JOURNAL
OF GEOPHYSICAL
RESEARCH, VOL. 92, NO. A6, PAGES 6133-6140, JUNE 1, 1987
Ion Distributionsin the DaysideMagnetosheaths
of Jupiterand Saturn
JOHN D. RICHARDSON
Centerfor SpaceResearch,Massachusetts
Instituteof Technology,Cambridge
Data from the Voyager1 and 2 PlasmaScienceexperimentin the daysidemagnetosheaths
of Jupiterand
Saturnare analyzed. The ion distributionsthroughoutthe daysidemagnetosheaths
of both planetsare well
modeledby a two-temperature
protondistribution.The two protonpopulationshave comparabledensities,
and temperatures
of 100 and 1000 eV. Similar two-temperature
protondistributions
are occasionally
observed in earth's magnetosheath. Ion temperatures,densities, and bulk velocities for the four
magnetosheath
crossingsare presented,continningthat sunwardflow of up to 100 km/s occurswhen the
magnetosphere
is expanding.
INTRODUCTION
outboundusing Pioneer 10 data and assumingthat the proton
distributions were MaxwellJan. Mihalov et al. [1976] found
The magnetosheathis the region between a planet's bow
that, althoughmany ion distributions
observedby Pioneers10
shock and magnetospherewhich contains shocked solar wind
and 11 in the Jovian magnetosheathare MaxwellJan,some have
plasma. The bow shocks at the earth and other planets have non-Maxwellian
characteristics with enhancements at low or
receivedextensiveattentionin the literature(see Greenstadtand
high
energy,
A
two-temperature
distribution
withcharacteristics
Fredericks [1979] and Russell [1985] for recent reviews). This
of both the unperturbedsolar wind and magnetosheath
plasma
work has generally focusedon the jump parametersacrossthe
(temperaturesof about 3 and 280 eV, respectively) was
shock and microstructureof the shock. Very little has been
sometimesobservedby Pioneers10 and 11 near the shock,but
written on the distribution function of plasma in the
these.spectramay indicate incompleteshock crossingsrather
magnetosheathregion, even though knowledge of these
than shock-produced
distributions.
distributions would seem essential for understandingthe
Little has been previouslyreportedabout the Voyager
processesoccurring in the shocks. Many early experiments
magnetosheath
plasmadata at Jupiter and Saturn,exceptfor a
noted the presenceof a non-Maxwellianhigh-energytail on ion
listing of the magnetosheath
boundariesat the four encounters
distributions in earth's magnetosheath [Howe, 1970;
[Bridge et al., 1979a, b, 1981, 1982]. This paper presents
Hundhausen et al., 1969; Wolfe and McKibben, 1968;
resultsfrom an analysisof ion spectraobtainedby the Plasma
Montgomeryet al., 1970]. These high-energytails containless Science instrument (PLS) on Voyagers 1 and 2 in the
than 10% of the total densityandbecomelesspronounced
away
from the immediate vicinity of the shock [Montgomeryet al.,
1970]. Formisanoet al. [1973] linked the presenceor absence
of high-energynon-Maxwellian tails in the proton distribution
function to upstream plasma conditions. More recent
observations
showthat somemagnetosheath
spectraexhibition
distributionswhich are well describedby two Maxwellians with
different temperatures.Observations
made from Apollo 15 in
the dusk magnetosheathshow that two-temperature ion
distributions sometimes occur during quiet times (low
geomagnetic activity), with the cold component having a
temperature
of 10-25eV anda density
of about1 cm-3, andthe
magnetosheathsof Jupiter and Saturn. Ion distributions
throughoutthe dayside magnetosheaths
of both planets are
found to be well simulatedby protonswith a two-temperature
distribution. The densities of these two MaxwellJan proton
populationsare comparable,and their temperatures
are about
100 and 1000 eV.
