Observations of Jovian electrons through six consecutive 13-month Jovian synodic periods from 1978 to 1984 have been made by the University of Chicago electron spectrometer onboard the ISEE-3 (ICE) spacecraft. The Jovian electron spectrum was determined from 5 to 30 Mev and was found to have a shape which is not a power law in kinetic energy, but cuts off at approximately 30 MeV. The average shape of the spectrum over each of the six intervals of best magnetic connection remains the same for all intervals within uncertainties

Evenson, P. A.

Meyer, P.

Moses, D.

English

NASA Technical Reports Server

201
SH 1.5-19
THEJOVIANELECTRONSPECTRUM:1978-1984
Paul Evenson +, Peter Meyer x, and Dan Moses x
+Bartol Research Foundation, University of Delaware
Newark,Delaware19716USA
XEnricoFermi Instituteand Departmentof Physics
Universityof Chicago,Chicago,Illinois60637USA
ABSTRACT. Observationsof Jovianelectronsthroughsix consecutive
13-month Jovian synodicperiodsfrom 1978 to 1984havebeenmade by the
Universityof Chicagoelectron spectrometeronboard the ISEE-3 (ICE)
spacecraft.The Jovianelectronspectrumwas determinedfrom 5 to 30 IdeV
and was foundto have a shapewhich is not a powerlaw in kineticenergy,
but cuts off at approximately20 MeV. The averageshapeof the spectrum
over each of the six intervalsof bestmagnetic connection remains the
samefor all intervalswithinuncertainties.
1. INTRODUCTION. Observation of interplanetaryelectronsof Jovian
originat I AU providesa uniqueopportunityto studythe propagationof
chargedparticlesin the innerheliosphere.Jupiteris a point sourceof
particlesimbeddedin the solarcavitywhich sweeps through a range of
magnetic connection to the earth with a well definedperiod. Previous
investigatorsof Jovian electrons at I AU during the period of solar
minimum (1,2) successfully applied a convection-diffusionmodel of
propagation,determinedthe value of the "cross-field"diffusioncoeffi-
cient, and demonstrated the modulation properties of corotating
interactionregions. The studypresentedin thispaper goes beyond the
previous work into the time of solar maximumwith observationsby the
Universityof Chicagospectrometeronboardthe ISEE-3spacecraft.
2. M_THOD. A detailed descriptionof the University of Chicago
electronspectrometeronboard ISEE-3 is given in reference (3). The
observations from ISEE-3are uniqueIn two aspects:the large changein
the level of solaractivityduringthe activelifeof the spacecraftand
the absenceof magnetosphericeffectsas the spacecraftwas positionedat
the innerLagragianpoint duringmost of the flight. Alsop the instru-
ment was very stable over the six years of observations. The single
factorrestrictingcontinuouscoverageof Jovianelectronsis contamina-
tion by solar flareelectrons. Eliminationof solarflare contamination
periodsis handledby monitoringthe spectralindexof electronsobserved
on IMP-8 at lower energy (1-3 MeV). A better solar flaremonitoris
beingdevelopedusingdata from the Max-Planck-lnstituteULEWAT instru-
ment on ISEE-3which is alsofree frommagnetosphericeffects.
The questionof instrumentstabilityin timeis critical for solar
modulation studies. The solid state detectors and plastic an-
ticoincidence detectors show stability at the 3% level in all calibration
and consistency checks over the duration of the flight. The ethylene gas
Cerenkovdetectorused for proton rejection has a small gas leak, and
loses approximately1 am of pressureper year from an initialpressure
Supported,in part,by NASA contractNAS5-26680and NASA grant
NGL-14-01-005
201 
* THE JOVIAN ELECTRON SPECTRUM: 1978-1984 
+ x x Paul Evenson , Peter Meyer , and Dan Moses 
+Barto1 Research Foundation, University of Delaware 
Newark, Delaware 19716 USA 
~nrico Fermi Institute and Department of Physics 
University of Chicago, Chicago, Illinois 60637 USA 
SH 1.5-19 
ABSTRACT. Observations of Jovian electrons through six consecutive 
13-month Jovian synodic periods from 1978 to 1984 have been made by the 
University of Chicago electron spectrometer onboard the ISEE-3 (ICE) 
spacecraft. The Jovian electron spectrum was determined from 5 to 30 MeV 
and was found to have a shape which is not a power law in kinetic energy, 
but cuts off at approximately 20 MeV. The average shape of the spectrum 
over each of the six intervals of best magnetic connection remains the 
same for all intervals within uncertainties. 
