In a recent paper by D. D. Barbosa, it is argued that electron bremsstrahlung is the most likely source of the auroral x-ray emissions that have been observed at Jupiter. Barbosa bases his argument on observational and theoretical studies of the production of secondary electrons in the Earth's aurora. It is argued here that Barbosa's interpretation is flawed because it ignors the constraint that the primary electron distribution parameters place on the parameters for the secondary electron distribution. As a result, Barbosa's postulated secondary electron fluxes are over three orders of magnitude greater than the theory of auroral electrons permits

Waite, J. Hunter, Jr.

English

NASA Technical Reports Server

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MEMORANDUM SEPTEMBER 1, 1991
TO:
FROM:
SUBJECT:
Dr. Donald K. West/GSFC
Dr. J. Hunter Waite, Jr./SwRI :_tM
Final Report for NASA Grant NAG5-1429
SwRI Project No. 15-3123-028
The primary purpose of this grant was m supportsimultaneous IUE and ROSAT observations of the
Jovian aurora. Two sets of observations were obtained. One set during the ROSAT all sky survey in
October of 1990 and the pointed ROSAT observations of GO Bagenai in April of 1991. In addition to
supporting the observations, theoretical calculations of the expected brighmess of I-I2 ultraviolet and
electron bremsswahlung were also partially supported by this work. A preprint of a related paper in press
in the Journal of Geophysical Research which reports this work is presented as an Appendix.
(NASA-CR-189493) 6REMSSTRAHLUNG _ RAYS FROM N92-13911
JOVIAN AURORAL ELFCTRO_S Find] Report
(Southwest Research Inst.) 13 p CSCL 03B
Unclas
G3191 0053222
https://ntrs.nasa.gov/search.jsp?R=19920004693 2020-03-17T13:44:55+00:00Z
Comment on "Bremsstratflung X Rays from Jovian Auroral Electrons"
by J.H. Waite, Jr., Southwest Research Institute, San Antonio, Texas 78238
The subject of this Comment is a recent paper by D.D. Bafoosa in which it is argued that electron
bremsstmtflung is the most likely source of the auroral x-ray emissions that have been observed at Jupiter
[Barbosa,1990].Barbosa baseshisargument on observationaland theoreticalstudiesof th_production
of secondary electrons in the Earth's aurora. As this Comment will show, however, Baxtmsa's
interpretation is flawed because it ignores the constraint that the primary electron distribution paramett_m-
place on the parameters for the secondary electron distribution. As a result, Barbosa's postulated
secondary electron fluxes are over three orders of magnitude greater than the theory of auroral dectrons
permits.
BACKGROUND
The identity of the particles involved in Jovian auroral activity has not been conclusively
established and remains a subject of some controversy. Data relevant to this question comprise both
observations of auroral emissions obtained by remote sensing at x-ray, UV, IR, and radio wavelengths and
in situ particle measurements made by the energetic particle detectors on Voyager. These data do not
permit the identification of any one single source for the Jovian auroral emissions. The UV data tend to
point to precipitating electrons (in the energy range of 10-50 keV) as the dominant source, while the in
situ measurements reported by Gehrels and Stone [1983] suggest that the precipitation of energetic heavy
ions (oxygen and sulfur ions in the energy range of 40-1000 keV) plays an important role in auroral
processes. X-ray observations have also been interpreted as evidence for heavy ionprecipitation [Metzger
et al., 1983]. Waite et al. [1988] have attempted to reconcile these interpretations by proposing that both
electrons and ions, depositing their energy at different altitudes and latitudes, play a role in the production
of the Jovian aurora. A definitive answer to the question of the parades and processes involved in the
production of the aurora at Jupiter, however, will require further remote-sensing observations in the
differentwavelengthregimesaswellasthemeasurementsto bemade by Galileo when it arrives at Jupiter
in early 1992.
INTERPRETATION OF THE X-RAY OBSERVATIONS
As noted above, further evidence in support of the heavy ion precipitation process was provided
by x-ray observations of the Jovian aurora carried out by Metzger et ak [1983]. The energy resolution
of the Einstein x-ray observatory used in the observations was not sufficient to distinguish between a
bremsstrahlung power law distribution and K-shell emission line spectra from sulfur and/or oxygen.
However, based on modeling the K-shell and bremsstrahlung mechanisms and their convoluted response
within the Einstein telescope, Metzger et al. [1983] inferred that the energy required to produce the
observed x-ray emission by means of electron bremsstrahlung was unreasonably large compared with that
required by the K-shell mechanism and thus argued in favor of heavy ion preeipitation as the source of
Jovian auroral x-rays.
