the role of zonal winds in the production of a pre ......en rp þ mi~g e þ~e þu~? ~b where s~ is...

The role of zonal winds in the production of a pre-reversalenhancement in the vertical ion drift in the low latitude ionosphere

R. A. Heelis,1 G. Crowley,2 F. Rodrigues,2 A. Reynolds,2 R. Wilder,2 I. Azeem,2

and A. Maute3

Received 24 January 2012; revised 25 June 2012; accepted 26 June 2012; published 7 August 2012.

[1] The evolution of the pre-reversal enhancement in the vertical ion drift in the equatorialF region is described via an examination of the current systems determined from a coupledionosphere thermosphere model. We find that the pre-reversal enhancement is producedby a reversal in the F region zonal wind that results in an additional upward current wherethe E region Pedersen conductivity is declining across the dusk sector. The continuityof the total current is maintained through an enhancement in the eastward zonal currentand an associated upward drift of the ions.

Citation: Heelis, R. A., G. Crowley, F. Rodrigues, A. Reynolds, R. Wilder, I. Azeem, and A. Maute (2012), The role of zonalwinds in the production of a pre-reversal enhancement in the vertical ion drift in the low latitude ionosphere, J. Geophys. Res., 117,A08308, doi:10.1029/2012JA017547.

1. Introduction

[2] Although changes in the interplanetary magnetic fieldmodulate shielding currents at high and middle latitudes,which allow electric potentials at high latitudes to penetrateto middle and low latitudes [Wolf and Spiro, 1997], the iondrifts at low and middle latitudes are primarily produced bythe dynamo action of neutral winds that drive currents due tocollisions with the ions [Richmond, 1995]. The direction andmagnitude of wind-induced currents depend on the detailsof the anisotropic electrical conductivity and the magneticfield as well as the neutral wind itself. Thus there are largevariations of these quantities in latitude, altitude and localtime that must be considered. This complex arrangement ofphysical processes makes it difficult to assess the relativeimportance of winds in different directions acting at differentaltitudes in the observed configuration of the electric fieldor the ion drift. The action of the wind dynamo at low andmiddle latitude has recently been included in global circula-tion models of the atmosphere and ionosphere [Richmondet al., 1992; Millward et al., 2001; Huba et al. 2010] allow-ing the self-consistent behavior of the ion and neutralmotions to be described. The altitude profile of the total ionconcentration in the ionosphere leads rather naturally to adivision into the E and F regions, and frequently the dynamoaction of the neutral winds in these regions is considered

separately. In the E region, diurnal and semi-diurnal tidesdominate the motion of the neutral gas, and the ion drifts(electric fields) consistent with these sources have beenstudied extensively [Fesen et al., 2000]. In the F region,neutral winds result from local heating of the thermosphereby the absorption of solar radiation, and the dynamo action ofthese winds has also been studied [Rishbeth, 1971; Heeliset al., 1974; Farley et al., 1986]. The F region dynamo isresponsible for enhanced zonal ion drifts in the nighttimeand is largely responsible for the so-called pre-reversalenhancement (PRE) in the vertical ion drift observed near theF region peak at the equator. Recent models that self con-sistently consider the dynamics of the charged and neutralparticles in the F region have successfully reproduced thepre-reversal enhancement in the vertical ion drift and itsdependence on solar activity and season [Fesen et al., 2000;Millward et al., 2001]. These studies have emphasized theimportant role played by the conductivity in the nighttimeE region and its magnitude relative to that in the F region[Crain et al., 1993].[3] In the E region, both zonal and meridional winds can

each drive both zonal and meridional currents perpendicularto the magnetic field because the Hall and the Pedersenconductivities are significant. In contrast, meridional andzonal winds in the F region drive only zonal and meridionalcurrents respectively, since only the Pedersen conductivityis significant. In the daytime, currents driven by winds inthe F region may close in loops along the magnetic fieldlines and through the highly conducting E region. Winds inthe dayside E region may drive much larger currents thatclose largely in the E region. The restricted altitude extent ofthe E region conductivity effectively suppresses the verticalcurrent (driven by zonal winds) leading to enhancements inthe zonal current. This effect is largest near the equatorwhere the so-called equatorial electrojet flows [Forbes,1981]. During the nighttime the E region conductivity is

1Hanson Center for Space Sciences, University of Texas at Dallas,Richardson, Texas, USA.