1. INTRODUCTION. Observation of interplanetary electrons of Jovian 
origin at 1 AU provides a unique opportunity to study the propagation of 
charged particles in the inner he1iosphere. Jupiter is a point source of 
particles imbedded in the solar cavity which sweeps through a range of 
magnetic connection to the earth with a well defined period. Previous 
investigators of Jovian electrons at 1 AU during the period of sol ar 
minimum (1,2) successfully applied a convection-diffusion model of 
propagation, determined the value of the "cross-field" diffusion coeffi-
cient, and demonstrated the modulation properties of corotating 
interaction regions. The study presented in this paper goes beyond the 
previous work into the time of solar maximum with observations by the 
University of Chicago spectrometer onboard the ISEE-3 spacecraft. 
2. METHOD. A detailed description of the University of Chicago 
electron spectrometer onboard ISEE-3 is given in reference (3). The 
observations from ISEE-3 are unique 1n two aspects: the large change in 
the level of solar activity during the active life of the spacecraft and 
the absence of magnetospheric effects as the spacecraft was positioned at 
the inner Lagragian point during most of the flight. Also, the instru-
ment was very stable over the six years of observations. The single 
factor restricting continuous coverage of Jovian electrons is contamina-
tion by solar flare electrons. Elimination of solar flare contamination 
periods is handled by monitoring the spectral index of electrons observed 
on IMP-8 at lower energy (1-3 MeV). A better solar flare monitor is 
being developed using data from the Max-P1anck-Institute ULEWAT instru-
ment on ISEE-3 which is also free from magnetospheric effects. 
The question of instrument stability in time is critical for solar 
modulation studies. The solid state detectors and plastic an-
ticoincidence detectors show stability at the 3% level in all calibration 
and consistency checks over the duration of the flight. The ethylene gas 
Cerenkov detector used for proton rejection has a small gas leak, and 
loses approximately 1 atm of pressure per year from an initial pressure 
* . Supported, 1n part, by NASA contract NAS5-26680 and NASA grant 
NGL-14-01-o0S 
https://ntrs.nasa.gov/search.jsp?R=19850026469 2020-03-20T17:02:40+00:00Z
202
SH 1.5-19
of 18 arm. The change in Cerenkov threshold is continually measured by a
change in the pulse height distributlon of the Cerenkov signal. The CsI
calorimeter also exhiblts a drlft due to gain changes in the photomul-
tiplier tube for which a correctlon is made uslng pulse height analysls
of the slgnal from minimum ionizing protons.
Excellent background rejection is obtained with the requirement of a
Cerenkov signal coincident with a stopping particle. The modulation of
the flux of Jovian electrons at I AU due to changes in the degree of
magnetic connection of the spacecraft to Jupiter can be used to further
improve background rejection of particles which are not electrons, as
well as electrons whlch are not Jovian in origin. The 10 MeV energy bin
was chosen to monitor the Jovian modulation because it is the bin with
the highest background rejection efficiency and because solar flares only
rarely emit electrons at 10 MeV. A regression of daily rates of each
energy bin relatlve to the 10 MeV bln determines the spectral shape of
the Jovian component.
