We present the first global empirical model for the quiet time F region equatorial vertical drifts based on combined incoherent scatter radar observations at Jicamarca and Ion Drift Meter observations on board the Atmospheric Explorer E satellite. This analytical model, based on products of cubic-B splines and with nearly conservative electric fields, describes the diurnal and seasonal variations of the equatorial vertical drifts for a continuous range of all longitudes and solar flux values. Our results indicate that during solar minimum, the evening prereversal velocity enhancement exhibits only small longitudinal variations during equinox with amplitudes of about 15–20 m/s, is observed only in the American sector during December solstice with amplitudes of about 5–10 m/s, and is absent at all longitudes during June solstice. The solar minimum evening reversal times are fairly independent of longitude except during December solstice. During solar maximum, the evening upward vertical drifts and reversal times exhibit large longitudinal variations, particularly during the solstices. In this case, for a solar flux index of 180, the June solstice evening peak drifts maximize in the Pacific region with drift amplitudes of up to 35 m/s, whereas the December solstice velocities maximize in the American sector with comparable magnitudes. The equinoctial peak velocities vary between about 35 and 45 m/s. The morning reversal times and the daytime drifts exhibit only small variations with the phase of the solar cycle. The daytime drifts have largest amplitudes between about 0900 and 1100 LT with typical values of 25–30 m/s. We also show that our model results are in good agreement with other equatorial ground-based observations over India, Brazil, and Kwajalein.

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Radar and satellite global equatorial F-region vertical drift model

We have derived patterns that describe the statistical interplanetary magnetic field (IMF) dependencies of ionospheric convection in the high-latitude region of the northern hemisphere. The observations of plasma motion were made with the HF coherent backscatter radar located at Goose Bay, Labrador, over the period September 1987 to June 1993. The area covered by the measurements extended poleward of 65°Λ to a working limit of about 85°Λ. Distributions of electrostatic potential have been derived and expressed as series expansions in spherical harmonics. The patterns are the first derived from direct ground-based observations of ionospheric convection that approach in completeness and level of detail the patterns derived in recent satellite studies [Rich and Hairston, 1994; Weimer, 1995]. We show the dependence of the convection on IMF angle in the GSM y–z plane for three intervals of IMF magnitude in this plane. Except for predominantly northward IMF, the convection is primarily two-cell. The dusk cell is larger in terms of both spatial extent and potential variation/The effect of IMF By is apparent in the global shaping of the cells and the orientation of the overall pattern in MLT; for By + (By−) the dusk (dawn) cell is more round (crescent-shaped) and the pattern more rotated toward earlier MLTs. The By effect on the nightside convection is pronounced and is hemispherically antisymmetric, like the well-known day side By effect. For IMF increasingly northward, the convection trajectories on the dayside become increasingly distorted, evolving through a three-cell to a four-cell circulation. The additional cells appear on either side of the noon meridian and result in sunward flow. The overall agreement with the results of the satellite studies is good and extends to quite fine detail in the case of the comparison with Weimer [1995]. There are significant differences with the statistical patterns derived from magnetometer measurements, which tend to show domination by the dawn rather than the dusk cell.

Statistical patterns of high‐latitude convection obtained from Goose Bay HF radar observations

