Analysis of Jovian Auroral H Ly-α Emission (1981–1991)

ICARUS 124, 350–365 (1996)ARTICLE NO. 0164

Analysis of Jovian Auroral H Ly-a Emission (1981–1991)

WALTER HARRIS1 AND JOHN T. CLARKE

Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, 2455 Hayward, Ann Arbor, Michigan 48109-2143E-mail: [email protected]

MELISSA A. MCGRATH

Space Telescope Science Institute, 3700 San Martin, Baltimore, Maryland 21218

AND

GILDA E. BALLESTER

Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, 2455 Hayward, Ann Arbor, Michigan 48109-2143

Received May 20, 1994; revised April 24, 1996

1. INTRODUCTIONWe present a new analysis of jovian auroral IUE spectra

The jovian aurora has been the subject of intensive studycovering the period from 1981 to 1991. To extract integratedsince it was first observed closely by the two Voyagerauroral H Ly-a emission from these spectra we have developed

a new extraction method that bins signal with wavelength while spacecraft and International Ultraviolet Explorer (IUE)preserving the spatial information provided by the IUE imaging nearly 17 years ago (Broadfoot et al. 1981, Clarke et al.spectrograph. This separates auroral emission from background 1980). After the Earth’s, the jovian aurora has receivedsources including the jovian dayglow, geocoronal H Ly-a emis- more attention than that of any other planet. The largersion, emission from hydrogen in the interplanetary medium moment of the jovian magnetic field, along with the lower(IPH), and grating scattered light in the IUE spectrograph.

solar wind ram pressure at 5 AU and the presence of aAuroral H Ly-a emission is compared with H2 emission coveringlarge internally generated plasma source associated withthe bandpasses from 1230 to 1300 A and 1550 to 1620 A. StudyIo, combine to make Jupiter’s auroral system an excellentof H2 and H Ly-a variability as a function of central meridiancandidate for comparative study with the terrestrial system,longitude (CML) and the results of linear correlative analysis

indicates that these features are produced by a common or which is controlled primarily by external influences. Fur-linked process with a similar spatial distribution. Further com- thermore, the intensity of jovian auroral emission and theparative analysis of the relative optical depth of the correlated relative proximity of the planet make high sensitivity ob-emission using a modified form of the H2 color ratio of Yung servations of the system with modest spatial resolutionet al. (1982, Astrophys. J. 254, L65–L69) suggest that H2 and possible with IUE. In the years since the Voyager encoun-H Ly-a emissions are subject to similar variability in CH4 extinc-

ters, the IUE has proven to be an extremely valuable andtion with CML. From this a conservative upper limit of 37%durable tool for determining the characteristics of the jov-is derived for the amount of H Ly-a emission that can beian aurora, and its archive has by far the largest collectionproduced above the altitude where the bulk of the H2 emission

is produced. We also discuss how the implied radiative transfer of jovian FUV auroral observations, with spectral dataenvironment for H Ly-a production and the local H/CH4 ratio covering a temporal baseline dating from 1979 to the pres-can limit the altitude distribution of the emission, the auroral ent. These spectra are an important reference for the gen-H Ly-a intensity, and the shape and information available from eral characteristics of the auroral system, because of theirthe emergent line profile. 1996 Academic Press, Inc. spatial coverage of the auroral zones and their high sensi-

tivity to diffuse auroral emission and because they simulta-neously sample the brightest features in the auroralFUV spectrum.

The study of ($100 kiloRayleighs[kR]) auroral emission1 Current address: Space Astronomy Lab, University of Wisconsin,from the H2 Lyman (B1S1

u-X1S1g ) and Werner (X1Pu-6281 Chamberlain Hall, 1150 University Avenue, Madison, WI 53706.

E-mail: [email protected]. X1S1g ) band systems in low dispersion IUE spectra from

3500019-1035/96 $18.00Copyright 1996 by Academic Press, Inc.All rights of reproduction in any form reserved.

ANALYSIS OF JOVIAN AURORAL H Ly-a EMISSION 351

1150 to 1650 A has been used to develop models for the (Ajello et al. 1984), and enhanced resonant scattering ofthe wings of the solar H Ly-a line in the upper atmospheregeneral auroral characteristics that include consistent mod-

ulations in auroral brightness with central meridian longi- (e.g., Ben Jaffel et al. 1993). Emission from these sourcesis produced at a large line center optical depth (t $ 105),tude (e.g., Clarke et al. 1980, Skinner et al. 1984, Livengood

et al. 1990, Herbert et al. 1987) and CH4 optical depth where multiple resonant scatterings will alter the emergentline profile and intensity (Gladstone 1992). Thorough in-(Yung et al. 1982, Livengood and Moos 1990) at both poles.

