It was proposed that the Jovian auroral emissions observed by Voyager spacecraft could be explained by energetic protons precipitating into the upper atmosphere of Jupiter. Such precipitation of energetic protons results in Doppler-shifted Lyman alpha emission that can be quantitatively analyzed to determine the energy flux and energy distribution of the incoming particle beam. Modeling of the expected emission from a reasonably chosen Voyager energetic proton spectrum can be used in conjunction with International Ultraviolet Explorer (IUE) observations, which show a relative lack of red-shifted Lyman alpha emission, to set upper limits on the amount of proton precipitation taking place in the Jovian aurora. Such calculations indicate that less than 10 percent of the ultraviolet auroral emissions at Jupiter can be explained by proton precipitation

Clarke, J. T.

Cravens, T. E.

Curran, D. B.

Waite, J. H., Jr.

English

NASA Technical Reports Server

N93-1 :'65
The Role of Proton Precipitation in Jovian Aurora:
Theory and Observation
J. H. Waite, Jr. 1, D. B. Curran _, T. E. Cravens _
and J. T. Clarke 3
!
Space Sciences Department
Southwest Research Institute
San Antonio, Texas 78228-0510
2 Department of Physics and Astronomy
The University of Kansas
Lawrence, Kansas
3 Department of Atmospheric, Oceanic, and Space Sciences
The University of Michigan
Ann Arbor, Michigan 48109
Abstract
Goertz [1980] proposed that the Jovian auroral emissions observed by Voyager
spacecraft could be explained by energetic protons precipitating into the upper atmosphere
of Jupiter. Such precipitation of energetic protons results in Doppler-shifted Lyman alpha
emission that can be quantitatively analyzed to determine the energy flux and energy
distribution of the incoming particle beam. Modeling of the expected emission from a
reasonably chosen Voyager energetic proton spectrum can be used in conjunction with
International Ultraviolet Explorer (IUE) observations, which show a relative lack of red-
shifted Lyman alpha emission, to set upper limits on the amount of proton precipitation
taking place in the Jovian aurora. Such calculations indicate that less than 10% of the
ultraviolet auroral emissions at Jupiter can be explained by proton precipitation.
https://ntrs.nasa.gov/search.jsp?R=19930001977 2020-03-17T09:41:21+00:00Z
Introduction
The first theoretical estimate of the contribution of proton precipitation to Jupiter's
aurora was offered by Heaps et al.[1975]. A further theoretical study based on in situ
observations of the Voyager plasma and fields experiments suggested the presence of
strong proton aurora on Jupiter's night side [Goertz, 1980]. However, since the time of
Goertz's original Voyager-inspired analysis, additional evidence has been gathered for the
contribution of heavy ion ( S_ and O ÷q ) precipitation, both from in situ observations of
energetic ion losses in the middle magnetosphere near the Io plasma torus [Gehrels and
Stone, 1983] and from inferences about the energy required to produce auroral X rays with
the intensity observed by the Einstein X ray telescope [Metzger et a1.,1983]. The efforts to
establish the identity of the precipitating auroral particles have been complicated yet further
by the lack of S and O recombination lines in the FUV H 2 auroral emission spectrum,
which suggests that the FUV aurora are largely electron-excited [Waite et al., 1988]. It
therefore appears that many different kinds of charged particles may be contributing to the
excitation of Jupiter's various auroral emissions.
This paper reports on theoretical calculations of the Lyman alpha line shape
expected from proton precipitation impinging on the H2 atmosphere in the Jovian auroral
zone and compares these predictions with high-resolution Lyman alpha line profiles of the
Jovian aurora obtained by Clarke et al. [1989] using the International Ultraviolet Explorer.
These comparisons are used to determine the role of protons in these auroral emissions.
Meinel [195 I] used ground-based high-resolution spectroscopy of Doppler-shifted Balmer
(H alpha ) emission to study the contribution of proton precipitation to the Earth's aurora
(cf. Rees, 1989). Similar techniques were applied by Clarke et al. [1989] to study the
Jovian aurora using high-resolution Lyman alpha spectra taken with the IUE telescope. No
red-shifted Lyman alpha emission with wavelength shifts as expected from energetic