3. RESULTS. An example of a Jovian electron ..... ' '
energy spectrum determined by the regression
technique during the first epoch of best con-
nection (Oct. 27, 1978 to Feb. 24, 1979) is
shown in Figure I. The spectrum is normalized
by the average flux in the I0 MeV bin with the
assumption that the flux in that bin is totally
free of background. One characteristic of the
Jovian electron spectral shape of partlcular 7
interest is the roll off at energies above 20_io-_
MaY. The existence of a hlgh energy limit to
the Jovlan interplanetary emission was first
suggested in (4) and has implications for the
acceleration of Jovian electrons, as w111 be I
discussed in a separate paper, o
The form of the energy spectrum of Jovian oo
electrons determlned by the regression tech-_ i
nique zs found to be the same for each epoch of E
best connectlon. Therefore, within statistical_
errors, the average spectral shape of the x10 -s
Jovian quiet-time increases is constant over,-y
the period of observatlon covering six Jovian
seasons. Further, the correlation of the daily
rates of each energy bln with the reference i0
MeV bin is better fit by a linear regression
than by any higher order polynomial during any
one epoch of best connection, implying that the
spectral shape is constant over a time scale as
short as one day. The chi-square goodness of
i I I J 1 I I i
fit parameter degrades wlth the separation in 4 40
mean energy of a bin from the reference 10 MeV Eleciron Energy (MeV)bin. Because of limited statlstlcs, the inter-
pretation of this uniform scatter about the Fi_____: Jovian electron
regresslon fit line as fluctuations of the spectrum determined by
spectral shape about an average shape cannot be regression fit technique
quantified. Specifically, I0 hour variations in during Dec. 1978 epoch.
202 
SH 1.5-19 
of 18 atm. The change in Cerenkov threshold is continually measured by a 
change in the pulse height distribut10n of the Cerenkov signal. The Csl 
calorimeter also exhib1ts a dr1ft due to gain changes in the photomul-
tipl ier tube for which a correct10n is made us1ng pulse height analys1S 
of the s1gnal fr  minimum ionizing protons. 
Excellent background rejection is obtained with the requirement of a 
Cerenkov signal coincident with a sto ping particle. The modulation of 
the fl ux of Jovian electrons at 1 AU due to change s in the degr e of 
magnetic co nection of the spacecraft to Jupiter can be used to further 
improve background rejection of particles which are not electrons, as 
well as electrons wh1ch are not Jovian in origin. The 10 MeV energy bin 
was chosen to moni tor the Jovian modulation because it is the bin with 
the highest background rejection e ficiency and because solar flares only 
rarely emit electrons at 10 MeV. A regression of daily rates of each 
energy bin relat1ve to the 10 MeV b1n determines the spectral shape of 
the Jovian compo ent. 
3. RESULTS. An example of a Jovian electron 
energy spectrum determined by the regre sion 
technique during the first epoch of best con-
nection (Oct. 27, 1978 to Feb. 24, 1979) is 
shown in Figure 1. The spectrum is normalized 
by the average flux in the 10 MeV bin with the 
a sumption that the flux in that bin is totally 
fr e of background. One characteristic of the 
Jovian electron spectral shape of part1cular, 
interest is the roll off at energies above 20>10-41-
MeV. The existence of a h1gh energy limit to Q) 
the JOV1an interplanetary emission was first ~ 
su ge sted in (4) and has impl ica tions f or the ~ 
a c leration of Jovian electrons, as w111 be I 
discu sed in a separate paper. ~ 
The form of the energy spectrum of Jovian (f) 
electrons determ1ned by the regression tech-~ 
nique 1S found to be the same for each epoch of E 
best connect10n. Therefore, within statistical ~ 
errors, the average spectral shape of the x 10-5 1-
Jovian quiet-time increa es is constant over ~ 
the per iod of observat10n covering six Jovian L.. 
