NASA/TM-1998-208756

Energetic Electron Measurements from

the Galileo Jupiter Probe

J. D. Mihalov

Ames Research Center, Moffett Field, California

H. M. Fischer and E. Pehlke

Institut fiir Experimentelle und Angewandte Physik (lEAP)

Leibnizstr. 11, D-24098 Kiel, Germany

L. J. Lanzerotti

Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey

National Aeronautics and

Space Administration

Ames Research Center

Moffett Field, California 94035-1000

December 1998
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Energetic Electron Measurements from the Galileo Jupiter Probe

J. D. MIHALOV, H. M. FISCHER,* E. PEHLKE,* AND L. J. LANZEROTTI t

Ames Research Center

Abstract

Energetic trapped electrons were measured with the Galileo

Jupiter Probe, with samples from inside Io's orbit, down

to just above the atmosphere. The energetic electron

fluxes and spectra agree well with the earlier results from

the Pioneer spacecraft, where comparison may be made

under the assumption of simple power law spectra. New

features from the Galileo measurements include direct

observations of the electron pitch angle distributions and

spectral softening, both as the atmosphere is approached

and at smaller pitch angles at each measurement location.

Introduction

On December 7, 1995, the Galileo Probe successfully

returned in situ atmospheric data from Jupiter. Prior to

atmospheric entry, beginning inside the orbit of Io,

magnetospheric energetic particle fluxes were measured

using a shielded, two-element solid state detector telescope

that viewed through the aft heat shield of the Probe

[Fischer et al., 1992]. Analyses of energetic electron

fluxes and spectra are reported here; proton fluxes and

spectra were also measured [Fischer et al., 1996].

Probe Trajectory

In figure 1 the Galileo Probe trajectory in magnetic

coordinates is shown superposed on Van Allen's [ 1976]

Pioneer contours of energetic electron count rates. This

figure is for illustrative purposes only, since the Pioneer

results from December 1973 and December 1974 were

organized using a tilted dipole magnetic coordinate

system, whereas the Probe trajectory is given using a

magnetic coordinate system based on 06 multipole

coefficients [Connerney, 1993 ]. Also, the results reported

subsequently here use the even newer VIP4 multipole

magnetic coordinate system [Connerney et al., 1998]. The

difference in the plot of the Probe trajectory when using

the two magnetic multipole systems is relatively small,

* Institut fiJr Experimentelle und Angewandte Physik (IEAP),

Leibnizstr. 11, D-24098 Kiel, Germany.

t Bell Laboratories, Lucent Technologies, Murray Hill,

NJ 07974-0636.

except within 0.4 Jupiter radii from the planet's surface.

This figure indicates that peak flux regions near the

magnetic equator were avoided by the Galileo Probe.

Results

Directional energetic electron fluxes were deduced from

spin-sectored, corrected counting rates above two energy

thresholds. A more heavily shielded, coincidence channel

provided a measurement with higher mean energy; a less

heavily shielded, non-coincidence channel provided a lower

energy threshold [Fischer et al., 1992]. Directional ion

contributions and rates due to particles penetrating the side

of the telescope were estimated using various assumptions, or determined from data, and subtracted. Rates due

to particles penetrating the sides could be determined

knowing rates in the inner, more heavily shielded telescope element due to directional particles; those rates were

known in turn from the telescope coincidence rates. The

responses of the two channels to energetic electron fluxes

exterior to the Probe heat shield were determined with

Monte Carlo simulation calculations. The energy and

angular dependence of sensitivities to electrons from the

direction of the entrance aperture were principally determined using the ITS code [Halbleib, 1988]. Omnidirectional responses to a suite of particles using the CERN

