Pedersen Currents Research Articles

Space Technology 5 observations of the imbalance of regions 1 and 2 field‐aligned currents and its implication to the cross‐polar cap Pedersen currents

In this study, we use the in situ magnetic field observations from Space Technology 5 mission to quantify the imbalance of region 1 (R1) and region 2 (R2) currents. During the 3 month duration of the ST5 mission, geomagnetic conditions range from quiet to moderately active. We find that the R1 current intensity is consistently stronger than the R2 current intensity, both for the dawnside and the duskside large‐scale field‐aligned current system. The net currents flowing into (out of) the ionosphere in the dawnside (duskside) are in the order of 5% of the total R1 currents. We also find that the net currents flowing into or out of the ionosphere are controlled by the solar wind‐magnetosphere interaction in the same way as the field‐aligned currents themselves are. Since the net currents due to the imbalance of the R1 and R2 currents require that their closure currents flow across the polar cap from dawn to dusk as Pedersen currents, our results indicate that the total amount of the cross‐polar cap Pedersen currents is in the order of ∼0.1 MA. This study, although with a very limited data set, is one of a few attempts to quantify the cross‐polar cap Pedersen currents. Given the importance of the Joule heating due to Pedersen currents to the high‐latitude ionospheric electrodynamics, quantifying the cross‐polar cap Pedersen currents and associated Joule heating is needed for developing models of the magnetosphere‐ionosphere coupling.

Ionospheric variability due to planetary waves and tides for solar minimum conditions

Large ionospheric variability is found at low to middle latitudes when a quasi‐stationary planetary wave is specified in the winter stratosphere in the National Center for Atmospheric Research thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model for solar minimum conditions. The variability includes change of electric field/ion drift, F2 peak density and height, and the total electron content. The electric field/ion drift change is the largest near dawn in the numerical experiments. Analysis of model results suggests that, although the quasi‐stationary planetary wave does not propagate deep into the ionosphere or to low latitudes due to the presence of critical layers and strong molecular dissipation, the planetary wave and tidal interaction leads to large changes in tides, which can strongly impact the ionosphere at low and middle latitudes through the E region wind dynamo. Large zonal gradients of zonal and meridional winds from the tidal components and the zonal gradient of electric conductivities at dawn can produce large convergence/divergence of Hall and Pedersen currents, which in turn produces a polarization electric field. The ionospheric changes are dependent on both the longitude and local time, and are determined by the amplitudes and phases of the superposing wave components. The model results are consistent with observed ionospheric changes at low and middle latitudes during stratospheric sudden warming events, when quasi‐stationary planetary waves become large.

Nightside ionospheric electrodynamics associated with substorms: PFISR and THEMIS ASI observations

Observations from the newly available Poker Flat Incoherent Scatter Radar have been used to study the ionospheric electrodynamics during the substorm expansion phase. Substorm events selected have been divided into three categories based on onset location relative to the radar, and repeatable features have been identified. West of onset, westward flows increased in response to the passage of the westward traveling surge (WTS) poleward of them. The enhanced flows occurred within the Sub‐Auroral Polarization Streams region of downward Region 2 currents, which extend equatorward from near the equatorward boundary of electron precipitation. Strong E region ionization increase associated with the WTS occurred poleward of the enhanced westward flows, suggesting that poleward Pedersen currents fed by the downward Region 2 field‐aligned current (FAC) further equatorward contribute at least partially to closure of the upward FAC of the WTS. An enhanced eastward flow channel developed east of onset. E region ionization decreased poleward of this flow peak, implying formation of downward FAC, and further increased equatorward of the flow peak, suggesting strengthening of preexisting Region 2 upward FAC. These observations indicate that the downward FAC formed at onset at least partially closes via the upward Region 2 currents. The enhanced eastward flows decreased after passage of the auroral bulge, and the reduced E region ionization increased again. When onset occurred within the radar fov, the thin breakup arc of onset occurred at the center of the Harang reversal, while if the optical onset was poleward of the radar fov, the Harang reversal was observed after the onset. A 2‐D picture of the evolution of ionospheric electrodynamics was generated by synthesizing observations from the three categories, which provides a schematic description of the relationship between Region 2 and substorm expansion electrodynamics.