IUE spectra have also been used to estimate the total terpretation of the line profile will require a detailed under-standing of the three-dimensional distribution of H Ly-a(1013–1014 W) energy budget (Waite et al. 1983), the alti-

tude distribution of the auroral emitting layer (Livengood emission, its relationship to the H2 emissions, the auroralatmospheric temperature profile, the altitude distributionet al. 1990, Gladstone and Skinner 1989), and the energy

spectrum of the secondary electrons believed to collision- of auroral emission, and the contribution of continuumabsorbers (Gladstone 1992). Once these characteristics ofally stimulate the H2 emission (Yung et al. 1982).

In addition to the H2 band emission features, the far the emission are determined, high resolution study of theH Ly-a line profile can be used to obtain additional infor-ultraviolet (FUV) Voyager and IUE spectra also contain

bright H I Ly-a emission attributed to the aurora. While mation about auroral dynamics and energetics and theincoming and secondary charged particle composition.the H Ly-a line is the brightest in the FUV spectrum of

Jupiter, little attention has been paid to it in IUE spectra This paper incorporates the extraction of auroral H Ly-a into the existing data base of H2 emission from IUE lowexcept at high spectral resolution (Clarke et al. 1989). There

are several reasons why significant study of this feature dispersion spectra. Our effort focuses on three specificquestions about the horizontal and altitude distribution ofhas not been pursued. Auroral H Ly-a emission is much

more difficult to separate from the various background auroral H Ly-a emission that will impose limits on thepossible source mechanisms and the altitude range oversources, such as the Earth’s geocorona, bright H Ly-a

dayglow emission, and solar H Ly-a backscattered by inter- which emission can occur and provide a basis for laterstudy of the line profile:planetary hydrogen (IPH). Auroral H Ly-a emission is also

very sensitive to wavelength dependent resonant scatteringthat multiply scatters and modifies the emission line. This (1) What is the relationship between auroral H2 and

H Ly-a emissions? Most auroral H2 emission is producedconfuses the interpretation of low dispersion data in thecase where little is known about the auroral atmosphere, by collisional excitation by secondary electrons generated

by incoming higher energy auroral primary particles (Yungthe spatial distribution of the emission, or the detailedshape of the emission line (e.g., Gladstone 1982, 1992). IUE et al. 1982, Gerard and Singh 1982, Gladstone and Skinner

1989). Electron collisional dissociative excitation of H2 alsoline profile measurements of the auroral H Ly-a emissionfeature do exist (Clarke et al. 1989), and these show evi- produces substantial H Ly-a emission, and a component

of the observed H Ly-a feature may be related to H2 ;dence of nonthermal broadening beyond the 0.14 A, lim-iting spectral resolution of the spectrograph. Due to the however, several different mechanisms could also make

substantial contributions to the emerging line that do notinstrumental limitations of IUE, however, these data con-tain little information about either the spatial distribution necessarily require the same spatial and temporal variabil-

ity. Establishing a direct correlation between the two fea-of the emission or the structure in the core of the line.Despite these difficulties, characterizing the three-di- tures implies a common source or distribution and permits

the use of the more easily observed H2 emission spatialmensional distribution of the H Ly-a emission and thecontribution of continuum absorbers, such as CH4 on Jupi- distribution as a proxy for H Ly-a.

(2) What is the role of extinction for auroral H Ly-ater, is a logical next step in the study of Jupiter’s UVaurora. Studies of the intensity, spatial distribution, and emission? IUE measurements of the relative intensity of

H2 emission at wavelengths above and below a substantialline profiles of resonant emission lines (e.g., OI 1304 A)have been used to characterize the thermal and nonthermal change in the CH4 cross section near 1450 A suggests that

the majority of energy deposition consistently occurs at anstructure and charged particle population of the Earth’saurora (e.g., Anger et al. 1987, Frank and Craven 1988), average altitude within one line of sight scale height of the