2ASTRA, LLC, Boulder, Colorado, USA.3High Altitude Observatory, National Center for Atmospheric Research,

Boulder, Colorado, USA.

Corresponding author: R. A. Heelis, Hanson Center for Space Sciences,University of Texas at Dallas, Richardson, TX 75083, USA.([email protected])

©2012. American Geophysical Union. All Rights Reserved.0148-0227/12/2012JA017547

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, A08308, doi:10.1029/2012JA017547, 2012

A08308 1 of 9

significantly reduced and it may no longer provide an easyclosure path for currents driven by F region winds. In thiscase the ions will drift with the neutral gas to reduce thecollisional force between them. Thus, the winds and con-ductivity distribution will subsequently modify the distri-bution of ionization and thereby change the ionosphericconductivity and the ion drag on the neutral gas. Modifica-tions to the wind, the wind driven currents and the ion driftoccur until a quasi-steady state is achieved. Observationallyit may be difficult to extract the separate contributions ofwinds and ion drifts to the final state. However, the use of amodel that self-consistently solves for the neutral dynamicsand the plasma electrodynamics can be used to extract thisinformation.

2. Calculations

[4] For this study we have performed a post-analysis of thewinds, the conductivity, the currents and the ion driftsderived from the NCAR TIEGCM [Richmond et al., 1992].This model self consistently solves the continuity, momen-tum and energy equations for the charged and neutral parti-cles and determines the electric potential required by thecondition of a divergence free current system. In thisapproach collisions between the ions and electrons areneglected and the momentum equations for the ions andelectrons are used to derive an expression for the currentdensity of the form

~J ? ¼ ~s � 1

eNrP þ mi~g

eþ~E þ ~U? �~B

� �

where ~s is the perpendicular conductivity tensor, N is theplasma density, P the plasma pressure, g is gravity and E, Uand B are the electric field, the neutral wind and the magneticfield respectively. Studies by Eccles [2004] and Maute et al.[2012] show that the pressure gradient and gravity forces aregenerally small compared to the collisional force from theneutral gas and in this work we consider only the currentfrom the electric field (or plasma drift) and from the neutralwind. In a quasi-steady state the plasma drift may be equiv-alently described in terms of the electric field E =�V� B andthe ion-neutral momentum exchange, proportional to V � U,may be equated to the J � B force.[5] The calculations are performed using the International

Geomagnetic Reference Field (IGRF) for which a modifiedmagnetic apex coordinate system is constructed [Richmond,1995]. A current continuity equation contains magnetic fluxtube integrals of the Pedersen and Hall conductivity, thespatial gradients of these flux tube integrated conductivitiesand the spatial gradients of the flux tube integrated neuralwind driven current. The wind system is calculated selfconsistently from all the applicable internal forces in thethermosphere, using, at the lower boundary at 97 km, tidalperturbations as defined by the Global Scale Wave Model(GSWM) [Hagan et al., 1999]. The model is run for a highsolar activity level with F10.7 = 220 sfu and the high latitudeforcing is imposed by a convection pattern specified byHeelis et al. [1982] for a cross-polar cap potential of 30 kV.[6] In this paper we address the role of wind and con-

ductivity gradients in the production of ion drifts near theequator. In the early evening the self-consistently calculated

