The energy carried by the solar wind can be fully deposited into the Earth’s upper thermosphere at high latitudes, and produce longitudinally extended waves (Bruinsma et al., 2007, 2009; Liu et al., 2018b). These waves then propagate away from the source region to other regions of the coupled Ionosphere-Thermosphere (IT) system. These waves are named as travelling atmospheric disturbances (TADs), which are a key factor in understanding the variability of the IT system. Exploring the possible mechanisms of TADs is critical in the modeling and forecasting of near-Earth upper atmosphere environment.
In the past decades, TADs during magnetic disturbed periods have attracted much attention (e.g., Bruinsma & Forbes, 2007; Liu and Lühr, 2005; Liu et al., 2010, 2014, 2018b; Oliveira et al., 2017; Otsuka et al., 2004; Shiokawa et al., 2003; Sutton et al., 2009; Thome et al., 1964; Zhang et al., 2019). Previous studies have reported that the storm time energetic particle precipitation and Joule heating (Rodríguez-Zuluaga et al., 2016), the gravity waves (Liu et al., 2014), and the sunset terminator (Beer, 1973) are critical in the generation of TADs.
As reported by Dungey (1961), the geomagnetic storms can be induced by the interaction between the IMF Bz in southward and the geomagnetic field. During disturbed periods, a large amount of energy and momentum carried by the solar wind are deposited into the Earth’s upper atmosphere, changing global circulation and producing TADs. TADs have been studied using meridional winds, neutral temperature, composition (O/N2) and air mass density data (e.g., Bruinsma and Forbes, 2007; Bruinsma et al., 2009; Fujiwara et al., 2006; Lei et al., 2008). Typically, TADs propagate with velocities of 400–1000 (100–300) m/s, periods of 0.5-3 (0.25-1) hour, and wavelength of thousands (hundreds) of kilometers in large-scales (medium-scales) (e.g., Bruinsma and Forbes, 2009; Hocke and Schlegel, 1996).
Liu and Lühr (2005) found that during storm times the enhancement of the thermospheric air mass density in the noontime showed two different features (see the upper panel of Fig. 1 in Liu and Lühr, 2005). One was the enhancement in the density at 0º~40º geomagnetic latitudes (MLat), which originated from the TAD propagating from the auroral region to low latitudes at around 20 UT on 30 October 2003. The other was the almost instant enhancement in the density at aurora and middle latitudes at about 20:00 UT on 30 October 2003. These two features overlapped at 20:00 UT and 40º MLat, forming broken mean circulation. This broken feature and its possible mechanisms have rarely been studied in the literature, which is one of the focus of the study.
Another interesting topic is raised by Sharma et al. (2011). They reported a longitudinal variation of the storm time O/N2 enhancement: it was stronger at Yibal (22.18º Geographic latitude (GLat), 56.11º Geographic longitude (GLon)) than at Kunming (25.03º GLat, 102.79º GLon) and Udaipur (24.67º GLat, 74.69º GLon). This longitudinal variation was attributed to the air upwelling due to the equatorward meridional wind (Sharma et al., 2011). The question is then whether TADs in the meridional wind also show notable longitudinal variations? Based on observations from all-sky airglow imagers, Otsuka et al. (2004) investigated the geomagnetic conjugacy of the medium-scale traveling ionospheric disturbances (TID). The conjugacy structures came from the polarization electric field that maps along geomagnetic field lines and moves the F region plasma upward/downward, producing the mirrored structures of plasma perturbations in the Northern and Southern Hemispheres. TID can be treated as the ionospheric counterpart of TAD with phase and time delay compared to TAD. Thus, the hemispheric conjugacy of the TAD is worthy of investigation.
While the TADs in thermospheric winds during storm time have been established in the literature, the exact longitudinal and hemispheric changes of the meridional wind responses to temporal oscillations of IMF Bz is still poorly understood, due to the fact that the thermospheric winds are balanced among many drivers (i.e., pressure gradient, ion drag, Coriolis). The thermospheric winds are important factors in the understanding of changes in ionosphere-thermosphere coupling system. In addition, the periodic oscillations of IMF Bz are a common phenomenon seen in the solar wind, which can produce periodic oscillations in the energy and momentum deposition in the upper atmosphere and TADs (e.g., Liu et al., 2018a; Zhang et al., 2019). Taking the abovementioned into consideration, the present work is to study the longitudinal and hemispheric patterns of the meridional wind changes at a fixed local time, which show some interesting signals of the broken mean circulation, standing feature, and the related longitudinal and hemispheric variations during the period of the oscillating IMF Bz. Furthermore, these thermospheric wind changes has not caught so much attention in the literature. The possible drivers for the noontime wind changes are revealed, including the possible effects of the geomagnetic topology. Electron density, plasma motion and neutral temperature changes are also examined.