Magnetosphere‐ionosphere/thermosphere coupling: Self‐consistent solutions for a one‐dimensional stratified ionosphere in three‐fluid theory

We study the local response of a model ionosphere to a change in the magnetospheric convection, on the basis of a three‐fluid (electrons, ions, and neutrals) approach to describing the dynamic processes of solar wind–magnetosphere–ionosphere/thermosphere coupling. The physical description, including the three‐fluid generalized Ohm's law, the plasma momentum equation, and the neutral momentum equation, as well as Maxwell's equations, takes into account electromagnetic coupling among the charged species and collisions among the three species; the geometrical configuration in this initial study, however, is highly simplified and approximates a localized region within the polar cap. We model the driver of the convection by a changing tangential flow of plasma, imposed at the top boundary of the ionosphere, and follow numerically the self‐consistent evolution of the entire system, which is assumed to be incompressible. A magnetic field distortion, corresponding to a horizontal (not field‐aligned) current, propagates from the magnetosphere to the lower ionosphere, producing at first a strong transient Pedersen current which then decreases to a steady state value. The transient time for the system to settle downscales as the Alfvén‐wave travel time between the E layer and the top boundary (verified by redoing the calculations with different heights of the upper boundary). Large perturbations occur during the first 10 Alfvén travel times, and it takes about 20 Alfvén travel times for the system to reach a quasi steady state. After the quasi steady state has been reached, the neutral wind continues to vary slowly (forces due to neutral pressure and effective viscosity have been neglected). When magnetospheric convection is reversed after 1 h, an overshoot of the Pedersen current occurs before the system settles into a new quasi steady state. The electrostatic approximation commonly used in the magnetosphere‐ionosphere coupling models remains poor for up to 10 Alfvén travel times (which could translate to more than 15 min in a more realistic geometry); the assumption that the neutrals remain at rest relative to the Earth is poor within the F layer.

Neutral wind control of the Jovian magnetosphere‐ionosphere current system

In order to clarify the role of neutral dynamics in the Jovian magnetosphere‐ionosphere‐thermosphere coupling system, we have developed a new numerical model that includes the effect of neutral dynamics on the coupling current. The model calculates axisymmetric thermospheric dynamics and ion composition by considering fundamental physical and chemical processes. The ionospheric Pedersen current is obtained from the thermospheric and ionospheric parameters. The model simultaneously solves the torque equations of the magnetospheric plasma due to radial currents flowing at the magnetospheric equator, which enables us to update the electric field projected onto the ionosphere and the field‐aligned currents (FACs) depending upon the thermospheric dynamics. The self‐consistently calculated temperature and ion velocity are consistent with observations. The estimated neutral wind field captures the zonally averaged characteristics in previous three‐dimensional models. The energy extracted from the planetary rotation is mainly used for magnetospheric plasma acceleration below 73.5° latitude while consumed in the upper atmosphere, mainly by Joule heating at above 73.5° latitude. The neutral wind dynamics contributes to a reduction in the electric field of 22% compared with the case of neutral rigid corotation. About 90% of this reduction is attributable to neutral winds below the 550‐km altitude in the auroral region. The calculated radial current in the equatorial magnetosphere is smaller than observations. This indicates that the enhancement of the background conductance and/or the additional radial current at the outer boundary would be expected to reproduce the observed current.

The Pedersen current carried by electrons: a non-linear response of the ionosphere to magnetospheric forcing

Abstract. Observations by the EISCAT Svalbard radar show that electron temperatures Te in the cusp electrojet reach up to about 4000 K. The heat is tapped and converted from plasma convection in the near Earth space by a Pedersen current that is carried by electrons due to the presence of irregularities and their demagnetising effect. The heat is transfered to the neutral gas by collisions. In order to enhance Te to such high temperatures the maximally possible dissipation at 50% demagnetisation must nearly be reached. The effective Pedersen conductances are found to be enhanced by up to 60% compared to classical values. Conductivities and conductances respond significantly to variations of the electric field strength E, and "Ohm's law" for the ionosphere becomes non-linear for large E.

