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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. A7, PAGES 13,537-13,540, JULY 1, 1994

Comment on "Low-energy particle layer outsidethe plasma sheet boundary" by G. K. Parks et al.
Ya. I. Feldstein
Insfitut·of Terrestrial Magnetism, Ionosphere Radiowave and Propagation, Troitsk, MoscowRegion, Russia

Yu. I. Galperm
Spae·Research Instituteof RussianAcademyof Sciences, Moscow

Abstract. We consider mappingschemes the wherethe plasmasheetboundary layer (PSBL) in the tail is projected the auroraloval as inconsistent to with the availabledata. We conclude that the low-energy layer (LEL) structure discovered Parkset a1.(1992)at the outeredgeof by the PSBL is consistent with the mappingscheme Feldsteinand Galperin(1985) wherethe by boundary plasmasheet(BPS) (whichincludes PSBL properandpresumably also the can includethe newly discovered LEL) is mappedto the polar diffuseaurora(PDA). Note that this definitionof BPS plasmadomainin the tail (which includesonly the PSBL and LEL, as distinctfrom the centralplasmasheet(CPS)), which projects the bulk of the regionof the to discrete aurora,or the auroraloval), differssubstantially from the BPS as definedby Winningham a1.(1975). In Winningham's et definitionthe BPS refersto the structured accelerated electrons precipitation regionat ionospheric altitudes, and it sometimes was improperly interpreted indicatingthe mappingof the nightside as auroraloval to the plasma sheet outerboundary.

In the analyzedISEE data by Parks et al. [1992] a new particle structure the tail was found:thelow energylayer in (LEL) composed an outwardelectronbeam with energies ~ of 100 eV and an earthwardbeam of low-energy ions. The structure, according to Parks et a/.[1992] (referred to hereafteras P92), is usually present at the outeredge the of plasmasheetboundarylayer (PSBL) and has a width of

precipitationstructures, such as inverted-V events. P92 claim that this schemeis an alternativeto those proposedby Eastman et a/.[1954, 1985] and by Feldstein and Galperin [1985], because the LEL was not known at that time. These two schemes, like the P92 model, relate the plasmadomains observed within the tail to the distinct regionsof auroral particleprecipitationand luminosity.
The main differences between the models of Eastman et al.

about(0.1-1.4) RE at distances (15-22) RE. The LEL
complements overall description the tail structure, the of which was establishedfrom the comprehensive situ measurements in by Eastmanet al. [1984, 1985]. It contains the centralplasma sheet(CPS), the main body of hot nearly isotropic plasmain the tail, and the PSBL with field-alignedvelocity-dispersed particlebeams,as observedat energies> 1 keV at the outer (lfigh latitude) edgesof the CPS. The lower energiesof the LEL, locatedat the outer edgeof the PSBL, may sicily some specific physicalprocess the distanttaft, or possibly in just an extension the PSBL generation of processes lower energies to and other locations. Anyway, the discovery LEL certainly of will play an importantrole in the theoreticaland modeling
studies of the distant tail.

Questions ariseaboutthe LEL mappingto the auroral ionosphere, comparisons with the corresponding measurements at lower altitudes, andthe relation of this new featureto previous models.P92 propose scheme which the LEL is mapped a in to the region of subvisual auroral luminosity excited by lowenergy electrons and located poleward of the discrete
Copyright1994by the AmericanGeophysical Union.
Papernund·r 93JA03269.
0148-0Z27/94/93JA-03269502.00

[1984,1985] and of Feldstein and Galperin [1985] were discussedat length in the latter paper and by Galperin and Feldstein [1989, 1991] (hereafter FG85, GF89, and GF91, respectively). short,the former model mapsthe PSBL to the In auroraloval, andthe centralplasma sheet (CPS) is mapped to the diffuseauroraequatorward from the oval. The modelFG85 (see Figure la taken from FG85) maps the boundaryplasma sheet (BPS) to the polar diffuseaurora(PDA), which is usually observedpolewardfrom bright discreteforms of the nightside auroral oval, while the tail CPS is mappedto the oval. A schematicglobal distribution of auroralfeaturesis shownin Figure la for a disturbed period% (Kp=5). In the captionthe magnetospheric plasma domains are identified, which are conjugate respective to auroralregions according FG85. to The equatorial boundaryof the oval, which was defined from brightdiscreteauroralforms, lies closeto the boundaryof stable trapping for high-energy particles (or the isotropic boundary).This boundaryplays a fundamentalrole in our mapping scheme because can be usedas it a naturaltracer it observable all altitudes.On the nightsideit definesa narrow at transitionshell region between the quasi-dipolarmagnetic field of the inner magnetosphere the taillike, stretched and field region farther in the tail. Evidently, this transition shell regionis due to the inner edge,or a strongoutwardgradient,of

