UCLA

Measurements of the neutral thermosphere within the postmidnight substorm recovery phase diffuse aurora show very large horizontal winds, and strong vertical structure. Rocket, satellite, and ground based observations during the ARIA (Atmospheric Response in Aurora) campaigns, and earlier dawn side rocket observations, indicate neutral winds of up to 200m/s, and a characteristic jet-like wind maximum around 110 to 120-km altitude, with strong shears above and below. The observed wind magnitudes are found to have a dependence on geomagnetic activity level, but recent modeling studies suggest that tides which propagate up from the troposphere and stratosphere may play an important role in generating the strong vertical variations in the neutral winds. The relative importance of auroral and tidal forcing in producing the measured wind structure is not known, however. Simulations have been performed using a three dimensional (3-D) high resolution limited area thermosphere model to understand the processes which generate the observed neutral structure within the postmidnight diffuse aurora. Parameters measured during the ARIA I observational campaign have been used to provide auroral forcing inputs for the model. Global background winds and tides have been provided by the CTIP (Coupled Thermosphere Ionosphere Plasmasphere) model. The sensitivity of the response of the neutral atmosphere to changes in different parameters has been examined. Variations in the amplitudes and phases of the propagating tides in the background winds are found to have significant effects on the neutral structure in the E region, and the wind structure below around I 10 km is found to be mainly produced by tidal forcing. Changes in the electric field and ion density affect the winds above around 120km, and the importance of auroral forcing is found to depend on background winds. Variations in the orientation of the aurora relative to the background field, which may be caused by changes in the interplanetary magnetic field, are also found to modify the wind structure. When both auroral forcing and propagating tides are included, many of the basic characteristics of the wind structure are displayed, although the great strength of the wind shears is not well reproduced. The strength of the shears may be related to a currently unmodeled process, or to different types of waves.