Energetic ion regimes in the deep geomagnetic ... - Wiley Online Library

GEOPHYSICAL RESEARCH LETTERS, VOL. 11, NO. 3, PAGES275-278,

ENERGETIC

ION

REGIMES

IN THE

DEEP

GEOMAGNETIC

TAIL:

MARCH1984

ISEE-3

S.W.H. Cowley, R.J. Hynds and I G. Richardson

The BlackettLaboratory,Imperial Collegeof Scienceand Technology,London SW7 2BZ, UK P.W. Daly, T.R. Sandersonand K.-P. Wenzel

SpaceScienceDepartmentof ESA/ESTEC, 2200 AG Noordwijk, The Netherlands J.A. Slavin and B.T. Tsurutani

Jet PropulsionLaboratory,4800 Oak Grove Drive, Pasadena,California91109,USA data from the Energetic Particle Anisotropy Spectrometer (EPAS) on ISEE-3, and the magnetic field data from the JPL magnetometer. Scholer et al. (1983a,b) have also recently reportedISEE-3 deep-tailenergeticparticlemeasurements, and while our resultsare broadly consistentwith theirs,the emphasis here will be on the relationshipbetweenthe energeticparticle and the plasmadata.

Abstract.During the periodJanuary1 to March 30, 1983,the ISEE-3 spacecraftwas in a previouslyunexploredpart of the distant geomagnetictail (60-220 RE). In two recent papers Slavin et al. (1983) have examinedthe grossmorphologyof the distanttail magneticfield, showingthat a well-orderedtwin-lobe structure is retained at these distances,while Bame et al. (1983)

have reported plasmameasurements for two 12-hour intervals and have identifiedthe plasmaregionsobserved.In this paper, for the latter time intervals, the energetic ion (35keV 1.6 MeV) and magneticfield measurements made by ISEE-3 are reported and are related to the plasma measurements.The plasma sheet is found to be a region of enhancedfluxes of energeticions, which are usually flowing unidirectionally tailward. These fluxes often extend outside the plasma sheet as identifiedin plasmadata to form a plasmasheetboundarylayer. A simplepicturewhichaccountsfor thesecharacteristics is given in which reconnectionoccursat a neutral line lying Earthward of the spacecraft.

Instrumentation

The EPAS instrument

on ISEE-3

measures the 3-dimensional

ion distribution about the spacecraftin 8 logarithmically-spaced energychannelsfrom 35 keV up to 1.6 MeV. The measurements are made by 3 particle telescopes mountedat angles30 ø 60øand 135øto the spacecraftspinaxis(whichis closelyperpendicularto the eclipticplane),with the telescopeoutputsbinnedinto 8 equiangularazimuthalsectorsduring each spacecraftspin. Further detailsare givenin Baloghet al. (1978).The magnetometerdata were obtained from the JPL instrument on ISEE-3, details of which are givenin Frandsenet al. (1978).

Introduction

The structureand extentof the magnetotailwell beyondlunar orbit (•60 RE) was the subjectof much debatein the 1960s(e.g. Dessler,1964;Dungey, 1965) but has been a topic in abeyance more recently, mainly as a result of the lack of detailed observations. Experimentalevidencehashithertobeenlimitedto brief tail traversalsat • 1000RE and •,3100 RE by Pioneer7 (e.g.Walker et al., 1975;Intriligator et al., 1979)and at •, 500 RE by Pioneer8 (e.g.Scarfet al., 1970;Bavassanoet al., 1974).These observationsshowed that while the distant tail may retain a coherentmagneticstructuresimilar to that near the Earth, the plasma properties differ considerably,consistentwith the spacecraftbeingpermanentlylocatedtailwardof the tail neutral line at these distances(Cowley, 1980; Cowley and Southwood, 1980). Using lunar gravitational assists (Farquhar, 1983) the International Sun-Earth Explorer (ISEE)-3 spacecraftwas put

Observations

Figures1 and 2 showparticleand magneticfield data for the periods 0000-1200 on January 24 and January 29, 1983 respectively.The particle data representthe 35-56keV ion omnidirectionalintensity.To providea measureof the direction of particleflow,a sphericalharmonicanalysisof the 3-D particle intensity distribution was made. In the secondpanel of each figure is shownthe A•0 componentwhich representsflow along the X-axis of the GSE coordinate system,i.e. the Earth-Sun line. A•0 positive correspondsto Earthward flow. Further information about the directionalpropertiesof the ion flux is givenin Figure3 where we present8-sectorplots of the data from the telescope closestto the eclipticplanefor 5 selectedintervals.The times of thesesectorplots are indicatedon Figures1 and 2 by the lettersA to E. The magneticfield data in Figures1 and 2 are

into a seriesof orbits, some of which take it down the tail to a

thethreecomponents Bx,By,Bz expressed in GSE coordinates,

distanceof • 220 RE, allowingcontinuoussamplingof the tailmagnetosheathregionsover the previouslyunmappedarea of

and total magnitudeB. The vertical lines divide Figures 1 and 2 into regions of

•, 60 to • 220 RE. Bame et al. (1983) have reported plasma electron measurements made in the tail at a distance •200 RE, and for two 12-

differingplasma characteristics, where for simplicitywe have adopted both the nomenclatureand the boundary locations givenby Bameet al. (1983).The four plasmaregimesidentifiedin Figures1 and 2 are: MS=magnetosheath, PS=plasma sheet, BL---boundary layer and L---lobe. The magnetometer data

hour periodson January 24 and 29, 1983 have indicatedthe plasmaregionsencountered. In this paper,for the sametwo 12hour periods,we report the energeticion (35 keV •