seasons. Further, the correlation of the daily 
rates of each energy b1n with the reference 10 
MeV bin is better fit by a linear regression 
than by any higher order polynomial during any 
one epoch of best co nection, implying that the 
spectral shape is constant over a ti e scale as 
short as one day. The chi-square g odness of, 
• 
-
-
f it parameter degrades w1th the separation in 
mean energy of a bin from the reference 10 MeV 
bin. Because of li ited stat1st1cs, the 1nter-
pretation of this uniform scatter about the 
regress10n fit line as fluctuations of the 
spectral shape about an average shape ca not be 
quant fied. Spec fically, 10 hour variations in 
4 40 
Electron Energy (MeV) 
Pig. 1: Jovian electron 
spectrum determined by 
regression fit technique 
during Dec. 1978 epoch. 
203
SH 1.5-19
the spectral shape were searched for using .....t
severaltechniquesandnonewerefound. A
4. DIS_JSSION.The spectralshape of Jovian A
electrons below I0 MeV presentedin thlspaper
is flatterthan the kinetic energy power law
index of 1.5 reported in (5). However, the
spectralindexreportedat lower energles(.2-1
MeV) of -1.3 or less (6) coupledwith the-
observed roll off above 20 MeV demonstrates'
that a power law spectrumis a poorapproxima->_I°-4_"
tion In this energy reglme. Relylng on the
background rejection efficiency of the instru- i
ment, an average spectrum over the same time_
interval can be generated which includes the o
electron component not modulated by the change oo
in the magnetlc connectlon of the spacecraft to i
Jupiter (and therefore excluded from the E
spectrum obtained by the regression technique)_
(Figure 2). This spectrum more closely matches x 10.5
that of Eraker and Simpson (5) and, in com-_
parison wlth the spectrum obtained by the'
regression technique, suggests the existence of
a significant non-Jovian component at lower I
energies which would probably be of solar
origln. A low energy solar electron component
could also explain the degradation of the
goodness of flt parameter at these energies.
The constancy of the spectral shape from
epoch to epoch during the onset of solar maxi ...... J .4 40
mum actlvlty demons_j_ates the lack of solar
cycle modulation of the Jovian electron ElectronEnergy (MeV)
spectrum. This lack of solar modulatlon is _: Spectrumof all
conslstentwith the conclusionsfrom the radlal electronsduringDec-
gradient measurements of cosmic rays (7) that ember 2978 epoch.
duringsolarmaximumactivlty99% of modulation occurs at a solar dls-
tance >31 AU. The differenceIn solarmodulationof galacticparticles
and the relativelylocalJovian particles is very clearly seen in the
intensity-timeplotsin Figure3. While galacticrelativisticprotonsof
signlficantlyhigherrigidityundergoan obvlous solar cycle intensity
variation, the Jovian electrons contlnuewith the same13-monthcycles
flt by the same convection-dlffusion model envelope as observed during
solar minimum activity. The only solar cycle effect in the Jovlan
electronsnotedto date is a more impulslverlse and fall of individual
increa ses.
REFERENCES
I. Conlon, T. 1978, JGR 83, 541.
2. Chenette, D. 1980, JGR 85, 2243.
3. Meyer, P. and Evenson, P. 1978, IEEE Trans. GEl____66,180.
4. L'Heureux, J. and Meyer, P. 1976, Ap.J. 20___99,55.
5. Eraker, J. and Simpson, J. 1979, Ap.J 232, LI31.
6. Mewaldt, R., Stone, E. and Vogt, R. 1975, Proc. of 14th ICRC, 2, 758.
7. McKlbben, B., Pyle, R. and Simpson, J. 1985, Ap.J 289, L35.
203 
the spe ctr al shape wer e se arched f or us i ng 
several techniques and none were found. 
