Browsing by Author "Roble, R. G."
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Item Restricted Modeling the low-latitude thermosphere and ionosphere(Elsevier, 2002-08) Fesen, C. G.; Hysell, D. L.; Meriwether, J. M.; Mendillo, M.; Fejer, B. G.; Roble, R. G.; Reinisch, B. W.; Biondi, M. A.The National Center for Atmospheric Research thermosphere/ionosphere/electrodynamic general circulation model (TIEGCM) is one of the few models that self-consistently solves the coupled equations for the neutral atmosphere and ionosphere. Timely questions are how well the TIEGCM currently simulates the low-latitude ionosphere and what modifications might bring about better predictions. Comparisons between data obtained in and around Jicamarca, Peru, near the magnetic equator, and simulations with the TIEGCM indicate good progress has been made but reveal some serious discrepancies. Good-to-excellent agreement is obtained for electron densities, electron and ion temperatures, and nmax. The agreement is fair to poor for hmax, zonal drifts, the oxygen nightglow, and the horizontal neutral winds. The most important discrepancy is in the simulated neutral temperature, which is at least too cold relative to Fabry–Perot interferometer observations. Increasing the EUV fluxes in the model to improve prediction of the model temperature also improves representation of airglow observations and of the ionosphere, for which the model typically underrepresents the electron densities. The disparity in neutral temperature is also present in comparisons with the empirical model MSIS which represents the largest database of thermospheric temperature measurements. Since the neutral and ionized atmospheres are tightly coupled at low latitudes, simultaneous measurements of neutral and ion parameters, preferably over an extended time period, would be invaluable to further the understanding of the region. Better knowledge of the EUV fluxes and the high altitude O+ fluxes may also help resolve some of the model/data discrepancies.Item Restricted Optical interferometric measurements of nighttime equatorial thermospheric winds at Arequipa, Peru(American Geophysical Union, 1986-05-01) Meriwether Jr., J. W.; Moody, J. W.; Biondi, M. A.; Roble, R. G.We obtained nighttime measurements of equatorial thermospheric wind dynamics at Arequipa, Peru, with an automated field-widened Fabry-Perot interferometer between April 1983 and August 1983 and reduced data from 62 nights. Significant seasonal variations in both zonal and meridional components of the thermospheric neutral wind vector were observed. Near the equinox, between 2000 and 2300 LT, the zonal wind component is eastward with an amplitude between 100 and 150 m/s that gradually ebbs to zero by dawn. The meridional component is generally small throughout the night. In the winter months (May–August) and at the winter solstice the zonal wind persists eastward throughout the night with speeds between 50 and 150 m/s. The meridional component is directed poleward (southward) toward the winter hemisphere with a speed of 50–75 m/s that decays to zero by midnight. Comparison with the predictions of the National Center for Atmospheric Research thermospheric general circulation model for equinoctial and solstice conditions shows good agreement. We conclude that the observed seasonal changes are caused by the changing nature of the solar forcing function. The Arequipa results found the day-to-day variability in the winter thermospheric winds to be less than that found for the summer equatorial observations obtained at Kwajalein. Interferometric measurements of the 630.0-nm intensity at equinox showed a major reduction of the emission lasting 1 or 2 hours in all directions but south shortly after evening twilight; this decrease was not observed during Winter.Item Restricted Simulation of the pre‐reversal enhancement in the low latitude vertical ion drifts(American Geophysical Union, 2000-07-01) Fesen, C. G.; Crowley, G.; Roble, R. G.; Richmond, A. D.; Fejer, B. G.Low latitude F region ion motions exhibit strong seasonal and solar cycle dependences. The pre‐reversal enhancement (PRE) in the vertical ion drifts is a particularly well‐known low latitude electrodynamic feature, exhibited as a sharp upward spike in the velocity shortly after local sunset, which remains poorly understood theoretically. The PRE has been successfully simulated for the first time by a general circulation model, the National Center for Atmospheric Research thermosphere/ionosphere/electrodynamic general circulation model (TIEGCM). The TIEGCM reproduces the zonal and vertical plasma drifts for equinox, June, and December for low, medium, and high solar activity. The crucial parameter in the model to produce the PRE is the nighttime E region electron densities: densities ≥ 104 cm−3 preclude the PRE development by short‐circuiting the F region dynamo. The E region semidiurnal 2,2 tidal wave largely determines the magnitude and phase of the daytime F region drifts.