A linear analysis of the hybrid Kelvin‐Helmholtz/Rayleigh‐Taylor instability in an electrostatic magnetosphere‐ionosphere coupling system

A linear analysis of the hybrid Kelvin‐Helmholtz/Rayleigh‐Taylor (KH/RT) instability in the magnetosphere‐ionosphere coupling system is presented. In the magnetosphere plasma particles can move by the electric (E × B) drift, magnetic drift (gradient B drift plus curvature drift), and inertia drift. Field‐aligned currents are generated primarily from divergence of the magnetic drift flux. They are closed via Pedersen currents in the ionosphere. As a simple model, there is a surface of discontinuity both in the azimuthal flow and the particle energy density, which extends longitudinally as projected to the ionospheric plane. The gradient of the energy density is directed inward or outward so that the plasma is RT unstable or stable, respectively. The present analysis shows that only a velocity shear cannot drive the system KH unstable when the growth rate, γKH, of the KH instability without Pedersen coupling is less than the inertial relaxation rate, ν, and that the presence of an energy density gradient allows a hybrid wave to grow even when γKH < ν. The wave growth is due to the active charge separation driving originally the RT instability. The picture of auroral deformations now changes drastically. The traditional picture is such that if the Pedersen coupling is properly taken into account, KH waves of long wavelengths (≳100 km at the ionospheric height) are found evanescent in the main body of the auroral oval. A new one is such that wavy structures therein can develop, only in the KH/RT hybrid mode, at the places where the particle energy density has an inward gradient. Similarly, the stability of the inner boundary of the low‐latitude boundary layer is controlled by the RT stability conditions more influentially than the KH conditions.

Separation of Cowling channel and local closure currents in the vicinity of a substorm breakup spiral

The substorm breakup spiral demarcates the westernmost terminator of the substorm current wedge. A long‐standing debate has been devoted to the question of to what extent the strong upward field‐aligned currents (FAC) in that spiral are either closed by downward FAC through Pedersen currents flowing radially to the center of the spiral (“local closure”) or by currents that flow westward though a Cowling channel which extends in the region eastward of the spiral (“Cowling closure”). In particular, studies using ground‐based and space‐based instrumentation came to systematically different results. In order to address this question, we reanalyze a pseudobreakup spiral event on 3 February 1999, ∼2204 UT, that has been observed with the MIRACLE network of ground‐based instruments in Fennoscandia and for which spatial results of electric fields, conductances, and currents have been calculated by Amm et al. (2001). We apply a new technique that allows, under certain geometrical assumptions, the current systems to separate in the vicinity of the breakup spiral into their local closure and Cowling closure parts. The result shows that for the event analyzed, ∼68% of the upward FAC in the spiral is closed via the local closure current system and the remaining ∼32% via the Cowling closure current system. Therefore, the local closure mechanism is the dominating provider of the upward FAC in the breakup spiral overall. However, a localized area at the western flank of the spiral, where conductance gradients are largest, exists in which the Cowling closure dominates and feeds more than 80% of the upward FAC in that area. Further studies are required in order to examine the generality of the results obtained for our specific event.

Ionospheric feedback effects on the quasi-stationary coupling between LLBL and postnoon/evening discrete auroral arcs

Abstract. We discuss a model for the quasi-stationary coupling between magnetospheric sheared flows in the dusk sector and discrete auroral arcs, previously analyzed for the case of a uniform height-integrated Pedersen conductivity (ΣP). Here we introduce an ionospheric feedback as the variation of ΣP with the energy flux of precipitating magnetospheric electrons (εem). One key-component of the model is the kinetic description of the interface between the duskward LLBL and the plasma sheet that gives the profile of Φm, the magnetospheric electrostatic potential. The velocity shear in the dusk LLBL plays the role of a generator for the auroral circuit closing through Pedersen currents in the auroral ionosphere. The field-aligned current density, j||, and the energy flux of precipitating electrons are given by analytic functions of the field-aligned potential drop, ΔΦ, derived from standard kinetic models of the adiabatic motion of particles. The ionospheric electrostatic potential, Φi (and implicitely ΔΦ) is determined from the current continuity equation in the ionosphere. We obtain values of ΔΦ of the order of kilovolt and of j|| of the order of tens of μA/m2 in thin regions of the order of several kilometers at 200 km altitude. The spatial scale is significantly smaller and the peak values of ΔΦ, j|| and εem are higher than in the case of a uniform ΣP. Effects on the postnoon/evening auroral arc electrodynamics due to variations of dusk LLBL and solar wind dynamic and kinetic pressure are discussed. In thin regions (of the order of kilometer) embedding the maximum of ΔΦ we evidence a non-linear regime of the current-voltage relationship. The model predicts also that visible arcs form when the velocity shear in LLBL is above a threshold value depending on the generator and ionospheric plasma properties. Brighter arcs are obtained for increased velocity shear in the LLBL; their spatial scale remains virtually unmodified. The field-aligned potential drop tends to decrease with increasing LLBL density. For higher values of the LLBL electron temperature the model gives negative field-aligned potential drops in regions adjacent to upward field-aligned currents.

