GEOPHYSICALRESEARCH LETTERS, VOL. 25, NO.10, PAGES1625-1628, MAY 15, 1998

The magnetosphericdriver of subauroralion drifts J. De Keyser,M. Rothand J. Lemaire BelgianInstitute for Space Aeronomy, Brussels, Belgium

Abstract. Subauroralion drifts (SAID) are narrow lay- Thedrift speedpeak exceeds 1 km s-•, correspondingto ers of intensewestward ionospheric flow observedduring an electricfield of 150mV m-• or more. The ionospheric substorms.We presentnumerical simulations showing that processesin SAID are well understood;they have a time the combinedeffect of thermo-electricand convectionelec- scale of a few minutes [Andersonet al., 1991, 1993]. tricfields in a magnetosphericcurrent sheetwwhen mapped Figure 1 sketchesthe ionosphericmagnetic and electric downto theionosphere•an account for thewestward di- fields Bi and Ei. The westward ion drift is Vi = Ei x rectionof the ion drift, the width and intensityof the drift Bi/B• becauseof thelow ionospheric collision rate. West- speedpeak, and the lifetime of SAID.The model can also warddrift correspondsto polewardelectric field (alsoin the explainwhy SAID occur mainly in thepre-midnight sector. southernhemisphere). The electricpotential drop across SAID is a few to tensof kiloVolts [Spiroet al., 1979;Smiddy et al., 1977]. Downwardfield-aligned currents flow equator- Introduction wardof the SAID; Pedersencurrents flow throughthe SAID (a resistiveload) in the ionosphere,and upward currents Sub-auroralion drift layers(SAID) are formedin the flow on the polewardside. The integratedcurrent per unit ionosphereduring substorms [Spiro et al., 1979].These lengthin theazimuthal direction is • 0.1 A m-• [Smiddy layershave a narrowlatitudinal extent and are character- et al., 1977; Anderson et al., 1993] and delivers to the iono- izedby an intense westward drift peak. SAID are observed morethan half an hourafter the onsetof a magnetosphericsphere a powerof m 1 kilowattm -• alongthe westward drift band. This power must be generatedby a magneto- substormand last generally less than 3 hours.Since SAID oftenseem to coincidewith the ionosphericprojection of sphericsource, since no ionosphericmechanism is known that canproduce so muchpower over such a narrowlatitudi- theplasmapause, previous models have attempted torelate nal range.Once an SAID is formed,the charge carriers in the theionospheric phenomenon to the structure of thespace- F-region are depleted,reducing the ionosphericconductiv- chargelayer (Alfv6n layer) at theplasmapause [Spiro et ity, thefield-aligned currents, and the powerdrawn from the al., 1981;Anderson et al., 1993;Ober et al., 1997].None source [Banksand Yasuhara,1978; Andersonet al., 1993]. of thesemodels, however, identifies the actualelectromo- tiveforce driving SAID. Nor do they explain why SAID are sometimesdetached from the plasmapause, whytwo or more Magnetospheric Current Sheet SAIDcan be present at thesame time, or whySAID occur We showthat a voltagedifference sufficient for the for- mainlyin thepre-midnight sector [Spiro et al., 1979].In mation of SAID is generatedacross a magnetosphericcur- thepresent paper we propose an alternative model, which rent sheetinterfacing hot injectedplasma and the relatively extendsthe ideas presented in [Lemaireet al., 1997].We cold plasmatrough.Evidence for the presenceof hot in- computethe electric structure of a magnetosphericcurrent jected plasma is foundin simultaneousobservations of SAID sheetinterfacing hot injected plasma and the cold plasma- trough.Mapped down to the ionosphere, theelectric poten- tialprofile leads to typical SAID ion drifts. The model also go accountsfor the lifetimeof SAID andtheir predominant oc- currencein thepre-midnight sector. SAID werediscovered by Galperinet al. [1973]. They havebeen identified in ground-basedradar data [Yehet al., 1991], aswell as in satelliteobservations of the ionospheric electricfield andion drift [Smiddyet al., 1977;Spiroet al., 1979;Anderson et al., 1991]. SAIDare observed in theF- regionor higher, and even in the magnetosphere [Maynard et al., 1980]. SAID invariantlatitudes (ILAT) rangefrom 50ø to 70ø (L = 2.5 - 8): betweenthe auroral zone and theplasmapause. Thelayer is typically 0.5ø-1 øILAT wide. Figure1. Geometryof theionospheric magnetic and elec- tric fieldsBi and Ei, and the drift Vi. Westwarddrift cor- Copyright1998 by the AmericanGeophysical Union. respondsto polewardelectric field. The open-headedar- Papernumber 98GL01135. rowsindicate the upwardand downward field-aligned cur- 0094-8534/98/98GL-01135505.00 rentsclosing through the SAID.