CH4 homopause. H Ly-a emission produced by collisionaland the study of resonant H Ly-a emission from Jupitercould lead to similar advances in our understanding of excitation of H2 , or any other process that occurs with a

similar altitude distribution, should also be attenuated bythe jovian auroral system. Potential sources for H Ly-aemission that could contribute to the jovian aurora include CH4 in addition to being scattered by local H atoms. Quan-

tifying the extinction of H Ly-a may provide useful limitsdirect excitation by protons, primary and secondary elec-tron impact, or heavy ion collisional excitation (Rego 1994, on the contribution of proposed high altitude H Ly-a gen-

eration mechanisms, the altitude distribution of auroralHoranyi et al. 1988), proton charge exchange (Clarke etal. 1989), collisionally induced dissociative excitation of H2 energy deposition, the type and energy spectrum of the

352 HARRIS ET AL.

auroral charged particle distribution, and the shape of the to the study of auroral H Ly-a. As shown in Fig. 1 a portionof the large aperture extends off the jovian limb when theemerging line profile.

(3) What is the effect of resonant scattering on the emer- center of the aperture is located near 608 latitude. TheIUE is an imaging spectrograph with a spatially resolvedgent H Ly-a line intensity and shape? Unlike optically thin

H2 emission, auroral H Ly-a emission is produced within axis close to parallel to the long axis of the large aperture.The reduced form of an exposure is a line by line spectrala thick column of resonantly scattering H atoms, and pho-

tons produced near line center will be subject to multiple (LBLS) image array with the large aperture spectrum cov-ering the central 20% of the image. An LBLS image pro-scatterings before escaping the atmosphere. Depending on

the shape of the emission line and the line center optical vides a simultaneous exposure of camera background, re-flected solar continuum and resonantly scattered H Ly-adepth where it is produced, the effect of multiple scatter-

ings will not only modify the shape of the emergent line from Jupiter, H2 and H Ly-a auroral emissions, and scat-tered solar H Ly-a from the geocorona and interplanetaryprofile, but can result in extinction when absorbing mole-

cules such as CH4 are present. This will result in a different hydrogen (IPH). All the nonauroral sources must be sub-tracted in the extraction of auroral emission; however,auroral H Ly-a CH4 optical depth vs altitude distribution

relative to H2 emission that could limit the altitude range the sky background sources are uniform across the entireaperture. Uniform emission is difficult to separate fromwhere correlated emission can be produced, especially in

an environment where the relative abundance of H and auroral emission as part of the more widely used spectro-scopically resolved reduction and was the major motivationCH4 vary with time or altitude.for the development of the spatially resolved techniqueused here.2. ARCHIVAL DATA

In the current reduction of this data we introduce a newtechnique that exploits the imaging capability of the IUEAll data used in this study were obtained from the Na-

tional Space Science Data Center (NSSDC) and are part spectrograph to facilitate removal of the uniform back-ground sources at H Ly-a and separation of the auroralof the large archive of IUE low dispersion jovian auroral

spectra that begins in 1979 (Clarke et al. 1980) and contin- emission from the jovian solar scattered continuum. Auro-ues to the present. The spectra were all preprocessed andreduced using the standard IUE Spectral Image ProcessingSoftware (IUESIPS). The most common observationalstrategy employed throughout the archive was to centerthe large aperture (21.40 3 8.90; Turnrose and Thompson1984) of the IUE short wavelength prime (SWP) camera(spectral coverage; 1150–1900 A) at 50–608 latitude at ei-ther pole on the jovian central meridian longitude (CML).Pointing accuracy for these observations was generally1–20 (Livengood 1991), which is well within the 3–40 pointspread function (PSF) of the IUE telescope (Cassatella etal. 1984). A sample aperture orientation is shown in Fig.1. The typical exposure in this data set is 15 min in duration,during which time Jupiter rotates by 98 in longitude. Thisis long enough to obtain good signal/noise (s/n) in both Hand H2 emission features and short enough that high lati-tude structures move less than the width of the IUE PSF.Including the 30 min that are required to read and reset theIUE instrument, individual exposures during an observingrun are separated by a minimum of 45 min, or 278 oflongitude. We used a subset of the archive including 14northern and 8 southern auroral observing sequences with153 individual spectra covering the period from 1981 to1991 (Table I) in our study of auroral H Ly-a emission.

