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niirir.1 .Nu.biI) S. MONITORING ORGANIZATION REPORT NUMBER(S)AFGL-TR-88-03 13

~. .~r~r r~i~rtA.. ~6u. OFFICE SYN16OL 7a. NAME OF MONITORING ORGANIZATIONJ~n Hc'"ns Univers4ty I.pi1coe Air Force Geophysics Laboratory

.'A~t Cly), )Ijfr. nAU 2ij1 CCAk 70. AOL~rESS Cty. Sla~e. ina e Cliv

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'DF ~ 'P~O G8t). OFFICE SYMBOL 9. PROCUREMENT INSTRUMENT IDENTIFICATION NUMBERoiiGANIZAf ON (it appicaole)

________________________ J___________ MIPR NOOO9-87-C-530lC(Y SJCC..U ZIP C7,JCJ 10 SOURCE OF FUNDING NUMBERS

PROGRAM PROJECT TASK WORK UNITELEMENT NO. NO NO. ACfSINNO.

2311 G5 AASO61102F G5 02

1 1 T17LL Linlhui S~cc,,ry 0I4u,tcion)lBirkeland Currents and Charged Particles in the High-Latitude Prenoon Region:

A New Interpretation

Q2 PERSOiNAL AUThOR~i,? .F. Bychrow, T.A. Potemra, R.E. Erlandson, L.J. Zanetti, D.M. Kiumpar*

i~d. TYPE OF REPORT 13o. TIME COvERED 14. DATE OF REPORT (YearMonth, Day) 15. PAGE COUNTRLPRINT t ROM TO 1988 November 17 1

16 UFPLEMENTARY NOTATION This research was partially supported by in-house work unit 2311G502.xQockheed Palo Alto Rsch Lab, Palo Alto, CAReprinted from J of Geoohvsical Research, Vol 93, #A9, pi) 9791-9803. 1 Sen IP881 7 COSATI CCOES 13. SUBJECT TERMS (Continue on reverse of necessary ad idientify-.by block numnber)

FIELD GROUP SUB-GROUP -. )Ionosphere; . Aucora .~Field~aligned currents - _

ii .aS~TR.ACT (Con(4raue an reverse it necess~y .*na identity by block nuffbeit)bmgc Nat-c a

- - ii~L~fo~w ~OlJ~~4CIfl.&,,CJDfU oi mfagrnki ficels and rbgu Iogde as Waic~~e i he wzin-5. ~~.3 Laiiiuac pircnoon w,,tor and the m ntIORcAL1I wert made with the 0D% P F? HLT- n Ac ManeO

..- t, j ,pticnc Paruclze Traxr E_%plarcrs (AjdpTE) CC e dse a.. I-.ti.94.Tst aa hwta ha~ -1Q.o-Litiit Portionl uf the IuIito"fla 'cippan'Vce VgaurfCis c~odtflu 'A abh ke prenoon region) I curew

... i4cnl I if bojnhoCrfl~phctcz And fOr both northwArd ind wouth'~ar inietpL petl a es nL%IF hoL..dhk.ILjWiiap' Baks cli..d ucurent arc . acd with the disperin region Of ct" oswe ~eIIFf.~c~ioruf jp6%xua mcasured by AZIPTE CCE in the as &rohz~lh near lC000 NILT aie $~ilagZ 'A shape I'

coery tOLha~ ~w~tl 4l~w titue bybothDMS j77a HILAT..TheC obsezvattons indicat thaif~-J bjt narthwvA.iI ,.~utLw..d IMF. the La.JLion"a .usP paruk digpnasurc is monadcnz wuh t rcVs

-4 Bg~cind ~rrnL ~stcs an m~ to aw J~tttlealon ieJ liea hatthread the dayside boundary kayet. ('Z:- The u-idational 4u~p- Btkdlad cwrcmn system flows along fidilli th" lie poiward Of she reg"o Of COiP

S* pd.LIe, Uld the region I cwrent syw=tn These field lines shrcad the PJAtma MankL Thus, we su9gsFha" ... '-

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uL)i~Ii Idw.jTIN/AVALAj)LTY OF ABSTRACT 21. ABSTRACT SECURITY CLASSIFICATION[3UNCLASSIFED.UNLIMiTEO C SAME AS RPT. CO TiC USERS Unclassified

.2.j. NAME OF KEi-OiS 7tLE aiNDIUAL 22b. TELIEPHONE (irsdude Area Code) 22c. OFFICE SYMBOLFrederick Rich AFL I PHG

00 FORM 1473, d4 mAR d3APRition my oused untieal a%6349. SECURITY CLASSIFICATION OF THIS PAGEAll other editions are obsolete.

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JOURNAL OF GEOPHYSICAL RESEARCH. VOL. 93. NO. A9. PAGES 9791 -9803. SEPTEMBER 1. 198h

AFGL-TR -88-0313Birkeland Currents and Charged Particles in the High-Latitude

Prenoon Region: A New Interpretation

P. F. BYTHROW., T. A. POTEMRA, R. E. ERLANDSON, AND L. J. ZANETTI

The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland

D. M. KLUMPAR

Lockheed Palo Alto Research Laboratory, Palo Alto, California

Simultaneous, conjugate measurements of magnetic fields and charged particles at low altitude in the high-latitude prenoon sector and the magnetosheath were made with the DMSP F7, HILAT, and Active Magneto-spheric Particle Tracer Explorers (AMPTE) CCE satellites on November I, 1984. These data show that thelow-latitude portion of the traditional "cusp" particle signature is coincident with the prenoon region 1 currentsystem in both hemispheres and for both northward and southward interplanetary magnetic fields (IMF). Thetraditional "cusp" Birkeland currents are associated with the dispersion region of cusp ions when the IMFis directed southward and with electron fluxes that are slightly enhanced over polar rain intensities. Finally,electron spectra measured by AMPTE CCE in the magnetosheath near 1000 MLT are similar in shape andenergy to those acquired at low altitude by both DMSP F7 and HILAT. These observations indicate that forboth northward and southward IMF, the traditional "cusp particle" signature is coincident with the regionI Birkeland current system and maps to low altitude along field lines that thread the dayside boundary layer.The traditional "cusp" Birkeland current system flows along field lines that lie poleward of the region of cuspparticles and the region I current system. These field lines thread the plasma mantle. Thus, we suggest thatthe traditional "',tsp" riirrent ystem might bc appiopriately renamed the imantie" Birkeland current system.

INTRODUCTION average energy of - 100 eV and ion average energy of - I keV.

