Singly ionized energetic helium has been observed in the solar wind by using the time of flight spectrometer SULEICA on the AMPTE/IRM satellite between September and December, 1984. The energy density spectrum shows a sharp cut off which is strongly correlated with the four fold solar wind bulk energy. The absolute flux of the He(+)ions of about 10000 ion/sq cm.s is present independent of the IPL magnetic field orientation. The most likely source is the neutral helium of the interstellar wind which is ionized by solar UV radiation. It is suggested that these particles represent the source of the anomalous cosmic ray component

Gloeckler, G.

Hovestadt, D.

Ipavich, F. M.

Klecker, B.

Moebius, E.

Scholer, M.

English

NASA Technical Reports Server

SH4. 6-5
176
OBSERVATIONOFPICK-UPIONSINTHESOLARWIND:
EVIDENCEFORTHESOURCEOFTHEANOMALOUSCOSMICRAYCOMPONENT?
D.HOVESTADT,E.MOBIUS,B.KLECKER,M.SCHOLER,
Max-Planck-Institut fi]r extraterrestrische Physik,8046 _RCHINO,Oermany
O.(3LOECKLERandF.M.IPAVICH
Dept.of PhysicsandAstronomy,University of Maryland, COLLEOEPARK, MD20?42, USA
ABSTRACT.Singly ionizedenergetichelium hasbeenobservedin the solar
wind by usingthe time-of-flight spectrometerSULEICAon theAMPTE/IRM
satellite between September and December, 1984. The energy-density
spectrum shows a sharp cut-off which is strongly correlated with the
four-fold solar wind bulk energy.Theabsoluteflux of the He+ionsof about
lo4ions/cm2-s is present independentof the IPL magneticfield orientation.
Themost likely source is the neutral helium of the interstellar wind which
isionizedbysolarUV-rediation.Itissuggestedthathesparticlesrepresent
thesourceoftheanomalouscosmicraycomponent.
I.Introduction.In 1972 anomalousfeatureeinthelowenergyquiet imecosmicrayenergy
spectrum havebeendetectedforhelium,oxygen,nitrogen,and neonby GARCIA-MUNOZet
el.(1972),HOVESTADTetel.(1973),andMc DONALDetel.(197,I).Thesefourelementsareknown
tohaveahighfirstionizationp tentialcomparedtootherelementslikecarbon,magnesium,silicon
andiron(e.gALLEN(1973)).ThisfactleadFISKetel.(1974)tosuggestthat hesourceofthe
particlesi theinterstellarneutralwindwhichbecomesionizedintheinnerheliesphereby
interactionoftheatomswithsolarultra-violettradiationand/orsolarwindions.Thenewlycreated
ionsthenarepicked-upbytheinterplanetarymagneticfield.Withtheirgyro-motioninthesolar
windframetheyrepresentadistinguishedpopulationwhichissubsequentlyconvectedintotheouter
heliospherewhilebeingacceleratedwithintheturbulentmagneticfieldsoftheheliosphere(e.g.
FISK1976a,b)orattheterminatingshock(PESSE8etel.(1981)).Theresultingenergyspectrum
isthenreshapedtotheobservedspectrumby modulationa dpropagationeffectsininterplanetary
space.Thispaperpresentsfirstdirectobservationsfthedistributionfu ctionffreshlyionized
heliuminthesolarwindwhichlikelyhasitsoriginintheneutralinterstellarwindandprobably
representthesourceoftheanomalousheliumcomponentincosmicrays.
2.ThePick-UpProcess.Freshlyionizedheliumatomsarelocallysubjectedtothecombinedforces
oftheinterplanetaryVswxB fieldofthesalarwindandtheLorentz-forceinthemagneticfield,B
whereVSW isthesolarwindvelocity;i.e.intheinertial-system(whichnearlycoincideswiththe
spacecraft-system)theparticlesundergoinitiallyacycloidalmotionina planeperpendicularto *
thelocalmagneticfield,B,witha minimumvelocitywhichisequaltothevelocityoftheneutral
