who named it a "dual-axis method".…”