The magnetospheric cusps (clefts) are neutral points (lines) on The region of intense precipitation was located between about 74*

the dayside magnetopause that divide lines of magnetic field that and 80° ILAT (invariant latitude) and extended from about 0800

drape the dayside magnetopause from those that thread the mag- to 1500 MLT (magnetic local time). Thus, they inferred that this

netotail lobes and mantle [see Crooker, 1977b, Figures I and 3]. plasma had unimpeded entry to the magnetosphere along field linesThe magnetospheric cusps derive from the deformation of the geo- that thread the magnetospheric cusps. The broad range in MLT

magnetic dipole by its interaction with the solar wind and are ap- and ILAT over which these particles were observed was attribut-

parent even in the earliest models of the magnetosphere [Spreiter ed to the extension of the axially symmetric cusp into a cleft and

and Briggs, 1962; Midgeley and Davis, 1963]. The solar wind in- to the porosity of the cusp magnetic bottle [Heikkila and Win-

teraction with the geomagnetic field results in the generation of ningham, 1971, and references therein].

a large-scale surface current, "the Chapman Ferraro current," Large-scale Birkeland currents detected by the TRIAD satellite

flowing along the magnetopause surface normal to the solar wind at high latitudes on the dayside were associated with the polar cuspvelocity vector [Chapman and Ferraro, 1933]. The polar cusp has by lijima and Potemra [1976]. These currents were found in rough-

been associated with the diversion of a portion of this current along ly the same region where magnetosheathlike particles had been

magnetic field lines and through the dayside high-latitude iono- statistically observed. Aptly named cusp currents, they lie poleward

sphere [Eastman et al., 1976; Jijima and Potemra, 1976]. How of and adjacent to the region I current system, and their flow

these currents may be related to the entry of solar wind particles is directed opposite that of region I at all local times between about

into the magnetosphere is as yet an unanswered question. 0900 and 1500 MLT. Since TRIAD did not carry a charged par-

Over the past two decades the cusp/cleft has been identified ticle detector, this study was unable to evaluate the relationship

by the p-esence of magnetosheath-like plasma in the low-altitude, between "cusp currents" and "cusp particles." Polemra et al [1977]high-latitude, dayside ionosphere. The charged particle detectors examined data from the low-energy electron detector (200-25 eV)

on IMP 5 observed magnetosheath-like plasma in the dayside mag- and photoclectron spectrometer (0-500 eV) on board the AE-C

netosphere at high latitudes from about 4 to 7 RE [Frank, 1971]. and AE-D satellites in the low-altitude cusp. From these data theyFrank concluded that this plasma had entered the magnetosphere concluded that two populations of electrons were present at highvia the magnetospheric cusp. Heikkila and Winningham [19711 latitudes on the dayside, a higher-energy population at low auroral

first defined the cusp at low altitude using data acquired at about zone latitudes and a lower-energy population at higher latitudes.

2000 km from the ISIS I SPS (soft particle spectrometer). They Based on the statistical pattern of large-scale Birkeland currents

noted that intense low-energy electron and proton precipitation they suggested that the higher-energy population was associatedhad spectra of energy versus number flux that were nearly identical with the region I currents and the lower-energy population wasto those of magnetosheath particles, i.e., Maxwellian with electron associated with the cusp currents. The first simultaneous measure-

ments of Birkeland currents and charged particles were made withISIS 2 in the early afternoon sector [Burrows et al., 1976). In acase study they showed that in this local time sector an upwarddirected Birkeland current was associated with cusp particle precipi-

Paper number 8A9534. tation. A later study of the magnetic field and charged particle0148-0227/88/008A-9534$05.00 data from ISIS 2 by McDiarmidet al. [1979] indicated that "cleft

9791 S8 11 28 041

9792 BYTHRO,, ET AL.: DAYSIDE CURRENTS AND CHARGED PARTICLES

electrons" overlapped the east-west reversal of the magnetic field were acquired from the HPCE (hot plasma composition experi-perturbation. ment) which provides ion composition measurements from very

In a recent statistical study of ion and electron precipitation using low energies, 0 eV/e (limited by spacecraft potential) to 17 keV/e,data from the Defense Meteorological Satellite Program (DMSP) and electron measurements from 50 eV to 25 keV [Shelley et al.,F2, DMSP F4, and the P78-1 satellites, Hardy et al. [1985] deter- 1985]. The DMSP F7 and HILAT satellites are both in low-altitudemined that a crescent-shaped region of low-energy electrons with (- 800 km) circular polar orbits and both carry similar magneticmagnetosheath-like properties extends several degrees in ILAT and field and charged particle experiments. The magnetic field experi-several hours in MLT either side of noon. This crescent of low- ments sample the ambient vector magnetic field at a rate of 20energy electron precipitation gradually blends into BPS (bound- samples/s using 13-bit analog to digital converters. Thus, a spatialary plasma sheet) electron precipitation in the dawn and dusk sec- resolution of -400 m and an amplitude resolution of - 12 nTtors. At the poleward edge of the crescent near noon, Hardy et is provided. For further information on the magnetic field experi-al. detected a localized region of minimum average energy. They ments, see Potemra et al. [1984] and Rich et al. [1985]. The chargedidentify this confined region of minimum average energy as the particle experiments on board DMSP F7 and HILAT are also oflow-altitude signature of the "polar cusp" or "entry layer" [Pasch- similar design. The DMSP F7 ion and electron spectrometers mea-mann et al., 19761. The crescent-shaped region was suggested to sure the precipitating flux of electrons and ions from the zenithbe the low-altitude projection of the dayside LLBL (low-latitude direction in 20 energy channels from 30 eV to 30 keV. Thus aboundary layer). The LLBL is likely to be one source of the day- 20-point ion and electron spectrum is completed in 1 s. HILATside region 1 current system [Eastman et al., 1976; Sonnerup, 1980; is equipped with three electron spectrometers oriented at the ze-Bvthrow et al., 19811. Analysis of data acquired with the magne- nith, nadir, and 450 from zenith look angles. These spectrometerstometer, ion drift meter, and low-energy electron experiment on measure the flux of electrons in 16 energy channels from 20 eVAtmosphere Explorer C showed that the region 1 current system to 20 keV and can provide four 16-point spectra each second fromin the dawn and dusk sectors spans the ion convection reversal each of three separate spectrometers. For further information onand is colocated with a low-energy electron population identified the charged particle spectrometers, see Hardy et al. [1984a,b].at having characteristics of boundar pL.,ma shc-,t electrons On November 1, 1984, the dayside nmagnetosphere was com-[Bythrow et al., 1981]. Due to the inclination of the AE-C orbit, pressed, and the bow shock was located unusually close (earth-the satellite's trajectory crossed the auroral oval at an oblique an- ward of AMPTE CCE) to the Earth due to the high solar windgie and, at best, skimmed the noon sector at high latitudes. For dynamic pressure. Data for this study were acquired between thethis reason the study did not attempt to address the cusp currents hours of 0700 and 1000 UT. IMF data from IMP 8 located 25or to differentiate between "cusp" and BPS particles. Birkeland RE upstream in the solar wind during this period are shown incurrents and charged particles have been examined separately and Figure 1. Fifteen-second averages are used in this plot of Bx, B,simultaneously in the dayside ionosphere, but the question still and B, in GSM coordinates. The shaded portions of the plot atremains as to what high-latitude current systems are most intimately A, B, and D denote low-altitude crossings of the prenoon sectorslinked with magnetosbeathlike particle populations. by the DMSP F7 and HILAT satellites. Times are corrected for

