Browsing by Author "Richmond, A. D."
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Item Restricted Post-Storm Middle and Low-Latitude Ionospheric Electric Fields Effects(Springer, 2016-12-05) Fejer, B. G.; Blanc, M.; Richmond, A. D.The Earth’s upper atmosphere and ionosphere undergoes large and complex perturbations during and after geomagnetic storms. Thermospheric winds driven by enhanced energy and momentum due to geomagnetic activity generate large disturbance electric fields, plasma drifts and currents with a broad range of temporal and spatial scales from high to equatorial latitudes. This disturbance dynamo mechanism plays a fundamental role on the response of the middle and low-latitude ionosphere to geomagnetic activity. In this review, we initially describe the early evidence for the importance of this process and the first simulation study which already was able to explain its main effects on the electrodynamics of the middle and low-latitude ionosphere. We then describe the results of more recent simulations and the extensive experimental work that highlights the importance of this mechanism for ionospheric space weather studies extending to post-storms periods, and present some suggestions for future studies.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.Item Restricted Theoretical effects of geomagnetic activity on low-latitude ionospheric electric fields(American Geophysical Union, 2005-05-27) Huang, C. M.; Richmond, A. D.; Chen, M. Q.The influence of geomagnetic activity on middle‐ and low‐latitude thermospheric winds and ionospheric electric fields is investigated using model results from the National Center for Atmospheric Research Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model. Model runs are made for different levels of geomagnetic activity. Model results show that the equatorward ionospheric currents produced by disturbance winds develop positive charge accumulation at low latitudes that maximizes in the premidnight sector. The local time of maximum electric potential perturbation depends significantly on universal time so that the local time of reversal of the equatorial zonal perturbation electric field varies with longitude by 2 to 3 hours, depending on the intensity of geomagnetic activity. The westward perturbation electric field in the postsunset period indicates that stronger geomagnetic activity will produce a lower driven height of the evening F region. After geomagnetic activity ceases, model results show that the zonal disturbance winds can last for many days in the postrecovery period, while the meridional disturbance winds decay more rapidly. The long‐lasting zonal winds, through the Pedersen currents they drive, help maintain meridional disturbance potential drops that decay much more slowly than the zonal disturbance potential drops after the activity ceases.