Galileo Plasma Wave Observations Near Europa - Radio and Plasma ...

GEOPHYSICAL RESEARCH LETTERS, VOL. 25, NO. 3, PAGES 237-240, FEBRUARY 1, 1998

Galileo plasma wave observations near Europa D. A. Gurnett,x W. S. Kurth,x A. Roux,• S. J. Bolton, 3 E. A. Thomsen,x andJ. B. Groene,X Abstract. In this paper we present results from the Galileoplasmawaveinstrumentduringthe first two flybys of Europa, which occurredon December 19, 1996, and February 20, 1997. Strongwhistler-modenoisewas observedin the vicinity of Europa during both fiybys. Emissionat the upper hybrid resonancefrequency,fun, and a propagation cutoff at the local electron plasma

2.

Observations

Spectrograms of the electricand magneticfieldsobtainedfromthe Galileoplasmawaveinstrumentduring the E4 and E6 fiybys are shown in Figures 2 and 3. For a description of the Galileo plasma wave instru-

ment, see Gurnett et al., [1992]. The top panelshows frequency,fpe,providedmeasurements of the localelec- the electric field intensitiesand the bottom panel shows tron number density. The electron density measure- the magnetic field intensities. The intensities are color ments show a region of highly disturbed plasma in the coded with red being the most intenseand blue being vicinity of Europa with density enhancementsranging the least intense. An intensity scalein dB is shownat from about 30 to 100 cm -3 above the ambient Jovian the top of each plot. As can be seen, major responses magnetosphericbackground, which in both caseswas are evident in both the electric and magneticfield specabout 80 cm -3. frograms around the times of closestapproach. Although the electric and magnetic field responses have someclosesimilarities, they do differ considerably in certain details, particularly with respectto the spec1. Introduction trum. Sincethe magneticfield spectrumis the simplest, we start with a discussionof the magnetic field. On The Galileo spacecraft, which was placed in orbit around Jupiter on December 8, 1995, is carrying out both fiybys the Europa-related magneticfield response a series of close fiybys of the four Galilean satellites extends over a very broad frequencyrange, from about [Johnsonet al., 1992]. Sincethe Galileansatellites ten Hz to as much as ten kHz. The peak magnetic are immersedin the rapidly rotating magnetosphereof field intensity on the E4 flyby occurred at about 06:50 Jupiter, the interaction of these satellites with the Jovian magnetosphericplasma has been of considerable IX•• :30 I CO-ROTATIONAL interest. In this paper we discussplasma wave observations obtained during the first two closefiybys of EuI PLASMA FLOW ropa, which occurredon December 19, 1996, and February 20, 1997. The spacecrafttrajectories during these fiybys are shown in Figure 1. A Europa-centeredcoor/"06:30 dinate systemis used with the +z axis alignedparallel to Jupiter's rotational axis and the +x axis alignedparz:oo/ allel to the nominal co-rotational plasma flow induced by Jupiter's rotation. The +y axis completesthe righthand coordinate system. The flyby on December 19, 1996, is labeled E4, which is the designationfor the GEOMETRI• .•"'"/;J/-X +Y Europa flyby that occurredon orbit 4. The flyby on February 20, 1997, occurred on orbit 6 and is labeled E6. As can be seen,the two flyby geometriesare quite different. The E4 flyby passedthroughthe wakeimmediately downstreamof Europa, and the E6 flyby passed through the region immediately upstream of Europa. The E4 closestapproachto Europa occurredat 06:52:58 ß

/

UniversalTime (UT) at a radialdistanceof 1.44Rs (the radiusof Europais takento be 1 Rs - 1565km), and the E6 closestapproachoccurredat 17:06:10UT at a

I

+Z JUPITER

07:30 •• II ... I I|

radial distance of 1.37 Rs.

I ,

,

E4>

i..• :

+Y

adept. of Physicsand Astronomy,Universityof Iowa. •'CETP/UVSQ, Velizy, France. sJet PropulsionLaboratory,Pasadena.

Papernumber97GL03706.

Figure 1. The Galileotrajectoriesduringthe E6 and E6 flybysof Europa. The E6 flyby p•sed throughthe wakeregiondownstreamof Europawith a closestapproachaltitudeof 692km. The E6 flybypassedthrough the regionupstreamof Europawith a closestapproach

0094-8534/98/97GL-03706505.00

altitude

Copyright1998 by theAmericanGeophysical Union.

237

of 586 km.

238

GURNETT

ET AL.-

PLASMA

2O

4O

6O

CLOSEST

ELECTRIC

ter duringthe E6 flyby [personalcommunication, M. Kivelson,1997],sincethe radial distancefrom Jupiter

APPROACH

fUH

105-

AT EUROPA

about 16:55 to 17:15 UT, again correspondingto a region slightly more than twice the diameter of Europa. Although no data were available from the magnetome-

dB

0

WAVES

is almost the same, one would expect the electron cy-

clotron frequencynear Europa to be comparableto the

E4 flyby (i.e., fee•" 12 kHz), whichagainindicatesthat z Io•

the noiseis propagating in the whistler mode. Simple inspection of the electric field spectrograrnsin Figures 2 and 3 showsthat the electric field response

a_ 102

frequency component that extends from a few Hz to

io!

consistsof three components:(1) a broadbandlowaboutten kHz, (2) a broadbandhigh-frequency component that

' • '"

extends

from

a few Hz to about

one hundred

kHz, and (3) a narrowbandemissionat about 100kHz.