In this paper we offer to answer this question by way of a set IMP 8's position in the solar wind. A 6-min time delay is appropri-of case studies made on November 1, 1984, with the DMSP F7, ate for the solar wind velocity of 450 km/s present at the timeHILAT, and Active Magnetospheric Particle Tracer Explorers of these measurements. The shaded region at C indicates the time(AMPTE) CCE satellites. On this day the magnetosphere was se- when AMPTE CCE electron data are examined. Consecutiveverely compressed to the extent that the bow shock was observed DMSP F7 crossings of the northern prenoon sector occurred whenwithin 8 RE with the AMPTE CCE. We survey simultaneous, the IMF was directed southward (A and D). The DMSP F7 pre-conjugate hemisphere measurements of magnetic fields and noon crossing denoted by B occurred in the southern hemispherecharged particles from the low-altitude prenoon sector and the when the IMF had been directed northward for over 50 min. I hemagnetosheath and examine consecutive passes of DMSP F7 HILAT satellite crossed the northern prenoon sector at a locationthrough the prenoon sector in the northern hemisphere. Our results nearly conjugate to DMSP F7 in the south during period B.demonstrate that in both hemispheres and for both northward andsouthward orientations of the interplanetary magnetic field (IMF)the lowest-latitude portion of the traditional "cusp particle" sig- Case A (Bz < 0)

nature is coincident with the prenoon region I current system. The Data depicted in Plate I were collected by the DMSP F7 satel-traditional cusp currents are associated with electron fluxes only lite as it traversed the northern hemisphere at latitudes greater thanslightly enhanced over polar rain intensities, and electron spectra 500 MLAT (magnetic latitude). As shown in Figure I, the IMFmeasured in the 1000 MLT magnetosheath are similar in shape Z component during this crossing was directed southward, andand energy to those acquired simultaneously at low altitude in both the Y component was positive (east). The top panel of Plate Ihemispheres. The implications of these observations are discussed is a plot of the transverse magnetic field perturbation vectors mea-in the context of a consistent model of the dayside magnetosphere. sured along the orbit track. The satellite was traveling from south

to north along its orbit, and the vectors are plotted on a MLAT/OBSERVATIONS MLT (magnetic local time) polar dial. The next two panels dis-

The magnetic field and charged particle data used in this study play the transverse components of the perturbation magnetic fieldwere acquired from four spacecraft, IMP 8, AMPTE CCE, DMSP in spacecraft coordinates for the prenoon sector portion of theF7, and HILAT, with IMP 8 located in the solar wind, CCE in orbit highlighted in the polar dial. Bz in spacecraft coordinatesthe magnetosheath, and both DMSP and HILAT in the low- is directed perpendicular to the orbit plane, which is essentiallyaltitude ionosphere. AMPTE CCE was launched on August 16, an east-west geographic orientation, while 3, is in the plane of1984, into an equatorial elliptical orbit with apogee of 8.8 RE and the orbit tra,"r-rse to the main field, i.e., a noi th-south geographicperigee of 1.1 RE. Charg- "-ie dats ii the magnetosheath orientation. Charged particle data, measured simultaneously with

BYTHROW ET AL.: DAYSIDE CURRENTS AND CHARGED PARTICLES 9793

1 Nov84 IMP-8 IMF (GSM)

20 A B C D

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Fig. 1. Interplanetary magnetic field data from IMP 8 for the period of interest. Each highghted segment represents a periodwhen low-altitude data were available.

the magnetic field, are displayed in the lower four panels. The weak (

1 NOV 84DMSP-F7 NORTHERN HEMISPHERE Bz < 0

12 MLT

15~ --. 9

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HR MIN MIAT MLT30 NT 4START 7 3 47.6 10.230 NT 4END 7 27 50.8 22.5

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UT 07:08:00 07:09:00 07:10:00 07:11:00 07:12:00MLAT 64.5 67.8 71.1 74.4 77.7 11/01MIT 09:55 09:53 09:49 09:44 09:35

B I ROSS I I \I .D CII)[ (tL RR-\ IS \\D ('|IixROI t) P \R IK I I-s 995

Between 0709:57 and 0710:17 L , the ion spectrogram shows to and after the rotation. Three large-scale gradients in the Z com-that precipitating ions exhibit a steeply sloped energy versus latitude ponent of the perturbation field indicate the three large-scale Birke-dispersion. The ion energy flux peaks near I keV at lower latitudes land currents involved in the rotation of the magnetic field. Theand decreases toward higher latitudes. This dispersion signature first of these, located at the highest latitude, extends from 0808:40has been cited by Reif et al. [1977, 19801 as evidence for particle to 0809:23 UT and is directed out of the ionosphere. The secondinjection resulting from dayside merging in the "polar cusp" when extends from 0809:23 to 0809:55 LIT and results from an earth-the IMF has a south\'ard component. Heikkila [19781 disputed 'ward directed region I current %kith a density of 2.5 1A, n. Ad-this interpretation and suggested that the same dispersion signature jacent to the region I current a weak region 2 current extends to-can result from plasma entry processes occurring on closed geo- wsard lower latitudes. As observed in case A for southvsard IMFmagnetic field lines. Recent MHD simulations by Galvez [19871 there is a narrow upward directed (',,,rent embedded in the re-hase sho\sn that polarization of streaming plasma and the ensu- gion I system at 0809:38 UT.ing shear instability of the polarization charge layer can provide Charged particle data show a traditional cusp signature betweenplisna sith nondiffusive access to closed field lines. 0809:18 and 0809:55 UT. At 0809:18 LT about 74' NILAT the

