Investigations of high latitude wind shears in the lower thermosphere

The Atmospheric Response in Aurora (ARIA) rocket campaign of March 1992 represented an effort to measure the effect of the prolonged heating due to a diffuse aurora on the neutral atmosphere. A previous model had predicted strong zonal winds in the lower thermosphere as the result of the strong coupling between the ionosphere and neutral atmosphere. An instrumented payload was launched into the diffuse aurora to measure neutral and plasma compositions, and two chemical releases provided the neutral wind measurements. In addition to this data, ground-based magnetometers and satellite measurements of the interplanetary magnetic field (IMF), electric fields, particle precipitation, and ram drift were incorporated into the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure. The AMIE procedure provided patterns of the electrostatic potential, electric fields, conductivities, and Joule heating on a global scale. The results of this analysis show the activity the chemical release of March 3 occurred in the region of greatest Joule heating. The neutral wind profile show strong wind shear existing at 115 km altitude, approximately the same altitude of the maximum electron density measured by the instrumented payload. This feature had a magnitude of approximately 150 m/s and was in the direction of the auroral electric field (southwestward).;These wind shears coexisting with maximum's in electron densities have been studied for many years from chemical releases done at midlatitudes. The explanation for the midlatitude wind shears however fails to explain the high latitude wind shears seen in the ARIA release due to the vertical nature of the magnetic field. At auroral latitudes the electron density does not build up like at lower latitudes. In order to investigate high latitude wind shears, 15 additional high latitude chemical release experiments, performed by Heppner during the 1970's, have been reanalyzed. A steady state solution called modified geostrophic adjustment is used in attempting to model these features. This model is simply a balance of the pressure gradient, Corolis and Lorentz forces. By incorporating the Time Dependent Ionospheric Model (TDIM) of Utah State, along with a model neutral atmosphere provided by MSIS90E, wind simulations were compared with sixteen chemical release experiments. Simulation results show the presence of wind shears around 120 km altitude whenever the electron density profile shows a maximum at approximately that height. The existence of this wind shear is shown to be dependent on the Hall conductivity, which can dominate the Pedersen conductivity at lower thermospheric heights. The magnitude of the wind shear produced in the steady-state solution is also observed to be dependent on the strength of the imposed electric field. Comparisons of these modeled velocities with the neutral wind profiles obtained from the Heppner chemical releases show varied results depending on the stability of the prevailing conditions, but results from the ARIA release compare quite well due to the prolonged heating due to the diffuse aurora.