3. THE SPATIAL EXTRACTION METHOD FOR THEFIG. 1. A representation of the IUE large aperture on Jupiter. Shown

REDUCTION OF IUE AURORAL SPECTRA in the aperture are the dispersion and spatial axes of the spectrally re-solved image and on Jupiter the rough boundaries of the auroral zones

The aperture targeting strategy employed for the major- are shown using a model outline based on the oval seen in WFPC2 images(Clarke et al. 1995).ity of the auroral spectra in the IUE archive proved critical


TABLE 1 including H Ly-a and H2 Werner band features. The otherIUE Spectra Used in the Auroral H Ly-a Reductiona two isolate H2 band auroral emissions over wavelength

ranges from 1230 to 1300 A (primarily H2 Werner bandfeatures) and from 1550 to 1620 A (primarily Lyman bandfeatures) that are similar to those used in the previousspectroscopic studies of H2 emission variability in the samespectra (e.g., Skinner et al. 1984, Livengood and Moos1990). They contain the brightest H2 auroral emission fea-tures that are detectable with the IUE, and, because ofthe change in the CH4 absorption cross section betweenthem, their ratio is also a diagnostic of the effective opticaldepth of the emission (Yung et al. 1982).

Separation of the spatially resolved auroral emissionfeature from the background involves successively sub-tracting the camera background, the jovian disk emission,grating scattered light, and, in the H Ly-a bandpass, thegeocoronal and IPH contributions. Figure 3 shows a sampleH Ly-a spatial profile with the different components la-beled. The camera background is due to the impact ofexternal charged particles into the IUE camera entrancewindow (Turnrose and Thompson 1984), and its intensityincreases roughly linearly with increasing line number. Itis removed as a first step in the reduction by subtractinga least squares fit to points on either side of the SWLA

a The spectral sequences correspond to a series of Short Wavelength image (Figs. 4a–4c).Prime (SWP) spectra in the IUE archive. The CML ranges are inSystem III (1965) magnetic coordinates for the sub-Earth longitude. Removal of Geocoronal and IPH Background Signal

from Auroral H Ly-a

The second step in the isolation of auroral H Ly-a isral emission is extracted along with the various background the subtraction of uniform sky background emissions, in-components from the LBLS image by binning in the disper-sion direction across three different bandpasses (Fig. 2).The first bandpass sums emission from 1200 to 1230 A,

FIG. 3. The various contributing sources in a spatially resolved ex-traction from an IUE spectrum from 1200 to 1230 A are shown, includingFIG. 2. A spectrally resolved extraction of IUE auroral spectrum

SWP40427, taken on 20 December, 1990. The bandpasses that contain camera background, geocoronal and IPH emissions, jovian dayglow andgrating scattered long wavelength continuum, and auroral emission fromH2 and H Ly-a auroral emissions used in the spatial reduction are shown,

as is the wavelength range used for the jovian center to limb profile. H Ly-a and the H2 Werner band series.

354 HARRIS ET AL.

FIG. 4. The different steps in the process of separating auroral emission from the different background sources. (a)–(c) The camera backgroundsignal is removed by using a least squares fit to points on either side of the aperture image. (d)–(e) The subtraction of a best fit averaged flat fieldassembled from spectra taken during the same year as the auroral observation. Geocoronal/IPH subtraction is not necessary in the H2 reduction.(g)–(i) The center to limb profile taken at 1824 A is fit to the disk/scattered light portion of the remaining feature and subtracted; leaving only theauroral component in (i).

cluding scattered solar H Ly-a from the hydrogen geoco- Uncertainty associated with the instrumental PSF is de-pendent on the intensity and latitude of the auroral emis-rona and by the IPH between the Earth and Jupiter. Be-

cause of its dependence on the Earth–IUE–Jupiter angle, sion feature. Emission in the wings of the p3.50 PSF ex-tends well beyond the limb of Jupiter, and thereforethe intensity of the geocoronal/IPH component of this

background must be determined for each individual obser- contributes to the sky intensity. For cases of low levelaurorae, the contribution to the geocorona/IPH is small,vation. We accomplish this in the spatial reduction by using

the portion of the IUE aperture that extends beyond the particularly at the end of the aperture where the sky andflat field are matched. In the case of very bright aurorae,Jovian limb in each spectrum. This part of the one-dimen-

sional image is matched to a scaled sky background image the emission in the wings can dominate the geocorona,making an effective determination of the exact intensitythat was constructed from flat field exposures taken during

the same year as the auroral observation and subtracted difficult. For these spectra it was necessary to make anestimate of the geocoronal contribution by using the scaleacross the entire aperture image (Fig. 4d–4e).