• ' '!' 'i........ 1 '• '"i' '

On the E6 flyby the low-frequencyelectric field component appears to be closely correlated with the lowfrequencymagnetic field noise, which is consistentwith the interpretation that this noise-is causedby electromagnetic whistler-mode waves. The peak electric field intensity occurred at 17:08 UT, shortly after closest approach. The broadband rms electric field strength at this point, integrated from 10 Hz to 10 kHz was

MAGNETIC

/WHISTLER-MODE

E•n• = 5.1 rnV/rn. On the E4 flyby, the correlation UT

0650

R(RE)

5.09

0640

5.12

0650

i.57

0700

2.08

GALILEO EUROPA 4 FLYBY, DAY 554,

0710

5.90

0720

5.92

DEC. 19, 1996

between the low-frequencyelectric and magnetic field components is not as good as on the E6 flyby. The low-frequency electric field noise, which extends from about 06:50 to 07:20 UT, extendsover a broader region

Figure 2. Spectrogramsshowingthe electricand magnetic field intensities detected by the Galileo plasma wave instrument during the E4 flyby. The strong

dB

0

magneticfield responsenear closestapproach(bottom panel)is believedto be dueto whistler-mode emissions. The narrowbandemissions labeledfuH (top panel)are

z,-, 105 •'-'

m IO 2

-' ,J'

ß

APPROACH

.....• .,."

...: - -• .... •

IO I

••

t MAGNETIC

WHISTLER-MODE--...• ' - •

, '• •.. - .•,•,':•..

well with the region where the Galileo magnetometer

detectedlargemagneticfieldperturbations [Kivelsonet • al., 1997]. The magneticfield detectedby the magneresponds to an electroncyclotronfrequency(fee= 28 B Hz) of about12.6kHz, anda protoncyclotronfrequency

CLOSEST

N

than twice the diameter of Europa, and correspondvery

tometer near Europa wasabout B - 450 nT, whichcor-

6O

,05

beledfpeis the propagationcutoffof free-spaceelectro-

UT, shortly before closestapproach. The broadband root-mean-square (rms) magneticfield strengthat this point, integrated over a frequencyrange from 10 Hz to 10 kHz, was Brms = 0.36 nT. Since the onset and termination of the magnetic field noiseis gradual, the boundariesof the regionof enhancedmagneticnoiseis difficult to accurately determine, but is roughly 06:45 to 07:00 UT. These times span a region slightly more

4O

ELECTRIC

upper hybrid waves, and the low-frequency cutoff la-

magnetic wavesat the local electronplasma frequency.

2O

-

---

1•.

io2

u. !oI UT

1640

1650

1700

17 I0

1720

1730

5.72 i.91 1.60 3.26 5.26 of about 7 Hz. Thus, the magnetic field noise is beR(RE) 5.74 tween the proton cyclotronfrequencyand the electron GALILEO EUROPA 6 FLYBY, DAY 51, FEB. 20, 1997 cyclotronfrequency.The only electromagneticplasma wave mode that can propagate in this frequencyrange Figure 3. Spectrogramsshowingthe electricand magis the whistlermode[Stix,1962]. netic field intensitiesdetectedduring the E6 flyby. The On the E6 flyby the peak magneticfield intensity oc- strong low-frequencyelectric and magnetic field noise curred at about 17:03 UT, again slightly before closest near closestapproach is believed to be due to whistlerapproach.The broadbandrms magneticfield strength, mode waves similar to those observedduring the E4 integratedfrom 10 Hz to 10 kHz, was Brans- 0.75 flyby (seeFigure2). Upperhybridresonance emissions nT, significantlystrongerthan for the E4 flyby. A dis- at full can be seennearly continuouslythrough the encernablemagneticresponsecan be seenextendingfrom tire flyby.