The region of tmaxim'im ion energy dispersion is colocated with electron and ion energy flux exhibit a marked increase. The electrona Birkeland current directed into tne ionosphere that is identified aserage energy is about 150 eV until 0809:55 LT and 72' Mi.ATas the region I current system, while at higher latitude the tradi- where the average energy increases to about I keV. Although thetional "cusp" current is associated with a less intense flux of ions IME had a northward component, the region I Birkeland currentthat exhibits a less steeply sloped dispersion signature. The elec- remained colocated with the cusp particle population as we ob-tron flux wsithin the region of traditional cusp current gradually served in the previous case for southward IMF. The traditionaldecreases with increasing latitude. Polar rain electrons are encoun- cusp current extends poleward of region I up to about 76' MLAT.tered at the equatorward edge of the "cusp current" and superim- The transition from an earthward directed region I current to anposed on this low%-energy electron precipitation. upward flowing current at higher latitude is marked by a sharp

The ions display a narross maximum in flux located in the center (< 50 km and 2 orders of magnitude) decrease in the energy fluxof the sharply sloped energy latitude dispersion. This flux maxi- of low-energy electrons, while the electron average energy remainedmum is colocated with a spatially limited pair of upward directed relatively constant at about 200 eV. At this transition the ion aver-and enhanced earthward directed currents embedded within the age energy and energy flux fell by 1 and 2 orders of magnitude,region I current system. The I-keV energy of this maximum is respectively. The transition to the region 2 current at the equator-comparable to the energ. of a proton flowing with a typical bound- ssard edge of the region I current is similarly marked by bound-ary layer bulk velocity of - 300 km/sec [Eastnan and Hones, aries in the electron and ion populations. At latitudes lower than1979]. A similar maximum in ion flux at about 0.7 keV is colo- the juncture between region I and region 2, the electron averagecated with a narrow earthward flowing current embedded within energy and energy flux both increase by an order of magnitude,the traditional cusp current at 0710:30 UT. while the ion energy flux and average energy undergo a gradual

decrease. These low-latitude plasma sheet-like ion and electronCase B (B. >0) D.SP F7 populations were undetectable in the previous case for southward

Here "e examine simultaneous and conjugate measurements ac- IMF although the magnetic local time was less than I hour later.quired from both the northern (HILAT) and southern (DMSP In the interval between 0809:18 UT and 0809:55 UT an energyF7) low-altitude ionospheres and from the 1000 MLT magneto- versus latitude dispersion of the ions cannot be discerned in thesheath at about 8 R, (AMPTE CCE). These data were acquired ion spectrogram. This observation is consistent with the predictionsabout 50 min after those data examined in case A. As shown in made by Reiff et al. [1980] for ion energy dispersion during periodsFigure 1, the IMF had a northward component for about 50 min of northward IMF, i.e., a diffusion signature. The energy fluxprior to the DMSP and HILAT crossings of the prenoon sector. of earthward directed ions and electrons in the region of the mostThe IMF Z component turned southward at about 0811 UT. intense flux is essentially the same for this case of northward IMFTherefore, during the period when AMPTE CCE data were col- as it was for the previous case when the IMF was directed south-lected the IMIF has been directed southward for about 20 min. ward. In the center of the most intense flux of low-energy elec-

In Plate 2 the magnetic field and charged particle data from trons about 0809:38 UT, the ion energy flux exhibits a maximumDMSP [-7 hase the same format as in Plate 1. At about 0900 MLT in the energy range between 0.7 and 2 keV. As before, this en-and 73' NILAT the perturbation magnetic field vector plotted on hanced flux corresponds to the upward directed current embeddedthe \I T MLAT polar dial rotates from an easterly to westerly within the region 1 current system.orientation corresponding to an earthward directed Birkeland cur-rent and an inferred reversal of E x B ion drift from a south- Case B (B: > 0) HILATea,,terl', to southwesterly flovs. In this case there is a significant Magnetic field and charged particle data acquired from the HI-polesard directed component of the perturbation field both prior LAT satellite between 0807:00 and 0811:00 UT on November 1,

1984, are shown in Plate 3. The HILAT data were collected inthe northern hemisphere at nearly the same MLAT, MLT, andUT as those data obtained from DMSP F7 in the southern hemi-

Plate I. (Opposite) The first of three consecutive crossings of the pre- sphere and displayed in Plate 2. The plots of HII.AT magneticnoon auroral zone by DMSP 1:7. This pass is from the northern hemi-phere wshen the IMF had a southward component. In the top panet mag- field data display the transverse components of the perturbationnetl, field vectors along the orbit track are plotted on a MIAT/ILT polar magnetic field in spacecraft coordinates. The X component isdial. The highlighted portion of :he orbit is displaved in detail below. The oriented in the east-west direction. The three gradients in the east-next two panels disptay the transverse components of the perturbation mag- west component of the perturbation magnetic field as measurednetic field. The next two panels are line plots of electron (white) and ion(orange) energy flux and average energy. The bottom two panels are spec-trograms of electron (lop) and ion (bottoml energy flux versus energy and those measured simultaneously by DMSP F7. Thus, the triplet oftime. Birkeland currents in the southern hemisphere measured by DMSP

1 NOV 84DMSP F7 SOUTHERN HEMISPHERE Bz >0

12 MLT

213HR MIN MLAT MLT

START 7 53 -49.4 23.124__ END 8 17 -53.7 10.2

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BN IHROS Li .51 DA SID CL RR-NIS -\N[D CHRGED) PARTIK I ES 9797

F' hase almost identical counterparts in the conjugate northern lighted in the MLT/MLAT dial are reproduced in detail in thehemisphere. Of the three gradients in B, the one associated with lower five panels. In the center of the highlight, the perturbationthe region I current between 0808:40 and 0809:26 UT is most near- magnetic field vector undergoes a shearlike reversal from a south-ly conjugate with its southern hemisphere counterpart, westerly orientation to an almost due easterly direction over less

