An Empirical Reference Model of the Quiet-Time Thermospheric Neutral Wind

Abstract:

The Horizontal Wind Model (HWM) provides a framework for summarizing five decades of satellite, rocket, and ground-based observations. The development of a statistical consensus of these data provided the means to accurately specify the climatological spatiotemporal variability of upper atmospheric winds as function of day-of-year, time-of-day, latitude, longitude, and altitude. HWM includes representations of annual and semi-annual variations of the general circulation patterns and migrating tides via Fourier modulated vector spherical harmonics, B-splines, and 17,4000 optimally estimated coefficients. The approach works well for the thermosphere which is dominated by forced periodic oscillations driven by in-situ solar heating under the influence of the earths rotation, tilt, and orbit around the sun. Where average climatological specification are adequate for basic research and engineering applications HWM provides a readily accessible observationally based alternative to the need to compute them from first principles. Results from the newly updated version of the model (HWM14) are presented. In this update new satellite- and ground-based observations fill data gaps in the prior observational database of the 250 km altitude region. Additional changes where made to the model formulation to rectify problems near the poles. The model parameter estimation procedure was also improved. In the new approach all 17,400 unknown model parameters are simultaneously estimated using error-weighted sequential optimal estimation theory. This mitigates difficulties from local-time aliasing of synoptically sampled single satellite datasets; however the utilization of multiple satellite- and ground-based datasets is essential. Construction of the reference model also provides the opportunity to inter-compare the new measurements. Results are presented highlighting important aspects of; 1) the thermosphere zonal mean general circulation, 2) equatorial local time variations, 3) the geomagnetic controlled high latitude wind patterns, and 4) day-to-day variability. Lastly examples of where existing data coverage hampers current observational specifications, and thus validation of the theoretical understanding, are presented.

This work sponsored by the Office of Naval Research.