GEANT code were also determined; calibrations using a

30 MeV linear accelerator at LLNL were done as well

[Fischer et al., 1992]. The relative responses of the two

channels to power law electron energy spectra were

determined from the simulations and matched with

electron count rate ratios deduced from the data from

Jupiter to obtain power law exponents. Directional fluxes

were then calculated using these power laws. The

directional fluxes were integrated over the ranges of pitch

angles observed, deduced from the VIP4 magnetic field

model, to obtain omnidirectional fluxes. In figure 2 these

omnidirectional fluxes, calculated for an electron energy

threshold of 21 MeV, are compared with those given in

figure 1, from the Pioneer spacecraft. These fluxes deduced

from the Galileo measurements appear to agree well with

the earlier results; they run somewhat lower in magnitude,

but not unreasonably so considering their uncertainties,

and they appear to peak at a higher altitude. These Galileo

omnidirectional electron fluxes may be accurate to within
afactorof 2.Animportantuncertaintyistheeffective

solidangleofsensitivity, affectedbyelectronscatteringin

theProbeaftheatshield,anddeterminedfromMonte

Carlosimulationsforindividual energiesanddirections.

Theeffectiveareasofthesolidstatedetectorsareaffected

bydetectionsensitivityattheedgesofthedetectors.

Effectsofdigitizationoftheexperiment's registervalues

arenegligible,asarecountingstatistics, exceptforthe

datasampleclosesttoJupiter'satmosphere. Thedecrease

influxshowninfigure2asJupiter'satmosphereis

approached couldnothavebeenmeasuredpreviously. The

figureomitsresultsfromanumberofadditional measurementlocationsatwhichonlyspin-averaged, notspinsectored, electrondatawereacquired.

Infigure3integral electronpowerlawexponentsdeduced

fromtheGalileoProbedataaregiven.Aspectral softeningwithdecreasingpitchangleisobservedatalllocations;thisisdiscussedfurtherbelow.Thespectral forms

forlocallymirroringelectronsappearslightlyharderthan

thosereported[Van Allen, 1976] from the Pioneer results

for omnidirectional fluxes at similar locations. The

gradual spectral softening as Jupiter's atmosphere is

approached may also be seen in the results of calculations

of equatorial diffusion of Jupiter's trapped energetic electrons [de Pater and Goertz, 1990]. Just above the atmosphere the electron spectra soften much more noticeably;

energy loss in the atmosphere may result in steeper, soft

spectra.

In figure 4 electron pitch angle distributions and the

behavior of the spectral softening observed at smaller

pitch angles are given for three locations. The pitch angle

distributions were obtained using directional fluxes calculated for energies above 21 MeV and assuming a sin n

dependence on pitch angle. A least-squares fit to spinsectored data was used to obtain both an orientation for the

local magnetic field and a value for the exponent n. The

field orientations obtained with this procedure differ from

those calculated using the 06 or VIP4 models, but they

tend to organize the spin-sectored data in a physically

meaningful way. The electron pitch angle distribution for

the most distant data shown does not follow closely a

power law, having excess flux apparently at the minimum

pitch angle sampled, as well as locally mirroring. The

pitch angle distributions closer to the planet are fit better

with power laws. The pitch angle distributions become

narrower, closer to the planet, where the loss cone is wider

(see also Fischer et al. [1996]). The spectral softening at

smaller pitch angles appears both to match a linear fit

better and to he a steeper function as the planet is

approached. A deconvolution of the experiment's angular

response was not performed when the above results were

dedw, ed.

Conclusions

Electron flux measurements from the Galileo Jupiter

Probe have provided a good sample from Jupiter's inner

magnetosphere. The fluxes and spectra agree reasonably

well with earlier results obtained with the Pioneer spacecraft where comparison may be attempted. A number of

new features have been found; these include a direct

observation of the decrease in flux and spectral softening

as the atnosphere is approached, direct measurements of

pitch an_le distributions, and persistent spectral softening

at smaller pitch angles.

References

Connerney, J. E. P.: Magnetic Fields of the Outer

Planets. J. Geophys. Res., vol. 98, 1993,

pp. 18659-18679.

Connerney, J. E. P.; Acufia, M. H.; Ness, N. F.;

and Satoh, T.: New Models of Jupiter's Magnetic

Fiehi Constrained by the Io Flux Tube Footprint.