ion drifts reveal a pre-reversal enhancement in the verticalion drift that is comparable to that seen by observations fromthe Jicamarca radar for similar conditions [Scherliess andFejer, 1999]. Various mechanisms for the generation of thepre-reversal enhancement have been examined by Eccles[1998]. The most commonly described feature is the exis-tence of a so-called fringing electric field associated with astrong local time gradient in the E region conductance at thedusk terminator (“edge” effect). In the simplest of config-urations this electric field, which is also described in theworks of Rishbeth [1971] and Farley et al. [1986], pointstoward the dusk E region terminator [eastward] from thedayside and is associated with an enhanced upward E � Bdrift at this location. We should point out that in the quasi-steady state examined here, a cause and effect associatedwith the pre-reversal enhancement cannot be established. Inthis work we examine a more complete evolution of thewinds, the conductivity and the electric currents associatedwith the pre-reversal enhancement in the vertical ion driftwith the goal of describing the configuration that establishesa force balance between the plasma and the neutral gas.

3. Results

[7] Before examining the current system associated withion drifts (polarization electric fields) in the equatorial regionand the F region zonal wind, it is useful to appreciate thelocal time variation of the conductivity in the E region andthe F region. The conductivity of the ionosphere and thewind driven currents throughout the ionosphere are contin-uous functions of altitude, latitude, longitude and local time.Therefore a division of the contributions from the E regionand the F region is somewhat arbitrary. However, it isconvenient to make a division, for this paper, between theF region and the E region at about 180 km altitude knowingthat above this level the Hall conductivity is negligible andat any given local time the zonal wind drives a current withno change in direction. To examine the electrodynamics inthe equatorial region for heights up to 500 km requires thatwe consider the flux tube integrated effects out to magneticlatitudes of �15� at 120 km. Over this small latitude rangethe features in the modeled ion drifts, the neutral winds andthe conductivity do not change significantly with latitudeand thus the flux tube integrated conductivity, or conduc-tance will vary in much the same way as the conductivity.At greater apex heights the magnetic flux lines can threadthe F region equatorial anomaly producing a flux tube inte-grated conductivity variation in apex height that differs sig-nificantly from the altitude variation of the conductivity.[8] Figure 1 shows the logarithm of Hall and Pedersen

conductivities at the magnetic equator as a function of alti-tude and local time. For the purpose of later comparison wehave chosen the longitude 75� west to illustrate the modeledbehavior. It is easily seen that during the daytime the Eregion may be considered as a thin layer occupying theregion from 100 km to 150 km in which the Hall conduc-tance is about 20 S and the Pedersen conductance is about15 S. The peaks in the E region Hall and Pedersen conduc-tivities appear at slightly different altitudes, and altitudevariations in the conductivity and winds (Figure 2) producecurrents that are driven by different wind components atdifferent altitudes. In this study we will defer a discussion of

HEELIS ET AL.: EQUATORIAL ION DRIFTS A08308A08308

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the details of the E region current systems and account forthem by assuming that they are reflected in the backgroundfeatures of the modeled ion drifts. Of critical importance tothis study is to note that the Hall and Pedersen conductivitiesin the E region reach a threshold minimum value a little after1900 local time and retain that constant level throughout thenight until 0500 local time.[9] In the F region the Pedersen conductivity is distributed

over a larger altitude range than in the E region. Thus, despitea smaller peak value than is seen in the E region, the daytimeF region Pedersen conductance is 10–50% of the daytimeE region Pedersen conductance and at night the F regionPedersen conductance exceeds that in the E region by afactor greater than 10. During the daytime the topside of theE region is marked by a rapid decrease in the conductivitybetween 150 km and 200 km altitude, but above 200 km thePedersen conductivity decreases very slowly with altitude upto about 400 km.[10] Across sunset the F region Pedersen conductivity

falls rather dramatically, which we will see later, is aresponse to a pre-reversal enhancement in the vertical iondrift that moves the F peak to higher altitudes. A large bot-tomside local time gradient in the conductivity is also seen