Electrodynamics of magnetosphere‐ionosphere coupling and feedback on magnetospheric field line resonances

We present a new dynamic model that describes coupling between standing inertial or ion‐acoustic‐gyroradius‐scale shear Alfvén waves, compressional modes, and auroral density disturbances. The model is applied to the excitation of field line resonances (FLRs) in dipolar and stretched geomagnetic fields in Earth's magnetosphere. Magnetosphere‐ionosphere coupling is included by accounting for the closure of magnetospheric field‐aligned currents (FACs) through Pedersen currents in the ionosphere. A second new aspect is that the height‐integrated Pedersen conductivity is treated as a dynamic parameter by electrodynamically coupling the two‐dimensional finite element wave model “Topo” to the ionospheric ionization model “Global Airglow Model (GLOW).” We demonstrate that field line stretching brings the equatorial plasma β above unity, where the reduced MHD formulism for low‐frequency plasma breaks down. As an application of our model, we study a specific FLR event observed on 31 January 1997, when the NASA FAST satellite was over the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) Gillam station. Using geomagnetic fields computed from the T96 magnetic field model, we show that auroral electron precipitation produces strong Pedersen conductivity enhancements that control the final amplitude and width of the excited FLR, along with the amplitude of associated density fluctuations. The predictions of the model are generally consistent with observations of this event.

Amplification and energy transformation of the magnetized Rossby waves in the ionosphere with an inhomogeneous zonal wind. II. Analysis of numerical solution

The specific features of the amplification and mutual transformation of wave modes in a linear regime have been revealed based on an analysis of the numerical solution to the set of equations describing the interaction of the magnetized Rossby wave and the inertial wave with the spatially inhomogeneous zonal wind (shear flow) in the ionospheric D, E, and F regions. It has been established that the presence of the geomagnetic field and Hall and Pedersen currents in the ionosphere improves the interaction and energy exchange between the waves and shear flow.

Equatorial spread-F initiation: Post-sunset vortex, thermospheric winds, gravity waves

We present experimental evidence and modeling results which indicate that eastward thermospheric wind is the primary controlling factor of equatorial spread-F initiation in the post-sunset ionosphere. Eastward wind-driven Pedersen currents are able to polarize F-region density perturbations with westward tilting wavefronts into rapidly growing modes to trigger the formation of spread-F bubbles. The described process is so rapid that seeding requirements of spread-F initiation by external factors such as gravity waves are effectively eliminated.

Electrodynamics of a split-transpolar aurora

[1] We report new results on the relation between drift velocity shear, average inverted V energy, and the separation of bifurcated high-latitude Sun-aligned polar arcs within a global transpolar auroral arc system. The two parallel arcs appeared at the location of converging −v × B electric field of two sunward flow channels that were separated by an intermediate region of much weaker antisunward flow. The flow shear consisted of differences mainly in the magnitude of the sunward drift velocity. The cross-track disturbance magnetic field was in phase with the drift velocity fluctuations in such a way that two upward field-aligned currents coincided with the peak average energy of the electron precipitation that generated the two polar arcs while a downward field-aligned current was observed in the intermediate region. The observed transverse arc separation is consistent with a reported linear dependence between the average energy of the precipitating electrons of the two arcs and the arc separation. We propose that this relation mainly follows from a local Pedersen current closure requirement. The greater the upward field-aligned current density of the two arcs, the wider the transverse region of the intermediate downward field-aligned current needs to be in order to supply a sufficient amount of current carriers to the bifurcated arc current circuit and thus to balance the Pedersen current.

