17, NO. 12, PAGES 2305-2308, DECEMBER 1990. ION COMPOSITION ... Lockheed Palo Alto Research Laboratory, Palo Alto, California. Abstract. Impulsive ...
GEOPHYSICAL RESEARCH LETTERS, VOL. 17, NO. 12, PAGES 2305-2308,DECEMBER 1990
ION COMPOSITION MEASU'RE••S
WITHIN MAGNETOSPHERIC FLUX TRANSFER EVENTS
D. M. K!umpar,S. A. Fuselier,andE.G. Shelley Lockheed PaloAltoResearch Laboratory, PaloAlto,California Abstract.Impulsivereconnection nearthesubsolar magne- plingalongthemagnetopause anditsadjacent plasmalayers, topause is a fundamental process of interaction betweenthe couldeffecta signaturethathasvirtuallythesamecharacterismagnetosphere andthesolarwindflow. Flux TransferEvents ticsasidentifiedwith FTEs. Onedistinguishing featurecould (FTEs)appearto be a manifestation of thisprocess. It is there- be theplasmacomposition.If FTEsarethefundamental interforeimportantto fully characterizethe FTE soasto betterun- connection of interplanetary !MF withtheterrestrial magnetic derstand the physicsof the interactionprocess.We usemag- fieldoneexpects to observea uniqueplasmacomposition conneticfield signatures fromtheAMPTE/CCE spacecraft to iden- sistingof ionsof bothsolarwindandterrestrial originandion tify two magnetospheric FTEs. Electrondistributionsmea- flowsthatreflecttheintermixing of thesetwocompositionally suredin the plane of the magnetopauseinside these FTEs differentplasmaregimes.Indeed,an ionospheric ion compocomplement previouslyreportedelectronmeasurements. We sitionsignature interpreted asionupwellingin thewakeof an report,for thefirsttime,ion composition measurements in the FTE has been reportedand discussedby Lockwoodet al. energyrange~0 to 20 keV/e within an FTE. We find that the [1988]. A fundamental question therefore is:is therea unique ion distributions,the ion composition,and the flow velocities areuniqueto the FIE and unlikeeitherthe adjacentmagnetosphere,the nearbyboundarylayer, or the nearbymagnetosheath.The H+, He+, andHe2+ distributionfunctionsin the
signature of an FTE that distinguishesit from a brief crossing/pressure pulseevent? We usemagneticfield andplasmasignatures measured by theAMFrE/CCE spacecraft to establish theobserved eventsas FTEs. The electronobservations FTEshavereversedtemperature anisot[opies andthe relative traditionalmagnetospheric He2+abundance is depressed withrespect to eitherthemag- with detailed2D distributionfunctionsin the planeapproxineosheath or the low latitudeboundarylayer. matelytangentialto the magnetopause complement andextend theresultspublishedby e.g.Thomsenet al. [1987],Farmgiaet Introduction al. [1988],Rijnbeeket al. [1988]. Havingidentifiedtheevents asmagnetospheric FTEs we examinethe ion compositionand the fn'stexaminationof ion Spacecrafttraversing the outer dayside magnetosphere ion flows. This paperrepresents commonly experience briefencounters withmagnetosheath-like compositionand speciesspecificflows in the energy•ge plasma. Suchencountersmay be characterizedas episodic from -43 to 20 keV/e within Flux Transfer Events. The AMPTE/CCE dataoffera new perspective to FFE obburstsof magnetosheath-like plasma,magneticfield perturbaorbit (inclination tions,andreductionof hot magnetospheric particlefluxeslast- servations.The CCE has a near-equatorial ing tensof secondsto a few minutes. Similarly, spacecraft 4.5ø), andan apogeejust inside9 Re. The satellitecarriesan outsidethemagnetopause currentsheetoftenobservebrief de- ion mass spectrometer(the Hot Plasma Composition HPCE)thatmeasures ionsfrom0 (spacecraft pocreasesof magnetosheath plasmaaccompanied by magnetic Experiment, fieldrotationsandstreamingmagnetospheric-like particledis- tentia!)to 20 keV [Shelleyet al., 1985]. For •e eventin this tributions. Such transientevents,whether inside or outsidethe paper,the HPCE wasoperatedin a modeto detectonlymajor ionspecies (H+,He+, He2+,andO+) sequentially every38ms currentlayer,may be dueto a numberof causesincludingraa spacecraft spin. dial motion of the Chapman-Ferraro currentsheet,surface whiletheenergywasfixedfor approx'anate!y Thus,a full 15pointvelocity•sffibufionforthefourmajorion waveson the currentsheet,local small-scaleboundaryirregularities,or Flux Transfer Events (FTEs).
species is obtained in lessthan2 m'mutes. h:cluded withthis
The latercategorywerefn'strevealedby magneticfield observations in thevicinityof themagnetopause havinga charac-
ion instrumentare8 fixed-energye!ec•n spectrometers cover-
ingtheenergyrange•m 50 eV to 25 keV. Both'msmanents viewradiallyoutwardwithrespect to thespinaxis,which,for approximately 80below increases andthendecreases (or vice versa)whilethe magnetic theeventsin thispaper,waspointing linein thedawn fieldstrength simultaneously increases. Thesemagnetic events theeclipticplaneand23ø fromtheEarth-Sun wereinterpreted by RussellandE!phic[1978]asthesignature direction.Thusduringonesatellitespinpefi•od(6-see) detectors sweep through 360øin a plane.that liesapproximately oftransient interconnection of magnetic flux between the!MF tothemagnetopause nearthesubso!•point.For magandthe geomagnetic field andtermedFlux TransferEvents tangent netospheric FTEs•sentiallyallpimhanglesare•rved every (FTE). alsoincludes a Whetheror not FTEsrepresent a fundamental interaction half spin. TheCCE ins•ment co..mplement whichmakes8 vectormeasurements of the processas envisionedby Russelland Elphichasbeenques- magnetometer fieldeverysecond [Potemxa etal.,1985]:. tionedmanytimes.RecentlySibeck[ 1990]hasarguedthatat magnetic
teristic signature in whichthebound• normalcomponent first
leastsomepublished FTEshavean alternative explanation.
O
Thealtemative,envisioned asa p•ssurepulse(or wave)ripCopyright 199,0 by the American Geophysical Union. Paper number 90GL02432 0094-8276 /90 / 90GL- 02432 $03. O0
On October! 8, 19'84theCCE .first
opause atapproximately !3• I•,, a• sho.wn in Figs. ! bya
drarnatic change in theelectron flux andby s.• .the magnetic field'from no•rthward .. t.o 2305