as the E region plasma rapidly decays. Later in the eveningand nighttime the ionosphere slowly drifts downward whilethe F region peak Pedersen conductivity falls only slightly,reaching its minimum height just before sunrise. During thisperiod the large bottomside vertical gradient in the Pedersenconductivity resides between 200 km and 250 km altitude.[11] The relationship between the winds and the electric

fields in the F region can be revealed by examination of thecurrents that result from the drivers in the E region and theF region. We first provide a large-scale view of the majorcontributors to the currents flowing perpendicular to themagnetic field in order to identify the features associatedwith the pre-reversal enhancement and the zonal driftreversal in the bottomside F region. In the equatorial regionabove about 120 km altitude the Pedersen conductivitydominates the Hall conductivity and only the zonal winddrives currents that are essentially vertical and perpendicularto the magnetic field.[12] Figure 2 shows, in a format similar to Figure 1, the

altitude and local time variation in the zonal neutral winddetermined in the TIEGCM at the geomagnetic equator and75� west longitude. We note that in the F region the zonalwind is westward by day and eastward at night with maxi-mum speeds that reach 100 m s�1. At any given local timethe wind is approximately constant with altitude above about250 km. However, with decreasing altitude below 250 kmthe wind decreases rapidly and during the evening hours itreverses sign at the lowest F region altitudes near 180 km.The wind variations with altitude below 250 km arise fromself-consistent calculations in the TIEGCM and are a resultof a transition from those produced by local forcing due toeuv heating above to the propagation of wavefields frombelow. Subsequent analysis of the current driven by thisF region wind and the winds in the E region can reveal howthe currents and ion drifts are arranged in association with theFigure 1. Modeled altitude and local time variations in the

Pedersen and Hall conductivities in the magnetic meridian at75�W longitude.

Figure 2. Modeled altitude and local time variations in thezonal (positive eastward) neutral wind in the magneticmeridian at 75�W longitude. A zonal wind shear exists inthe nighttime near 200 km altitude.


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pre-reversal enhancement in the vertical ion drift and the dayto night difference in the F region zonal drift.[13] Figure 3 illustrates the altitude and local time varia-

tion in the vertical (left panel) and geomagnetic east-west iondrift (Figure 3, right) produced in the model at the geo-magnetic equator and 75� west longitude. These parametersequivalently describe the zonal and vertical electric fieldrespectively that are associated with the dynamo action ofneutral winds in the E region and F region. Superimposed onthis figure is a white curve representing the local timedependence of the E region Pedersen conductivity at 150 kmaltitude. The time at which the E region conductivity attainsa steady state nighttime level is easily identified and pro-vides the most reliable way to locate the local time at whichthe E region is in darkness. We refer to this location as theE region terminator. At any given local time the vertical iondrift decreases slowly in the altitude range 100 km to550 km, but it varies in local time with upward drifts duringthe daytime and downward drifts at night. The daytimeupward drift shows a maximum about 20 m s�1 near localnoon and is terminated by a pre-reversal enhancement thatplaces a peak upward drift of about 24 m s�1 just prior to theE region terminator. This pre-reversal upward drift is com-monly observed over a broad longitude extent during theequinox period [Fejer et al., 2008]. Downward ion driftsare almost uniformly distributed across the nighttime sectorwith a magnitude near 25 m s�1. A maximum in the down-ward drift of about 30 m s�1 occurs just prior to sunrise.These features in the downward drift are also seen in directobservations made from satellites [Pacheco et al., 2010] andground-based radar [Fejer et al., 1991].[14] The modeled zonal ion drift variations (right panel) in

the F region above about 300 km altitude are also consistentwith observations from satellites [Pacheco et al., 2011] andground-based radar [Fejer et al., 1991]. In the F region thedrifts are largely westward during the daytime and eastwardat night, with the reversal from westward to eastwardoccurring near 1800 LT and the reversal from eastward to