1625 1626 DEKEYSER ET AL.: SUBAURORAL ION DRIFTS

andmagnetospheric particle flux dropouts (low density, high andthe traveling time of electromagneticdisturbances along temperatureregions) [Andersonet al., 1993;Shiokawa et a fieldline withthe Alfv6n speed (seconds to minutes),the al., 1997]. Otherevidence consists of directmeasurement of evolutionof theinterface is soslow that TD equilibriumcan SAID-likeelectric fields in themagnetosphere [Maynard et bereestablished continuously. For the sake of simplicity,we al., 1980]. takethe TD to be locallyplanar, with its normalin theradial Finitegyroradius effects cause a chargeseparation in the direction.We usea 1-D kineticequilibrium TD model--a interfacelayer. Field-aligned currents flow as a consequencesimplified version of themore general model discussed by of the thermo-electricpotential and try to neutralizethe Rothet al. [ 1996]--tocompute the electric field in themag- chargeseparation (see, e.g., [Willis,1970] for a discussionnetospheric current sheet. We adopt a right-handedreference of thisprocess for the magnetopause interface layer). The frameco-moving with the plasmatrough (x points away from chargedistribution, however, is maintainedas charges are Earth, z to the north). continuouslyreplenished from the plasma reservoirs on ei- Becauseof thelow plasmabeta in theplasmatrough, the therside of thecurrent sheet because of plasmamotion. The changein magneticfield magnitudeand orientation across structureof thetransition is not essentially modified by the the interfaceis rathersmall. However,a significantveloc- field-alignedcurrents as long as the injected plasma reser- ity shearcan exist acrossthe interface. The thermal veloc- voiris notexhausted and as long as its inwardmotion con- ity of plasmatroughions (1.5 eV protons)near the plasma- tinues.This long-lived dynamic equilibrium can therefore pauseis 20 km s-t. Theinjected plasma velocity cannot be bemodeled in a firstapproximation by a tangentialdiscon- muchlarger, or instabilitieswould develop. As SAID move tinuity(TD) equilibrium which ignores the canceling effects closerto theionospheric projection of theplasmapause in of ionosphericdischarge and constant replenishment. This theearly stages of a substorm[Yeh et al., 1991], theinter- correspondsto an unloaded electric circuit: only currents face approaches Earth. The sunward velocity of theinjected perpendiculartothe electric field flow in a (planar)TD, i.e., plasmaand the corotation velocity of theplasmatrough near no energyconversion takes place. The aboveargument is theplasmapause are anti-parallel at thedusk side, and par- corroboratedby the following order-of-magnitude calcula- allel at the dawn side. Therefore, azimuthalflow shearis tion(cf. [Rothet al., 1993]).With a plasmatroughden- largestin thepre-midnight sector. sityof 10cm -3 atL = 4, a magnetosphericfluxtube with The structureof the TD dependson the electricfield in- unitcross-section inthe equatorial plane contains 10•5 par- sidethe layer. There are two main contributions: a polariza- ticles(using an r -4 plasmasphericdensity profile). Assum- tion and a convectionelectric field. Because the gyroradii ingthermal particle transport inthe flux tube away from the of plasmatroughand injected ions and electrons are all dif- equatorand toward the ionosphere, themaximum particle ferent, the length scales œ describing thepenetration depth evacuationrate is 6 x 10TM m -2 s-• for0.75 eV plasma-of eachplasma population across the interfaceare different troughelectrons and 2 x 10TM m -2 s-• for1.5 eV protons. too. In the absenceof velocityshear, the thermo-electric A field-alignedpotential drop can enhance this rate by an or- fieldpeaks inside the layer (Figure 2a), butthere is no net derof magnitude.The density gradient at the current sheet is potentialdrop. When a pre-midnightshear flow is present, thereforesmoothed on a timescale > 103s: the time needed theconvection electric field enhances the electric field peak to emptythe cold plasma flux tube. Field-aligned currents as in Figure2b; theconvection and the thermo-electric fields canalter the structure ofthe interface only on a longertime tendto cancel in the post-midnight configuration (Figure 2c). scale.(SAID lifetime is indeed of theorder of hours.) Cold The relationbetween the electricfield insidethe TD andthe plasmaflux tube evacuation continues aslong as the current orientationofthe shear flow is closely linked to the asymme- sheetmoves inward. The inward motion will be abruptly de- tryin theconditions for which TD equilibriacan exist [De celeratedatthe plasmapause, where the particle content of a Keyserand Roth, 1997]. coldplasma flux tube sharply increases: SAID are therefore Figure3 showsthe computed structure of a magneto- notexpected topropagate very far across the plasmapause. spheric interface atL = 4 (whereB •-,490 nT) in the pre- Asthe lifetime of theinterface is longcompared to the midnightsector. The velocity shear is 10km s -• . Theplas- ionosphericphysico-chemical timescale (a fewminutes) matrough region 0close to theplasmapause (number den-

(a) Ex (b) (c) E X

..