GURNETT

ET AL.: PLASMA WAVES AT EUROPA

and occurs later than the low-frequencymagnetic field noise, which extends from about 06:45 to 07:00 UT. The poor correlation is most likely due to the presence of a electrostatic noise that partially obscuresthe electric field of the whistler-mode waves, which were not as intense as on the E6 flyby. After closestapproach, at about the time the spacecraftis passingthrough the wake, a strong broadband high-frequency component can be seenextendingup to nearly 100 kHz from about 06:57 to 07:02 UT. This noiseis almost certainly electrostatic, since it extends well above the electron cyclotron frequency, and no comparable responsecan be seen in the magnetic field data. The narrowband emissionat about 100 kHz is a commonfeature of planetary magne-

tospheres [Walshet al., 1964;Mosieret al., 1973;Warwicket al., 1979;Kurth et al., 1980],and is causedby electrostaticwavesat the upper hybrid resonancefrequency, fUH. The upper hybrid resonancefrequency is

given byfun= (•+ f•2e)•/2, where fpe- 8,980v• Hz is the electronplasmafrequency(N• - electronnumber densityin crn-•), andf• is the electroncyclotron fre-

2]]9 I'- CLOSEST APPROACH

'05 I EUROPA 4 '

UT

R(RE)

i

"

J'

0650 5.09

'

0640 5.12

I

'

I

0650 1.57

I05

'

I

0700 2.08

'

0710 5.90

0720 5.92

•CLOSEST APPROACH EUROPA 6

FEB. 20, 1997

z

.

i

UT

1640

R(RE) 5.74

'

I

1650

5.72

'

I

'

1700

1.91

I

'

I

'

I

1710

1720

1750

1.60

5.26

5.26

'

quency. The upper hybrid emissionis particularly clear Figure 4. The electron number density,N•, computed duringthe E6 flyby (Figure3), andhasseveraldistinct from the upper hybrid resonancefrequency, fun, and enhancementsnear closestapproach, from about 17:02 the propagation cutoff at the electronplasma frequency,

to 17:12UT. Duringthe E4 flyby (Figure2), the upper fp•. The dashedline givesour best estimateof the amhybrid emission line is more difficult to identify and, with the exception of a brief strong intensification at about 80 kHz from 06:57 to 06:59 UT, can be clearly identified only in the downstream region, after about 07:01 UT. The brief, strong intensification from about 06:57 to 06:59 UT has a peak electric field intensity of

bient backgrounddensity of the Jovian magnetospheric plasma.

As can be seen, large disturbancesare present in the electron density in the immediate vicinity of Europa.

about4 mV/m. In theregion before 06:57UT, from Because ofthehighJovian magnetospheric plasma denabout06:30to 06:55UT, a sharp,low-frequency cutoff sitythat existsat the orbitof Europa,approximately canbeseenat 80to 90kHz. Thiscutoffisbelieved to be 80 electrons cm-a, the densityenhancements associdue to the propagationcutoff of flee-spaceelectromag- ated with Europa are difficult to identify. To help idennetic radiation at the local electron plasma frequency tify Europa's contributi.on to the total electron density, straight dashedlines have been drawn in Figure 4 that (seeStix[1962]),andis labeledfp•. are asymptotic to the magnetosphericelectron density

3. Electron Density

beforeand after the Europafiybys(i.e., in the region R >• 3 Rs). Using thesedashedlinesas guides,the

electron density enhancementsassociatedwith Europa Two methods can be used to compute the electron are estimated to be about ANe = 50 to 100 electrons number density from these plasma wave observations. cm-a duringthe E4 flyby,andANe - 30 to 50 electrons When the radio emission cutoff can be identified, as

from 06:30 to 06:55UT in Figure 2, the electrondensity can be computeddirectly from the plasmafrequency

cm-• duringthe E6 flyby.

usingNe = •pe/(8980) 2 cm-•, where fpeis thecutoff4. Discussion frequency in Hz. In principle,thiscutoffonlyprovides

anupper limittotheelectron density. However, since Wehave shown thatanenhanced level ofplasma wave thecutoff isverysharp webelieve a good case canbe emissions occurs inthevicinity ofEuropa. Theprimary made thatthecutoff iscontrolled bythelocalelectron emission consists ofelectromagnetic noise froma few density. When theupper hybrid emission canbeiden-tens ofHztoabout tenkHz,narrowband upper hybrid rifled, asit canafterabout 06:57 UTinFigure 2,andemissions at about 100kHz,andbroadband electroinmost regions ofFigure 3,theelectron density canbe static noise. Thefrequency range ofthelow-frequency computed using Ne- (faun•e)/(8980) 2cm -•, whereelectromagnetic noise, between theproton cyclotron frefunistheupper hybrid emission frequency andfeeisthe quency andtheelectron cyclotron frequency, strongly electron cyclotron frequency, both inHz.Thisequation suggests thatthisnoise consists ofwhistler mode emisfollows directly fromthedefinition oftheupper hybridsions. Comparisons ofthemagnetic-to-electric field ra-

frequency, faun - •pe+ •e' FortheE4flyby, weused tiowith therefractive index ofthewhistler mode supmagnetic field measurements from theGalileo magneport this interpretation. Forexample, at17:04 UTdurtometer tocompute fee.Since nomagnetic field dataing theE6 the magnetic and electric densities atflyby, 300Hz wereB /Af=3x 10-field nTspectral Hz2

4

z

1

were available during the E6flyby, weused the nomi-and E2/Af- 5x10-9V2m-2Hz , which nalvalue ofB=450 nT, which stillgives good accuracy gives acB/E since •e