The two major sources of uncertainty inherent in this factor obtained for the fitted flat field measurement inspectra taken before and after the bright event and themethod are the improper determination of the contribution

from IPH scattered solar H Ly-a and the effect of auroral known variability of geocoronal intensity with IUE view-ing geometry.emission scattered beyond the limb of Jupiter by the IUE

instrumental psf. In the former, because much of the neu-tral IPH is found beyond Jupiter, this component is slightly

Subtraction of Scattered Solar Continuum from thegreater in the part of the aperture than looks past the limb.

Jovian DiskSince this is what is used to remove the feature, it will tendto oversubtract a portion of the 300 to 1000-Rayleigh (R) In cases of very bright auroral emission, there is an

identifiable separation of the intensity peaks for the auroraIPH signal. In practice the effect of this on our observationsis negligible, because, while the fraction of interplanetary and the profile of the jovian disk near the limb (Fig. 4);

however, the features always overlap due to the low spatialhydrogen from beyond Jupiter is large, its contribution toback scattered solar H Ly-a is small due to the much lower resolution of the IUE. Subtraction of the part of the jovian

center to limb profile that crosses into the auroral featuresolar flux in the outer Solar System (Wu and Judge, 1980).


requires a template that is also resolved at the IUE PSF.This template is obtained using a fourth bandpass fromthe spectrum centered at either 1824 or 1750 A, where thesolar continuum has increased, the IUE camera is unsatu-rated, and no auroral emission is observed (Fig. 2). Aswith the sky background subtraction, this profile is scaledto correct for the brighter solar continuum at longer wave-lengths and then moved to adjust for a slight tilt in thespectrum relative to the long axis of the image (Fig. 4)until a best fit is obtained. Along with the planet continuumsubtraction of the template also removes grating scatteredlight, which the same spatial shape as the center to limb

FIG. 5. A test for the accuracy of the disk emission subtractionprofile. This subtraction method proves highly sensitive to process for a spectrum (SWP15836) in which no auroral emission waslow intensity auroral emission that is otherwise unresolv- observed. From 1230 to 1300 A the non-auroral signal is almost entirely

grating scattered light from longer wavelengths, because solar continuumable spatially or spectrally from background sources, withis very low and what reaches the planet is absorbed by CH4 . The fit ofa limiting sensitivity of p0.4 kR averaged over the 8.90 3the long wavelength profile is accurate to the level of the noise in the2.1480 standard area used for this reduction.camera background.

This technique is limited by the wavelength dependencein the center to limb profile and by the contribution of thewings of the auroral emission feature in the scaling of

The H Ly-a bulge is a nonthermal emission feature cen-disk emission. The differences in the center to limb profiletered along the magnetic dip equator caused by enhancedbetween the filters can lead to subtraction errors at highresonant scattering in the wings of the solar H Ly-a linelatitudes, where aerosol hazes combine with hydrocarbons(Clarke et al. 1991, Ben Jaffel et al. 1993). The intensityto attenuate much of the incoming solar radiation at shorterof the bulge is longitudinally and temporally variable withwavelengths (Wagener et al. 1985). This is mitigated bya broad intensity maximum near 1108 longitude and 158the poor angular resolution of the IUE telescope, whichnorth latitude. While the IUE aperture does not reachleaves the instrument insensitive to sharp discontinuitiesequatorial latitudes for polar observations, emission fromin the center to limb profile, by the low brightness of thethe bulge can be observed in some auroral spectra in thedisk relative to the aurora at high latitudes, and by themost equatorward part of the aperture. The exact contribu-substantial contribution of grating scattered NUV light,tion of equatorial emission is a complicated function ofwhich has a center to limb profile similar to that of theboth the intrinsic variability of the emission and the uncer-template. Because of the very low solar flux below 1700tainty in the pointing of the large aperture. Using a centerA the fraction of grating scattered continuum is very largeto limb profile contaminated by bulge emission will resultfor the H2 bandpasses, and between 1230 and 1300 A it isin an oversubtraction of H Ly-a emission relative to thethe only nonauroral contribution at high latitudes. Figurelong wavelength limb darkening curve for longitudes where5 shows a sample spatial reduction of a spectrum in whichthe bulge is bright. Comparison of on and off bulge extrac-no H2 band auroral emission was observed using a spectro-tions suggest that the effect of this on the current analysisscopic extraction; no evidence of subtraction errors are ev-is small.ident.