The conclusions of Metzger et ak have been called into question recently by the work of Barbosa
[1990]. Barbosa states that his
"aim is to examine critically the conditions under which the x-ray
observations of Metzger et al. [1983] can be plausibly accounted for in
the framework of an electron excited aurora. We find that electron
bremsstrablung gives a most credible explanation of the x-ray data and
one which is consistent with electron generation of UV, infrared, and
radio emissions from the auroral regions as well. The main conclusion
drawn from the analysis is that the precipitating auroral electrons should
have a beamlike distribution with a bump on the tail in the maximum
emissivity layer. This result implies the existence of field-aligned
potential drops above the auroral region which give rise to characteristic
electron energy spectra similar to those observed over the terrestrial
2
aurora [Frank and Ackerson, 1971]. The theory here relies heavily on
auroral electron measurements made at Earth [Rees and Maeda, 1973]
and corresponding theories of such measurements [Banks et al., 1974;
Evans, 1974] for its proper interpretation in terms of primary and
secondary auroral electrons."
The purpose of this Comment is to present electron transport calculations similar to those
performed for the terrestrial aurora by Banks et al. [1974]. These calculations will demonsuate the
inconsistency between the primary and secondary electron distribution parameters chosen by Barbosa
[1990]. The decreased magnitude of secondary electron fluxes that result from calculations using the
transport equations suggests that electron bremsstrahlung is not likely to be the source of Jovian x-rays,
if terrestrial auroral electron theory is applicable.
THE MODEL
A serf-consistent calculation of the primary and secondary precipitating electron distributions form
the basis for the two-stream electron transport calculation used in the present model, which is derived from
a Jovian auroral electron model introduced by Waite et al. [1983]. The model solves the one-dimensional
chemical diffusion equations for atomic hydrogen, the major hydrocarbon species CH4, CzH2, CzH4, Call,,
and CH3, and the major ionospheric species H ÷ and 1-13÷. The neutral temperature structure adopted in the
present study is an equatorial profile determined from the Voyager UVS occultation experiments [Festou
et al., 1981]. Although auroral energy input is expected to modify this profile, there is at present little
indication as to the effects of this input. Furthermore, increases in the auroral thermal structure produce
little change in the calculations apart from changes in the relative altitude scale of the atmosphere.
Hydrocarbon results are consistent with the recent work of Gladstone and Allen [private communication]
using an eddy diffusion coefficient at the homopause (I_) of 2 x 106 cm = sz. However, the presence of
hydrocarbons has no effect on the present work apart from acting as an appropriate guide in determining
the valid range of the primary electron beam parameters based on the relative absorption of I-I2 band
emissions by hydrocarbon species [Livengood et al., 1990].
The auroral electron distributions as a function of altitude and energy are found by using a two-
stream electron transport code modified for Jupiter [Walte et al., 1983] and extended to electron energies
of 2 MeV using the relativistic _ cross sections of Garvey et al. [1977]. Input parameters of the primary
incident electron distribution were chosen to be consistent with the cases A, B, C that were presented by
Barbosa( 1990] and are shown in Table I. The differential bremsstrahlung cross sections were taken from
the work of Koch and Motz [1959] (formulas 3BN and II-6). X-ray absorption effects were calculated,
but were less than 10% at all photon energies above 100 eV for the primary electron beam cases
considered (10 to 100 keV).
RESULTS AND DISCUSSION
The difference between the auroral electron distribution at the altitude of peak auroral energy
dissipation and that assumed by Barbosa [1990] is shown in Figure 1. The Barbosa distribution is over
3 orders of magnitude different from that of the two-stream electron transport calculation at an electron
energy of 1 keY. The theories of auroral electron measurements [e.g., Banks et _, 1974] establish a
strong correspondence between the primary electron beam parameters and the secondary beam parameters,
since the secondary electron spectrum is created from ionization by the primary electron beam and the
collective transport of the secondary electrons formed from this process. This suggests that there exists
a strong coupling between the primary electron beam parameters and the secondary electron distribution
parameters within the context of terrestrial auroral theory. However, this constraint is ignored in the
calculations that are presented by Barbosa [1990]. Table 1 is a representation of the auroral electron
distribution form and the free parameters for specifying the primary and secondary electron distribution
function of Barbosa [1990]. The primary electron beam parameters in Babosa's study were chosen to
represent both the total power constraints and the spectral characteristics of the observed H2 band
emissions [cf. Livengood et al., 1990]. The secondary electron distribution parameters were then
independently chosen to satisfy the observed x-ray spectrum [Met-zger et al., 1983], while at the same time
being loosely constrained by the overall power dissipation of the observed Jovian auroral emissions.