INSTRUMENT AND ANALYSIS
ThePLSinstrument
consists
of fourmodulated-grid
Faraday
cups which measureion and electroncurrentsin an energy-per-
charge range of 10-5950 eV (for completedetails on the
hot componenthaving a temperatureof 70-150 eV and a instrumentsee Bridge et al. [1977]). The low-resolution(L)
density of 7-10 cm-3 [Sanders et al., 1978, 1981]. mode coversthis range with 16 contiguousvoltage "windows,"
Measurements
in earth'sdawnsidemagnetosphere
by the ISEE 1
or channels,and the high-resolution(M) mode covers the same
spacecraftshow that during at least one quiet time the ion voltagerange with 128 channels. A completeset of L and M
distributionis well describedby two thermal populationsof mode spectra is obtained every 96 and 192 seconds,
nearly equal density with temperatures
of 60 and 500 eV respectively.Three of the detectors(A, B, and C) are oriented
[Petersonet al., 1979]. Althoughno statisticalstudieshave in a cluster whose central axis pøints toward earth and are
been done, these two-temperatureion distributionsappearto be
ideally oriented for measuringsolar wind and magnetosheath
rare at the earth, having been reportedonly during quiet times flow. The D cup is at right anglesto this directionbut still
by two experiments.
Observationsof the magnetosheaths
of Jupiter and Saturn
have beenmade by the Pioneerand Voyager spacecraft.Wolfe
et al. [1974] calculatedprotondensities,temperatures,and bulk
velocities in the Jovian magnetosheathboth inbound and
detectsion fluxes in the magnetosheath
where the plasmais
subsonic.
A sample set of M mode spectra is shown in Figure 1.
Currentin femtoamps
(10-•s A) is plottedversusa logarithmic
Papernumber6A8835.
energyscale. Currentsare measuredsimultaneously
in all four
cups. The plasma flow is directly into the A, B, and C cups,
The D cup is at a right angle to the flow direction,so currents
measuredin this cup are due to the thermal spreadin the ion
0148-0227/87/006A-8835502.00
velocities.
Copyright1987 by the AmericanGeophysical
Union.
6133
6134
RICHARDSON:
BRIEBREPORT
A-CUP
104'
B-CUP
_-
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• 10•
c• 102
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,o'oo
( volts
Fig.
1. A sample
setofM mode
spectra
fromVoYager
2 at69.5Rj. Overlaying
thedata
isthebest
fii tothedata
ob•ined
using
a two-temperature
proton
distribution
with(VR,Vr,V•v)= (-155,9, -6)loth/s,
n•t• = 0.74cm-3,TcOt.D
= 53eV,niaoT
= 0.86cm-3, andTrio
T = 389eV.
theconclusion'
tha
t bothion populations
arepredominately
Plasma
fluid
parameters
areobtained
byusing
aleast
squares
composedOf protons. It is certainthat an alphapopulation
fittingmurine'tofindthedensity,
velocity,
andtemperature,
5-10 %' of the totalion densitydoesunderlythe
which, combinedwith the instrumentresponse(see Barnett containing
[1984] and Barnett and Olbert [1986]), best simulatesthe data. main proton population,but this shouldhave a minimal effect
It is assumedthat plasma distributionsare convectedisotropic on the results presented here. The assumption that ion
MaXwelliansand that both ion populations
havethe samebulk distributions
areisotropic
is largelyvindicated
by theabilityof
velocity.
the simulations
to matchthe datain all four detectors,
although
the presenceof a small degree of anisotropycannot be ruled
out.
DATA AND RESULTS
Figure 2 shows the persistenceof the two-temperatureion
Figure 1 shows the data and best fit to a set of M mode distributions throughout the Jovian and Saturnian
spectrain the dayside magnetosheathtaken by the Voyager 2 magnetosheaths
..assampled
by bothVoyagers
1 and2. The
PLSinstrument
about
69.5Jovian
radii(Rj)fromJuPiter.