westward occurring near 0600 LT. In the F region the day-time westward drift has a maximum near 80 m s�1 at noon,while the maximum nighttime eastward drift is in excess of100 m s�1 occurring near 2100 LT. The local time extentand magnitude of the westward and eastward drifts are suchthat the equatorial ionosphere superrotates in the F region[c.f. Pacheco et al., 2011]. A strong altitude shear in thezonal ion drift develops near 1800 local time at 300 kmaltitude in association with the pre-reversal enhancement inthe vertical drift. The maximum westward flows in thebottomside F region occur at the E region terminator. Com-parison of the zonal ion drifts with the zonal neutral wind inFigure 2 shows that in the altitude region between 250 kmand 200 km and the local time region from 1800 to 2000 h theion drift remains westward while the neutral wind now blowsto the east. By 2200 h local time the shear in the ion drift hasdescended to altitudes between 250 km and 200 km where itpersists throughout the night and corresponds to the samefeature in the zonal neutral wind.[15] The broad similarities between the major features in

the model ion drifts and the features described in observa-tions provides a high level of confidence that the drivers forthe major electrodynamic features, including the pre-reversalenhancement, are adequately represented in the model.Thus, we are confident that the configuration of winds andcurrents associated with the pre-reversal enhancement can beexposed. In the equatorial region the zonal neutral wind isthe principal driver of currents and ultimately the ion drift.Being perpendicular to the magnetic field this wind com-ponent drives vertical current in the F region and verticaland zonal current in the E region.[16] Figure 4 shows the vertical current density as a

function of altitude and local time at the magnetic equatorand 75� west longitude. The upper scale refers to theFigures 4a and 4b labeled, J-Efield and J-Wind represent-ing the two contributions to the zonal collisional force pro-portional to V � U weighted by the conductivity. Figure 4c,

Figure 3. Modeled altitude and local time variations in the vertical (positive upward) and zonal (positiveeastward) ion drifts in the magnetic meridian at 75�W longitude. The Pedersen conductivity at 150 km altitude(white line) is also shown as a function of local time allowing easy identification of the E region terminator.


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labeled J-Total, shows the total current density representa-tive of the zonal J � B force.[17] Here positive current density in red, is directed

downward and negative current density in blue, is directedupward. Currents that arise from ion motions induced by thegravitational force and the plasma pressure gradient force are

also included in the model but make almost no contributionto the vertical current density in the magnetic meridianshown here.[18] A description of local current loops in the E region is a

topic for a separate study and in this work the color scalesare chosen to illustrate features in the region above 180 km.We note that in the daytime E region, below 180 km thevertical current density at the magnetic equator is upward(blue) with a local time variation that mimics that of theelectrojet current seen in Figure 5. The vertical current isclosed locally by poleward and downward currents that flowaway from the magnetic equator (not shown). During thenight the current density is small due to the low conductivityresulting from rapid recombination of the layer. However, thewinds and ion drifts (electric fields) may be of comparablemagnitude to those seen during the daytime. In the F region,across noon and in the afternoon the vertical current densityis quite small as the westward plasma drift is almost equal tothe westward neutral wind. However, in the pre-noon hoursand across the dusk sector the vertical current density isinfluenced by changes in the F region zonal wind, whichchanges the collisional force and results in correspondingchanges in the vertical current that provides the balancingJ � B force. In the pre-noon hours, from 0700 to 1000local time, the rising prominence of the wind driven currentis due to the rapid increase in the westward wind in theF region (Figure 2) and the increase in the F region Pedersenconductivity in the F region (Figure 1). Across dusk theupward current density is due to the maintenance of theF region conductivity and a reversal in the F region windfrom west to east so that it now requires a force in additionto that required by the winds in the E region.[19] As we will see, the enhanced downward current in the

pre-noon hours is accompanied by an increase in the east-ward zonal current associated with the rising F region Ped-ersen conductivity. More importantly for this study, theenhanced upward current across the dusk sector cannot be

Figure 4. Modeled altitude and local time variations in thevertical current density (positive downward) in the magneticmeridian at 75�W longitude. (a) The contribution attribut-able to the polarization electric field and (b) the contributionattributable to the zonal neutral wind. (c) The net verticalcurrent density.