Mechanism of the planetary Rossby wave energy amplification and transformation in the ionosphere with an inhomogeneous zonal smooth shear wind

The generation and further dynamics of planetary magnetized Rossby waves and inertia waves are investigated in the rotating dissipative ionosphere in the presence of a smooth inhomogeneous zonal wind (shear flow). Magnetized Rossby waves appear as a result of the interaction of the medium with the spatially inhomogeneous geomagnetic field and are an ionospheric manifestation of usual tropospheric Rossby waves. An effective linear mechanism responsible for the intensification and mutual transformation of Rossby and inertia waves is found. For shear flows, the operators of the linear problem are not self‐adjoint, and therefore the eigenfunctions of the problem maybe nonorthogonal and can hardly be studied by the canonical modal approach. Hence it becomes necessary to use the so‐called nonmodal mathematical analysis which has been actively developed in recent years. The nonmodal approach shows that the transformation of wave disturbances in shear flows is due to the nonorthogonality of eigenfunctions of the problem in the conditions of linear dynamics. Thus there arises a new degree of freedom and a new way for the evolution of disturbances in the medium. Using numerical modeling, we illustrate the peculiar features of the interaction of waves with the background flow as well as the mutual transformation of wave disturbances in the D, E, and F regions of the ionosphere. It is established that the presence of a geomagnetic field, Hall and Pedersen currents in the ionospheric medium, improves the interaction and mutual energy exchange between waves and a shear flow.

On the profile of intense high-altitude auroral electric fields at magnetospheric boundaries

Abstract. The profile of intense high-altitude electric fields on auroral field lines has been studied using Cluster data. A total of 41 events with mapped electric field magnitudes in the range between 0.5–1 V/m were examined, 27 of which were co-located with a plasma boundary, defined by gradients in particle flux, plasma density and plasma temperature. Monopolar electric field profiles were observed in 11 and bipolar electric field profiles in 16 of these boundary-associated electric field events. Of the monopolar fields, all but one occurred at the polar cap boundary in the late evening and midnight sectors, and the electric fields were typically directed equatorward, whereas the bipolar fields all occurred at plasma boundaries clearly within the plasma sheet. These results support the prediction by Marklund et al. (2004), that the electric field profile depends on whether plasma populations, able to support intense field-aligned currents and closure by Pedersen currents, exist on both sides, or one side only, of the boundary.

Modeling and analysis of solar wind generated contributions to the near-Earth magnetic field

Abstract Solar wind generated magnetic disturbances are currently one of the major obstacles for improving the accuracy in the determination of the magnetic field due to sources internal to the Earth. In the present study a global MHD model of solar wind magnetosphere interaction is used to obtain a physically consistent, divergence-free model of ionospheric, field-aligned and magnetospheric currents in a realistic magnetospheric geometry. The magnetic field near the Earth due to these currents is analyzed by estimating and comparing the contributions from the various parts of the system, with the aim of identifying the most important aspects of the solar wind disturbances in an internal field modeling context. The contribution from the distant magnetospheric currents is found to consist of two, mainly opposing, contributions from respectively the dayside magnetopause currents and the cross-tail current. At high latitudes the field-aligned component is of partidular interest in connection with internal field-modelling. In the altitude regime of 400–800 km (typical for low Earth orbit satellites) the ionospheric currents are found to contribute significantly to the disturbancance, and account for more than 90% of the field-aligned disturbance. The magnetic disturbance field from field-aligned currents (FACs) is basically transverse to the main field, and they therefore contribute with less than 2% to the disturbance in total field intensity. Inhomogeneity in ionospheric conductance is identified as the main cause of main-field disturbance in the field-aligned direction. These disturbances are associated with the ionospheric Pedersen currents, and may introduce systematic errors in internal field models.