The spectraShownspanthe radial range from 48 to 86 R• and 62 to
two-temperatUre
characterof the distributionis apparentin the 97 Rj for Voyagers 1 and 2, respectively,at Jupiterand cover
datafromthe.A,B, andC cups.Thefit shownOverlaying
the the range from 19.4 to 26 Rs at Saturn. The fits shown
datamodels
theiøndistribution
using
twoproton
components,
a superimposedon the data in all cases assume both ion
areProtons
andarea resultof obtaining
thebestfit
coldcomponent
with a densityof 0.74 cm-3 anda temperaturecomponents
of53eV,anda hotcbmponent
withadensity
of0.86cm
-3and in all four cups,althoughonly one cup from each set of spectra
in the
a te'mperature
of 389eV. Thesimulated
current•
obtained
using is shownin the figure. Althoughthereare variations
thesefit parameters
fit thedatawellin all fourcups.Thissetof shapeof the o.f the spectraand the relative densitiesand
of thecoldandhotcomponents,
the•haracteristic
spectrahas also been fit assumingthat the lower-energytemperatures
component.
is protonsandthe,higher-energy
component
is alpha two-temperaturesignatureexists throughoutall four Voyager
throughthe daysidemagnetosheaths
of Jupiterand
particles. In this case,goodfits are againobtainedin the A, B, passages
and C cups with a proton density of 0.84 cm-• and a Saturn.
An ion distribution
consisting
of two Maxwellian
proton
temperature
of 57 eV, and an alphadensityof 0.42 crn-* and
.,
temperature
of 649 eV. The simulatedcurrentsin the D cup, populationshas been used to simulate ion data•and derive
however, are a factor of 2-3 lower than the observed currents. plasmafluid parametersthroughoutthe daysidemagnetosheaths
The ratio between the proton and alpha temperaturesand of Jupiter and Saturn. At Jupiter, ion spectra from all the
densitiesare also much differentthan one would expectbased dayside Voyager magnetosheathcrossingsare used to derive
on Upstreamsolar wind parameters.In the solar wind, alpha plasma properties. The spectraobtained at Saturn are often
particles
makeup 5'-12 % Of the totalsolarwinddensity,and contaminatedby noise at high energiesdue to damagethe PLS
the proton and alpha temperaturesare usuallywithin a factor of instrumentsuffered in the Jovian radiation belts. This,
2 (see Figures 3-6). Thus fitting the two magnetosheathcombinedwith the lower flux of ions due to Saturn's larger
fromthe sun,lowersthe signal-to-noise
ratioin the
components
usingprotons
andalphaswoul
d requirethatthe distance
percentageof alphaparticlesincreaseby a factorof 3 acrossthe spectra obtained, causing the spectra to be generally more
shockand that alphasbe heated5-8 timesas muchas the difficultto analyzeand the fit parameters
obtainedtcibe more
protonsin the shock.This seemsunreasonable
basedon our Uncertain.
Forthisreason,
lessspectra
havebeenanalyzed
at
regionsencounteredby Voyager
knowledgeof earth's bow shockand magnetosheath,
justifying Saturn,and the magnetosheath
RICHARDSON:
BRIEFREI•RT
Voyager
I,
Jupiter
Voyager
•_
6135
2,
R:85.5o
•
Jupiter
Voyager
I,
Saturn
R = 97.52
R:
26.01
-
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4
io4
R = 24.57
io 3
io 2
R:
23.18
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t ,o-, -•58
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Voyager
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Saturn
R: 23.44 --
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_
,,%
o'b
io
,
ioo
t
iooo
io
ioo
VOLTAGE
iooo
IO
IOO
IOOO
(V)
Fig.2. Sample
ionspectra
spanning
themagnetosheaths
of Jupiter
andSaturn
during
theVoyager
encounters
showing
the
persistent
bimodal
temperature
structure.
Also
shown
are
the
best
fit
to
the
data
and
hot
and
cold
proton
densities
in
units
of
cm-3.
2 between
26.6 and28.0 Rs andbetween
29.0 and31.6 Rshave the spacecraft
is in the sheathor the percentage
of alpha
beennot beenincluded
in thisstudybecause
of particularly
particlespresentif the spacecraftis in the solar wind. The
severenoiseproblems.
bottom panel gives the temperatureof the two plasma
Figures
3-6 showplasmaparameters
fromVoyagers
1 and2 components
present;the hot and cold protontemperatures
are
in the daysidemagnetosheaths
of JupiterandSaturnandin the givenfor themagnetosheath,
andtheprotonandalphaparticle
adjacent
solarwind. The location
of thespacecraft
(solarwind, temperatures
are givenfor the solarwind. The top five panels
magnetosheath,
or magnetosphere)
is indicated
on thefigures,as usecirclesand trianglesto indicateparameters
obtainedfromL
arethelocations
of themagnetopause
andbowshockcrossings.
and M mode spectra,respectively.To avoid confusion,the
Whenthe spacecraft
is in the solarwind,therearelargeradial temperature
plot usesthe samesymbolfor temperatures
derived
velocities,
low densities,
andlow temperatures.
