A kinematic analysis of the high‐latitude thermospheric neutral circulation pattern

Abstract

Averaged measurements from the Dynamics Explorer 2 (DE 2) satellite have been combined with theoretical model calculations to provide “synthesized” thermospheric neutral wind fields for solar maximum, December solstice conditions for both quiet (Kp ≤ 3) and active (3 + ≤ Kp ≤ 6) levels of geomagnetic activity. Specifically, high‐latitude DE 2 wind data obtained from multiorbit averages were merged with modeled winds from the National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere general circulation model (NCAR‐TIGCM), using an objective analysis scheme based on the vector spherical harmonic (VSH) spectral technique of Killeen et al. (1987). A “kinematic analysis” was then performed to decompose the merged semiempirical wind fields into their respective divergent (irrotational) and nondivergent (rotational) components at high latitudes and to calculate the corresponding potential and stream functions. This decomposition provides quantitative insight into the sources of momentum that contribute to each of the wind components. The principal conclusions of the study are (1) the nondivergent component of the high‐latitude thermospheric neutral wind is representative of the convection‐driven component of the neutral wind at F region altitudes. This component of the neutral wind is primarily driven by the ion‐drag and Coriolis forces. (2) The nondivergent wind component makes up a large percentage of the total wind field for both quiet and active geomagnetic conditions. In fact, the nondivergent wind component has stronger sunward directed winds in the dawn and dusk sectors than observed in the total wind field but weaker antisunward directed winds over the polar cap. Vortex formation of the nondivergent wind component is consistently more dominant in the dusk sector than in the dawn sector. (3) The irrotational component of the high‐latitude thermospheric neutral wind is representative of the solar‐driven component of the neutral wind at F region altitudes, directed primarily along the 14 ‐ 2 MLT plane. This component of the neutral wind is mostly driven by the pressure‐gradient‐, Coriolis‐, and Pedersen‐drag (nonconvective‐ion drag) forces. (4) The irrotational component complements the nondivergent antisunward flow over the polar cap but inhibits the nondivergent sunward flow in the dawn and dusk sectors. The total wind field is therefore observed to have a strong antisunward surge over the polar cap with modified sunward flow in the flanks of the polar cap. The development of sunward winds in the dawn sector of the total wind field is capricious, owing to the near vector cancellation of the irrotational and nondivergent wind components in that sector. (5) The above results are also observed in the kinematic analysis of the NCAR‐TIGCM simulations. Poor formation of dawn sector sunward winds is observed in both simulations and indicates other nongeomagnetic forces are responsible, given knowledge of the input ion convection pattern having equal potential between convection cells. (6) Overall, the NCAR‐TIGCM simulations were in reasonable agreement with the DE 2 observations for the nondivergent wind component but typically underestimated the structure in the irrotational wind component. This analysis indicates the possible underestimate of matched power input to the convection patterns chosen to represent the two geomagnetic activity conditions in the NCAR‐TIGCM simulations, thus causing the underestimate of the irrotational wind component.