13,537


13,538

FELDSTEIN

AND

GALPERIN:

COMMENTARY

the integrated cross-tail current of the plasma sheet FG85 and thus cannot agree with the alternative mapping (including the neutral sheet). Thus it divides the two proposed the formerpaper. in magnetospheric regions with grossly different energetic In Figure la and lb we compare the schemesof auroral particlemotions and currents. is the main physicalbasisof luminosity It structure their mappings the magnetospheric and to our scheme and notations. plasma domains duringdisturbed times according FG85 and to The regionof the inner magnetosphere extending inwardfrom P92. In these paperbrightdiscrete auroral forms are located the trapping boundary(the outer radiationbelt) to the soR between the polar and equatorial zones of diffuse electron precipitation boundary(SEB), a convection bound- precipitation. The bordersof the zonesin Figure la are given in local time ary, or the plasmapause, thus mapped to the diffuse approximately the invariantlatitude-- magnetic is auroral precipitation equatorward from the nightside oval. This coordinate frame for disturbed time (Kp -· 5) according to the outer part of the trapped zone where the large-scale various experimental data. Under disturbedconditions convection particles from the plasma sheet that are polar diffusezone width oRen shrinksto - 50 - 100 km at the continuously convected still exists, was named the remnant ovalpoleward border.In Figure lb (taken from P92) only a pattern withouta coordinateframe is presented for layer in FG85. Low-energy and/or injected here during qualitative recovery phase. substorms, form the diffuseprecipitation zone,with dispersed a substorm Let us evaluateand comparethese two schemes. The LEL "plasma clouds"[ de Forest and Mcllwain,1971] that gradually decay. Thereforethey can be considered remnantsof the width at high altitude,as estimated P92, is from - 41 to as by to plasma sheet hot plasma within the region of energetic 8900 km. This LEL width, when mapped the ionosphere particletrapping. usingthe Tsyganenko-87 model for Kp = 3 ( Tsyganenko, The definition of the BPS in FG85, GF89, and GF91 was 1987),gives width of 0.55 to 121 km. Usinga typicalLEL the a somewhatbroader thanthat of PSBL by Eastmanet al. [1984, width of 450 km, one calculates projectionof about5 km. This is clearly too small to comprise the regionof discrete 1985] becauseit included lowerparticleenergies. Electrons of < 1 keV were considered to extend fm·er outward then those of aurora,and it is probably also too small for, or sometimes to, > 1 keV. This is consistent with the generally largerobserved comparable the typical width of the PDA, evenfor disturbed In PDA width at lower precipitating electronenergies than the conditions. FG85, GF89, and GF91 it was notedthat the PDA expandsduring quiet times, which fm·er increasesthe PSBL projection. This led us to introduce term BPS. Now, the we believethat the BPS may incorporateboth the PSBL and inconsistency with the P92 mapping the recovery for phase. that the PDA includes LEL. It is interesting notethatin manyrecent to papers term At the same time, the supposition the PSBL is used in the same broad sense as we used the term BPS. precipitation regionsboth from the PSBL and LEL, is not
It remains to be seen whether the LEL is due to a different
inconsistent with the data.

physicalprocess from that of the PSBL proper,and until this time we would prefer to use the term BPS as includingboth
the PSBL and LEL. We were unable to find contradictions

betweenthe new data described P92 and the model in by

As to the precipitating particlesof the PDA, in FG85 and GF89 it wasalsonotedfromthe results low-energyparticle of data on the COSMOS 261 and AUREOL 1, 2, and 3 satellites, thatlow-energy electrons with energies 30--150 eV arethe main

a

s·'

DiffusePrecipitation (Plasma Sheet)

Discrete Aurora(PSBL?)
Subvisual Auroral

Lurninoaity (LEL)

oo

Figure1.Schematic pictures summarizing spatialdistribution different of auroralluminosity typesaccording to the two schemes: From Feldstein (a) and Galperin[1985] the A -- MLT frame (fromtheir Figure24b for in Kp=5). Auroravoid -- polar cap area free from brightauroralluminosity, projected the tail lobes;polar to diff·e aurora(PDA) projected the boundary plasma sheet(BPS) in the tail; SA, structured(discrete) to aurorasof the auroraloval,projected the central to plasma sheet(CPS); D, diffime auroras withinthe auroral ovalof discrete forms; DA, diff·e aurora equatorward fromthe auroral oval,projected the outer radiation to belt region till the convection boundary (plasmapause); PSPA,postsubstorm plasmaspheric diffime auroras (withinthetrappedradiationregion); (b) From Parks et al. [1992].


