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Title: Radar and Airglow Studies of F-Region Composition and Dynamics at Low Latitudes
Authors: Nicolls, Michael
Keywords: Ionosphere
F region
Issue Date: 20-Jul-2006
Abstract: This thesis presents a series of studies investigating the composition and dynamics of the low latitude F-region ionosphere using airglow imagers and the Arecibo and Jicamarca incoherent scatter radars (ISRs), along with supporting instrumentation. Some dynamical aspects of the low latitude ionosphere are investigated. The effects of a large-scale traveling ionospheric disturbance (TID) are studied. The TID characteristics are consistent with an atmospheric disturbance caused by high latitude Lorentz forcing associated with periodic substorms. Plasmaspheric coupling is necessary to explain the observed electron densities and airglow intensities. Evidence is presented for post-midnight uplifts during which the low latitude ionosphere is lifted by tens of kilometers in mild events and by over a hundred kilometers in severe events. The uplifts are not caused by a reversal of the zonal electric field, instead occurring as a response to a decreasing westward field in conjunction with sufficient recombination and plasma flux. The midnight pressure bulge may play a role in the dynamics and the disturbance dynamo may enhance the uplifts. The spectral properties of daytime penetration fields are analyzed using ground-based magnetometer data. The spectrum deviates from a power law under disturbed conditions, and the integrated power is a strong function of geomagnetic activity. The transfer function of the system is estimated using simultaneous interplanetary electric field data. Case studies support the notion of long-duration responses and of a weakly resonant system. Models of the airglow emission rates are used to investigate the nighttime F-region ion composition. The intensities calculated including the molecular ions given by the International Reference Ionosphere (IRI) model are much higher than measurements indicate, implying that IRI overestimates the molecular ion fraction. A revision of the IRI ion composition is needed, which could be constrained with airglow measurements. The neutral thermosphere over Arecibo is investigated using ion energy balance to estimate the neutral density and temperature. The so-called Burnside factor, associated with errors in the O+/O collision cross section, is estimated. In contrast to ion momentum studies, which typically lead to a Burnside factor greater than one, ion energy balance studies tend to result in low values for the Burnside factor. However, the ratio derived here is close to theoretical simulations of the collision cross section and may be a first step in reconciling the discrepancy between momentum and energy balance results.
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