Magnetosheath
fromL andM modespectra.Dependingon the locationof the
regionshave lower velocitiesand higher temperatures
and spacecraft, diamonds and crosses represent either the
densities.The densities
in theoutermagnetosphere
aretoolow temperaturesof protons and alphas in the solar wind or
for plasmaparametersto be obtained.
temperatures
of the hot and cold protonpopulationsin the
The top three panels of Figures 3-6 show the three magnetosheath.Sometimesthe alpha peak in the solar wind
components
of the plasmabulk velocityin RTN coordinates,outsideSaturn'sbow shockis overwhelmed
by noise;in these
whereR is radiallyoutwardfromthesun,T completes
a right- cases,only the protontemperature
anddensityareplotted. The
handedcoordinatesystemdefinedby R and N, and N is formal 1-sigmaerrorsfrom the fits to theseparameters
(see
perpendicular
to the eclipticandpointsnorthward.The fourth Bevington
[1969])are comparable
to or lessthanthepointsize
panelshowsthe totalion density.The fifth panelgiv,
es the on the plots.
percentage
of thetotaldensitycontained
in thehotcomponent
if
The radial velocitydecreases
by abouta factorof 4 acrossthe
6136
RI•ARDSON: BRIEFRF.I•RT
600
qoot
MS
200
0
-200
lO0
0
-100
z
loo
-100
•
00
v
z lO-1
i
'
i
lO• _
lO2 _
101 _
ii
lOø _
ß
I
ß
' ß
•e+++++ ß+• +
i
+
10-1
qO.O
50.0
60.0
70.0
80,0
90.0
RJ
Fig. 3. Ion bulk velocity,total ion density,percentage
of totaldensityin the hot proton(magnetosheath)
or alpha(solarwind)
component,and temperatures
of the two ion populations
in the solarwind and Jovianmagnetosheath
duringthe Voyager 1
encounter.The bow shockand magnetopause
locationsare given by alternatinglong- and short-dashed
lines and by shortdashedlines,respectively.The regionsof solarwind (SW), magnetosheath
(SH), andmagnetospheric
(MS) plasmaare labeled
at the top of the plot.
bow shockfrom the solar wind to the magnetosheath,
as magnetosphere
to themagnetosheath
andbeforecrossings
from
predictedby the Rankine-Hugoniot
relationsfor a supercriticalthe magnetosheath
to the solarwind). The spacecraft
velocityis
(magnetosonic
Mach number(Msts)> 3) shock(seeTable 1 for small comparedto the expectedspeedof the boundaries[Siscoe
shockparameters). The flow is diverted at the shockso as to et al., 1980]. This effect is most apparent near the
flow around the magnetosphere,resulting in Vr being magnetosheath
boundariesat Jupiter. The first Voyager 1 bow
predominatelypositive at Jupiter where the magnetosheathshock crossingat Jupiter at 85.6 Rj occurred during an
crossingsoccur in the morning sectorand negativeat Saturn expansionphase,and radial velocitiesjust insidethe shockare
where the magnetosheath
crossingsare in the afternoonsector. near zero. This expansionreverses, and radial velocities
Normal velocities are generally small except close to approach200 km/s before the shockis recrossedat 82.3 Rj.
magnetosheath
boundariessince the spacecraftare generally Similarly,the Voyager1 radial velocitiesdecreasebeforethe
close to the eclipticplane. The exceptionis Voyager 1 at magnetopause
crossings
at 46 and67 R• andincrease
beforethe
Saturn,which left the ecliptic to encounterTitan and observes shockcrossinginto the solarwind at 58 R•. The motionof the
southward
magnetosheath
flow.