Figure 5. Modeled altitude and local time variations in thezonal current density (positive eastward) in the magneticmeridian at 75�W longitude.


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completely closed by field-aligned currents to the E regionand is therefore also fed by an eastward zonal current that isassociated, in the presence of a declining conductivity, withan enhanced eastward electric field, which may be identifiedas the pre-reversal enhancement in the vertical ion drift.Across the dusk sector, in the region between 200 km and300 km the Pedersen conductivity increases with altitude asdoes the eastward neutral wind. But the increase in the ver-tical current density is moderated by a decreasing verticalelectric field consistent with a decrease in the relative ion-neutral zonal drift and the ability of the E region to close thecurrent. Above 300 km the wind driven current continues toincrease with altitude, but in the absence of an efficientclosure path through the E region it is opposed by a down-ward current requiring an eastward ion motion in the samedirection as the neutral gas. Between 1900 and 2100 localtime the peak in the F region conductivity decreases inaltitude (Figure 1), as does the reversal in the zonal ion drift(Figure 3). After 2100 the zonal motion of the ions andneutral gas is the same throughout the F region and thevertical current is very small.[20] Figure 5 illustrates the features of the zonal current

density described above. Here is shown the zonal currentdensity as a function of altitude and local time for the mag-netic equatorial plane at 75�W longitude. The format is thesame as that used in the panels of the previous figure and inthis case the zonal current density is shown with positivecurrent directed to the east. In this display the current densitydriven by pressure gradients and gravity is removed to revealonly the current in the F region associated with the neutralwind and the electric field. Note that, except for the region ofhigh Hall conductivity below 120 km, the neutral windcontributes very little to the zonal current density and thus adivision by the ion and neutral drifts does not contribute anyadditional insight in the region of interest. In the F region,

above 180 km altitude the current is eastward during thedaytime and westward at night. This is consistent with thecurrent required to balance the collisional force produced bythe upward motion of the ions during the day and theirdownward motion at night. We note the strongly increasingeastward current indicated by the almost vertical contours inthe pre-noon hours is connected to the downward currentseen in Figure 4. Also note, that by comparison with thevertical ion drift in Figure 3, the F region zonal currentacross the dusk sector is collocated with the enhancedupward ion drift (eastward electric field) in the pre-reversalenhancement.[21] Figure 6 shows the evolution of the electric fields and

currents at 250 km altitude in the local time period from1600 to 2400 h to reveal more quantitatively the featuresdescribed in Figures 4 and 5 in the bottomside F region.Labeled in the top panel are the local time variations of thePedersen conductivities in the bottomside F region at250 km and at 150 km in the E region. The two middlepanels show the total vertical and zonal current densities at250 km altitude as solid curves and the contributions to thecurrent density at 250 km altitude from the wind and theelectric field shown by dotted and dashed curves respec-tively. The lower panel shows the zonal neutral wind and thezonal and vertical ion drifts, which are also indicative of theelectric field components at 250 km altitude. Vertical linesare drawn through all panels and labeled A, B and C to markspecific events germane to the evolution of the pre-reversalenhancement.[22] Consider first the evolution of the vertical current