wind(assumedtobe < 20 kmls andthereforeb ingneglectedin thefollowing).Themaximum
velocitywhichtheseionscanobtaininthepick-uprocessisdeterminedbythesolarwindvelocity o
andtheanglec(betweenitsflowdirectionandthedirectionofthelocalmagneticfield:
v(max,loc)= 2-VSW. sin(x (I) . Thisleadstoanenergyof
E(max,loc)= 4- M/2-VSW2. sin2 _ (2)
Inthe8clapwindframethemotioninitiallyeadstoa conicalpitch-angledistributionwiththe
pitchangleo(.Thevelocitiesparallelandperpendiculartothelocalmagneticfielderegivenby
https://ntrs.nasa.gov/search.jsp?R=19850026730 2020-03-20T16:51:43+00:00Z
177
SH4.6-5
v(par) =V6w-COS o_ (3) and
v(perp)= VSW. sin c¢ (4) .
Thereforethe total ion velocity in thesolar wind frame is givenby
v = (Vpar2 + Vperp2)1/2 = IVswl (s)
If the interplanetary mediumwere homogeniouswith parallel magneticfield lines and if there
were noscattering imposedby intrinsic or self-generatedwaves,the distribution function would
remain gyrotropic andring-shaped in the three-dimensionalvelocity space.
In the real world ,however, the motion of the particles is subjected to effects generatedby
temporal and spatial magnetic irregularities in the expandinginterplanetary medium. While
energy-changingwave-particle interactions can be neglected,pitch-angle scattering andadiabatic
deceleration can greatly influence the particle distribution. Within a scattering mean free
path-length X the initially ring-type velocity distribution is expected to be reshaped by
pitch-angle diffusion into a spherical-shell type distribution which is fully canvectedwith the
solar wind. Theorbital velocity Ivl= IVswlremains constantin the solar wind frame as long as
adiabaticdecelerationdoesnot play a significant role. In the inertial systemthe expectedvelocities
rangefrom zero to 2. YSWwith correspondingobservableenergiesbetweenzero andfour times the
solar wind bulk energy (0 < Epick..up< 4- M/2. VSW2).Theenergyspectrumshouldshowa clean
cut-off at that energy value. Thespectrum below cut-off should
reflect the effect of adiabatic deceleration in the expanding AHPTE/IRH SULEICA
interplanetary mediumupstreamof theobserver. 20 20keV/e
S. INSTRUMENTATIONandSATELLITE.Thedatapresentedhere are _-SW
obtainedwith the suprathermal particle spectrometerSULEICAof 1£ He+n '
the Max-Planck-lnstitute/ University of Maryland onboard the j',11
IRM spacecraft of the A_,ctive-Magnetospheric-P_.article-Iracer
Explorer project (AMPTE), launchedon 11. August 1984 into a
highly elliptical orbit with an apogeeof 18.9 earthradii. During _6.skev_eHe+
thetime period from launchuntil December1984 the S/Cspenta z_
large fraction of eachorbit in the solar wind upstream of the 1(
bow-shockof the earth. TheSULEICAspectrometeris basedon the
techniquesof electrostatic deflection followedby a time-of-flight o
and residual energy measurement(for details see MOBIUSet
el. 1985). Theelectrostatic deflectionanalyser, representedby two 0 n, f "1111/*.1keV/e
concentricsegmentsof a sphere,selectsincomingionsaccordingto
their energyper chargein 24 logarithmically spacedvoltagesteps --_r_+- __
correspondingto anenergy rangefrom 5 to 226 keY/charge.After 10
passing through the analyser the ions enter the time-of-flight
sectionwhere the velocity of the ions is measured.The ions are