However, this independent specification of the primary and secondary electron distributions is inconsistent
with theoretical [Banks et al., 1974] and observational [Fung and Hoffmau, 1988] characteristics of the
terrestrial aurora and corresponding electron transport calculations of the Jovian aurora presented in Figure
1.
X-ray flux as a function of photon energy as seen from Earth is shown in Figure 2. The solid lines
indicate the two-stream calculation and the dotted lines the calculations of Barbosa [1990]. The
observational data points of the Einstein Jovian x-ray observations are shown by the black dots with
corresponding error bars. The excellent agreement of Barbosa (1990] is due to the arbitrary choice of the
free parameters specifying the secondary electron distribution, the source of contention in the present
Comment. Note that the x-ray spectrum produced by the two-stream model is both harder in spectral
content and over an order of magnitude smaller in x-ray intensity in the region of the Einstein x-ray
observations.
Similar calculations of the predicted Jovian x-ray production from bremsstrahlung electrons have
also been carried out by Martin Walt [private communication 1991] using a more sophisticated model of
the electron energy degradation and subsequent x-ray production {Walt et al., 1979]. The agreement
between the Walt [private communication 1991] and Waite (this model) calculations is within a factor of
2 at all energies, assuming the same energetic electron spectrum [BaIbosa, 1990] and the same neutral
atmosphere model described in this paper.
CONCLUSIONS
Two possibilities appear to exist that could rescue the bremsstrahlung hypothesis of Jovian x-ray
emissions: 1) the auroral electron energy flux during the Einstein observations exceeded 500 ergs em _ s"t,
a deviation of over 3 sigma from the average value as determined by Livengood et al. [1990], and 2) the
Jovian auroral secondary electron spectrum is enhanced by over two orders of magnitude from that
expected from degradation and ionization from auroral primary electrons; whereas the terrestrial analog
suggests that standard electron-transport calculations explain observations of secondary electrons at Earth
to better than 50% [Fung and Hoffinan, 1988]. In addition to the changes required to increase the auroral
x-ray intensity to the desired levels, the calculated bremsstralalung x-ray flux also has a much harder x-ray
spectrum than that observed by Einstein. Preferential forward scattering of electrons at higher energies
(not ineorperated in the present calculations, since isotropie emissions were assumed) would have a
tendency to soften the _, but even so, calculations by Martin Walt [private communication 1991]
indic.ate that significant modifications to the auroral secondary electron speemnn would be required to
produce the soft speetmm observed by Einstein.
Therefore, the results of the two-stream calculations reported here suggest that bremsstrahiung x-
rays are not the likely source of Jovian x-ray emissions. However, ROSAT observations obtained in April,
1991 [Bagenal et al., 1989] should help to quantify and clarify the source of Jovian x-rays. ROSAT has
over a factor of two increase in sensitivity and in energy resolution in the energy range of interest (up _o
2 keV). Clearly, additional multispectral observations (x-ray, U'V, IR), modeling, and in situ particle
observations may be necessary to sort out the source of the Jovian auroral particles.
6
Table 1.
AURORAL ELECTRON BEAM MODELS
BAR B OSA,J G R,95(Ag), 14969,1990
E os -E/E o ..E E/Eo p
J(E) = Jos E e s +Jop Eop
Y = 2
CASE A E
op
Eos
J
op
J
08
= (10keY)
= (10keY)
= 108cm "2 s.1 keV
= 1.9 x 108cm "2
-1
s-lkeV -I
CASE B Eop = (30keY)
Eos = (10keV)
J = 107cm "2 s"I keV
op
J = 2.4 x 107cm "2
OS
-1
s'lkev -1
CASE C Eop = (100keY)
Eos = (10keY)
J = 106cm "2 s "I keV
op
J = 2.8 x 106cm "2
O$
-1
s-_keV -I
14
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LI..
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BARSOSA'S. JGR.B6|ABI. 14 gEE. 1890
PRIMARY AND SECONDARY ELECTRON
SPECTRA TAKEN FROM TABLE 2 CAIIllll
A(lOkeV|. 8($0keVt, AND C_100teVl
_IOOEL TWO-STREAM ELECTRON
FLUZEII AT THE ALTITUDE OF
PEAK ENERGY DEPOSITION
USING 8ARBO|A'II |tSI|0_
PRIMARY BEAM PARAMETERS.
