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Sources and Losses of Ring Current Ions: an Update Ade. Space Res. Vol.9. No. 12. pp. (12)171—112)152. 1989 0273—1177i89 50.00 + .50 Printed in Great Britain. All rights reser~ed. Copyright © 1989 COSPAR SOURCES AND LOSSES OF RING CURRENT IONS: AN UPDATE J. U. Kozyra Space Physics Research Laboratory, Departmentof Atmospheric, Oceanic and Space Sciences, The University of Michigan, Ann Arbor, Ml 48109—2143, U.S.A. INTRODUCTION Majorprogress has been made in thelast threeanda halfyears.inourunderstandingof the roleof ionospheric plasma as asource for theplasma sheet and ring current. Observations by the Dynamics Explorer 1 satelliteover thepolar ionosphere and concurrent modelling efforts havetraced thepath of heated ionospheric ions from a localized source associatedwith the polar cusp along trajectories that energize theions anddeposit them in thenear-earth plasma sheet AMPTE ion composition measurements in the ring current havebegunto clarify the relativecontributions of theionospheric and solarwind ions in the plasma sheet and ring current andcharacterize differences inthe energization processes whicheach population undergoes. The role of charge exchange as amajorloss process for the ring current has beenconfirmed by all sky observationsonboard the ISEE Isatellite of energeticneutral atoms resulting from this interaction and associatedmodeling efforts. The oxygen ion component of the ring current has beenfound to be asource ofheating via Coulomb collisions for the thermal plasmas in theouter plasmaspherewith associatedloss of energy from thering current. The roleof wave-particle interactions in ring current decay, however, is still unclear. In thepresent paperno attempt is made to comprehensively review thelargebody of literature describing ouraccumulated knowledge of ring current ion sources andlosses. A number of excellent reviews haveappeared in thelast few years discussing thedevelopment of the ring current and its dynamics /1,2,3,4/ The presentreview will concentrate on new developments in our understandingof sources andlosses of ring current ions that havetaken placeduring theperiod 1985 - 1988, overlappingslightly some ofthelatter reviewscited above with reference to earlierstudies as required. The populations comprising theearth’s ring current are determined by the strength andcomposition of ion sources, transportprocesses, and loss mechanisms. The intent of this review is to focus on processes which supplyions to the ring current and plasma sheet and the decayof the ring current. The transportof ions from theplasma sheet earthward and the subsequent formation of the ring current will not be discussed. RING CURRENT SOURCES Ionospheric Sources of Plasma Sheet Ions It is generally considered that thering current formation is atwo-stepprocess (cf. Williams /4,5/ andreferencestherein). A series of injection eventsmove plasma inward from the plasma sheet to theouterradiation belts (25 RE). In response to enhanced storm-time, dawn-dusk electric fields, this outer trappedpopulation is convected inward (—2.5 -5 RE) to form the ring current. Therefore processes whichpopulate either the plasma sheet or these near earthregions with plasma are considered, for the purposesof this review, sources ofthe ring current plasma. Plasma in these regions is likely amixture of solar wind and ionospheric populations. Observations fromthe Dynamics Explorer 1 satellite of upflowing ionospheric ions (H~,He, O~’N~’and~ alongwith theoretical models investigating the transportof these ions into the polar magnetosphere, haveestablished thedayside cusp region as acopious and persistent source ofupflowing heated ionosphericplasma f7,8,9,10.ll,l2,13.l4,15,l6,17.l8,19/. The characteristics of this ionosphericsource axe listed belowso that these may later be compared with measurements of the ion composition of the plasma sheet and ring current and the changes in this composition with solarcycle and magneticactivity. These ion upflowshave beenlabelled ‘upwelling ion events’because all ion speciesparticipating in theevent display theeffects of equal ion heating to parallel andperpendicular temperatures of about 2 and 10ev, respectively i9/. Upwelling ion eventshave been statistically correlated with field aligned currents/4/. For oneparticular event studied in derail, the ionospheric heating was coincident with magnetic field signaturesof strong field-alignedcurrentson the boundaryof thedayside polar cap /13/. Also present on these field lines was astrong, localized convection channel orjet. Observations and theoretical models point Out the importance ofion heating (which aids the heavy ions in overcoming the gravitational barrier) in producing heavy ion outflows in the polar magnetosphere. Studies of transient polar wind outflow in thepresence of low altitude ion