protons was observed. However, blue-shifted emissions resulting from fast atomic
hydrogen with ten's of eV of translational energy were observed to make up around 50%
of the auroral Lyman alpha emission. The lack of significant red-shifted emission suggests
that protons are not the primary precipitating particle responsible for the bulk of the
observed ultraviolet aurora at Jupiter. On the other hand, the presence of significant blue-
shifted emission suggests significant energization and outflow of protons and H atoms
and/or significant thermospheric winds as a result of auroral energy dissipation. For
further discussion of the blue-shifted emission we refer the reader to Clarke et al. [1989]
and Clarke et al. [1991] and for present purposes we concentrate on using the lack of
significant red-shifted Lyman alpha emissions to set limits on the energy flux of allowable
proton precipitation into the Jovian auroral atmosphere.
The Model
The model employs a continuous slowing-down approximation for an equilibrated
beam of energetic hydrogen atoms and protons incident on H2. The energy loss is given
by
d-_--_.zl--nn(z) Lt/2(E)see(0),
t:
where LH,(E) is the total energy loss function in H 2 at energy E, theta is the mean pitch
angle of the incoming particles with respect to the vertical, and nm(z) is the number
density of H__at altitude z. The energy loss function for protons in H 2 as a function of
energy used in the model is that of Anderson and Ziegler ]1977].
Two processes for the production of Lyman alpha photons by the interaction of the
beam with the H__ atmosphere are considered:
H + + H 2 ---> H* + H2+ (1)
H + H 2 ---> H* + H 2 (2)
Both protons and hydrogen atoms are present in the beam since electron stripping and
charge exchange processes between the energetic beam and the H: gas are constantly
modifying the charge state of the beam (ie., the H÷/H ratio). Due to the energy
dependence of these cross sections the beam changes charge state fraction as it dissipates
energy in the H 2 atmosphere. The energy-dependent proton-to-hydrogen atom fraction used
in this calculation is taken from the work of Allison ]1958]. Cross sections for production
of Lyman alpha by process (1) at energies below 10 keV are taken from the work of Van
Zyl et al.[1990] and above 10 keV from extrapolating using the energy dependence of the
ionization cross section as measured by Birely and McNeal ]1971].
Cross sections for process (2) are taken from Van Zyl et al. [1990]. Once again a
reasonable extrapolation with energy above 10 keV is added on to model processes at
higher energies. These cross section values for processes (1) and (2) are shown in Figures
l a and lb, respectively. An estimate of the beams interaction with the dissociated (atomic)
hydrogen component of the suggest that less than 5% of the emission can be attributed to
such a source since at the altitude of maximum H÷/H beam energy deposition nil2 _ n H.
The volume production rates as a function of altitude and energy were calculated by
introducing an incident proton/hydrogen beam with a known flux within a specified energy
bin. Each beam was then individually tracked as it deposited its energy within the
atmosphere. The charge state of the beam (ie., proton to hydrogen ratio) was determined
from the beam energy at each altitude step and the volume production rates for processes
(1) and (2) were calculated at each altitude during the process of ion beam dissipation
using the formulas
VPn" (z, Ei,,it,Ez) = nH2(Z) fH(Ei,it'Ez) _11) (Ez)
VP H (z, Ei,it, E z) -- ntt(z ) fit(Einit, Ez ) (Yl2)(Ez)
where: VP i (i=H ÷ or H) is the volume production at altitude z, initial beam energy Einit,
and present beam energy at altitude z given by Ez, fint is the i=H ÷ or H flux from the initial
beam of energy Ei°_t now at the altitude - dependent energy E_, and _ is the cross section
for Lyman alphaexcitation by processi=(l) or (2) at energyEz. The contribution to the
Lyman alphaproduction asa function of energy and altitude is binned to allow
computationof the Doppler-shiftedLyman alpha line profile. The production of Lyman
alpha that resultsfrom secondaryelectronproduction is not included in the present
calculation sincetheseemissionsarecreatedvirtually in the rest frame of the background
gasand thus do not contain anobservablered-shift.
A precipitatingenergeticproton spectrumis modeled by taking the Jovian
magnetosphericproton spectrumfrom the Voyager LECP dataof Krimigis et al. [1981],