FELDSTEIN

AND GALPERIN:

COMMENTARY

13,539

Thusit seems the mapping the BPS (including that of PSBL), with the general softening electrons of towards outwardedge, its to the regionof "subvisual auroralluminosity" poleward from the oval (PDA), is well documented supported various and by measurements. is, however, at odds with the mappingin It Figurelb, wherethe PSBL (with a question mark) is mapped to the discrete aurora region, despite the statementin P92 (p.2952) that "The streaming low-energy ion beams are the velocity dispersedion streamsthat Zelenyi et a/.[1990] observedin the ionosphere." According the mappingschemein Figure l a, the band of to the LEL projectionto the ionosphere must be located at the outer part of the PDA (i.e., in the outer part of the BPS were used in GF89 and GF91 as the natural tracers for the projection)or on its outer border. We believe that this new mapping the PSBL to the PDA. of structure the tail now can be incorporated the scheme in into VDIS--II struc·es that consistof ion energies much greater of Figurela aspart of the BPS withoutany changes its other in thanreported the LEL havebeen observedat low altitudes parts. Theoreticalarguments the physical distinctionbein for from the AUREOL 3 in only about 11% of passes. higher tweenthe PSBL andLEL are still not very clear. At The VDIS-II structures within the PSBL are well described altitudesand at lower ion energiesthis percentage much is of higher,up to - 40% as was recentlyshownby Saito et by the model calculations Ashour--Abdallaet al. [1991, ion field a/.[1992] from AKEBONO data. These high-altitude 1992] as due to nonadiabatic motionsin the magnetic reversal within the distant neutral sheet. The nonadiabatic measurementsalso show that the typical difference in electric potentials alongthe field lineswhichcontain VDIS--1I effectsfor ions crossingthe neutral sheet give rise to fieldfor function. ion structures, negligible. It cannot si·ificantly limit alignedacceleration a part of the ion distribution is the latitudinal width of VDIS--11 structures and extend them The resultingion beams are velocity dispersed the E x B by
above the inverted-V structuresof the auroral oval, as was redrffi in the dawn--dusk electric field. The model results of

contributors the excitationof the auroralemission to (mostly 630-nm oxygen lines) in this region.Numerous citations to previous ground-based airborne and measurements supporting this inference were also given in the above papers. These particles believedto be the low-energy are part of the BPS population couldalsoinclude LEL particles. and the Velocity-dispersed precipitation ion structures the second of type(VDIS-ID, whichappear sporadically withinthe PDA, were reportedby Kovrazhkin [1987], Bosqued [1987],and Zelenyiet al [1990].As stated the latterpaper 12,121),the VDIS-1I in (p. structures observedat ionosphericaltitudeswere identified, using the mappingproposed earlier in FG85 and GF89, with the ionospheric precipitation signatures the dispersed of earthward beamsof the BPS, or the PSBL, as observed ion in situ by Takahashi and Hones[1988]. The VDIS-II structures

accounted for as a contribution from the LEL soft electrons at

the outward part of the BPS.

resultof the same nonadiabatic accelerationprocess which takes place duringthe first encounter the plasma mantle of tailward streaming ions with the distant neutral sheet. In this first encounter a si·ificant angular deflection of streaming ionscan occur with only minor acceleration accordingto Ashour-Abdalla a/.[1991, 1992]. et Onsageret a/.[1991] constructed simplekinematic model a of the PSBL formationfrom the velocity filter effect with a particle sourcedistributedalong the distanttail. This model of mapping BPS(or PSBL)of 1--2RE thickto the whole reproduces very well the particle distributionfunctions the within the PSBL, if the plasmatemperature within width of the oval (including inverted-V events) which is observed usuallyseveral hundred km wide, and thus subtendsmuch the CPS is supposed be higherthan in the lobe(whichmay to with the nonadiabatic heating found by Ashourhighermagnetic flux. This difficultyis alsonotedin P92, but be consistent didnot leadto a modification Figurelb. of Abdalla et al. [1992] for the plasma sheet ions). This to Otherarguments mapping BPS, including PSBL, to kinematicmodel is complementary the more rigorous ion for the the
the PDA are describedin FG85, GF89 and GF91. One of the