, stoppedin a Silicon surfacebarrier detectorwhere the residual
energy is_determined.Thegeometrical factor of the instrument is 100 200. 300
4.3.10 -2cm 2srandtheenergyresolutionisAE/E = 0.097. TIME-0F-FLIGHT[NSEC]
. For the investigationpresentedhere theenergyof the pick-up ions _: Typical TOF-histogram!
is too low to create a sufficiently high energy signal in the at three different energy steps,',
solid-state detectors.Thereforewe identify the ion-speciesonly by takenin the solar sector.Dataare
combiningthe electrostaticdeflection( E/Q )and thetime of flight obtainedon Nov.11,I 984 at = 18
signal (TOF). For a given E/Q stepthe TOFhistogramas shownin REin frontofth8bow-.,hock
fiqure 1 therefore represents a mass-per-charge histogram
which is takenin thedirection of the solar wind.
178 SH4. 6 - 6
4. OBSERVATIONS. A limited number(ten) of AMPTE//IRM 84-11-11
observational periods were chosen in the solar wind lo:50:oo - 11;29:.39
between launch of the 8/C and December,15, 1984. AZIMUTH: 0,0- 0.0
During all periodswe observeda peak in the i.o_+o8
TOF-histogramsatMIQ = 4 at EIQ stepswhichare J)E
significantlyhigherthanwe wouldexpectforgenuine I
solarwindparticles.DependingontheEIQstepandthe ,_E+os
solarwindvelocity,we also bserveinadditiona broadn,
andvariablepeakatTOFvalueswhichcorrespondtothe
',solarwindvelocity.TheseionswithMIQ valuesabove5 _ ,.oE+o4
correspondtosolarwindheavyions.FiqureI showsa
typicalexampleofTOF histogramsforthreedifferent
, l, I
E/Q steps obtainedon November11, 1984. It should be ½_.oE+o_ I \\noted,that the He+ peak is visible at all orientations of 0
the interplanetary magnetic field no matter wether \ I .* _XL3
bow-shockacceleratedionsare presentor not. -1.oE+o2 "--
Fiqure2 showstheenergy-densityspectrumof theM/Q I -- 7"' t
: 4 peakas measuredduring a sample period of 40 I i
minute duration on Navenber11,1984, whenthere are ;.oE+o; ......... I _
nobow-shockacceleratedparticles present.Thespectra ;.oE+oo ;.oE_ol +o2
are taken from the directional sector which alsocontains ENERGY[ReV/Q]
the solar wind. Except the two neighbouringsectors all Fiqure 2:Exampleof an energy-density
ether sectors do not ahow any counts in the M/Q = 4 spectrum of the M/Q=4 ion channel
bin.There is a sharp cut-off at an energy of about 23 accu- mulatedover 40 minutes. Dashed,
keV/ewhich correspondsto four time thebulk-energy of curved line representsthe 1 count/step
the solar wind. Belowthecut-off energyweobservea level.
more or less flat distribution which endsat a sharp rise of the spectrum at about 8 keY/e. We
attribute the rise to solar wind particles of M/Q = ,t which will benotdiscussedin this paper.
5. Discussion.Following theargumentsin section2 we tried to correlate the experimental cut-off
energies, firstly (_) with the full four-fold solar wind bulk-energy for mass M = 4
(Helium), and secondly(figure 4) with the four-fold componentperpendicular to the local
interplanetary magneticfield (eq.(2)).
There is an excellent correlation with the full energy (correlation coefficient = 0.98) while the
correlation with the perpendicularcomponentis rather poor (correlation coefficient = 0.26). This
result suggeststhat the originally injected ions havelost their directional information in the solar
wind frame due to pitch angle scattering, while maintaining their injection energy. The poor