Figure2. Structure ofthe current sheet: (a)thermo-electric field(no shear flow); (b)convection (thinline) and total (thick line)electric field for pre-midnight shearflow; (c) the same for post-midnight shearflow. DE KEYSER ET AL.' SUBAURORAL ION DRIFTS 1627

impliesa thicknessof • 1000 km. Given the gyroradius p+ of the hot protonsof 30 km, we thereforefix the tran- sitionlength œ+ - 50p+ for bothproton populations; we takeE- = œ+/5 for theelectrons (œ+/œ- shouldnot be too large to avoid excessivelystrong, unstable electric field if \\ configurations[Roth et al., 1996]). As the referenceframe co-moveswith the plasmatrough,the electricfield is zero there;the convectionelectric field is positivein the injected 10'• plasmaregion. A strongpeak about 1000 km thick is formed 10a insidethe layer (cf. Figure3c). We havesimilarly computed v + 10• the structureof a transitionwith a slightly smallershear of

10• 6 kms -1 in theopposite sense to mimicthe post-midnight situration; we found a smooth transition of the electric field 10ø between the convection electric field values on either side,

c as in the qualitativepicture of Figure2c.

E•5 IonosphericSignature The electricpotential in the magnetosphericcurrent sheet canbe mapped down into the ionosphere, at leastif themag- I , 20O00 25O00 3O00 netosphericconductivity is infiniteand/or if thefield-aligned x (km) currentsare small. As the dipolarmagnetic field linescon- 150 vergetoward the ionosphere,the electricfield acrossthe magnetosphericinterface at L - 4 is amplifiedby a factor 100 2Lv/L - 1 m 14. The ionosphericelectric field peaknow

E 50 exceeds100 mV m-1 (Figure3d); the ion drift (Figure 3e) v ._ of morethan 2 km s-1 is westward,concentrated in a region o 1ø wide, locatednear 60 ø ILAT--very typicalof SAID.

-50 Discussion and Conclusions

o The purposeof this paperwas to identifythe proper- tiesof a magnetosphericcurrent sheet that maps down into the ionosphereas an SAID. We performedsimulations of a currentsheet interfacing the plasmatroughwith hot in-

i jectedplasma. Due to plasmasphericcorotation the az- 56 58 60 62 ILAT (degrees) imuthalshear velocity at the interfaceis largestin the pre- midnightsector. Additionally, analysis of the internalstruc- Figure 3. Computedstructure of a magnetosphericTD ture of the interfaceshows that particularlyintense electric at L = 4 in the pre-midnightsector: (a) protondensities (dashedline: plasmatrough,solid line: injectedplasma); (b) fieldsare generatedfor the pre-midnightshear flow sense. meanproton temperature; (c) electricfield (comparewith One of the meritsof the presentSAID modelis the realiza- Figure2b); (d) ionosphericelectric field; (e) drift speed. tion thatthe shearflow controlsthe sizeand sense of the po- tential variationsacross the currentsheet. This can explain the predominantoccurrence of SAID in the pre-midnight sity 10 cm-3) is takento consistof 1.5 eV protonsand sectorvery well. The simulationsreproduce the morphol- 0.75 eV electrons[Comfort, 1996]. For the injectedplasma ogy of the SAID drift speedprofile (westward orientation, we adoptplasmasheet temperatures (10 keV protons,! keV intensityof thedrift speedpeak, thickness, and location). It electrons)and a numberdensity ! cm-3. (Numericalex- is remarkablethat both SAID thicknessand peak intensity perimentsconfirm that the results are not very sensitive to matcha singlevalue of 1;+/p+. thesechoices, as long as the plasmatroughdensity outnum- The modelused in this paperignores several features of bersthe injected particles, while the latter are much hotter). the real physicalsystem: the geometryis simplified,cur- The order-of-magnitudecalculation enables us to estimate vatureeffects are not accountedfor, the neutralizingeffect the layerthickness: with the L = 4 flux tubeevacuation of field-alignedcurrents and the associated polarization cur- rate< 1012m -2 s-1, a particleinflux of 1018m -1 s-1 rentsis ignored.Also, we havenot incorporated the interac- (inwardvelocity (m 10 kms -1) timesdensity (10 7 m-3) tion betweenthe shieldingelectric field in the inner mag- timesthe projected surface of theflux tube in theinward di- netosphereand the thermoelectricand convectionelectric rection(107 m 2 perunit length in theazimuthal direction)), fields inside the injectedplasma interface. 1628 DE KEYSER ET AL.: SUBAURORAL ION DRIFTS