density. At 1600 LT the E region Pedersen conductivityremains high compared to that in the F region. At this timethe local F region zonal wind is directed to the west and isassociated with a downward current. However, the net ver-tical current in the bottomside F region is upward, beingdominated by the upward/poleward electric field from theE region that is associated with a westward ion drift inexcess of the neutral velocity. Between 1730 and 1830 localtime (between times A and B), both E region and the F regionconductivities continue to decline. The vertical upward cur-rent associated with the vertical electric field (zonal ion drift)decreases as the drift remains almost constant. However, theF region zonal wind reverses from westward to eastward,thus requiring an upward current. A large bottomside gradi-ent in the conductivity develops as the E region conductivitydeclines (Figure 1). At the same time the neutral wind has anincreasing eastward component with increasing altitude(Figure 2). Thus, between locations A and B, from about1730 to 1830 local time, both the wind and the electric field(ion drift) require current that is directed upward and thatincreases with increasing altitude (Figure 4). This significantgradient in the vertical current density exists, while thedeclining E region conductivity makes it increasingly moredifficult to accommodate that gradient with field-alignedcurrents that flow through the E region. Thus, an eastwardzonal current must supply the increasing vertical current.An upward ion drift recognized as the pre-reversal enhance-ment associated with an elevated zonal electric field providesthe collisional force that balances the J� B force of the zonalcurrent.[23] In the local time region between B and C, from 1830

to 1930 local time the F region conductivity reaches a

Figure 6. Modeled local time variations in the Pedersenconductivity (top), the vertical current density (middle) andthe zonal current density (lower) as a function of local timeat the magnetic equator and 250 km altitude. (bottom) Thelocal time variations in the ion drift and the zonal neutralwind in the F region.


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minimum resulting from the enhancement in the upwardE � B drift and the current density is everywhere at a mini-mum as the E region reaches a low nighttime conductivitylevel. After 1930 local time, on the nightside of the E regionterminator, the current in the F region flows west toward theterminator and results almost exclusively from the E regionwinds. The F region vertical current remains very small sincethe relative zonal motion between the plasma and neutrals isnow very small.

4. Discussion

[24] We have illustrated how the pre-reversal enhance-ment appears in a region where the E region conductance isnot large enough to provide a closure path for gradients inthe F region current density resulting from the F regionwind, and not small enough to ensure that F region polari-zation currents will largely oppose the wind-driven current.[25] We first consider the likely situation in the F region

in the absence of F region winds that produce a pre-reversalenhancement. In this case, across the dusk terminator thedeclining vertical ion drift is accompanied by a decliningeastward current in accord with balance between collisionalforces proportional to the plasma velocity and J � B forces.This zonal current divergence is accompanied by a down-ward field-aligned current into the E region that subse-quently flows through the daytime E region to close theupward field aligned current across dawn associated withthe increasing eastward electric field (vertical ion drift) andincreasing Pedersen conductivity. Some small fraction ofthe field-aligned current also closes through the nighttimeE region in accord with the westward electric field (down-ward E � B drift).[26] Now consider the additional current resulting from

the F region zonal wind. During the daytime, the wind is tothe west providing a force in that direction that is matchedby a downward current that is closed along the magneticfield in the magnetic meridian through the E region. Duringthe nighttime, when the E region conductivity is a minimum,the F region current cannot be readily closed through theE region. Then the ions and neutrals in the F region move tothe east at about the same speed in order to reduce thecollisional force between them. A transition between thesetwo limits occurs between 1700 and 1900 h as both theE region and F region conductivities decline. In this localtime region the F region zonal wind reverses and drives anadditional upward current that is partially closed in the localmeridian by currents that flow along the magnetic field linethrough the E region. However, zonal currents that flow fromthe dayside into the region also contribute to the currentclosure. This zonal current enhancement is associated with acorresponding enhancement in the zonal electric field fromthe dayside to the nightside, which is associated with a so-called pre-reversal enhancement in the vertical ion drift in theF region.[27] Figure 7 schematically shows the currents and electric

fields as a function of local time that result when the wind-driven current across this local time region is upward, as isthe case modeled here. In this figure the currents (in red) andelectric fields (in blue) resulting from the F region zonal windare shown with the color intensity indicating the magnitudeof the current and electric field. This configuration would be