Charged particle data from HILAT are limited to electrons only, thai 0.5* of magnetic latitude. At latitudes greater than aboutbut unlike DNISP F7, pitch angle information is available. Pitch 730 MLAT the vector rotates gradually to the southeast. The shearangle information was provided by sampling the electron flux with reversal in the magnetic field vector indicates the presence of anthree ftxed detectors at different orientations to the magnetic field. intense region I current flowing into the ionosphere, while theThe three spectrograms of number flux versus time versus energy gradual rotation at higher latitude is associated with the outwardare from the zenith, 40, and nadir pitch angles. Between 0808:40 directed "cusp" current. As mentioned previously, the nominaland 0809:30 UT there is an enhanced flux of low-energy electrons direction and velocity of ion drift can be inferred from the mag-meas :red by the zenith and nadir detectors that can be associated netic field vectors and an ionospheric conductivity model. Thus,with the traditional cusp, but as was observed by DMSP in the the ion drift undergoes a shear reversal from a dominantly sun-southern hemisphere, these electrons are colocated not with the ward to antisunward flow, the shear reversal being centered withincusp current but with the region I current. The most intense flux the region I Birkeland current. Heelis et al. [1976] have shownot lo s-energy electrons at 0808:55 UT is colocated with a plateau that these reversals occur within a region of "cleft particle" precipi-or slightly reversed gradient in the magnetic field embedded within tation.the region I current. This feature is very similar to the one ob- The gradient in B. between 0850:35 and 0851:00 UT, seen insersed simultaneously by DMSP in the conjugate hemisphere. The the top line plot, is about 20 nT/s. From this we derive a regiontraditional cusp current observed between 0809:30 and 0810:26 1 current density of 2 pA/m 2 with flow directed into the iono-LT extends to higher latitudes than region I and is associated with sphere. This case shows very clearly that the equatorward edgea less intense flux of electrons. At the interface between region of the region I current is precisely colocated with the onset of low-I and cusp currents (0809:30 UT), the electron pitch angle distri- energy electron precipitation that has generally been associated withbution transitions from relatively isotropic to field aligned, suggest- the polar "cusp/cleft." The high-energy tail of the cusp electronsing a change in magnetic field topology from dayside to tail lobe clearly visible in the spectrogram is bounded by the limits of theor plasma mantle field lines. The region 2 current between 0807:55 region I current system, while a classic polar rain electron signa-and 0808:40 UT is associated with a plasma sheet-like population ture with essentially no flux of electrons at energies above I keVof electrons that extend from about 680 MLAT up to the region 1/ has its onset at the poleward edge of the region I current. Theregion 2 interface at about 72.5' MLAT. onset of polar rain is colocated with a reversal in the magnetic

field gradient signifying an outward directed cusp Birkeland cur-Case C Spectra rent with a density of about 0.6 jA/m2 . Ions associated with the

region 1 current display a steeply sloped energy versus latitude dis--igure 2 shows three typqical electron spectra: one obtained from

HILAT in the northern hemisphere at 9808:55 UT, another from persion signature with a maximum flux at about I keV. The total

DMSP F7 in the southern hemisphere at 0809:38 UT, and a third energy flux of ions is of the same order of magnitude as is the

energy flux of electrons within the region I current. Ions associatedfromAMPE CE wen te saceraf wasin he 000MLT with the cusp current appear to have convected poleward from

magnetosheath. The data are plotted as log differential number w e tus an t a sha e soped ener v r a

flux versus log energy. The three spectra are nearly identical in tude dispes a nd a s ine as e Anthat frsu th-

shape, peak flux, and average energy. The similarity between these wad iMei winte i f t s instrumation,thre secta cnfims hateve drin peiod ofnorhwad IF, ward IMF, within the limits of the DMSP F7 instrumentation,three spectra confirms that even during periods of northward IMF, there is no discernable indication of a region 2 current or of plasma

magnetosheath particles are relatively unimpeded as they pene- sheet-like electron precipitation equatorward of the region cur-

trate into the magnetosphere and along magnetic field lines into ret.Tis iseconi ita dayside gap in the region 2 cur

the avsde arorl zne. anddi nd Mng 1981 hae sown rent. This is consistent with a dayside gap in the region 2 currentthe dayside auroral zone. C'andidi and Meng [1984] have shown sytm[jmanPoer,971ndithdfuevsblwv-system [lijima and Potemnra, 1978] and in the diffuse visible wave-that near-conjugate measurement of precipitating electrons in the length aurora [Meng, 1981 and references therein).prenoon sector yields nearly identical spectra. Our observationsshow that during a period of northward IMF, conjugate iono- DiSCUSSioNspheric spectra are essentially indistinguishable from nearly simul-taneously measured magnetosheath spectra. Thus, an efficient plas- The relationship between Birkeland currents and charged par-

ma entry mechanism other than dlayside merging may be opera- ticles in the ionosphere at low altitude is not well understood for

tire when the IMF is directed northward, the late morning through early afternoon hours, yet this relation-ship has significant bearing on the generation of large-scale Birke-

Case D (B: < 0) land current systems and on mechanisms of charged particle entryinto the dayside magnetosphere. Statistically, the region I Birke-

DMSP F7 next crossed the prenoon sector in the northern hem- land current system is a persistent feature of the dayside iono-isphere when the IMF had once again acquired a southward com- sphere at all magnetic local times and generally has current flowponent. The magnetic field and charged particle data collected dur- directed into the ionosphere in the morning hours and out of theing this traversal are displayed in Plate 4. The display format is ionosphere in the afternoon hours. A recent study by Erlandsonthe same as for Plates I and 2. Data from the orbit segment high- et al. (IMF B, dependence of region I Birkeland currents near

noon, this issue 1988), using magnetic field and charged particledata from the Viking spacecraft, has revealed that near noon the

Plate 2. (Opposite) The same as Plate I but in the southern hemisphere region I current flow direction is strongly dependent on the Y com-

about 45 min later with a northward component of the IMF. The data ponent of the IMF. The so-called "cusp Birkeland currents" liepresented in this plate are simultaneous with and conjugate to those present- adjacent to region I at higher latitude and are somewhat moreed in Plate 3. limited in local time [lijima and Polemra, 1976].

9798 BYTHROW ET AL.: DAYSIDE CURRENTS AND CHA.RGED PARTICLES

1 NOV 84HILAT NORTHERN HEMISPHERE Bz > 0

- 400

-.- 200

0

200

400 __________ _____________ _________ ____

-400

-200 R

x

200

400

10, z102

* 10'

>-. 1o' 400a,

LNN

10'

UT 08:07 08:08 08:09 08:10MLAT 68.3 70.6 72.8 74.6MLT 08:13 08:40 09:12 09:50

Plate 3. Magnetic Field and electron data collected with the HILAT satellite in the northern hemisphere simultaneously withthose data presented in Plate 2. The upper two panels are the transverse components of the perturbation magnetic field. Thelower three panels are electron ,pertrograms from the 7enith, 40', and nadir look angles.

BN IHRON% ET m D\),lDt CLRRIFN'F -\1M) CH\RGEI) P-RriCL F 999

Hilat DMSP AMPTE CCE and oer a fe%, degrees in invariant latitude in the dayside iono-1 Nov 84 sphere [Hardy et al., 1985 and references therein]. Vasyliunas

r [19791 suggested that only a spatially limited region located near

noon and embedded within the larger region of magnetosheath-7 - like particles was indeed associated with the magnetospheric cusp.