The difference between the auroral and the templateApplication of a Yearly Averaged Flat Field to

limb profiles is more profound at H Ly-a for two reasons,Auroral Ly-a

limb brightening and the H Ly-a bulge. Due to a line ofsight enhancement in the column of H atoms above the An important consideration in the extraction of auroral

H Ly-a is accounting for the IUE camera degradation atCH4 homopause, a greater fraction of the incoming solarH Ly-a line is resonantly scattered more effectively at high 1216 A. Due to the cumulative effect of repeated over

exposures of the IUE camera at H Ly-a, the detector haslatitudes. This effect is particularly prominent in a confinedarea right at the limb, where the H column becomes very developed moderate to severe deviations from the normal

IUE intensity transfer function (ITF) at this wavelength.large (Clarke et al. 1991, L. Ben Jaffel, personal communi-cation 1994) and results in a local peak that is much brighter The most prominent manifestations of this are a general

depression in sensitivity at the center of the SWLA andthan the planet background. Although its effects are spreadout by the IUE PSF, it does result in an undersubtraction the presence of several ‘‘hot’’ pixels (McGrath and Clarke

1992). This is complicated by the migration of the H Ly-of the disk component, leaving an artificial residual in theauroral H Ly-a intensity. The implications of this for the a spectral feature on the IUE camera face in response

to differences in the charge distribution in the Vidiconinterpretation of the H Ly-a results are discussed below.

356 HARRIS ET AL.

phosphor for the readout of different spectra. This means bandpass and a 100-eV electron impact model spectrum(model data provided by Y. J. Kim) that had been matchedthat the hot pixels and low sensitivity regions will also

migrate by small noninteger numbers of pixels between to laboratory data. From this method we obtain the rela-tionshipexposures, making them very difficult to remove when

other emission sources are in the aperture.To address this problem we employ a method similar to IH2(1200–1230) 5 0.51 3 IH2(1230–1300). (1)

that used in the McGrath and Clarke (1992) study of Ly-a from Saturn. Several sky background images taken at We used short wavelength H2 bandpass for this despitedifferent exposure levels were co-added to create a series lower s/n compared to 1550–1620 A, because there is noof ‘‘yearly averaged’’ spatial flat fields that track the devel- significant change in the CH4 absorption cross section be-opment of the camera degradation. These same flat fields tween 1200 and 1300 A.are also used earlier in the reduction to subtract geocoronal Some attenuation of auroral emission by resonant scat-emission in the auroral spectra. The individual sky expo- tering off of hydrogen between the Earth and Jupiter andsures are extracted over the H Ly-a bandpass using the in the Earth’s geocorona is also possible because the linetechniques discussed above, scaled for differences in inten- center scattering optical depth of the IPH approaches t psity, and co-added. By binning in wavelength the effect of 1 at infinity (Ajello et al. 1987). However, the auroralthe uncorrectable camera defects is diminished by averag- emission line is broad (Clarke et al. 1989) and Dopplering with other normal pixels. The resulting flat field is then shifted relative to the directed IPH flow direction (Lalle-normalized to the average value across the profile and ment et al. 1993). This combination leaves most of the linedivided line by line through the auroral H Ly-a feature. optically thin to this medium, and we are able to ignore IPH

scattering in our analysis. The same holds for geocoronalscattering effects for similar reasons, and because the highExtraction of Flux, Conversion to Rayleighs, andgeosynchronous orbit of the IUE keeps it above the bulkRemoval of Unresolved Backgroundof the scatterers.