SECONDARY FLUX SELF-CONSISTENTLY
CALCULATED FROM THE TWO-IITREAM
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6
10 100 1000 10 _ 10 _ 10
ENERGY (eV)
Figure L
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JOVIAN X-RAY FLUXES
T
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b EINSTEIN X-RAY OBSERVATIONS
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.... 8ARBOSA (19901 CALCULATION
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100 1000 104 I0 _
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106
ACKNOWLEDGEMENT
Partial support for this work has been provided by a NASA Planetary Atmospheres grant NAGW-
1657, NASA IUE and ROSAT Observations of Jupiter's Aurora gram NAG5-1429, and by an SwRI
Internal Research project 15-9634. I would also like to acknowledge helpful diseussiom with Dr. Marlin
Walt of LPARL.
i
REFERENCES
Bagenal, F., et al., ROSAT observations of Jovian auroral x-rays, a proposal to the National Aeronautics
and Space Administration for the ROSAT Guest Observer Program, 1989.
Banks, P.M., C.R. Chappelh and A.F. Nagy, A new model for the interaction of auroral electrons with the
atmosphere: Spectral degradation, backscatter, optical emission, and ionization, J. Geophys. Res.,
79, 1459, I974.
Barbosa, D.D., Bremsstrahlung X rays from Jovian auroral electrons, J. Geophys. Res., 95, 14969, 1990.
Festou, M.C., S.K. Atreya, T.M. Donahue, B.R. Sandel, D.E. Shemansky, and A.L. BroadfooL
Composition and thermal profiles of the Jovian upper atmosphere as determined by the Voyager
ultraviolet stellar occultation experiment. J. Geophys. Res., 86, 5715, 1981.
Frank, L.A., and K.L. Ackerson, Observations of charge particle precipitation into the auroral zone, I.
Geophys. Res., 76, 3612, 1971.
Fung, S.F., and R.A. Hoffman, On the spectrum of secondary auroral electrons, J. Geophys. Res., 93,
2715, 1988.
Garvey, R.H., H.S. Porter, and A.E.S. Green, Relativistic yield spectra in H 2, J. Appl. Phys., 48, 4353,
1977.
Gehrels, N., and E.C. Stone, Energetic oxygen and sulfur ions in the Jovian magnetosphere and their
contribution to the auroral excitation, J. Geophys. Res., 88, 5537, 1983.
10
Koch,H.W.,andJ.W.Motz,Bremsstrahlungcross-section formulas and related data, Rev. Mod. Phys.,
31,920, 1959.
Livengood, T.A., D.F. Strobel, and H.W. Moos, Long-term study of longitudinal dependence in primary
particle precipitation in the north Jovian aurora, J. Geophys. Res., 1, 10375, 1990.
Metzger, A.E., D.A. Gilman, J.L. Luthey, K.C. Hurley, H.W. Sctmopper, F.D. Steward, and J.D. Sulivart,
Tim detection of X rays from Jupiter, J. Geophys. Res., 88, 7731, 1983.
Recs, M.H., and K. Maeda, Auroral electron spectra, J. Geophys. Res., 78, 8391, 1973.
Waite, J.H., Jr., T.E. Cravens, J. Kozyra, A.F. Nagy, S.IC Atreya, and R.H. Chert, Electron precipitation
and related aeronomy of the Jovian thermosphere and ionosphere, J. Geophys. Res., 88, 6143,
1983.
Waite, J.H., Jr., J.T. Clarke, T.E. Cravens, and C.M. Hammond, The Jovian aurora: Electron or ion
precipitation?, J. Geophys. Res., 93, 7244, 1988.
Walt, M., L.L. Newkirk, and W.E. Francis, Bremsstrahlung produced by precipitating eleetrorls, J.
Geophys. Res., 84, 967, 1979.
FIGURE CAPTIONS
_: A plot of the differential electron flux as a function of electron energy. The dashed lines refer
to cases A, B, and C from Barbosa [1990]. The solid lines are from calculations using the two-stream
electron transport model of Waite (this paper) with initial electron fluxes at the top of the atmosphere set
by the primary beam parameters of Barbosa's [1990] cases A, B, and C. The small drop outs in the
11
modeledelectronfluxes are due to discrete changes in the energy bin su_cture and do not otherwise affect
the results.
Fi_. 2: A plot of the Jovian differential x-ray flux as a function of photon energy as viewed from an Earth
orbiting observation platform such as Einstein. The Metzger et al. [1983] Einstein observations are shown
by the data points and the empirical fit of Baxbosa {1990] by the dashed line. Results of the serf-
consistent model of Walte (this paper) for input cases A, B, and C are shown by the solid lines. Again
the discrete changes in the energy grid introduce small drop outs that do not otherwise affect the results.
12


Bremsstrahlung x rays from Jovian auroral electrons

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