The coupling of the plasma sheet to the solar wind is studied statistically using measurements from various satellite pairs: one satellite in the solar wind and one in either the magnetotail central plasma sheet or the near-Earth plasma sheet. It is found that the properties of the plasma sheet are highly correlated with the properties of the solar wind: specifically that (1) the density of the plasma sheet is strongly correlated with the density of the solar wind, (2) the temperature of the plasma sheet is strongly correlated with the velocity of the solar wind, (3) the particle pressure and total pressure of the plasma sheet are strongly correlated with the ram pressure of the solar wind, (4) B y in the plasma sheet is strongly correlated with the B y in the solar wind, (5) B z in the plasma sheet is weakly correlated with the B z in the solar wind, (6) E y in the plasma sheet is weakly correlated with the E y in the solar wind, and (7) plasma sheet earthward-tailward flow velocity is weakly anticorrelated with the solar wind velocity. After removing these solar wind dependencies, the dependencies of the properties of the plasma sheet on geomagnetic activity are reduced and changed. The time lags between the solar wind density and the plasma sheet density are investigated statistically and on a case-by-case basis; it is found that solar wind material reaches the midtail plasma sheet in ∼2 hours, reaches the near-Earth nightside plasma sheet in ∼4 hours, and reaches the dayside plasma sheet in ∼15 hours. The pathway for mass transport into the plasma sheet is explored; it is suggested that solar wind material is transported rapidly up the tail to the near-Earth region via the plasma sheet boundary layer, followed by poloidal transport via E x B convection in the near-Earth (low β) plasma sheet or followed by eddy diffusion in the not-too-distant (high β) plasma sheet. Analogies are drawn between the high Reynolds number plasma sheet and a high Reynolds number fluid wake.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JA02986

The driving of the plasma sheet by the solar wind

An integrated treatment of observations and models of the undisturbed equatorial electrojet is presented. Previous reviews have concentrated on particular aspects of the electrojet phenomenon, such as ground magnetic variations, the counterelectrojet, or E region electron density irregularities and associated plasma instabilities. Here accomplishments during the past 15 years involving all aspects of the phenomenon are covered and, where possible, interrelated. Observational and theoretical studies during recent years have emphasized the local time, longitude, vertical, and latitudinal structure of electric fields and currents in the electrojet; this review similarly places its emphasis along these lines and by necessity involves a detailed examination of electrodynamic coupling with the neutral atmosphere tidal oscillations in temperature, composition, and winds. The review concludes with an assessment of current knowledge concerning the equatorial electrojet and suggestions for the future direction of observational and theoretical research on electrodynamical processes in the equatorial ionosphere.

The equatorial electrojet

Electrodynamics in the low and middle latitude ionosphere: a tutorial

The characteristics of the large-scale electrodynamic parameters, field-aligned currents (FACs), electric fields, and electron precipitation, which are associated with auroral substorm events in the nighttime sector, have been obtained through a unique analysis which places the ionospheric measurements of these parameters into the context of a generic substorm determined from global auroral images. A generic bulge-type auroral emission region has been deduced from auroral images taken by the Dynamics Explorer 1 (DE 1) satellite during a number of isolated substorms, and the form has been divided into six sectors, based on the peculiar emission characteristics in each sector: west of bulge, surge horn, surge, middle surge, eastern bulge, and east of bulge. By comparing the location of passes of the Dynamics Explorer 2 (DE 2) satellite to the simultaneously obtained auroral images, each pass is placed onto the generic aurora. The organization of DE 2 data in this way has systematically clarified peculiar characteristics in the electrodynamic parameters. An upward net current mainly appears in the surge, with little net current in the surge horn and the west of bulge. The downward net current is distributed over wide longitudinal regions from the eastern bulge to the east of bulge. Near the poleward boundary of the expanding auroral bulge, a pair of oppositely directed FAC sheets is observed, with the downward FAC on the poleward side. This downward FAC and most of the upward FAC in the surge and the middle surge are assoc iated with narrow, intense antisunwqard convection, corresponding to an equatorward directed spikelike electric field. This pair of currents decreases in amplitude and latitudinal width toward dusk in the surge and the west of bulge, and the region 1 and 2 FACs become embedded in the sunward convection region. The upward FAC region associated with the spikelike field on the poleward edge of the bulge coincides well with intense electron precipitation and aurora appearing in this western and poleward protion of the bulge. The convection reversal is sharp in the west of bulge and surge horn sectors, and near the high-latitude boundary of the upward region 1, with a near stagnation region often extending over a large interval of latitude. In the eastern bulge and east of bulge sectors, the region 1 and 2 FACs are located in the sunward convection region, while a spikelike electric field occasionally appears poleward of the aurora but usually not associated with a pair of FAC sheets. In the eastern bulge, magnetic field data show complicated FAC distributions which correspond to current segments and filamentary currents.