Once the H2 and H Ly-a emission profiles are isolatedfrom the limb and other background sources, they are Photometric Ratios of Extracted Emissionsummed and their total intensity is converted from ergs/

In previous studies of auroral emission from Jupiter thecm2 sec A to Rayleighs. Because the widths of the auroraloptical depth of the emission to hydrocarbon attenuationfeatures are more indicative of the width of the IUE psfhas been measured through the use of a photometric, orthan the actual auroral zone, we assume an angular extent‘‘color,’’ ratio (Yung et al. 1982, Livengood and Moosof a fixed area equal to the 8.90 width of the SWLA by1990). The technique involves comparison of the ratio ofthe 2.1560 height of two pixel lines first used in Clarke etintegrated emission in the bandpasses above and belowal. (1980). The actual dimensions of the auroral emittingthe change in the CH4 cross section near 1450 A withregions that have been seen in FOC (e.g., Dols et al. 1993,unattenuated laboratory spectra of electron excited H2Gerard et al. 1994), WFPC2 (Clarke et al. 1996), and H1

3(e.g., Ajello et al. 1984), such that(e.g., Baron et al. 1991) images are typically narrower

north/south than we use here. However, the intensity andCRH2 5 IF1/IF2 5 IF1/[IF2(0) exp2t] 5 CRunattenuated exp2t ,angular extent of these features have been observed to vary

(2)sharply on short time scales. We employ a conservativeoverestimate of the area that reflects this uncertainty, pro-vides continuity with the dimensions used in previous anal- where IF1 is the emission in the unattenuated bandpass

(H2(1620–1550 A)), IF2 is the emission in the attenuated band-yses, and reflects our desire to not impose any externalareal estimate that is not directly indicated by the IUE data. pass (H2(1230–1300 A)), t is the optical depth of the emission,

and CRunattenuated is the color ratio in the absence of CH4The two final considerations for the reduction are thecontributions of unresolved H2 Werner band emission and absorption for the representative impacting electron en-

ergy. Comparison of the unattenuated ratio with the samethe attenuation of auroral H Ly-a by hydrogen betweenthe Earth and Jupiter. To fully extract the H Ly-a emission ratio in IUE spectra has been successful in showing the

presence of hydrocarbon attenuation and its variabilityfeature a wavelength range equal to 3 times the width of theSWLA is used. This bandpass also contains a substantial with CML in H2 emission from Jupiter’s aurora (e.g., Liv-

engood et al. 1992).contribution of H2 Werner band emission that is unre-solved both spectrally and spatially. This contribution must Making use of a similar technique to characterize the

effective optical depth of auroral H Ly-a emission is lessbe inferred from H2 emission in a different bandpass andsubtracted from the auroral intensity. We accomplished straightforward. Key to the use of the color ratio in the

study of H2 emission is the fact that the emissions in boththis by using the auroral intensity in the 1230 to 1300 A


bandpasses are attributable to one molecule and a single nificantly on time scales of less than one jovian rotation(e.g., Livengood et al. 1990, Prange et al. 1993). These highexcitation mechanism and that the resulting emission fea-

ture is optically thin to scattering. Using a similar method activity events appear to represent a departure from thenormal characteristics of auroral energy deposition andfor H Ly-a emission is complicated because there is no

longer wavelength atomic hydrogen emission feature in longitude distribution. Over longer time scales the H2 au-rora also varies, but not in a systematic way that mightthe IUE SWP bandpass to use in a photometric study,

and because any H Ly-a emission produced near the CH4 reflect a long term persistent change in conditions eitherwithin or beyond the jovian magnetosphere (Livengoodhomopause will be at a very large resonant scattering opti-

cal depth (t $ 105) that will further modify the emergent 1991). To determine the response of auroral H Ly-a withtime and relative to H2 emission, we compared the bright-line. As an alternative we compare the intensity of auroral

H Ly-a with emission in the H2 bandpasses used in CRH2 . est integrated auroral intensities for the H2(1550–1620 A) andH Ly-a bandpasses of each observing sequence versus timeThese two ratios are represented byin Fig. 6. To improve the temporal coverage we also include

CRa1 5 [[I(Ly-a(1200–1230 A))/I(H2(1550–1620 A))](3) the brightest spectra from several additional sequences

2 0.51/CRH2]21that were not completely reduced in this study (Table II).Our findings indicate that both the H2 and H Ly-a emis-

and sions sources respond similarly to changing auroral condi-tions on all time scales, including the very short response