boundaries
is fastenoughthatthe flow reverses
andbecomes
as
The measured
radial velocityin the magnetosheath
results high as 100 km/s sunward.The presence
of sunwardflow in
from a superposition
of the velocityof the shocked
solarwind the magnetosheath
duringthe Voyager1 passage
throughthe
flow andthe velocityresultingfromthe movement
of the bow magnetosheath
waspointedout by Siscoeet al. [1980];these
shockand magnetopause
due to changesin the solarwind resultsprovidequantitative
verificationof their conclusions.
pressure. Thus one expects the measuredmagnetosheath
The Voyager2 radialvelocitiesalsoshowthiseffect,increasing
velocityto be lowerwhenthe magnetosphere
is expanding(after before the shock crossingat 68.8 Rj and decreasingwhen
crossings
from the solarwind to the magnetosheath
and before approaching
the magnetopause
crossingat 58 R•. There is no
crossingsfrom the magnetosheath
to the magnetosphere)
and obviouschangein the spectraat the boundaries
betweeninward
higher when it is contracting(after crossingsfrom the and outwardflow. The Voyager2 spectraat 62.1 Rj has an
R•••N:
BRI•
REPORT
6137
600
_MS
•00
200
0
-200
100
0
-100
z
0
-200
00
10-1
lO3_
102_
-
101
:
I
I
-
1oø
E
_
10-• 55.0
85.0
75.0
85.0
95.0
105.0
BJ
Fig. 4. Ion bulk velocity,total ion density,percentage
of total densityin the hot proton(magnetosheath)
or alpha(solarwind)
component,
and temperatures
of the two ion populations
in the solarwind and Jovianmagnetosheath
duringthe Voyager2
encounter.The bow shockand magnetopause
locationsare givenby alternatinglong- and short-dashed
lines and by shortdashedlines,respectively.The regionsof solarwind (SW), magnetosheath
(SH), andmagnetospheric
(MS) plasmaare labeled
at the top of the plot.
outwardflow velocityof 15 km/sbut is not markedlydifferent Jupiterdatabut alsopresentin the Voyager1 Jupiterdata. The
in appearancefrom other spectrashown. At Jupiterwhere the total densityis not correlatedwith either of theseparameters.
magnetosheath
may be encountered
over a distanceof up to 50 For comparison,the ratio of the alpha to proton temperaturein
Rj the radial velocities could be useful tools for monitoring the solar wind varies from 1.5 to 7, with a median of about2.
solar wind pressurevariationsupstreamif direct monitoringis
not possible.
DISCUSSION
The densitiesincreaseby abouta factorof 4 acrossthe shock
from the solar wind to the magnetosheath,
also in accordwith
The preceding section demonstrated that observed ion
the Rankine-Hugoniotrelations. The percentageof the density distributionsin the daysidemagnetosheath
of Jupiterand Saturn
in the hot protoncomponentvariesbetween20 and 50% for all are well simulatedby two MaxwellJanprotondistributionswith
four magnetosheath
crossings. This comparesto an alpha comparabledensitiesand temperaturesof about 100 and 800
populationwhich makesup 4-12% of the solar wind density. eV. The only similar distributionsreported in the earth's
The temperature of the cold proton population in the magnetosheathoccurred during a quiet time in the dawn
magnetosheath
varies from 50 to 400 eV, with a median magnetosheathwhen the magnetosheathflow was stable over
temperatureof about 100 eV. The temperatureof the hot many hours [Petersen et al., 1979]. The two-temperature
protoncomponent
variesfrom 400 to 2000 eV, with a median distributionsmay result from a type of shockinteractionwhich
of about 800 eV. Thus the temperatureof the hot protonsis is rare at earth but more common at Jupiter and Saturn. The
6-10 timesthat of the cold protons,and the temperature
profiles shock crossing distance in planetary radii, the magnetosonic
of eachcomponent
trackeachotherquiteclosely,maintaininga Mach number Msts, and the ratio of thermal pressure to
fairly constanttemperature
ratio. The percentage
of ionsin the magneticpressure([•) are shownin Table 1. All shockscrossed
(the angle between the solar wind
hot componentis anticorrelated
with the temperature
of both are quasi-perpendicular
protonpopulations,
an effectmostclearlyseenin theVoyager2 magneticfield and the shocknormalis greaterthan 50). The
6138
RICHARDSON:
BRmFILSPORT
VOYRCER
5OO
1
SRTURN
1
i
'1
--- 300
_
MS
SH
A
m
0o
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0
-
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0
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-513 _
o
'
'i
o o o o•oo •b•' •,•,•
o
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o
o
i
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z
-50
_
0
o
0
0
0
0
0
o
_
-150
I
•
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•
I
I
z
-
•"
.5-
r,D
z
o
o
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o oo o •oo•o....