Effect of the acceleration current on the centrifugal interchange instability

The centrifugal interchange instability is the primary driver of radial plasma transport in the magnetospheres of Jupiter and Saturn. Most previous theoretical treatments of this instability have ignored the role of the acceleration current and have assumed that the divergence of the centrifugal drift current in the magnetosphere is closed by Pedersen currents in the underlying ionosphere, through the connection provided by Birkeland (magnetic‐field‐aligned) currents. This is a reasonable approximation when the radial transport speed is small compared with the rotation speed. However, the exponential growth of the instability inevitably leads to the eventual violation of this condition. I analyze a simplified model of the Io plasma torus to show that when the radial transport speed becomes comparable to the rotation speed, the acceleration current becomes the primary mechanism for closure (actually local cancellation) of the centrifugal drift current, and the connection to the planetary ionosphere therefore becomes irrelevant. An immediate consequence is that the growth rate of the instability does not exceed the planetary rotation rate.

Signature of Saturn's auroral cusp: Simultaneous Hubble Space Telescope FUV observations and upstream solar wind monitoring

Model simulations by Bunce et al. (2005a) have shown that direct precipitation of electrons in Saturn's dayside cusp regions is not capable of producing significant FUV aurora. Instead, they suggested the possibility that the FUV bright emissions sometimes observed near noon are associated with reconnection occurring at the dayside magnetopause, possibly pulsed, analogous to flux transfer events seen at the Earth. Pulsed reconnection at the low‐latitude dayside magnetopause when the IMF is directed northward (antiparallel to Saturn's magnetic field lines) is expected to give rise to pulsed twin‐vortical flows in the magnetosphere and hence to bipolar field‐aligned currents centered in the vortical flows closing in ionospheric Pedersen current. In the case of southward IMF and high‐latitude lobe reconnection the model predicts that the vortical flows are displaced poleward of the open‐closed field line boundary with reversed field‐aligned currents compared with the former case. During January 2004, a unique campaign took place during which magnetic field and plasma instruments on board the Cassini‐Huygens spacecraft measured the in situ solar wind and embedded interplanetary magnetic field while the Hubble Space Telescope simultaneously observed the far ultraviolet aurora in Saturn's southern hemisphere. The IMF was highly structured during this interval. The electric potential at Cassini is estimated from solar wind magnetic field and velocity measurements for the case of low‐latitude or lobe reconnection. We show that a dayside FUV signature of intense electron precipitation is found poleward of or along the main oval during a period of minor compression period when the dayside reconnection voltage is estimated to be ∼30–100 kV. Overall, we find that the conceptual model of Bunce et al. (2005a) provides a good estimate of the UV brightness and power for the case of northward IMF but somewhat underestimates the power for the southward IMF case, except if the speed of the vortical flow is larger than its value in the nominal model.

Comment on “Nonlinear electron heating by resonant shear Alfvén waves in the ionosphere” by J. Y. Lu et al.

[1] Lu et al. [2005] want to limit the amplitude of parallel currents in standing Alfven waves by having a new ionization term triggered by strong electric fields. The underlying logic seems to be that if the parallel currents are unable to close because the Pedersen conductivities are small, a perpendicular electric field, E?, will build up. The strong E? will pump up the electron temperatures, Te, through frictional heating from collisions with the neutrals. The hot electrons will then introduce appreciable ionization by overcoming the ionization potential of the ambient atoms and molecules. For large enough E? the ionization would become so effective that the conductivity would increase at the same pace as E?, causing the Pedersen currents to increase rapidly, thereby strongly slowing down any further increase in the E?. [2] I see many difficulties with this thesis. For one thing, the authors argue that 40 to 50 mV/m perpendicular fields heat the electrons to 10000 K (1eV). This result flies in the face of ionospheric observations and is obtained because Te is not properly calculated. Secondly, as far as I am concerned, the ionization mechanism is overestimated substantially: even if Te was as high as the authors claim, there would still be only an infinitesimal amount of extra ionization created. Thirdly, the nature of the parallel currents is not clear: I would think that with Alfven waves, we deal with soft electrons. However, soft electrons are stopped at 200 km and above. It would therefore seem that current closure would have to take place at 200 km or above. If closure is done lower down, then, how are the currents reaching 120 km? Are parallel ionospheric E fields required? Finally, if parallel fields are not used and closure is to take place in the F region, we have another problem: when E? becomes large, the F region ionization goes down, not up, owing to the fact that ion frictional heating with large E? fields produces hot O ions which rapidly turn into NO and recombine.

Theoretical effects of geomagnetic activity on low‐latitude ionospheric electric fields

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.