Showing papers by "Arthur D. Richmond published in 2003"

Long-lasting disturbances in the equatorial ionospheric electric field simulated with a coupled magnetosphere-ionosphere-thermosphere model

Abstract: [1] The Magnetosphere-Thermosphere-Ionosphere-Electrodynamics General Circulation model of Peymirat et al. [1998] is used to investigate ionospheric-wind-dynamo influences on low-latitude ionospheric electric fields during and after a magnetic storm. Simulations are performed with time-varying polar cap electric potentials and an expanding and contracting polar cap boundary. Three influences on equatorial electric fields can be of comparable importance: (1) global winds driven by solar heating; (2) direct penetration of polar cap electric fields to the equator that are partially shielded by the effects of Region-2 field-aligned currents; and (3) disturbance winds driven by high-latitude heating and ion-drag acceleration. The first two influences tend to have similar magnetic local time (MLT) variations in a steady state, while the disturbance-wind influence tends to have the opposite MLT variations. The nighttime disturbance winds at upper midlatitudes that affect the global ionospheric wind dynamo are predominantly westward after the simulated magnetic storm. The nighttime winds drive an equatorward dynamo current that tends to charge the low-latitude ionosphere positively around midnight, which can lead to reductions or reversals of the normal equatorial night-side east-west electric fields. The simulations partly support the theories of the so-called “disturbance dynamo” [Blanc and Richmond, 1980] and “fossil wind” [Spiro et al., 1988], both of which predict long-lasting disturbances in the equatorial eastward electric field associated with magnetic storms. However, the simulations do not support the element of fossil wind theory that links the disturbance-wind influence on equatorial electric fields to polar cap contraction following the storm. The simulations show a stronger wind-produced enhancement of steady state shielding than predicted by the model of Forbes and Harel [1989], due to the fact that the disturbance winds extend well equatorward of the Region-2 currents.

Winds in the high‐latitude lower thermosphere: Dependence on the interplanetary magnetic field

Abstract: [1] Wind observations in the summertime lower thermosphere at high southern latitudes, measured by the Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite, are statistically analyzed in magnetic coordinates and correlated with the interplanetary magnetic field (IMF) to determine influences of IMF-dependent ionospheric convection on the winds. Effects are clearly detectable down to 105 km altitude. Above 125 km the wind patterns show considerable similarity with ionospheric convection patterns, and the speed of the averaged neutral wind in the polar cap often exceeds 300 m/s. The correlation between the IMF Bz component and the diurnal harmonic of the winds is generally best when the IMF is averaged over the preceding 1–4.5 hours. The magnetic-zonal-mean zonal wind below 120 km correlates best with the IMF By component when the latter is averaged over approximately the preceding 20 hours. The wind has a significantly stronger rotational than divergent component. Around and above 120 km a dusk-side anticyclonic wind vortex is prominent, consistent with earlier findings. Around 140 km and higher the dusk-side vortex intensifies for negative Bz, but around 120 km it is the dawn-side cyclonic vortex that responds more strongly to Bz variations. The dependence of the wind on the IMF is nonlinear, especially with respect to IMF Bz. For positive Bz the difference winds are largely confined to the polar cap, while for negative Bz the difference winds extend to subauroral latitudes. A significant correlation between the diurnal Bz-dependent neutral and convection velocity components exists above 108 km, when the convection velocity is suitably rotated in magnetic local time (MLT) with respect to the wind. The rotation that maximizes the correlation ranges from −1.5 hours at 130 km (wind preceding convection) to nearly +6 hours at 108 km (wind lagging convection). The rotated diurnal Bz-dependent wind pattern projects onto the diurnal Bz-dependent ionospheric convection pattern with about 60% the amplitude of the latter above 125 km, decreasing to about 17% at 108 km. On timescales of ∼20 hours, a By-dependent magnetic-zonal-mean zonal wind generally exists, with maximum wind speeds at 80° magnetic latitude, typically 10 m/s at 105 km, increasing to about 60 m/s at 123 km and 80 m/s at 200 km. In the southern hemisphere the wind is cyclonic when the time-averaged By is positive and anticyclonic when By is negative; the wind direction is expected to be opposite in the northern hemisphere.

High-latitude ionospheric electric field variability and electric potential derived from DE-2 plasma drift measurements: Dependence on IMF and dipole tilt

Abstract: [1] In this study the characteristics of electric field variability are investigated by using the sample standard deviations estimated from plasma drift measurements obtained during the Dynamics Explorer 2 (DE-2) mission. The spatial distribution of the standard deviation over the area poleward of 45° magnetic latitude and its climatological behavior with respect to the magnitude and orientation of the interplanetary magnetic field (IMF) and the dipole tilt angle (season) are examined. In comparison with past studies based on ground-based measurements and with results from a data assimilation model, this study quantifies the electric field variability with more complete spatial coverage and with more extensive climatological information and therefore is of importance to the problem of the global Joule heating estimation in thermospheric general circulation modeling. In general, the magnitude of the standard deviation exceeds the strength of the mean electric field in most of the polar area, especially under northward IMF conditions. In contrast to the climatological electric field, whose magnitude tends to be most intense in the polar cap, the standard deviation generally intensifies in the vicinity of the convection reversal and the cusp. Under most IMF clock angles the area of the largest electric field variability lies near the cusp; under the southward BZ condition the area extends toward the potential maximum on the dawn side and toward the region of strong sunward convection in the afternoon, while under the positive BY condition the area extends poleward of the potential maximum on the dawn side. The analysis reveals that electric field variability varies with magnetic latitude, magnetic local time, IMF, and season in a manner distinct from that of the climatological electric field. This indicates that empirical models and data assimilation models designed to reproduce the average electric potential or the average electric fields correctly are not necessarily well-suited to represent the squared electric fields or the electric field variability correctly.

Ionospheric control of the magnetospheric configuration: Thermospheric neutral winds

Abstract: [1] In this study we present the first results from the University of Michigan's coupled magnetosphere-ionosphere-thermosphere general circulation model. This code is a combination of the Michigan MHD model with the NCAR thermosphere-ionosphere-electrodynamics general circulation model (TIEGCM). The MHD code provides specification of the high-latitude ionospheric electric potential and the particle precipitation pattern, while the TIEGCM provides the divergence of the height-integrated neutral wind multiplied by the conductance. This can be easily incorporated into the electric potential solver in the MHD code. We show in this study that the neutral winds cause an approximately 6% increase in the cross polar cap potential when the IMF is strongly southward. This causes the magnetospheric field aligned currents to decrease by a small amount. In the magnetosphere, the flow speeds are increased by only a small amount while the IMF is strongly southward, but when it turns northward the differences become 10–20%. When the IMF is northward, the pressure on the dayside magnetosphere is reduced while the pressure on the nightside is increased by ∼10% of the total pressure.