o
o
z
70.0_
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I--
o
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50-
_
o o
o
o
o
o
0
Oat:)0
I
_
X
-
0-
i
_
_
li-- -- --
_
10•_
_
•
102-
+ +
+
+
+
_
101 -
i
_
_
_
_
100_
_
i
_
10-1 -
I
22.0
I
23.0
I
2q.O
+
-
tl
25.0
26.0
27.0
BS
Fig. 5. Ion bulk velocity,total ion density,percentage
of total densityin the hot proton(magnetosheath)
or alpha(solarwind)
component,
and temperatures
of the two ion populations
in the solarwind and Saturnianmagnetosheath
duringthe Voyager1
encounter.The bow shockand magnetopause
locationsare given by alternatinglong- and short-dashed
lines and by shortdashedlines, respectively.The regionsof solarwind (SW), magnetosheath
(SH), and magnetospheric
(MS) plasmaare labeled
at thetop of the plot.
solar wind M mode spectra immediatelypreceding(or on observations
between0.45 and 4.6 AU is usedto estimate
succeeding)
the shockcrossingis usedto determinethe flow electrontemperatures
for use in computingtheseparameters.
velocities,plasma densities,and ion temperatures
used in The shockparameters
givenhere are roughestimates
intended
calculatingthe parameters
in Table 1. In calculating
Mststhe to indicatethe wide rangeof shockparameters
whichresultin
shockis assumedto be stationary;actualshockvelocitiesmay two-temperature
proton distributions. At the inbound bow
be ashighas 100km/s,sotheactualvaluesof Mstscoulddiffer shocks
at Saturnfor whichgoodmagnetosheath
datais available
from the tabulated
valuesby up to 25%. The magneticfield theupstream
Mstswas14.0and7.7 and• was1.4 and0.54for
used is the 96s averagefrom the VoyagerMagnetometer
Voyagers1 and2, respectively.
At Jupiter,valuesof • in the
instrument
coveringthe time periodwhenthe plasmadata is solarwind nearshockcrossings
rangefrom 0.18-11, andMsts
measured. The solar wind electronsare often too cold for the valuesrange from 4-19. Thus all the shocksare supercritical
Voyager instrumentto measureat thesedistances,so the (M•s > 3), andthe datasetcoversthe rangefromlow to high
polytropic
relationof SittlerandScudder
[1980]whichis based[•. Thelowervaluesof • andMstsarein a rangewhichis not
Table1. Upstream
Magnetosonic
MachNumbers
andPlasma
[3at the
Jovian and Saturnian Bow Shocks
V1Jupiter
V2Jupiter
Rj
MMs
I•
Rj
85.6
82.3
71.7
57.8
8.7
11.8
11.6
17.6
0.26
0.72
0.79
6.6
98.6
97.3
86.6
68.8
MMS
3.9
12.1
17.0
12.1
I•
0.18
0.71
2.8
1.1
V1 Saturn
Rs
26.1
MMS I•
14.0
1.4
V2 Saturn
Rs
23.6
MMS
7.7
I•
0.54
RICHARDSON:BRIEF RFJ•RT
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2
v
I
i+
+
+
+
+
+
+
+
-
+
+++++1
I
I
lO 1
-
i
I
_
_
i
i
_
-
I
I
-
-
i
I
I
I
-
_
I
-
10ø 18.0
+
+
I
Jl
19.0
I
I
I
20.0
21.0
22.0
I
23.0
•
I
2LJ,.0
25.0
BS
Fig. 6. Ion bulk velocity,total ion density,percentage
of total densityin the hot proton(rmgnetosheath)
or alpha(solarwind)
component,
andtemperatures
of the two ion populations
in the solarwind and Saturnianmagnetosheath
duringthe Voyager2
encounter.The bow shockand magnetopause
locationsare given by alternatinglong- and short-dashed
lines and by shortdashedlines,respectively.The regionsof solarwind (SW), magnetosheath
(SH), andmagnetospheric
(MS) plasmaare labeled
at the top of the plot.