" DMSP He hypothesized that the region of low-energy charged particle0313 Hiat precipitation that is distributed over a broad range of local times

6 AMPTE is the low-altitude signature of the low-latitude boundary layer.Observations of plasmas and magnetic field made by the HEOS2 satellite confirm the entry of magnetosheath plasma into the mag-

E 5 netosphere at high latitude via the entry layer [Paschmann et al.,19761. Crooker [1977a], using high-latitude charged particle andmagnetic field data from the Explorer 33 spacecraft, extended the

4interpretation of the entry layer to include high-latitude dipolarfield lines that may extend as far as 8 RE down the tail. Theseresults are consistent with our definition of the dayside boundary

3 -, layer. The IMP 6 spacecraft made measurements of plasmas andmagnetic fields in the LLBL (low-latitude boundary layer) nearnoon which show that ions and electrons that are in the boundary

21 layer near the inner edge of the magnetopause appear to be on-2 -1 0 1 2 closed field ines and are virtually indistinguishable from adjacent

Energy (log keV) magnetosheath particles [Eastman and Hones, 1979]. Thus, day-Fig. 2. Three near-simultaneous electron spectra taken from HILAT in side magnetosheath plasma has relatively unimpeded entry to thethe northern hemisphere, DMSP F7 in the southern hemisphere, and magnetosphere at low as well as high latitudes. This plasma hasAMPTE, C(E in the 1000 MLT equatorial magnetosheath at 8 Earth radii. a significant component of flow directed across the magnetic field

and is therefore a likely source of the region I currents.

Region I Currents and "Cusp" Particles Conjugate MeasurementsThe data presented in Plates I through 4 provide evidence that The similarity between the simultaneous, conjugate measure-

the region I current system in the prenoon sector is colocated with ments of magnetic fields and charged particles displayed in Plateslow-energy electrons and ions apparently of magnetosheath origin. 2 and 3 indicates that for conditions of northward IMF either day-This is in agreement with Erlandson et al.'s recent results from side merging can proceed in a hemispherizally symmetric mannerViking. Moreover, in each case the equatorward edge of the re- or the region of Birkeland currents and magnetosheathlike particlesgion I current is very nearly coincident with the lower-latitude can reside on closed field lines or both. Owing to the large positiveboundary of these particle fluxes. In Plates 2 and 3 the equator- B, component of the IMF it is unlikely that a dayside mergingward edge of the region I current in both the northern and southern process would produce hemispherically symmetric results. There-hemispheres coincides with the poleward edge of the region 2 cur- fore, under these conditions the boundary layer is likely to con-rents. Electrons associated with region 2 have hotter non-Max- sist of magnetosheath particles that reside primarily on closed fieldwellian spectra than those associated with region I and are likely lines. A comparison by McDiarmid et al. [19761 between low-en-to originate in the dayside extension of the plasma sheet. ergy particles measured with the SPS and more energetic particles

There is mounting evidence that the dayside region I currents measured with the EPD (energetic particle detector) on ISIS 2 re-are generated as a direct result of the solar wind/magnetospheric vealed that a significant fraction of the low-altitude cusp particlesinteraction and that they couple the electrodynamic processes be- do in fact reside on closed field lines. From this observation theytween the solar wind, .agnetosphere and the ionosphere. If mag- concluded that diffusion of magnetosheath plasma onto closednetosheathlike particles in the dayside ionosphere are colocated field lines could be one source of cusp particles. Evidence for bothwith the region I Birkeland currents as these data show, then it merging and diffusive entry processes was given by Reiff et al.is unlikely that this plasma originates in a stagnant region of the [1977]. They observed energy versus latitude dispersion signaturesmagnetosheath. On the contrary, an MHD generator that is the in the ion spectrograms acquired from the AE-C and deduced thatsource of the dayside region I currents is best suited to regions an inverse latitude versus energy relationship indicated mergingof high-velocity shears associated with "viscous" flow in the low- and that a V-shaped signature was indicative of diffusion. It waslatitude boundary layer [Eastman et al., 1976; Bythrow et al., 1981; determined that the merging signature dominated when the IMFLundin and Dubinin, 1984). In this view the region I current and had a southward component [Reiff et al., 1980]. In case B thethe cusp particles both originate in the boundary plasma which DMSP F7 spectrogram of ion energy flux versus energy and timeextends over the entire dayside, i.e., a dayside boundary layer. shows no evidence of an energy versus latitude dispersion, whichIn this context we define the "dayside boundary layer" as the mag- supports the conclusion that the dayside boundary layer and thusnetospheric plasma regime associated with those field lines whose the sheathlike particles reside on closed field lines for B, > 0.ionospheric foot points lie in the region of cusp particle precipi- Candidi and Meng [1984] used electron flux data from thetation, although their equatorial crossing points may lie tailward DMSP F2 and DMSP F4 satellites and solar wind data from ISEEof the dawn-dusk meridian. (See, for example, Figure 1 of Vasyliu- 3 and IMP 8 to examine the relationship between sheathlike elec-nas [19791.) tron fluxes in the dayside ionosphere and solar wind parameters.

Charged particles with magnetosheathlike characteristics are ob- They found that the density of the solar wind was well correlatedserved at low altitude over a broad range of magnetic local times with the intensity of low-altitude electron flux, while the orienta-

1 NOV 84DMSP-F7 NORTHERN HEMISPHERE BZ

N 1 H O 0 " D A D .s , D E C L " p" 7 \S A 'N 1 C H A R E L F A R II E 9 8 0

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the

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repre.hresh ,linrrents",

are rersn h ego a LnOf the ted as~o 2. cnurrents aerpe

[iee o0f the IM4F appeared atbest to hav ae muchMdng4 C , d " O A e1 984. i e i o M c ailer effect 7 1e

nerl that the total energy f lu~ j -11 nPae ,2 n dl Clr e,7th s O hah m~ ~ ~ o b u d

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Intoan the xtend

B)I I RkkS\ I I \1 t)X sttI 1 0RtK Ix P,\s) ('If sk(.1) P' R11 1i 1 Is

cres cotniponts io t he shecat hike preci pit at'on disappear. Gio- .lioa Aeontics and Space Adinmist rat ion in pros ding I he sensor fotst'lho cen cor a/ [19841 risi ng eleCt ron data from the [)NSP 1:2 sat- tIle I ) ISI F' magnetis tield experitneni is gratefulls ack,10i edged. \\eellitec cotncluided that polar raini electrons, orig inate in hie plasma aie graitUt to I .J1 Rich, I).A. Hard\ and J.1 J ames of Thle Air 1- orce