The SAID model discussedhere has an advantageover observationof large sub-auroralelectric fields, Geophys.Res. earliermodels that identify SAID with the Alfv6n layer, Lett., 7, 881, 1980. OberD.M., J.L. Horwitz, andD.L. Gallagher,Formation of density sinceit canalso explain SAID thatare detached from the troughsembedded in the outer plasmasphereby subauroralion plasmapause.It is alsoconsistent with the observation of drift events,J. Geophys.Res., 102, 14,595-14,602, 1997. energeticparticle precipitation from the auroral region down Roth, M., D.S. Evans, and Lemaire, J., Theoretical structure of a to the SAID latitude [Andersonet al., 1993]: the precipita- magnetosphericplasma boundary: application to the formation tiontraces the region where hot injected plasma is present. of discreteauroral arcs, J. Geophys.Res., 98, 411, 1993. Roth, M., J. De Keyserand M.M. Kuznetsova,Vlasov theory of the equilibriumstructure of tangentialdiscontinuities in space Acknowledgments. Part of the TD modeldevelopment was plasmas,Space Sci. Rev.,76, 251-317, 1996. supportedby a PRODEXcontract with ESA in theframework of Shiokawa, K., C.-I. Meng, G.D. Reeves, EJ. Rich, and K. Yu- theUlysses Interdisciplinary Study on Directional Discontinuities. moto, A multieventstudy of broadbandelectrons observed by We acknowledgethe supportof the BelgianFederal Services for the DMSP satellites and their relation to red aurora observed Scientific,Technological and Cultural Affairs. at midlatitudestations, J. Geophys.Res., 102, 14,237-14,253, 1997. References Smiddy,M., M.C. Kelley,W. Burke, F. Rich, R. Sagalyn,B. Shu- man, R. Hays, and S. Lai, Intense polewarddirected electric Anderson,EC., W.B. Hanson,and R.A. Heelis, The ionospheric fieldsnear the ionosphericprojection of the plasmapause,Geo- signaturesof rapidsubauroral ion drifts,J. Geophys.Res., 96, phys.Res. Lett., 4, 543, 1977. 5785, 1991. Spiro,R.W., R.H. Heelis,and W.B. Hanson,Rapid sub-auroralion Anderson, P.C., W.B. Hanson, R.A. Heelis, J.D. Craven, D.N. driftsobserved by AtmosphericExplorer C., Geophys.Res. Lett., Baker and L.A. Frank, A proposedproduction model of rapid 6, 657, 1979. subauroralion drift andtheir relationship to substormevolution, Spiro,R.W., M. Harel, R.A. Wolf, andP.H. Reiff, Quantitativesim- J. Geophys.Res., 98, 6069-6078, 1993. ulationof a magnetosphericsubstorm 3. Plasmasphericelectric Banksand Yasuhara,Electric fieldsand conductivityin the night- fieldsand evolutionof the plasmapause,J. Geophys.Res., 86, time E-region- A new magnetosphere-ionosphere-atmosphere2261-2272, 1981. couplingeffect, Geophys. Res. Lett., 5, 1047-1050, 1978. Willis, D.M., The electrostaticfield at the magnetopause,Planet. Comfort,R.H., Thermal structureof the plasmasphere,Adv. Space SpaceSci., 18, 749-769, 1970. Res., 17, 175-184, 1996. Yeh, H.-C., J.C. Foster, EJ. Rich, and W. Swider, Storm time elec- De Keyser,J. and M. Roth, Equilibriumconditions for the tangen- tric field penetrationobserved at mid-latitude,J. Geophys.Res., tial discontinuitymagnetopause, J. Geophys.Res., 102, 9513- 96, 5707-5721, 1991. 9530, 1997. Galperin,U.I., Y.N. Ponomarov,and A.G. Zosinova,Direct mea- J. De Keyset, M. Roth and J. Lemaire, Belgian Institute for surementsof ion drift velocityin the upperatmosphere during a magneticstorm, Kosm. Issled., 11,273, 1973. SpaceAeronomy, Ringlaan 3, B-1180 Brussels,Belgium. (e-mail: Johan.DeKeyser@ oma.be) Lemaire, J., M. Roth, and J. De Keyser,High altitudeelectrostatic fieldsdriving subauroral ion drifts,in MRAT Proceedings,T Lui (ed.), COSPAR Colloquiaseries, 1997. In press. (ReceivedDecember 1, 1997; revisedMarch 20, 1998; Maynard, N.C., TL. Aggson,and J.P. Heppner,Magnetospheric acceptedMarch 25, 1998.)