scaling it to the desiredenergyflux, and introducing it into the top of the atmosphere(see
Figure 2). The model H2atmospherewas taken from the earlier auroral electronmodeling
of Waite et a1.[1983]and is shown for referencein Figure 3. Also shownin Figure 3 is
the approximatealtitude rangefor the Lyman alphaemissionsourceand an approximate
indication of the methanehomopausebelow which CH4absorptionof Lyman alphacould
affect the results. The beam flux hasbeennormalizedto producean integratedenergyflux
of 20 ergscrn-2sl' which is approximatelythe flux that would be requiredto producethe
observedH2Lyman and Wemer bandsystems,UV emissions [Horanyi et al.,1988]. A
meanangleof 30° betweenthe magneticfield direction and the IUE view direction was
adoptedfor the observationalviewing geometry. The broadeningof the line emissionfrom
the "actual" pitch angledistributions of the ions (and chargeexchangedneutrals)were not
accountedfor, but were not expectedto add considerablebroadeningbeyondthe effects
brought on by the assumedinitial beamenergydistribution and subsequentenergydecay
within the upperatmospherewhich areproperly accountedfor by thesecalculations.
Results and Conclusions
Red-shifted emission intensities that results from a proton distribution with a total
energy flux of 20 ergs cm -2 s-I are shown in Figure 4 along with representative IUE Lyman
alpha spectra from Clarke et al. [1991]. The location of the peak of the red-shifted Lyman
alpha emission is determined by the convolution of the energy dependence of the Lyman
alpha production cross sections at low proton/hydrogen energies and the tail at longer
wavelengths (above 1220/_,) is directly related to the initial beam distribution. The location
and shape of the red-shifted Lyman alpha peak from our calculations has been compared to
similar observations of terrestrial Lyman alpha from the auroral zone [Ishimoto et al.,
1989[ and has been shown to be consistent with their results. Not shown in this figure is a
15-30kR Lyman alpha emission at line center (1215.7,_,) which would result from
secondary electrons produced by the beam atmosphere interaction impinging on
atmospheric H and H 2. These secondary electron-generated emissions were not explicitly
calculated since they result in no "red-shifted" emission. Such emissions would be easily
observable by the IUE telescope. Clarke et al. [1989] did not, however, observe such
emission intensities at these wavelengths. Clearly, therefore the observed aurora does not
contain a proton energy flux large enough to produce the observed H2 Lyman and Wemer
bands. However, a smaller flux of protons is possible given the constraints of the IUE
Lyman alpha line profiles. The upper limits of proton precipitation allowed by the
observations can be calculated by retaining the same form of the proton energy distribution
asdescribedaboveand by scalingdown the energyflux to matchthe levels of red-shifted
emissionseenin the observations. Comparisonof the observationswith the model line
profile suggestthat protonscomprise5% or lessof the particlesresponsiblefor the bulk of
theJovian ultraviolet aurora [cf., Broadfoot et al., 1981]. We note, however, that these
resultsarewetly dependenton the energyspectrumof the precipitating protons. Given
the presentavailabledataand the model, it is difficult to envision a scenariowhereprotons
would be responsiblefor over 10% of the observedauroral ultraviolet emission. More
energeticproton beams(>> 1 Mev) that deposit the bulk of their energybelow the
hydrocarbonabsorptionlayer arenot ruled out by the presentobservations,but they also
cannotcontribute to H2 bandultraviolet auroral emissions.
The resultsreportedhere setuseful constraintson magnetosphericprocesses
responsiblefor auroral particle precipitation and add yet a further pieceto the ongoing
puzzleas to the identity of the particlesresponsiblefor Jovianauroral observations.
Perhapsin situ confirmation of theseresultswill be possibleduring the high-latitude
encounterof Ulysseswith Jupiter in January-Februaryof 1992. In addition, high-resolution
spectraat Lyman alphaby HST may provide additional observationalconstraintson auroral
proton precipitation.
References
Allison, S.K,; Experimental Results on Charge-Changing Collisions of Hydrogen and
Helium Atoms and Ions at Kinetic Energies Above. 2keV Reviews of Mod Phys.,
vol. 30, p. 1137, 1958.
Anderson & Ziegler, Stopping and Ranges of Ions in Matter, vol. 3: Hydrogen Stopping
