Winds in the high‐latitude lower thermosphere: Dependence on the interplanetary magnetic field

Abstract

[1] Wind observations in the summertime lower thermosphere at high southern latitudes, measured by the Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite, are statistically analyzed in magnetic coordinates and correlated with the interplanetary magnetic field (IMF) to determine influences of IMF-dependent ionospheric convection on the winds. Effects are clearly detectable down to 105 km altitude. Above 125 km the wind patterns show considerable similarity with ionospheric convection patterns, and the speed of the averaged neutral wind in the polar cap often exceeds 300 m/s. The correlation between the IMF B z component and the diurnal harmonic of the winds is generally best when the IMF is averaged over the preceding 1-4.5 hours. The magnetic-zonal-mean zonal wind below 120 km correlates best with the IMF B y component when the latter is averaged over approximately the preceding 20 hours. The wind has a significantly stronger rotational than divergent component. Around and above 120 km a dusk-side anticyclonic wind vortex is prominent, consistent with earlier findings. Around 140 km and higher the dusk-side vortex intensifies for negative B z , but around 120 km it is the dawn-side cyclonic vortex that responds more strongly to B z variations. The dependence of the wind on the IMF is nonlinear, especially with respect to IMF B z . For positive B z the difference winds are largely confined to the polar cap, while for negative B z the difference winds extend to subauroral latitudes. A significant correlation between the diurnal B z -dependent neutral and convection velocity components exists above 108 km, when the convection velocity is suitably rotated in magnetic local time (MLT) with respect to the wind. The rotation that maximizes the correlation ranges from À1.5 hours at 130 km (wind preceding convection) to nearly +6 hours at 108 km (wind lagging convection). The rotated diurnal B z -dependent wind pattern projects onto the diurnal B z -dependent ionospheric convection pattern with about 60% the amplitude of the latter above 125 km, decreasing to about 17% at 108 km. On timescales of $20 hours, a B y -dependent magnetic-zonal-mean zonal wind generally exists, with maximum wind speeds at 80°magnetic latitude, typically 10 m/s at 105 km, increasing to about 60 m/s at 123 km and 80 m/s at 200 km. In the southern hemisphere the wind is cyclonic when the time-averaged B y is positive and anticyclonic when B y is negative; the wind direction is expected to be opposite in the northern hemisphere.

FPI observations of nighttime mesospheric and thermospheric winds in China and their comparisons with HWM07

“…Figure 4 shows that the distributions of the midnight zonal and meridional winds at ∼ 250 km vary greatly from one day to another. The large variability in the thermospheric winds can be induced by the solar activity, geomagnetic activity and interplanetary field (Smith et al, 1994;Killeen et al, 1995;Richmond et al, 2003;Emmert et al, 2006a, b;Drob et al, 2008). In contrast, the variability of the model wind is not as significant as the FPI wind.…”

On the generation/decay of the storm‐enhanced density plumes: Role of the convection flow and field‐aligned ion flow

“…The nominal dayside thermospheric wind at the latitude of PFISR is poleward and <200 m s À1 at 276 km [Richards et al, 2009]. During geomagnetic storm times, the thermospheric wind pattern changes dramatically compared with quiet time [e.g., Richmond and Matsushita, 1975;Richmond et al, 2003;Lu et al, 2012;Wang et al, 2008]. Wang et al [2008] used the Coupled Magnetosphere-IonosphereThermosphere model to study the horizontal wind variations during quiet and storm times.…”

Contributions of Heating and Forcing to the High-Latitude Lower Thermosphere: Dependence on the Interplanetary Magnetic Field

“…This period is a characteristic of conditions for wind observations analyzed by Richmond et al (2003) and previous analyses of forcing ) and vorticity (Kwak et al 2008a,b). On this date, the daily F10.7 index was recorded as 102.7 × 10 -22 W/m 2 /Hz.…”

“…To investigate the response of the thermosphere to Richmond et al (2003). A 2-min time step is used for the entire simulation, and linear interpolations of input values are made at a given time step.…”

“…Thus the highlatitude thermospheric wind which are influenced by interaction with the ionospheric plasma, forcing on the wind and Joule/particle heating, are also strongly modulated by the IMF variation (e.g., McCormac & Smith 1984, McCormac et al 1985, 1995, Meriwether & Shih 1987, Thayer et al 1987, Rees & Fuller-Rowell 1989, 1990, Sica et al 1989, Hernandez et al 1991, Niciejewski et al 1992, 1994, Won 1994, Richmond et al 2003, McHarg et al 2005, Zhang et al 2005. That is, one can expect there is an intimate relationship between the IMF variation and the relative strength of momentum forcing / heating responsible for maintaining thermospheric dynamics at high latitudes.…”