heating i20/ indicate that significant heavy ion outflows will result 120/. Lockwood et a!. /21/have pointed out that ion outflows from this region couldbe enhancedby theionosphericeffectsof flux transfer events (FTE) on thedayside magnetopause whichmap to the polar cap boundary. Theroughlyequal hearingof thevarious ion species implies that field aligned flow velocities will be inversely proportional to thesquare root of the ion mass. This mass dependent velocity, in combination with theExB convection andthe localized nature of the upwelling ion source, results in adispersionof these ions by mass andvelocityas they convectacross the polar cap /12/. This effect has beentermed the “geomagneticmassspectrometer” and results in the lowest-energyand highest (12)171 (12)172 J. U. Kozyra mass ions extending turthest toward tile mgtltside trom thecleft ionosphere. This upwelling ion source regionhas also been termedthedeft ion fountain/221 because as convection becomes antisunward andstrong, ions are dispersedfurther onto the nightside, analogous to a fountainblowing in the wind. 0~outflows increase both with increasingsolar radio flux at 10.7cm (FlO.7) and with increasingmagneticactivity; H~outflows exhibit no markedvariation with solarcycle anda muchsmaller variation with magnetic activity than that shown by the0~outflows 123/. Figure 1 is a schematic illustrationof the ion ouflows from the polar cleftandpolar cap regions taken from Horwitz 124/. The question of how these ionosphericoutflows from the cleftion fountain reach theplasma sheet was addressed by Qadis125 / using akinetic, guiding-centertrajectory-based code with ion motion computed only near the noon-midnight meridional plane. Theions areaccelerated alongtrajectoriesdue to the centrifugalforce arising from theion convectionvelocity andtheenergy supplied by thecomponent of thecurvaturedrift of the ion along theconvection electricfield. Figure 2a displays ion guiding center trajectoriesprojected on the noon-midnight plane from Cladis 125/for fourdifferent conditions of themagnetic andelectric field. 0+ ions can reach thecenterplane of theplasma sheet even at times of moderate convection.Total energyof 0+ ions along these trajectoriesare shown in Figure 2b as afunctionof time. During magnetically disturbed conditions the O~’trajectories intersect the centerplane ofthemagnetosphere more earthward than during quiet times. The 0+ ions can be energized to typical plasma sheet energies alongthese trajectories with travel times betweenthe ionosphere and thecenterplane of 1-2 hours. Figure 3a from Cladis /25/illustrates thedependence of the ion motion on its initial locationin thepolar ionosphere andFigure 3b demonstrates this dependence on initial ion velocity. Ions gain higherenergiesandcross the centerplane atlarger radialdistances as their initial positions approachthecusp field lines. A similartrend results from increasing the ion parallelvelocities. Thenet result is an increase in heavy ion content of theplasma sheet with decreasing radial distance. The combination ofthesatellite observations detailed above andthetrajectory calculationsof Cladis /25/ indicates alargeproportionof the ionospheric ions observed in theplasma sheet during magnetically disturbed periods originate in thecleft ion fountain. In addition aadis /25/has shown that these ions reach appropriateenergiesprior to injection in theplasma sheet and require no additional energization. The origin of theH+ injected from this source region, however, is still ambiguous, since theH+ population in thedayside cusp region at theenergiesand altitudes of interest, is likely a mixtureof solarwind andionospheric ions. On appropriate trajectories Lockwood etal. fl/have suggested that heavy ion outflows will act as a source for thenighrside aurora! acceleration region. Figure 4 illustratesschematically how this might occur. N, MEASUREMENT OF POLAR CAP MASSGEOMAGNETIc DENSITYELECTRON SPECTROMETER R PROFILES H.+ PROPOSED NIGHT, ACCELERATING ~ \\~( ~, O+POLAR WIN7 4 HIGH—ALTITUDESUPERSONIC ~ “NCLEFT~I~.V/ION ~ /~‘~~ PARA8OLICN. O~.FLOWN OF LIGHT ION IN4. H.1 FOUNT’~IN CLEFT LOW—ENERGY POLAR WINO IN ~ ~ HEAVY IONS ~ Fig. I. Schematic illustration of ion outflows in thepolar ionosphere taken from 124/. ioll _~5 IS 0 -S -10 ;S -20 10 2.000 0,000 8.000 8,000 10.000 X.1R 0) TIME (sJ Fig. 2. (a) Guiding-center trajectories projected on theX-Zplane of ions flowing out of the polar cleft region during different levels of magnetic disturbance. In the simulation, the ions were started at 1.2 RE, e = 22°,p = 0°with V = 12 km/s and Cr = 30°.Disturbed conditions in curves 1-4, respectively, arerepresented by parameters (Ei, Kp) of (23.3 mV/rn, 2-), (40mV/rn, 2+),(56.7 mV/rn, 3-to3+),and (80 mV/rn, >3+). Arrival times in stepsof 1000 seconds are denoted
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