Comment on “B_y fluctuations in the magnetosheath and azimuthal flow velocity transients in the dayside ionosphere” by Newell and Sibeck
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Lockwood, M., Cowley, S. W. H. and Smith, M. F. (1994) Comment on “B_y fluctuations in the magnetosheath and azimuthal flow velocity transients in the dayside ionosphere” by Newell and Sibeck. Geophysical Research Letters, 21 (17). pp. 18191820. ISSN 00948276 doi: https://doi.org/10.1029/94GL01360 Available at http://centaur.reading.ac.uk/38813/
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CentAUR Central Archive at the University of Reading Reading’s research outputs online GEOPHYSICAL RESEARCH LETTERS, VOL. 21, NO. 17, PAGES 1819-1820, AUGUST 15, 1994
Commenton "Byfluctuations in the magnetosheathand azimuthal flow velocitytransients in the daysideionosphere" by Newell and Sibeck
M. Lockwood RutherfordAppleton Laboratory, Chilton, Oxfordshire,U.K.
S.W.H. Cowley BlackettLaboratory, Imperial College, London,U.K.
M.F. Smith Laboratoryfor ExtraterrestrialPhysics, Goddard SFC, Greenbelt,Maryland, U.S.A.
Newelland Sibeck[ 1993] (hereafterN&S) list someobjec- ionosphereinduces flow in the rest of the polar cap" and tions to our interpretationof daysideauroral transients and proceedto demonstratethat the motionof suchtubes through associatedazimuthal flow burstsin termsof pulsedrecom•ec- an otherwisestagnant ionosphere induces flow only in the tion [e.g.Lockwood et al., 1989; 1993a].They presentwhat immediatevicinity of the tube. This discussioncompletely they terman "apparentlyoverlooked" alternative explanation misrepresentsthe C&L model. In fact, C&L describein in terms of steady reconnectionand fluctuationsin the detail how (eg.) a pulse of daysidereconnection alters the magnitudeof theBy component of themagnetosheath field. equilibrium configurationof both the region of open and The objectionsof N&S can all be answeredby referenceto closed flux, generatinglarge scale flows in the magneto- our previouspublications and their alternativeexplanation sphere-ionospheresystem. The "idea" quoted above is was only "overlooked"in so far as it fails to explain the N&S's, is not part of the C&L model and is incorrect. observations.Here we discussjust someof the reasonswhy 3. N&S dismissa large body of evidencethat the solar the objectionsof N&S areinvalid, and then give reasons why wind electricfield doesnot simplymap into the ionosphere, theevents are not simply due to magnetosheathIBylchanges. on timescalesshorter than severalsubstorm cycles, with the phrase"this idea is said to replacethe inapplicableidea of Objectionsof N&S to Bursty Reconnection mapping the solar wind electric field to the ionosphere". They later admit to the possibilityof "dampingof high- 1. N&S state"If the sheathparameters have not changed frequencychanges due to self inductancelimitations". This sincethe openfield linesjust polewardof the NMR merged is true but, without an attemptto define "high frequency", then thoselines too were exposedto preciselythe same meaningless.In this respect,N&S fail to point out that the forcesand shouldmove in the sameway". This we agree inductive circuit analogy they cite [Sanchezet al., 1991] with, given that N&S have renamedwhat we termed the predictsthat the relevanttime constantfor ionosphericflow "newly-openedflux" as a "newly-mergedregion" (NMR). excitation is about 20 min. (Similar values have also been However,despite a correctstatement in their abstract,N&S derived from line-tying arguments,from the EISCAT- incorrectlycontinue their argumentin the main text: "There AMPTE data on ionosphericflow responsesand (by C&L) should be no relative motion observed between the new from the time for newly-openedflux to evolve into the tail mergingand the openlines just poleward.... unless either the lobe). Sanchezet al. stressthat it is even inappropriateto IMF or the magnetosheathflow has changedin the interval map the electricfield over 3-4 hour time scales. betweenthe two mergingepochs". This secondstatement is 4. The argumentsused by N&S aboutpersistence of the in errorbecause it ignoresthe fact that the motion of an open cuspare