J. G_,_ophys. Res., vol. 103, 1998,

pp. 11929-11939.

de Pater, I.; and Goertz, C. K.: Radial Diffusion Models

of Energetic Electrons and Jupiter's Synchrotron

Radiation 1. Steady State Solution. J. Geophys.

Res, vol. 95, 1990, pp. 39-50.

Fischer, H. M.; Mihalov, J. D.; Lanzerotti, L. J.;

Wilzberenz, G.; Rinnert, K.; Gleim, F. O.; and

Bacl, J.: Energetic Particles Investigation (EPI).

Sp. Sci. Revs., vol. 60, 1992, pp. 79-90.

Fischer, H. M.; Pehlke, E.; Wibberenz, G.; Lanzerotti,

L. J ; and Mihalov, J. D.: High-Energy Charged

Part cles in the Innermost Jovian Magnetosphere.

Science, vol. 272, 1996, pp. 856-858.

Halbleib J.: The Integrated Tiger Series. In: Monte Carlo

Tral.sport of Electrons and Photons, T. M. Jenkins,

W. __. Nelson, and A. Rindi, eds., Plenum Press,

N.Y., 1988, pp. 247-284.

Van Allen, J. A.: High-Energy Particles in the Jovian

Mal.netosphere. In: Jupiter, T. Gehrels, ed., U. of

ArB ona Press, Tucson, 1976, pp. 928-960.
5Rj

4

3

2

f

o

-I

-2

-3

-4

-5

i Z 3 ,t 5 6 t 6 9 • to tt t2 t3 t4 I_Rj

Reprinted with permission from figure 3 on page 460 of the article by J. A. Van Allen, B. A. Randall,

D. N. Baker, C. K. Goertz, D. D. Sentman, M. F. Thomson, and H. R. Flindt, entitled "Pioneer 1 1

Observations of Energetic Particles in the Jovian Magnetosphere," in the May 2, 1975, issue of

Science, vol. 188. Copyright 1975 American Association for the Advancement of Science.

Figure 1. Illustration of Galileo Probe trajectory, in magnetic coordinates, superposed on energetic electron count rate

contours (s-_) obtained from Pioneer 10 and 11 measurements [Van Allen, 1976]. The Probe trajectory was calculated

using the 06 magnetic coordinate system [Connemey, 1993], whereas the count rate contours were drawn using

coordinates from a centered, tiffed magnetic dipole.

'to

10 r

10 s

A

x= 10 s

10 4

13

_p

1000

.... I ' ' '..2.' I '..2.2''_' I .... I .... 1 .... I .... I .... 1'''.

i i "' i ° ; _ 4..

.......... -i................ i......................... i................................................................... i........

i

i [ i i

i r

................ .

......" i i .... i .... i .... t .... t .... i .... i, ,

1.5 2 2.5 3 3.5 4 4.5 5

L, R i

Figure 2. Omnidirectional electron fluxes above a 21 MeV

energy threshold, deduced from Galileo Probe measurements, assuming power law energy distnbutions. A

comparison with earlier Pioneer results is given by the

dashed line which was obtained from figure 1. The magnetic

shell parameter values (L) along the abscissa were

obtained using the VlP4 model for Jupiter's magnetic field

[Connemey et al., 1998].

8 .''''1''''1 .... !'''' ''''1'"''1 .... I .... I'''

+ , i i i i

¢:

_- 6

4 - : ..........

"'...". : " :.' i"

2 _ i ,I .... T,,,, I,,,_T,;-T;T, ,;;I, , ,TF; ,;;I+i_L

• . . . .+

1 1.5 2 2.5 3 3.5 4 4.5 5

L, R i

Figure 3. Values of power law exponents for energetic

electrons, from Galileo Probe measurements. The solid

symbols are for locally mirroring electrons, and the hollow

symbols for the minimum pitch angle observed at each

location. At each location, the spectra are softer for the

smaller pitch angles. The abscissa was obtained the

same as for figure 2.
-=

^

w

, , , , , , , ,

210 s

107 ......

