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Eccles J.V.,Space Environment Corporation Providence | St Maurice J.P.,University of Saskatchewan | Schunk R.W.,Utah State University
Journal of Geophysical Research A: Space Physics | Year: 2015

The evening prereversal enhancement (PRE) of the vertical plasma drift has important consequences for the Appleton density anomaly and the stability of the nighttime ionosphere. Simplified simulations were used to review the three competing theories of the PRE origin, to explore their relative importance, and to indentify their interdependence. The mechanisms involved in the generation and climatology of the PRE are, first, a curl-free electric field response to rapid changes in the vertical electric field associated with the nighttime F region dynamo; second, a divergence of Hall currents in the E region away from the magnetic equator; and, third, the moderating effect of the large Cowling conductivities in the equatorial E region. The simulations indicate that the equatorial Cowling conductivity creates an important current path that limits the other two mechanisms prior to equatorial sunset and releases them after equatorial sunset. The curl-free mechanism is the dominant mechanism when the terminator and magnetic meridian are aligned in part due to the accelerating F region zonal wind. When the solar terminator is not aligned with the magnetic meridian, there is an interaction involving all three mechanisms contributing to the magnitude and timing of the PRE. Finally, the altitude profile of the PRE decays more quickly with altitude when the curl-free mechanism dominates as compared to when the Hall current mechanism dominates. © 2015 American Geophysical Union.

Rice D.D.,Space Environment Corporation Providence | Sojka J.J.,Space Environment Corporation Providence | Eccles J.V.,Space Environment Corporation Providence | Redmon R.,National Oceanic and Atmospheric Administration | Hunsucker R.D.,National Oceanic and Atmospheric Administration
Radio Science | Year: 2014

The study of long-term changes requires researchers to identify relevant historical measurements; quantifying and quality controlling these measurements becomes central to their utilization by broader communities. This pilot study reports on the evaluation of ionograms and their inversion from the decade prior to the Space Age. The ionograms were recorded on 35mm film reels and archived at the National Geophysical Data Center World Data Center A in Boulder, Colorado. For this study, ionogram film from one of the earliest operational sites in North America, Fort Belvoir (near Washington, DC), is analyzed. This analysis generates two distinct products: a digital ionogram with virtual height and frequency coordinate registration, and scaled values including an electron density profile. Validation of the ionogram registration and its resolution are described. The scaling and inversion processing, including uncertainties for the parameters and quality assessment, are explained. This study demonstrates how the archived film ionograms can have extensive value as historic measurements of the ionosphere once they are digitized and coordinate registered. Furthermore, modern analysis of the ionograms shows that a complete bottomside ionospheric specification may be obtained at better time resolution than is typically available from original scaled data. This research has set the stage for an effective recovery of ionospheric information for almost a full solar cycle prior to International Geophysical Year. In addition, between Fort Belvoir and Wallops Island archives, an East Coast midlatitude ionospheric data set can be generated from the late 1940s through the present. ©2014. American Geophysical Union.

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