(icophs sic' Lahorabor\ for pros ding thle data for- this Ituds, to C. Tossni-Iniatit I fulli , tilie 0oitset Of polar ratit electron,, on thle da side mnay send and D. Hollanid of th-- Applied Physics Laborat or\ for their insal-he I dentified as, Ihe loss-fat it ude honndar\ of thle plasma mantle. able prostraniming elfforts aid to) t h, 11 W DI Space D~epartmt lorFtlas ma ttlant Ic i ons are e Ciudled from- thle tail lobes due to their tifrc design, construction, and test iris of' thle DM SP1 I- Tand t A t mag -lows t hernial pressure [G~ussenh/oven e cit., 19841. Thus, in the ion- fiic field experimlents.ospitere tilte po lessard lini t 01tai lward eonkectin a sheathike ions rite Editor thaniks R. I tijii arid . D. 1'iiniiingltait (-or thfei r asiai~c L

represets thtle IiUct-tat it ude lintit to thle Plasmia mantle. In eaeh tteaufn hsppr

casec ss, e has e exalittind. thle highe1St-latitude Birkeland current is Ri If t Ri ( IS

a-ocate wih tiN olewrd xtesio ofnlazi,.noseatlik ios itrross s. .1. R.. NI. 1). Ws'ilson, anid I. B. Nlcliarinid, Sitiultaneous field-anrd thierefore tlowss alone field title,, That thread the plasma mantle. alignted current and charged particle nea'sUrements in the cleft. iiiAn gI[lie electric: field rnadients required b\ these currentts imply a uieosphu'ri i'wiicles and lields. edited by\ 1. MI Nlcformac. D. Reidel,

di~reet 1(m f Iotiospheric plasitta ,i high lati tude oi he day- HinlaiMs.,1-23196side Bstioss. P. F., .RA. Heelis. ss\. B. Hanson, R.N. Power. arid R. A.

Hoftutan. Observational evidence for at boundar\ laver source of dlavside(t~t t S~ INSregion I field-aligned currents, I. Geophrs. Res., 86, 5577, 1981.

lie elaionhipof he arg-scle irk~an curen', o mg (atdidi. NI.. and C-. Meng, Nearly simultaneous obsersations of' theI li reat unxiip01 li laec-saleI~ikelnd urrntsto ag- conjtugate polar cusp regions, Planet. Space Sci.. 32. 42, 1984.itetosplieric boundaries attd plasma regintes as sse pereise them Chiapmani. S.,anid V. C. A. Ferraro, A ness theory of- magntetic storms.is sltoi fit [' late 5. This is a grossly simplified schemnatic thtat L1 Geoph 'rl-. Res., 378. 79, 1933.shtow~s tile projection of ionospheric Birkeland currents anid charged (rooker, N. U., Explorer 33 entry layer observations, J. Geophrs. Rex..

partcle ono tle oonnudii~i meidin ad euatria plne. 82, 515. 1977a.

paricls oto he oontuiitihtittridan nd quaoril patt. rooker. N. U i.. The magnectospheric boundary layers: A geonletricallyThe folloss nit list summinariz~es oitr results anid conclusions con- explicit mtodel, . Geophi-s. Res., 82, 3629, 19'77h.

cerning the relationship betwseen das side Birkeland currents and Eastman, T. E., and E. W. Hones, Jr., Characteristics of the magneto-Ilarged particles. These coniclusion,, htaw beeni drawn from a limit- spheric boundlary layer and magnetopause layer as observed by IMIPd let of !oss-altiude obsers atioris and from comparison with 6, . Geophcs5. Res., 84, 2019, 1979.

preIOU stuies bu we ropse hat he\are eneall appicale. Fastmian, T. E.. E. W. Hon es. Jr., S. J. Bamre, and J. R. Asbridge, Thepresion sttdis. bt sc popoe tht tey re enerllyappicale. magnetospheric boundary layer: Site of plasma, momentum and energy1 -or periods wshen the INIF has either positive or negative tranisfer fronm the magnetosheath into the niagnetospliere, Geophrs.

/cotmpontents, the prertoori regiotn I currents, are colocated with Rex. Lett., ., 685, 1976.charted particles that have characteristics, similar to those found Fratnk. F. A., Plasma in the Eartb's polar magnetosphere, . Geolphrs.

iii tc agntoshathAltoug thee prtilesbasebee trdi- Re_., 76, 5202, 1971.ill he navletsheth. lthughthee prtices avebee trdi- Galsez, M., Computer simulations of a plasma streamning across a mag-tiottallv asse :iated wiith rthe polar cusp, the region I currents flow itetic field, Phrvs. Fluids, 30, 2729, 1987.alonp field lines that thread the davside boundary layer. Thus, (Jussenhiosen. MI. S., D. A. Hardy, N. Heinemanni, and R. K. Burkhardt,this region is likely to be tlie soutrce of' the sbeatblike particles. Niorpftology of the polar rain. J. Geopmrs. Res., 89, 9785. 1984.

2. Ions, that are colocated ws ith the regiotn ICurrent systemn Hardy, D. A., L. K. Schmntt, MI. S. Gussenhoven, F. J. M~arshall, H.C. Yelt, T. IL. Schumaker. A. Huber, and J. Pantazis, Precipitating

base cssemitialls the same enerizs flux for both northward and electroni attd ion detectors (SSJ,'4) for black S/D flights 6-10 DMSPsouthwsard INIU. When thle INIF is directed soutbsskard, these ions satellites: Calibration and data presentation, Pubi. AFGL-TR-84-0317,exhibit al n ervs ser-SuS latitude energY\ dispersion. For northw-ard Air Force Geophys. Lab., Bedford, Mass.. Nov. 21, 1984a.I\111 this dispersion swictature is absent. Therefore, we conclude Hardy, D. A., A. Huber, and J. A. Panazis, The electron flux J sensor

thatfornonltsardI~hF patice aces to he assde ounary for HILAT. APL Tech. Dig., 5 (21, 125, 1984b.later notwr IM ,pril cestoted-ieh~dr Hard\. D).M. S. Gussenhoven, and E. Holeman. A sIultstical model

leris UIInipeded. bitt thle mechartisnm of entry may differ from of auroral electron precipitation, . Geophns. Res., 90, 4229, 1985.whlentile [.\I[ is directed sou1.tfisard. Heelis. R. A.. W. B. Hanson, and J. L. Burch. Ion convection velocity\

3. I[lie traditionral ''cLIp Birkelatid Currents'" lie poleward of' resersals in the dayside cleft, J. Geophyvs. Rex.. 81, 3803, 1976.thle mtost ititetnse sheailike particle precipitation. Hence, it is likely Hcikkila. W_ ., and J. D. Winningham, Penetration of magnetosheath

plasma to low% altitudes tfhrough the dayside magnetospheric cusps. J.tha~t thtese currett map to thle plasmia mantle arid arc associated fGeoptts. Rex., 76. 883, 1971.\%till disergent fli)\ of ionospheric plasma near noon. Heikkila, W. J., "'Comiment on 'Solar wind plasma injection at the mag.