uncommon
at earth. Thussomeothereffectparticularto Jupiter outboundmagnetopauseboundary. Spectrafrom the Voyager 1
and Saturn must be responsiblefor producing these two- and Voyager 2 outboundmagnetosheath
crossingsat Jupiterand
temperature proton distributions. This mechanism was from the Voyager 1 outboundcrossingat Saturn have been fit.
apparentlynot operativeat the time of the Pioneer encounters It is difficult to determineif the two-temperaturedistributionis
with Jupiter,when many of the protondistributionswere single still present for several reasons. One is a noise problem; the
Maxwellians [Mihalov et al., 1976].
PLS instrument suffered damage from its encounter with the
We have looked for similar ion distributions in Uranus's
Jovianradiationbelts beforepassingthroughthe magnetosheath
daysidemagnetosheath.They are not present. The Mstsand [t outbound,causinga noise problem a higher energies. Another
values at the only dayside bow shock crossingat Uranus are is that the observedion distributionsvary in time. Some are
about 15 and 3 [Bagenalet al., 1986], similar to valuesat some best fit with single proton Maxwellians, some with two proton
of the Jovian and Saturnian shock crossings. Despite this Maxwellians, some with protons and alphas, and some cannot
similarity, most of the spectra obtained in the Uranian be simulated at all using Maxwellians. This could be the result
magnetosheath
are fit fairly well by a singleMaxwellJan;some of interactionswith the magnetopause
or of samplingplasma
do have high-energytails, but the persistenttwo-temperature which has crossed the shock under different shock conditions
distributions seen at Jupiter and Saturn are not seen (see (that is, a different shock normal angle for solar wind plasma
Bagenal et al. [1986] for sample Uranian magnetosheathentering at the sides of the magnetosphere).Thus it will take
more analysis of outbound spectra to determine if the twospectra).
The last point to consider is the evolution of these temperaturedistributionsobservedin the daysidemagnetosheath
distributionsas the magnetosheathplasma moves past the persist to the tailward magnetosheathor if these distributions
planet. This problem can be addressed by looking at have time to relax to single Maxwellians.
distributions in the nightside magnetosheath. While it is
Acknowledgments.I would like to thank R. L. McNutt, Jr., for his
difficult to map the plasma from the dayside to the work on the data analysisroutinewhich madethis studypossibleand F.
magnetosheath,one can naively assume that magnetosheathBagenaland J. Belcher for helpful commentson the manuscript.The
plasma from near the subsolar point ends up close to the magneticfield data used in this study were obtainedby the Voyager
6140
RICHARDSON:BRm• REPORt
Magnetometer
instrument
(N. Ness,
Princi•lInveStigator).
ThisworkHundhausen,
A. J.,S.J.Bame,
andJ.R.Asbridge,
plasma
flowpattern
J. Geophys.Res., 74, 2799-2806, 1969.
was supported
by NASA undercontract953733 from the JetPropulsion in the earth's magnetosheath,
Laboratoryto the Massachusetts
Instituteof Technology.
Mihalov, J. D., J. H. Wolfe, and L. A. Frank, Survey for nonThe Editor thanks W. C. Feldman and J. D. Mihalov for their
Maxwellianplasmain Jupiter'smagnetosheath,
J. Geophys.Res., 81,
assistance
in evaluating
thisPaper.
3412-3416, 1976.
Montgomery,M.D., J. R. Asbridge,and S. J. Bame, Vela 4 plasma
observationsnear the earth's bow shock, J. Geophys. Res., 75,
1217-1231, 1970.
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(ReceivedNovember 17, 1986;
revisedMarch 3, 1987;
accepted
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