invalid. N&S state"Observations thus lead us back field line depends on the time elapsed since it was to the conceptthat the cusp is always present".We agree, reconnected.Initially, a newly-openedfield line movesaway with the caveatthat "always"is somethingmore than 80% of fromthe X-line under magnetic "tension". If the IMF IBylis the time [Smithet al., 1992]. However,N&S continue"Day- largethis will yield a strongazimuthal (east/west) flow in the side mergingmust then occur constantly".This is not so. ionosphere.But as the field line evolvesit straightensand Smithet al. pointout that precipitation termed "cusp" persists hencethe tensionforce decays while the sheathflow speed on a newly-openedfield line for over 10 min after it is it experiencesincreases. Hence N&S are not correct when reconnected.Hence for the cuspto be absentrequires that no they statethat the newly-openedflux will move with the reconnectiontake placefor a periodof greaterthan this. The samevelocity as the flux that was openedat someearlier persistenceof a cuspin the observationsN&S cite doesnot time, such that there is no relative motion between them. provethe reconnectionis evencontinuous, let aloneconstant. 2. N&S discussthe Cowleyand Lockwood[1992] (C&L) 5. Despiteciting the Lockwoodet al. [1993a] paperwhich model of ionosphericflow excitation.They state"The idea discusses the differences between observations of 557.7nm- is thatdragging tubes of recoxmectedfield linesthrough the dominant and the associated 630nm-dominant transients, N&S confusethe two. For example,multiple brighteningsof Copyright1994 by the AmericanGeophysical Union. a 557.7nm transient are not a problem for the transient reconnectiontheory - they simply representchanges in the Paper number 94GL01360 stabilityof the requiredupward field alignedcurrent: they are 0094-8534/94/94GL-01360503.00 not seen in the more extensive 630 m-dominant transients.
1819 1820 LOCKWOOD ET AL: COMMENT ON IONOSPHERIC TRANSIENTS
Rather,the 630 nm-dominanttransient continuously fades as [Lockwoodet al., 1993b].In addition,Elphic et al. [1990] the newly-opened field lines evolve and fewer sheath reportdayside auroral transients and azimuthalflow enhance- particlesgain accessto the ionosphere.Our modeldescribes mentsseen by EISCAT when the sheathwas directly the 630 nm transientfading at the sunwardedge of the polar observedby ISEE-2at thedayside magnetopause: IBylwas cap whenthe newly-openedflux hasbecome appended to the stable,even during the FTE events(defined by bipolarB N tail lobe: it doesnot penetratefurther into the polar cap [e.g. with a rise in B) that Elphic et al. associatewith the iono- Lockwoodet al., 1993a, figure 1]. N&S state that this is spherictransients (see BM variation in theirFigure 2). These contraryto observation.The 630nm-dominanttransients we eventscannot be interpretedas being due to changesin the have studiedall faded at the sunwardedge of the polar cap magnetosheathBy. [Sandholt et al., 1992]. In summary,the objectionsto ourmodel raised by N&S 6. N&S revive the "rigid moving-cloud"concept, despite areincorrect. The observationsthey cite which fail to detect a full discussionof why that couldno longerbe considered thatthe persistent cusp is madeup of a seriesof poleward- applicablein the paperby Smithet al. [1992] citedby N&S. movingtransients do not provethat this is not the case.The Given this discussionby Smith et al., N&S need to justify alternativeexplanation offered by N&S cannotexplain the their assumptionthat the region of newly-openedflux (for observationsof daysideauroral transient/flow burst events enhancedBy) is ellipticaland remains elliptical, despite being whichmove repetitively in oneazimuthal direction, nor those draggedpast other field lines. The changeof shapeof the eventswhich move poleward with little azimuthalmotion and newly-openedflux as the systemreturns to an equilibrium constantplasma flow direction.Because all our observations configurationis an integralpan of our theory [Smith et al., fall into one of thesetwo classes,the model of N&S fails to 1992] and is a vital part of our predictionof the observed explainany of the eventswe have observed. cusp