Characterization of Thermospheric Neutral Winds Using Empirical Modeling and Observational Validation
An understanding of the coupling between the neutral winds of the thermosphere and the ionosphere is critical to understanding ionospheric dynamics. Measurements of upper atmospheric neutral air motion are required for any in-depth study of ionosphere-thermosphere coupling. The neutral wind is a maj...
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| Format: | Dissertation |
| Language: | English |
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ProQuest Dissertations & Theses
01-01-2017
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| Online Access: | Get full text |
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| Summary: | An understanding of the coupling between the neutral winds of the thermosphere and the ionosphere is critical to understanding ionospheric dynamics. Measurements of upper atmospheric neutral air motion are required for any in-depth study of ionosphere-thermosphere coupling. The neutral wind is a major contributor to space weather through its effect on many of the observable quantities and physical processes of the ionosphere, including the density profiles of the ionospheric F region and the generation and maintenance of electric fields. The behavior of neutral winds is one of the most important and poorly known factors affecting the day-to-day variations in ionospheric electron and ion densities because it controls the whole electron density profile by altering the rate at which the ions diffuse along magnetic field lines. In fact, truly quantitative modeling of F region densities is not possible without an accurate specification of neutral winds. Measured neutral winds are important for validating winds produced from general circulation models. In the mid-latitude regions, thermosphere neutral winds are the primary driver of the altitude of the peak ionosphere (hmF 2) due to the forcing of ionospheric plasma up and down Earth’s magnetic field lines. In this thesis, we (1) optimized and validated a technique for deriving magnetic meridional thermospheric neutral winds using hmF2 observations from bottomside sounders. Compared derived neutral winds from ionosonde data were compared with wind observations from Fabry-Pérot interferometers (FPI) in order to adjust the model parameters to regularly generate physically-realistic winds. This technique was then used to (2) investigate the rate and causes of the sudden descent (midnight collapse) of the ionosphere hmF 2 at Townsville, Australia over a ten-year period. The study included a harmonic analysis of the tidal components of the meridional neutral winds in order to study long-term trends and how the individual wind tides may contribute to the collapse of hmF2. A global database of ionosonde observations spanning three decades was then used to (3) develop a new model of the horizontal equivalent neutral winds in the mid-latitude regions. The result of task (1) was a new, consistent method for regularly generating realistic meridional neutral winds from hmF2 observations in the mid-latitude regions. The modeled winds compared well with FPI wind observations and performed better than another thermospheric neutral wind model. The application of this method in task (2) determined that the diurnal (24-hour), semidiurnal (12-hour), and terdiurnal (8-hour) tidal components of the meridional neutral wind all play a significant role in the regular midnight collapse of the ionosphere at Townsville, but the effect of the quatradiurnal (6-hour) wind component was minimal. A spectral analysis of the tidal wind components over the full decade revealed that the relative strength of wind tides varies widely with solar flux and that the terdiurnal wind component becomes dominant during solar maximum. Task (3) resulted in the generation of a new empirical wind model that generates neutral winds as a function of the year, day of year, solar local time, solar flux, and geographic latitude and longitude. The new empirical winds compared well with FPI wind observations over short time periods and performed well (statistically) with the observed winds over four seasonal 30-day periods spanning an entire year. The new model also matched wind observations better than the current most widely-used thermospheric wind model, and it accounts for changes in the solar flux, which the other model does not yet do. |
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| ISBN: | 9780355411713 0355411717 |