4. DISCUSSION. The spectral shape of Jovian 
electrons below 10 MeV presented in thlS paper 
is flatter than the kinetic energy power law 
index of 1.5 reported in (5). However, the 
spectral index reported at lower energles (.2-1 
MeV) of -1.3 or less (6) coupled with the_ 
I 
observed roll off above 20 MeV demonstrates- • 
h 1 . . > lO-t at a power aw spectrlUIl 1S a poor approx1ma- Q) 
tion ln this energy reglme. Relylng on the ~ 
background rejection efficiency of the instru- ~ 
ment, an average spectrum over the same time ~ 
interval can be generated which includes the u 
Q) 
electron component not modulated by the change (I') 
in the magnet1c connectl0n of the spacecraft tONI 
Jupiter (and therefore excluded from the E 
spectrum obtained by the regression technique) U 
(Figure 2). This spectrlUIl more closely matches -; 10-5 f-
that of Eraker and Slmpson (5) and, in com- ~ 
parison w1th the spectrum obtained by theU-
regression technique, suggests the existence of 
a significant non-Jovian component at lower 
energies which would probably be of solar 
origln. A low energy solar electron component 
could also explain the degradation of the 
goodness of f1t parameter at these energies. 
SH 1.5-19 
• 
• 
• 
-
• 
The constancy of the spectral shape from 
epoch to epoch during the onset of solar maxi-
mum act1v1ty demons~ates the lack of solar 4 40 
cycle modulation of the Jovian electron Electron Energy (MeV) 
spectrum. This lack of solar modulat1on is Fig. 2: Spectrum of aU 
cons1stent with the conclusions from the rad1al electrons during Dec-
gradi ent measurement s of cosmic rays (7) that ember 1978 epoch. 
during solar maximlUIl activlty 99% of modulation occurs at a solar dlS-
tance >31 AU. The difference 1n solar modulation of galactic particles 
and the relatively local Jovian particles is very clearly seen in the 
lntensity-time plots in Figure 3. While galactic relativistic protons of 
sign1ficantly higher rigidity undergo an ObV10US solar cycle intensity 
variation, the Jovian electrons contlnue with the same 13-month cycles 
flt by the same convection-dlffusion model envelope as observed during 
solar minimum activity. The only solar cycle effect in the JOV1an 
electrons noted to date is a more impulS1ve rue and f all of individual 
increases. 
REFERENCES 
1. Conlon, T. 1978, JGR 83, 541. 
2. Chenette, D. 1980, JGR jii, 2243. 
3. Meyer, P. and Evenson, P. 1978, IEEE Trans. GE16, 180. 
4. L'Heureux, J. and Meyer, P. 1976, Ap.J. 1Q2, 955. 
5. Eraker, J. and Simpson, J. 1979, Ap.J 232, L131. 
6. Mewaldt, R., Stone, E. and Vogt, R. 1975, Proc. of 14th ICRC, .1, 758. 
7. McKlbben, B., Pyle, R. and Slmpson, J. 1985, Ap.J 289, 135. 
204
SH1.5-19
(a)
12000
2000
I I I I I I
1979 1980 1981 1982 1983 1984
(b)
f/)
(.-.
0 600
L
-0--
u
_D
I_ 400
0 200 =
I
I I I I I
1979 1980 1981 1982 1983 1984
Fig. 3: Daily rates of protons (a) and electrons (b) observed by
ISEE-3. No flare subtraction has been made in these plots.
12000 
~ 10000 
0 
-0 
~ 
a.... 
> 
Q) 
(!) 
0 
. 
tr) 
I 
Lf) 
. 
0 
VI 
C 
8000 
6000 
..000 
2000 
o  
~ 
- 
Q) 
w ..  
> 
Q) 
~ 
o 200 
 
1979 
4 
SH 1.5-19 
(aJ 
~~~ ~~ ~~J~~~t ~I~I 
0     .. 
( J 
980  82 
Fig. 3: Daily rates of protons (aJ and electrons (bJ observed by 
ISEE-3. No flare subtraction has been ade in these plots. 


The Jovian electron spectrum: 1978-1984

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