ion energyjumps. Note that Lockwoodet al. [1993a] use an ellipse only for an order of magnitudeestimate of References event area:N&S's conclusionthat only local flow is excited is wholly dependenton their assumptionsof a rigid elliptical Cowley, S.W.H. and M. Lockwood,Excitation and decayof tubedown which the motional solar wind electric field maps. solar-winddriven flows in the magnetosphere-ionosphere 7. N&S argue that sunward-movingevents away from system,Annales Geophys., 10, 103, 1992. nooncatmot map to the noonmagnetosphere where FTEs are Cowley, S.W.H., et al., Dependence of convective observed.In fact, an explanationof suchmapping was given flows and particleprecipitation in the high-latitudedayside by Cowleyet al. [1991]. Likewise,N&S arguethat the large ionosphereon theX andY componentsof the interplanetary longitudinalextents of somebreakup events cannot map to magneticfield, J. Geophys.Res., 96, 5557, 1991. this pan of the magnetopause.Explanation of the variability Crooker N.U., et al., Opening the cusp, J. Geophys. Res., 96, 34907-3503, 1991. of this mappingfactor was given by Crookeret al. [1991]. Elphic, R.C. et al., Signatures of flux transfer That N&S shouldraise such objectionsis curiousbecause, eventsat the daysidemagnetopause and in the ionosphere: given that FTEs can occurwherever reconnection can occur, combinedISEE, EISCAT andoptical observations, Geophys. they would also apply to the N&S interpretation. Res. Lett.,17, 2241, 1990. Lockwood,M. et al., Interplanetarymagnetic field controlof daysideauroral activity and the transferof momentumacross The interpretation of N&S the daysidemagnetopause, Planet. Space Sci, 37, 1347, 1989. Lockwood, M., H.C. Carlson and RE. Sandholt, There is a simple reasonwhy we did not interpretthe The implicationsof the altitudeof transient630 nm dayside events as steadyreconnection with varying magnetosheath auroralemissions, J. Geophys.Res., 98, 15571 1993a IByl.This mechanism would produce changes in theazimuthal Lockwood M. et al., Ionospheric signatures of flow componentin the ionosphere;however, newly-opened pulsedmagnetic reconnection at the Earth'smagnetopause, flux tubes,down which sheathelectrons stream and generate Nature, 361 (6411), 424, 1993b. 630 nm aurora,would be producedat a constantrate. Hence Newell, P.T. and D.G. Sibeck, By fluctuationsin the the regionof 630 nm aurorawould continuouslybe present magnetosheathand azimuthalflow velocity transientsin the and would "wag" back and forth in the east-westdirection daysideionosphere, Geophys. Res. Lett., 20, 1719, 1993. withthe flow direction changes due to theIByl changes. For Sandholt, RE. et al., Periodic auroral events at the example,increasing By (>0) wouldcause westward migration midday polar cap boundary:implications for solar wind- of the plume of 630 nm aurorain the northernhemisphere magnetospherecoupling, Geophys. Res. Lett., 19, 1223, 1992. (as the flow becomesmore westward),but the subsequent Smith, M.F. et al., The statistical cusp: a simple flux transferevent model Planet. SpaceSci., 40, 1251, 1992. decreasein Bywould cause the plumeto returneastward. This is not observed. What is seen is a series of events which S.W.H. Cowley, Blackett Laboratory,Imperial College, move in one direction acrossthe field of view [Lockwoodet London,SW7 2BZ, UK. (e-mail: SPAN 19773::cowley) al., 1993a]. This point is particularlyclear for thosesmall M. Lockwood, RutherfordAppleton Laboratory,Chilton, eventsimaged in full by one 630 nm camera[e.g. Sandholt Didcot, OXll 0QX, UK. (e-mail: Internet. et al., 1992].In suchcases, the eventsare seen to repetitively mike•eiscat.ag.rl.ac.uk) form, propagateazimuthally in only one directionand fade. M. F. Smith, Laboratoryfor ExtraterrestrialPhysics, NASA An east-west"wagging" of a cuspplume is not observed. GoddardSpace Flight Center, Greenbelt,MD 20771. (e-mail: Neithercan the theory of N&S explainthe many observa- [email protected]) tions of daysidetransients which move poleward,with no azimuthalmotion, nor that these are unaccompaniedby (ReceivedSeptember 21, 1993; revised February22, 1994; changesin the flow direction,as is often seenby EISCAT acceptedMay 9, 1994.)