Showing papers by "Arthur D. Richmond published in 2000"

Simulation of the pre-reversal enhancement in the low latitude vertical ion drifts

Abstract: 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.

Electrodynamic coupling of high and low latitudes: Simulations of shielding/overshielding effects

Abstract: The penetration of the electric field and associated magnetic perturbations from high latitudes to low latitudes is studied with the Magnetosphere-Thermosphere-lonosphere-Electrodynamics General Circulation Model (MTlEGCM) of Peymirat et al. [1998] in response to variations of the polar cap potential drop. For a sudden decrease of the polar cap potential of ∼ 40 kV, the model reproduces the well-known overshielding phenomenon corresponding to a sudden reversal of the low-latitude electric field. For quasi-periodic oscillations of the polar cap potential drop of ∼ 40 min period, the model predicts that the poleward electric field and the eastward Hall current in the auroral zone lag slightly in phase (< 1 min), while the eastward electric field and current at the magnetic equator are advanced slightly in phase, with respect to the potential-drop oscillations. These phase differences are interpreted as the consequence of the succession of shielding and overshielding episodes induced by the response of the region-2 field-aligned currents to the polar cap variations. The phase differences among the polar cap potential and the auroral and equatorial electric fields and currents increase with the plasma sheet pressure. The amplitude of the associated magnetic perturbations is very dependent on the distribution of the potential along the polar cap boundary. The model predictions are tested against the observations of Kobea et al. [this issue] for two events, one representing simple overshielding and the other associated with polar cap potential oscillations. The model underestimates the decay time of the magnetic perturbations by a factor of 2 during the overshielding event, and the model gives results compatible with the observations during the second event. The disagreements may be due to limitations of the model and uncertainties of the input parameters.

Electrodynamic coupling of high and low latitudes: Observations on May 27, 1993

Abstract: The penetration of disturbance electric fields from the polar region to the magnetic equator on the dayside of the Earth is examined with geomagnetic data on May 27, 1993. First, we examine a dayside equatorial disturbance that followed the rapid recovery of magnetic activity from a storm and that has the characteristics of overshielding caused by persistent region-2 field-aligned currents. It lasted *0 3 hours. Second, we analyze a series of fluctuations with periods of 25-75 min, to determine the variations of amplitude and phase with magnetic latitude and magnetic local time. The fluctuations were highly coherent at all latitudes between the magnetic equator and the auroral zone, but the coherency decreasedin the polar cap. A northward fluctuation at the equator during midday hours accompanied auroral zone fluctuations that were southward before noon, eastward around noon, and northward after noon. The amplitudes decreased away from the auroral zone toward midlatitudes but were amplified under the equatorial electrojet. No detectablep hased ifferencesa re found, indicating that any temporal lags which might be inducedb y persistencein the region-2f ield-alignedc urrentsa re less than i min for fluctuations having periods like those examined here. A synoptic inversion analysis of the high-latitude magnetic data to estimate the time-varying high-latitude electric potential patterns shows that fluctuations of the high-latitude east-west potential gradient tended to be concentrated around midday, where they were in phase with fluctuations in the midday east-west potential gradient at the magnetic equator.