Feb 1, 1995 - Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland .... plane. Identical SSJ/4 instrumental packages were installed.
JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 100, NO. A2, PAGES 1837-1846, FEBRUARY
1, 1995
Interplanetary magnetic field control of mantle precipitation and associated field-aligned currents Dingan Xu and Margaret G. Kive!son Instituteof Geophysicsand PlanetaryPhysicsand Departmentof Earth and SpaceScience,University of California, Los Angeles
Ray J. Walker Instituteof Geophysicsand PlanetaryPhysics,Universityof California,Los Angeles
Patrick T. Newell and C.-I.
Meng
Applied PhysicsLaboratory,JohnsHopkins University, Laurel, Maryland
Abstract. Dayside reconnection,which is particularly effective for a southward interplanetarymagnetic field (IMF), allows magnetosheathparticles to enter the magnetosphere where they form the plasmamantle. The motionsof the reconnectedflux
tubesproduceconvectiveflowsin the ionosphere.It is knownthat the convectionpatterns in the polarcap are skewedto the dawnsidefor a positiveIMF By (or dusksidefor a negativeIMF By) in the northernpolarcap. Correspondingly, one wouldexpectto find asymmetricdistributionsof mantle particleprecipitation,but previousresultshave been unclear.In this paperthe correlationbetweenBy and the distributionof mantleparticle precipitationis studiedfor steadyIMF conditionswith southwardIMF. Ion and electron data from the DMSP F6 and F7 satellitesare usedto identify the mantleregionand IMP 8 is usedas a solarwind monitorto characterizethe IMF. We studythe local time extension of mantle precipitationin the prenoonand postnoonregions. We find that, in accordance
with theoreticalexpectations for a positive(negative)IMF By, mantleparticleprecipitation mainly appearsin the prenoonregion of the northern(southern)hemisphere.The mantle particle precipitationcan extend to as early as 0600 magneticlocal time (MLT) in the prenoonregionbut extendsover a smallerlocal time regionin the postnoonsector(we did not find mantle plasmabeyond 1600 MLT in our data set althoughcoverageis scantin this area). Magnetometerdatafrom F7 are usedto determinewhetherpart of the region 1 currentflows on open field lines. We find that at times part of the region 1 sensecurrent extendsinto the regionof mantleparticleprecipitation,and is thereforeon open field lines. In other cases,region 1 currentsare absenton open field lines. Most of the observed featurescan be readily interpretedin terms of the open magnetosphere model.
Introduction
After Heikkilaand Winningham [1971], Winningham and Heikkila [1974] and Winninghamet al. [1975] reported on polar region particle precipitation,low-altitudepolar region satelliteobservations of precipitationwere accepted asevidencefor an openmagnetosphere. Becausethe various domainsof the magnetosphere map to the ionosphere,the precipitatingplasmameasuredin low altitudepolar orbits may serve as a probe of the global magnetosphere and its dynamics.In particular,the spatialdistributionof regions of precipitationin the ionosphereis expectedto respond systematicallyto solar wind conditions.In this paper, we report on interplanetarymagneticfield (IMF) control of the
Copyright1995 by the AmericanGeophysical Union. Paper number 94JA02037. 0148-0227/95/94JA-02037505.00 1837
distributionof mantle plasmaprecipitationin the ionosphere and on associatedfield-aligned currents(FACs). The mantle was originally observedat mid altitude and in the tail [Rosenbaueret al., 1975; Sckopkeet al., 1976; Hardy et al., 1975, 1979]. According to Rosenbaueret al. [1975], the plasma mantle is a persistentlayer of tailward flowing deenergizedmagnetosheathplasma inside of and adjacent to the magnetopause.It is formed of plasma on tailward drifting flux tubesfrom the cuspregion. Since ions on theseflux tubesmove along the magneticfield at a speed that increaseswith energy while moving across the field at a speed independentof energy, the lower-energy ions spend a longer time transmittingto and reflecting from the ionospherethan do the higher energyions. Therefore,in the magnetosphereat a fixed distancefrom the polar cusp, the energy of the plasma decreaseswith distanceinward from the magnetopauseboundary. The correspondingsignature of the mantle layer in the polar ionosphereshould be a decreaseof the density and energy toward higher latitudes
1838
XU
ET AL.:
IMF
CONTROL
OF MANTLE
along the antisunward direction. Recently, Sanchez et al. [1990b] useddata from low-altitude polar orbital satellites DMSP F6 and F7 and Newell et al. [1991a] used data from DMSP F7 andF9 to studymantleparticleprecipitation.They found regionsshowingion data with smooth,continuousand monotonic decreasesin average energy and density with increasing latitudes. They identified these regions as the plasma mantle from the energy dispersiondiscussedabove. Newell et al. [ 1991a] also comparedthe densities,energies and temperaturesof the identified mantle plasma at low altitudeswith high altitude mantle observationsand obtained consistentresults. These results support the expectations basedon the descriptionsgiven by Rosenbaueret al. [1975] and establishthe usefulnessof the approachto identifying mantle plasma at low altitudes. The distributionof mantle plasmais expectedto depend
PRECIPITATION
rotation of the whole magnetotail as proposedby Cowley [1981].
The only study using polar orbital satellitesto investigate the relationbetweenthe IMF By and the mantleprecipitation was a brief paper by Sanchezet al. [1990b]. From F6 satellitedata taken in a dawn-duskmeridianplane near local noon, they found that the monotonicdecreasingenergytrend of the mantle ions could be from dawn to dusk or from dusk
to dawn dependingon the sign of IMF By. However,the spatialdistributionof the mantleprecipitationappearedto be
uncorrelated with IMF By,althoughtheyemphasized thatan
expandeddatabasewould allow more accuratedetermination of a possible asymmetry. In this paper, we report on a more direct study that expandson the preliminary work of Sanchezet al. [1990b]. In the daysidereconnectionmodel for IMF BzO,there are 35 passesin the prenoonregionof northernhemisphere and 24 passesin the prenoonregionof southernhemisphere (Figure 2a). For IMF By(O, there are 33 passesin the
Tablel. UTIntervals Covered forFigures 2aand2b IMF By
Hemisphere
northern >0
southern
prenoonregion of southernhemisphereand 24 passesin the prenoonregion of northernhemisphere(Figure 2b). In tryingto choosesymmetricallydistributedpassesaroundthe
northern 0(N:35S:24)
(b)
By0shownin Figure2a. Amongthe 35 casesin the prenoonregionof northernhemisphere, 31 passes includeclearmantlecrossings like the oneshownin Plate la. Amongthe 24 passesin the prenoonregion of southernhemisphere, only threepasses includemantle crossings.Figure 2c showsthe distribution of the mantlecrossings for the passesof Figure2a.
Mantle Crossings
(c) By>0 (N:31/35 S:3/24) (d) By0 + 29 IMF By0 and 1 for IMFB,,