910"

8104

710 `=

6104

5104

4104 L ........

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

B/Bo

3.8 ""'"''r.... ].... I.... !.... i.... I.... I....

° ! !

3.6 '. o i i '. ...............

slope: -_024 ± .002 Ideg

3.4 ............. _................. !.............. _................... _.............i.............. + ................

i o

i i i i i

3.2 1

I, ! i o "

3 ........_. .i i o

i : i o

2.8 '

_5 50 55 60 65 70 75 80 85

_ch angle,deg

-=

A

¢=

2

5 lOs

4 10s

3105

2105

, , , , , , ,

o(BI Bo)'1"°7"_

, _ i t i i i ; i t i i i i J L = L

1 2

B/Bo

_' ' I ' ' ' ' I .... I'' ' ' ' ' ' ' I ' .... 2.9 -; .............._ ............... _........................................ -_..............

i .7 : -.D13 ± .001 tdeg

2.6 _ i "_

2.32.42.5 _,_--"'_i)!i!i iii iiiiiiii!_i! ....................... ! i _'.................. o-:....................... °°ii ......................... o

40 50 60 70 80 90

pitch angle, deg

8105

710 s

_ 6105

^ 5 10 _'

_= 410 s

1 2 3

B/Bo

o

i

4 5

''''! .... ] .... I .... ! .... _ .... I .... .

2.95 -..-__----.......... : __---- ............... .. "-..- !................ .'---ii ............... .............

M

2.9 :.--o_ .................. ,..... !..... _...............i .................

2.85 F........ 9:_.00i5 iideg -_

2.8

2.75

2.65 ..........

2.6 ............i ,, i,, i I .... i°, ,. ,i ..... "-

25 30 40 50 60 70 80 90

pitchangle,deg

Figure 4. Electron pitch angle distributions and power law exponents for/arious pitch angles, for three locations along the

Galileo Probe trajectory. The top data are for L = 1.44, the central plots f( r L = 1.8, and the bottom plots for L = 3.3. The

pitch angle distributions become narrower as the planet is approached at d the loss cone becomes larger. Power law

exponents for electron pitch angle distributions are given on the plots. At L -- 3.3 the highest and lowest points were

omitted when the power law exponent was calculated. The spectral softening with decreasing pitch angle seems to

become steeper, closer to Jupiter. Slopes from least-squares fits are given on the plots.
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December 1998 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Energetic Electron Measurements from the Galileo Jupiter Probe

6. AUTHOR(S)

J. D. Mihalov, H. M.Fischer,* E. Pehlke,* and L. J. Lanzerotti t

7. PERFORMINGORGANIZATIONNAME(S)ANDADDRESS(ES)

Ames Research Center

Moffett Field, CA 94035-1000

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

889-54-44-16-00

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REPORT NUMBER

A-9900546

110. SPONSORING/MONITORING

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NASA/TM-1998-208756

11. SUPPLEMENTARY NOTES

Point of Contact: J. D. Mihalov, Ames Research Center, MS 245-3, Moffett Field, CA 94035-1000

(650) 604-5516

*IEAP, Leibnizstr. 11, D-24098 Kiel, German_'.

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Availability: NASA CASI (301) 621-0390

tBell Laboratories, Lucent Technologies, Murray Hill, NJ 07974-0636.

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13. ABSTRACT (Maximum 200 words)

Energetic trapped electrons were measured with the Galileo Jupiter Probe, with samples from inside Io's

orbit, down to just above the atmosphere. The energetic electron fluxes and spectra agree well with the

earlier results from the Pioneer spacecraft, where comparison may be made under the assumption of simple

power law spectra. New features from the Galileo measurements include direct observations of the electron

pitch angle distributions and spectral softening, both as the atmosphere is approached and at smaller pitch

angles at each measurement location.

14. SUBJECT TERMS

Jupiter magnetosphere, Trapped radiation, Outer planets

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