Feb 1, 1981 - character. We have also .... batic processes. The error bars shown in all figures were esti- .... candy different at each energy step. OMy He ++ ...
JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 86, NO. A2, PAGES 761-767, FEBRUARY
1, 1981
EnergeticIon Compositionof the PlasmaSheet W. K. PETERSON,R. D. SHARP, E.G.
SHELLEY, AND R. G. JOHNSON
LockheedPalo Alto ResearchLaboratory, Palo Alto, California 94304 H.
BALSIGER
Universityof Bern, Bern, Switzerland
Data obtainedfrom the energeticion massspectrometer experimenton Isee 1 in the distantplasma sheetarepresented. Thesedatashowthat(1) theplasmasheethasa significant andvariableionospheric component (H+ andO+) representing from 10%to morethan50%of thetotalnumberdensityand(2) thereis morethan oneprocess responsible for the energization of solarwind plasma(H + and He++) to plasmasheetenergies.
INTRODUCTION
The quasi-stable plasmasheetin the earth'smagnetotailis an importantand puzzlingtopologicalfeature of the outer magnetosphere. There have been many observationsand model calculationsmade in the attempt to understandthe sourceand dynamicsof the plasmasheet[e.g.,Honeset al., 1971;Hill, 1974; Jaegerand Speiser,1974; Schindler,1979; Pilippand Morrill, 1978;Frank and Ackerson,1979]and its intimaterelationshipwith magnetospheric substorms(see,for example,Hones [1979] and the extensivereferencelist of McPherron [ 1979]).
Until quite recentlyit was assumedthat energeticions in the plasmasheetwereprimarilyof solarwind origin(H + and He++)witha possible smalladmixtureof terrestrialionsfrom the polar wind (H + and He+; see,for example,Hill [1974]). Recentcompositionmeasurements in the equatorialplasma sheetnearsynchronous altitude[Balsigeret al., 1980]have revealedthe presenceof substantialfluxesof 0% The observations of intense fluxes of O+ streamingup high-latitude auroral field lines at altitudes of •1 Re [Shelley et al., 1976a; Ghielrnettiet al., 1978] and the inferencesof O + streamsdeep
on plasmaof solarwind origin. Finally, we make a rough extrapolation in energyof the observedmasscompositiondata
and estimatefor selected intervalsthe relativeterrestrial(ionospheric)contribution to the plasma sheet. INSTRUMENT
AND
OPERATIONS
The energeticion massspectrometeron the Isee I spacecraft and the Isee 1 orbit have been describedin detail previously [Shelleyet al., 1978; Ogilvie et al., 1978]. The Isee 1 spacecraftis in a highly elliptical orbit inclined22 degreesto the eclipticwith apogeeat approximately23 Re. The spacecraft spinswith a period of approximately3 s with the spin axis approximately perpendicularto the ecliptic plane. The energeticion massspectrometer viewsoutwardapproximately in the spin plane. Becauseof a preaccelerationfeature, the Iseeenergeticion massspectrometer sensitivityis significantly greaterat energieslessthan approximately1 keV/e than typical electrostaticanalyzers.The instrumentalresponseis approximately constant,independentof energy,having a value of approximately0.6 cm2 eV sr at 200 eV/e, comparedto
•0.03 cm2 eV sr at 200 eV/e for the quadraspherical Lep-
in the magnetotailboundarylayerby Frank et al. [1977]and edea'son Isee 1 and 2 [Frank et al., 1978]. For studiesof the earth's magnetotaftplasma, special inHardy et al. [1977]suggestthat there might alsobe measurstrument modeswere used to acquire energy-angledistribuable fluxesof energeticO+ ionsof terrestrialorigin deepin the tions of selectedmassesas rapidly as possible.Becauseof explasma sheet. After detailedintercomparisons of energeticion massspec- perimental limitations a typical 15-min cycletime is required. trometer data obtained from Isee 1 deep in the plasma sheet, Most of the cycle is devoted to obtaining complete angular we have found that there is a significant ionospheric com- scansover logarithmically spacedenergy stepsspanning the ponent(H + and O+) that hasa widelyvaryingintensityand range 10 eV/e