Powers and Ranges in all Elements, Pergamon Press. 1977.
Birely, J.H., McNeal, R.J. "Lyman Alpha Emission Cross Sections for Collisions of H,
and H with N 2 and O 2. JGR, 76, 3700, 1971.
Broadfoot, A.L., et al., Overview of the Voyager ultraviolet spectrometry results through
Jupiter encounter, J. Geophys. Res., 86, 8259-8284, 1981.
Clarke, J. T., J. Trauger, and J.H. Waite, "Doppler shifted H Ly _ emission from Jupiter's
aurora", Geo. Res. Lett., 6, 587, 1989.
Clarke, J.T., G.R. Gladstone, and L. Ben-Jaffel, Dayglow H Lyman alpha emission profile,
Geophys Res. Lett., 18, 19, 1991.
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.
Goertz, C.K., "Proton aurora on Jupiter's nightside", Geo. Res. Lett., 7, 365, 1980.
Heaps,M.G., B.C. Edgar,and A.E.S. Green,"Jovian proton aurora", h'arus, 24, 78, 1975.
Horanyi, M., T.E. Cravens, and J.H. Waite, Jr., The precipitation of energetic heavy ions
into the upper atmosphere of Jupiter, J. Geophys. Res., 93, 7251-7271, 1988.
Ishimoto, M., C.-I. Meng, G.R. Romick, and R.W. Huffman, Doppler shift of auroral
Lyman oc observed from a satellite, Geophys. Res. Lett., 16(2), 143, 1989.
Krimigis, S.M.. J.F. Carbary, E. P. Keath, C.O. Bostrom, W.I. Axford, G. Gloeckler, L.J.
Lanzerotti, and T.P. Armstrong, "Characteristics of hot plasma in the Jovian
magnetosphere: Results from the Voyager Spacecraft", J. Geophys. Res., 86, 8227,
1981.
Meinel, A.B., "Doppler-shifted auroral hydrogen emission", Astrophys. J., 113, 50, 1951.
Metzger, A.. et al.. "The detection of X-rays from Jupiter", J. Geophys. Res., 88, 7731,
1983.
Rees, M.H., Physics and chemistry of the upper atmosphere, pages 45-52, Cambridge
University Press, Cambridge, U.K., 1989.
Van Zyl, B., M.W. Gealy, and H. Neumann, Lyman-c_ emission from low-energy H + H 2
and H ÷ + H z collisions, accepted for publication in Phys. Rev. A., 1990.
Waite, J.H., T.E. Cravens, J. Kozyra, A.F. Nagy, S.K. Atreya, and R.H. Chen, "Electron
precipitation and related aeronomy of the Jovian thermosphere and ionosphere", J.
Geophys. Res., 88, 6143, 1983.
Waite, J.H., J.T. Clarke, T.E. Cravens, and C.M. Hammond, "The Jovian aurora: electron
or ion precipitation'?", J. Geophys. Res., 93, 7244, 1988.
Figure Captions
Figure 1: Cross sections for Lyman alpha excitation of a) protons on H 2 and b)
hydrogen atoms on H z taken from the work of Van Zyl et a1.[1990].
Figure 2: Detailed spectral fit to the low-energy ion channels. Plotted (closed
circles) are the intensities measured in sector f (-90 ° from convection direction) of the
PL02-PL07 channels. In this direction, the detector response is thought to be due to
protons only. The dotted curve shows the thermal distribution obtained using parameters
listed in the figure. The dashed curve indicates a power law fit with a spectral index of
2.8. The closed square is from the LEPT detector channel which is sensitive only to
protons. [Krimigis et al., 19811.
Figure 3: H 2 model atmosphere altitude profile from Waite et a1.[1983]. Also
indicated on the figure are the altitude of the doppler shifted Lyman alpha emission and the
approximate altitude of the methane homopause below which altitude Lyman alpha
absorption by methane could significantly affect our results.
Figure 4: The brightness numbers as a function of wavelength for both the model
and the IUE SWP spectra (December, 1986). The brightness numbers assume that the
emitting region is an auroral zone which is diffuse East-West (i.e. fills the 9 arc second
large aperture) and is less than the IUE spatial resolution of 5 arc seconds North/South (i.e.
is unresolved). Figure 4(a) shows the IUE SWP 29880 spectra data compared to a proton
aurora energy flux of 20 ergs cm-2s l which is roughly that required to account for the H 2
Lyman and Werner band emissions that were observed. Figure 4(b) shows a comparison
of new IUE SWP spectra 44340 and 44342 with a 5% intensity of the 20 erg cm2s -_ aurora
(1.0 ergs crn-Zs -_) to illustrate the emission allowed by the present observations.
Acknowledgements
Partial support for this work has been provided by NASA Planetary Atmospheres
grant NAGW-1657, NASA IUE and ROSAT Observations of Jupiter's Auroral grant
NAG5-1429, and by SwRI Intemal Research project 15-9634.
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APPENDIX D


The role of proton precipitation in Jovian aurora: Theory and observation

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