Correlation with the perpendicular componentshows that the ions have lost their directional
information and,thereforesuggeststhat the sourceregionof the ions extendsbeyondonemeanfree
scattering length upstreamof the observer. Themagneticrigidity of suprathermal singly ionized
heliumofafewkeY/nucleonisintheorderof5 toI0 MY.The meanfreepathisknowntobeofthe o
orderof 0.05AU duringquietinterplanetaryconditions(e.g.compilationinMASON etel.(1983)).
Theapparentlyargextentofthesourceregionexcludesaterrestrialoriginoftheions.
Theeffectofadiabaticdecelerationsexpectedtobehighduetostrongcouplingoftheparticlesto
thesolarwind.Thissetsanupperlimitforthedistanceb tweenpointoforiginandobserver.Ions "
originatingtoofarupstream(ortcoclosetothesun)loosesomuch energythattheybecome
indistinguishablefromthesolarwinditself.
Togeta roughestimateofthesourcestrengthofthepick-upionswe usetheenergyspectrum
(_) andarguethat heenergychannelsbelowthecutoffenergyarepopulatedviaadiabatic
deceleration,therateofwhichisassumedtovaryasIIrwithheliocentricd stance.Themeasured
179 SH4. 6 - 5
quantityE.djldE-d_ineachenergychannelinfigure3 isrelatedtotherelevantsource-quantities
as: E-djldEdR _ E •S(r)•Ar •4-AC2SULEICAI(4mAE-A_) = 5(R).ArlTT-(EIAE)
where S(r)= NHe • _ion isthe sourceCut-0ffEnergy(keV) AMPTE/SULEICA
strengthofsinglyionizedhelium.For a 3o
relativeenergywidthoftheinstrumentof
AE/E = 0.Ithelengthofthesourcecolumn 25 /
oftheSIC translatesintoAr = 20 /'/"upstream
0.2AU = 3-I012cm.Usinganaveragevalue . i...-_'
of 7 .I05 fortheenergydensityofthe _5 _
pick-upions(fromfigure2J we obtaina
sourcestrength8(r=I AU) _ 8 • 10-10 io ,-,"I"I
IcmSs.Foran ionizationrateof8.10-81s
5
(e.g.Holzer1977) we arriveata neutral
interstellarheliumdensityNO=10-2Icm5, o
a valuewhich is fullycompatiblewith s 1o _5 2o 25 3o
resultsfromopticalEUV measurementsof 4-MI2.Vs2w ( key)
He II resonantlyscatteredEUV-linesFiQure3:Correlationbetweenexperimenmtalcut-off
(WELLER andMEIER 1974;DALAUDIERet energiesandthefour-foldsolarwind bulk-energy
ai.1984) in the heliosphere.These(Correlationc efficient= 0.98).
measurementsrepresenthe firstdi.reCtcut_offEnergy(keY) AMPTE/SULEIC.A
observationof energetichelium ]ons30
picked-upby the solarwind from the
interstellarneutralwindtherebyfilling25 "
onemoregapinthehypothesestoexplain
the anomalouscosmicray component. 20 •
• Iii II •
AcknowledqementThe authors are grateful 15 -. . .
to the many individuals at the Max- • •
Planck-lnstitut and the University of _o
Marylandwho contributed to the successof •
theexperimentandto theAMPTE project as 5
a whole.
O I I I I I "
REFERENCES 0 5 10 is 20 25 30)
Alle._._B_n1973,ASTROPHYSICALQUANTITIES, 4.M/2'V2vsin2oc(keY )
The AthlonePress,London FiQure4:Sameasfigure3,butabscissaisvertical-to-B'
Dalaudieretal.1984, componentofthe8W bulk-energy(Corr.coeff.=0.26).
Astron.Astrophys.,14 171
Fisketel.1974,Ap.J.Letters190,L35
- .FiskL.A.1976a, Ap.J.20G, 333; 1976b, J.Oeophys.Res.8!, 4633,
_rcia-Munoz et el. 1973, Ap.d.Letters! 82, L81
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Pesseset el. 1981, Ap.d.Letters,2_16,L85
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Observation of pick-up ions in the solar wind: Evidence for the source of the anomalous cosmic ray component?

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