added to the electric field and currents originating from theE region. On the dayside, large downward currents andsmall upward electric fields result from the F region cur-rents closing in the magnetic meridian directly through theE region. On the nightside (after 1900 local time), the smallupward electric current, but large downward electric field, isassociated with the most complete state of polarization inthe F region when the plasma and neutrals drift togetherto the east. In the region between these two extremesthere is a transition from downward to upward current inthe F region supported by a zonal electric field (vertical iondrift) associated with the diversion of current from the day-side F region, via the E region. These enhanced upwardplasma drifts in the F region are commonly referred to as thepre-reversal enhancement. In addition the vertical electric field(zonal ion drift) in the lower F region is modified to producewestward drift in the lowest regions of the bottomside (below200 km) and eastward drifts above. This phenomenon hasbeen referred to as the bottomside vortex [Tsunoda et al.,1981; Kudeki et al., 1981; Rodrigues et al., 2012].[28] Descriptions of the electrodynamics of the evening

sector and pre-reversal enhancement were originally putforward by Rishbeth [1971] and modeled with variousdegrees of sophistication by Heelis et al. [1974], Farley et al.[1986], Haerendel et al., [1992] and Eccles [1998]. It isimportant to emphasize that the velocity perturbation thatdefines the pre-reversal enhancement will appear followingthe reversal in the zonal wind in the F region, and while rapidrecombination after sunset limits the ability of the E region toclose field-aligned currents that would otherwise flow inresponse to gradients in the current in the F region. Themagnitude of the vertical ion drift will depend upon threefactors: first the magnitude of the wind, which increases withlocal time until about local midnight, second the F regionionospheric conductivity itself, which decreases with localtime throughout the night, and finally the E region conduc-tivity, which decreases with local time to reach somethreshold low value by about 1900 local time. Furthermorethe upward drift perturbation is superimposed on drifts drivenby currents and associated electric fields in the E region thatreverse from upward to downward across the E region ter-minator. Thus the pre-reversal enhancement will be largelyconfined to local times before the E region attains the lowestnighttime conductivity. After this time there exists a verticalgradient in the bottomside F region conductivity and a ver-tical electric field. But the vertical current density is smallsince the zonal drift of the ions and neutrals is about the sameand the collisional force between them is small.[29] The field responsible for the pre-reversal vertical

drift has often been referred to in the past as a fringingfield, by analogy with the field associated with the edgeof a charged plate. The charged plate is in this case thebottomside of the F region. However, since the entire volumecontains a magnetized plasma it should more correctly beviewed as the plasma drifts that evolve to maintain a localdivergence free current density in the presence of changes inthe neutral wind and conductivity.

5. Conclusions

[30] A detailed examination of the currents driven byzonal winds and the associated ion drifts in the equatorial


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ionosphere shows that the pre-reversal enhancement appearswhen the F region zonal wind demands a vertical current in aregion of reduced E region Pedersen conductivity. In thiscase the enhanced vertical current is accommodated in twoways. First it is reduced by modifications in the zonal iondrift and an associated vertical electric field to reduce thecollisional force between the plasma and the neutral gas.Second it is fed by an increase in the zonal current withcorresponding enhancements in the zonal electric field(upward plasma drift). The electric field associated with thepre-reversal upward drift resembles that produced by thefringing effects of a charged plate. However, there are nosuch discontinuous boundaries in the ionosphere and the iondrifts and electric fields simply evolve in a way that main-tains a divergence free current density in the presence ofchanges in the neutral wind and conductivity.

[31] Acknowledgments. Support by NASA grant NNX07AT82G tothe University of Texas at Dallas allowed R.A.H. to spend an extended visitat ASTRA to complete this work. R.A.H. is grateful to Geoff Crowley for host-ing his visit. The work at ASTRA was supported by NASA LWS ContractNNH10CD49C. A.M. was supported in part by NASA grant NNX09AN57G.

The National Center for Atmospheric Research is sponsored by the NationalScience Foundation.[32] Robert Lysak thanks the reviewers for their assistance in evaluat-

ing this paper.

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