Electrodynamic parameters in the nighttime sector during auroral substorms

Ionospheric electric current measurement, determining vertical current density distribution and electron number density for geomagnetic field study

Measurements of ionospheric currents off the coast of Peru

Plasma and electric field observations from two satellite encounters with equatorial plasma bubbles updrafting at velocities of about 2 km/s are presented. These large, upward velocities are consistent with an adaptation of Chandrasekhar's model for the motion of plasma blobs supported against gravity by a magnetic field. Vector magnetic field measurements, available during one of the bubble encounters show a perturbation of about 150 nT, directed radially outward from the earth, near the western wall of deepest plasma depletion. This magnetic variation is too large to be caused by simple shunting of the g x B current along the bubble's edge. Rather, it is Alfvenic in nature, radiating from a generator located near the magnetic equator, in the plasma outside the bubble's leading edge. A heuristic model of a depleted flux tube with constant circular cross section moving upward through a background plasma predicts most of the measurements' qualitative features.

Equatorial bubbles updrafting at supersonic speeds

While convection patterns in the high-latitude ionosphere are usually presented in a corotating frame of reference, those of the magnetosphere are given in inertial coordinates. In the corotating representation the convection throat, which is frequently associated with the cusp, opens between 1000 and 1100 MLT. Cusp precipitation, however, centers about noon. We find that transforming the convection patterns of Heppner and Maynard (1987) (hereinafter H-M) into inertial coordinates aligns the throat region with local noon. We present projections of the H-M patterns to the magnetosphere in both corotating and inertial coordinates using the magnetic field model of Tsyganenko (1989). In inertial coordinates the mapped H-M convection throat opens at noon. Consistent with predictions of the Rice convection model for magnetospheric electric fields late in the substorm cycle, only a small fraction of the equipotential contours penetrate to the subsolar region. This suggests that a significant portion of flux tube merging occurs on magnetic field lines whose equatorial mapping is on the flanks of the magnetosphere. Nonconjugacy between the mapping of H-M patterns for both positive and negative interplanetary magnetic field BY, especially in the 1400-1600 LT sector, may explain the BY dependence of the electron precipitation “hot spot” discovered by Evans (1985). A separate lobe cell is not required to explain the central, equipotential contours of the large convection cell.

Mapping ionospheric convection patterns to the magnetosphere

San Marco D electric field measurements have been averaged in terms of the equivalent plasma drifts for days near the 1988 autumn equinox. The observed plasma drifts provide a satellite-based composite in altitude and local time of the evening enhancements in the zonal and vertical drifts and the shear in the zonal drift. The satellite data reveal details of the postsunset plasma drift vortex, which have been studied in radar and chemical release experiments. The averaged drift data show a proportional relationship between the prereversal enhancement of the vertical drift and the zonal derivative of the zonal drift. This relationship suggests that the vertical drift enhancement is caused by a curl-free response to the rapid increase of the zonal drift, which in turn is tied to the F region zonal winds and the E region sunset. The data also show a curl-free drop-off in altitude of the vertical plasma drift above and below the peak drift at 450 km altitude during the prereversal enhancement. Comparisons of the averaged data with results of coupled ionosphere-electrodynamics models are used to examine the underlying causes of the evening plasma drifts. The evening plasma drift vortex is shown to be a single drift structure with a single source. The source of the evening drift vortex resides on the bottomside of the integrated F region, just as originally supposed by Rishbeth [1971]. This dominant single source, which is produced by the divergence of the wind-driven current of the F region, causes the evening enhancement of the eastward plasma drift, the shear in the eastward drift, and the prereversal enhancement of the vertical drift.

Nelson C. Maynard

Papers

Electrodynamic parameters in the nighttime sector during auroral substorms

Measurements of ionospheric currents off the coast of Peru

Equatorial bubbles updrafting at supersonic speeds

Mapping ionospheric convection patterns to the magnetosphere

Study of the evening plasma drift vortex in the low‐latitude ionosphere using San Marco electric field measurements