4. Tile re' Lin I ecurrett sflowi alottg field lines, that map to netospheric cusp' bs P. H. Reiff, T. W. Hill, and J. L. Burch,'" .[tic'dssd hoitmdars, la~cr-' wh ile the 'cusp cutrrents"~ floss, on (;eo/ltvs Rex., 83, 227, 1978.

lijita, T., and 1. A. Potemtra, Field-aligned ctirrents, in the dayside cusptitos field lines that miap tailssard itito thle plasmna mantle. Thus, observed by TRIAD, I. Geoph/ox. Res., 8), 5971, 1976.ssc suggest that tfte loss-altit tde represetation of the magneto- lijima. T., and T. A. Potemra, ILarge-scale characteristics of field-alignedspheric cleft might be best idemtified ats the interface betwveen thle currents associated w~ithi substorms,.I1 Geophlvvx. Res.. 83, 599, 1978.region I amid hiefter-latittide Currents. We also suggest that a mnon Lundin. R.. and E. IDubinin, Solar witnd energy transfer regions inside theappropriate am for thi flg-laide curretste is the "mtait- magnetopause. I. Evidence for magnetosheath plasma Penetration. Plan-

ti":ret.an hieih-l.t ic Space So_, 32, 745, 1984.t~c'' curent. NcIiarmid, 1. BI., J. R. Burrows, and F. E. Budinski. Particle proper-

itt tite dayside cleft. .1. Geophvs Res., 81, 221, 1976.t~ ~ ~~~\ A i'i d'toi ei ih to thatnk Patrick Newsell anid D~avif Si- NMcIi rmid, 1. B., J. R. Burrows, and MI. D. Wilsont, Comparison of mag-

hcsk. for their rnatt helpfuil oittitenis andix suiggestionis concerning this netie field perturbations at high latitudes with charged particle and 1\11s..ik. e , ,ils, express itir gratitude iio R. I epping and .1. King oft the measumremrents, .1. GeoltVs. Res., 83. 681, 1978,6(i No im ri dire 1\11 data frot IMPI 11. Tbis research uias supported NleDiarmid, 1. B., .1. R. Burrows, and M. 1). Wilson, Large-scale mag-hi fic \aitinial ,.ciinawics,,itd 'Space N1dtitstsratioti, the LSA! Nir Forc itelic fietd pertarbatiotis atid particle measurements at 1 4W1 km ott the

ipisisI ahi rai)rs I tic Defentse Nuclear .Ngettc, I[tie National Soi daysidc, ./. Geophtrs. Res., 84, 1431, 1979.etice I mi daii i. anid [the (Pitt c~ it Naisal Rcsearcfi I fte I tuckliteed pr- MettgC .I.-lect ron precipitation in the tniudav autroral ox al, .. (ico-tion of this is irk \% a, u ppiicd hifthe Natitonal Necroi taffi Cs arid Spacein p/i u . Re%. 86, 2149. 1981.Admintistrationi tinder coitraco N .\S 5.28"9. [he assistance of the Na N1idglc\, -1. F., and I . IDasis, JIr., Calculatiot by a moment tecfhnique

BYTHROW ET AL.: DAYSIDE CURRENTS AND CHARGED PARTICLES 9803

of the perturbation of the geomagnetic field by the solar wind, J. Geo- tion experiment (HPCE), IEEE Trans. Geosci. Remote Sens. 23 (3),phys. Res., 68, 5111, 1963. 241, 1985.

Paschmann, G., G. Haerendel, N. Sckopke, and H. Rosenbauer, Plasma Sonnerup, B. U. 0., Theory of the low-latitude boundary layer, J. Geo-and magnetic field characteristics of the distant polar cusp near local phys. Res., 85, 2017, 1980.noon: The entry layer, J. Geophys. Res., 81, 2883, 1976. Spreiter, J. K., and B. R. Briggs, Theoretical determination of the form

Potemra, T. A., W. K. Peterson, J. P. Doering, C. 0. Bostrom, R. W. of the boundary of the solar corpuscular stream produced by interac-McEntire, and R. A. Hoffman, Low-energy particle observations in tion with the magnetic dipole field of the Earth, J. Geophys. Res., 67,the quiet dayside cusp from AE-C and AE-D, J. Geophys. Res., 82, 37, 1962.4765, 1977. Vasyliunas, V. M., Interaction between the magnetospheric boundary layers

Potemra, T. A., P. F. Bythrow, L. J. Zanetti, F. F. Mobley, and L. Scheer, and the ionosphere, Proceedings of the Magnetospheric Boundary Lay-The HILAT magnetic field experiment, APL Tech. Dig., 5 (2), 120, ers Conference, Alpbach, 11-15 June 1979, Eur. Space Agency Spec.1984. Publ., ESA SP-148, 387, 1979.

Reiff, P. H., T. W. Hill, and J. L. Burch, Solar wind plasma injectionat the dayside magnetospheric cusp, J. Geophys. Res., 82, 479, 1977.

Reiff, P. H., J. L. Burch, and R. W. Spiro, Cusp proton signatures andthe interplanetary magnetic field, J. Geophys. Res., 85, 5997, 1980. P. F. Bythrow, R. E. Erlandson, T. A. Potemra, and L. J. Zanetti,

Rich, F. ' , D. A. Hardy. and M. S. Gussenhoven, Enhanced ionosphere Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20707.magnetosphere data from the DMSP satellites, Eos Trans. AGU, 66, D. M. Klumpar, Lockheed Palo Alto Research Laboratory, 91-20, 3251513, 1985. Hanover Street, Building 255, Palo Alto, CA 94304.

Rosenbauer, H., H. Grunwaldt, M. D. Montgomery, G. Paschmann, andB. Sckopke, HEOS 2 plasma observations in the distant polar magne-tosphere: The plasma mantle, J. Geophys. Res., 80, 2723, 1975. (Received January 28, 1988;

Shelley, E. G., A. Glielmetti, E. Hertzberg, S. J. Battel, K. Altwegg- revised April 20, 1988;VonBurg, and H. Balsiger, The AMPTE/CCE hot-plasma composi- accepted April 28, 1988.)

Accession For

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