cently suggested Lyons[1992]. by As to the low-energy tailward streaming electrons the LEL, of their locationon the PDA field lines is apparently accepted in P92. However, in FG85 the whole structureof the BPS is mapped the PDA, whereasin the schemeof P92 only the to LEL, that is, a narrowplasmalayer at the outerpart of the BPS, is mappedto the wholePDA region. In the P92 scheme PSBL properis mapped, in the work the as by Eastman a/.[1984, 1985], to the oval (but with a question et mark in Figure lb). Many arguments againstsucha mapping are described detail in FG85, GF89 and GF91. In particular, in the magnetic flux argument was used to showthe impossibility

Ashour-Abdalla a/.[1992] are very similar to the observed et VDIS--II, includingtheir fine structures. We note that the
location of the LEL at the outer border of the PSBL could be a

trajectory calculations Ashour-Abdalla al. [1991, 1992]. by et
In both of these models the PSBL is located on closed field

most convincingof them is basedon the directmeasurements of the PSBL observedfrom ISEE, and its projections, using the Tsyganenko [1987] model, to the bright arc at the polar borderof the extended oval usingsimultaneous auroralimages fromthe DE 1 [ Frank and Craven,1988]. As the field-aligned currents were not included in the tracing model, it is difficultto decidewhetherthe PSBL was projectedexactly to the bright auroral or sometensof kilometers arc poleward from it. In this particularexamplethe 1.5-keV electrons extended
further outward within the PSBL than the electrons of 5 keV.

linesof the CPS at largedistances. Anotherpossibility for the LEL generationis a distinct ion acceleration regionand/ora specific process the far tail in (e.g.,at the distant neutral line or turbulent plasma

sheet). Bothpossibilities consistent are with the mappingof the BPS (whichincludes PSBL)to thePDA. the Summarizing, conclude we that the discovery the LEL by of Parks et al.[1992] is fully consistent with the mapping scheme proposedby Feldstein and Galperin [1985], and

From our Thusit was natural to suppose the lower-energy that electrons with the low-altitude data available on the PD· that constitute the main part of the precipitatingelectrons schemethe LEL, located the outerboundary the PSBL, at of
within

thatis withinthe BPS, mustbe mapped the outerpart,or to to this way a wider PDA region (and the observedaverage the boundary, the PDA, thuscomplementing scheme. of this polewardsoftening electronenergies of within it), could be We conclude that the previouslyknownplasmadomains, the

the PDA

extend

even further outward in the BPS. In


13,540

FELDSTEIN AND GALPERIN' COMMENTARY donshipto magnetospheric plasma domains,in: .Auroralphysics, editedby C.-I. Meng, M.J. Ryereft, and L.A. Frank, p. 207,
Cambridge UniversityPress,New York, 1991. Kovrazhlcin, R.A., J.-M. Bosqued,L.M. Zelenyi and N.V. Jorjio,Observations evidence reconnection plasma of of and acceleration at

BPS (including PSBL and,now also the LEL), the CPS, the and the region of stabletrapping(outer radiationbcR) are

mapped according Feldstein to and Galperin [1985] (in
particular,for disturbed timesin accord with Figurela).

Acknow _led·mmmts. workhas This been suppo. attheIZMIRANby r· Russian Foundation Fundamental for Research grant'93-05-8722.

a ·stancoof about500000km in the tail of ·e Earth's magnetosphere, Lett. Soy. JETP, Engl. Trans·l., 377,1987. 45,
Eur.Space Agency Spec. Publ.,ESA$P-335,257, 1992.

L.R., Inferences concerning magnetospheric the source region TheEditor thanks Borersky a seco· referee their Lyons, Joe and for forauroral breakup, SUBSTeRMS edi,'tod C.Mattok, in: 1, by assistance in evaluating paper. this

Onsager, M.F. Thomsen, Elp·c, andJ.T.Gosli·,g, T.G., R.C. Model
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