CRa2 5 [[I(Ly-a(1200–1230 A))/I(H2(1230–1300 A))] 2 0.51, times exhibited during the bright auroral events.(4) Previous attempts to correlate H2 emission variability

with solar activity indicators have yielded negative resultswhere the rightmost terms in each equation account for(Livengood 1991), and our comparison suggests that thethe contribution of H2 band emission in the 1200 to 1230-same is true for H Ly-a. It is worth mentioning, however,A bandpass. The CRa ratios will give information identicalthat the implied result of this, namely that short time scaleto the CRH2 for the case where all of the H Ly-a emissionauroral variability at Jupiter is not solar activity dependent,is produced by dissociative excitation of H2 . This is at leastmay be misleading when the number of samples is consid-partially the case, since laboratory spectra do show brightered. Invoking the terrestrial solar dependent auroral vari-H Ly-a associated with dissociative excitation of H2 (Ajelloability as a paradigm suggests that, if an analogous processet al. 1984, 1991), and this will be a portion of the observedoccurs at Jupiter, it would not be manifested as an increaseauroral H Ly-a. However, since other possible sourcesin overall activity, but increased probability of auroralmight contribute to the emission, this case cannot be as-events as the number of solar driven plasma disturbancessumed to hold completely. On the other hand, the CRaincreases. Solar dependent auroral events have long beenratios may still provide results quantitatively similar toknown to occur on the Earth throughout the solar cycle,CRH2 , if the H2 and optically thin H Ly-a emissions arewith increasing frequency correlated to higher solar activitycorrelated, even if no other relationship between the emis-(e.g., Greaves and Newton 1928). Since the archive con-sions is evident. The degree to which these emissions aretains observations of only a few rotations in any given yearcorrelated is discussed in the next section.in our data set, we have temporal coverage of no moreThe CRa1 ratio measures the relative attenuation of Hthan 1–2%, which is not sufficient to provide a reliableLy-a and the nonattenuated H2 from 1550 to 1620 A andindicator of a solar activity relationship at Jupiter. Therewill be similar to the CRH2 if the H Ly-a emission ishave also been no consistent in situ measurements of solarattenuated and unscattered. The CRa2 compares H Ly-awind data near Jupiter over the period covered by thewith the attenuated H2 emission from 1230–1300 A andIUE archive, and no positive correlations between auroralwill produce results similar to those for CRH2 in the caseevents and solar wind disturbances. A single possible ex-where optically thin H Ly-a is less attenuated than H2ception to this occurred in December 1990, when a solaremission near the same wavelength. Individually thesewind disturbance was detected by the Ulysses spacecraftratios can suggest the presence or lack of hydrocarbonat a distance of 1.4 AU from the Sun, and a bright auroralattenuation in correlated H Ly-a, and combined they areevent was later observed on Jupiter at a time close toa useful indicator of the relative line of sight optical depththe predicted encounter with the solar wind disturbanceof the H and H2 emissions.(Prange et al. 1993). Again, while an interesting coinci-

4. RESULTS AND DISCUSSION dence, a single event cannot supply the statistical basisneeded to show a relationship.

Temporal Behavior of Auroral H Ly-a IntensityThere are also some previous measurements of evolution

of solar wind features. Measurements at 1, 2.0, and 4.6 AUOver the history of IUE observations the intensity ofjovian auroral H2 emission has been shown to change sig- using the Pioneer 10 and 11 spacecraft show evidence of

358 HARRIS ET AL.

FIG. 6. The brightest auroral H Ly-a emission recorded during different spectral sequences between 1980 and 1991 (Table 2) are displayedwith time to show any temporal behavior. While substantial short term changes in the intensity with time are observed, there are no statisticallysignificant long term trends that could relate the short term variability with a solar cycle related behavior.

changes in solar wind streams over this distance (Siscoe greater rotational symmetry and a more extreme viewingangle in the south. The observed asymmetries with CMLand Intriligator, 1993). Given these data, it is not clear that

a measurement of a solar wind feature from the distance are believed to be due to a combination of viewing geome-try toward the auroral zones, the high latitude of the emis-of Ulysses in December 1990 was applicable to conditions

at Jupiter. Clearly more frequent synoptic observations of sions, the effects of the 3.50 PSF in the IUE telescope(Prange et al. 1996, Ballester et al. 1996), and to intrinsicboth auroral emission and solar wind parameters will be

required to isolate the mechanism controlling the jovian longitudinal structure in the energy deposition rate alongthe zones (Livengood and Moos 1990).auroral variability.

A CML dependence similar to that seen in the mergeddata set is observed from each individual sequence. How-

Variation of Jovian Auroral H2 and H Ly-a Emissionever, in certain scans an intensity maximum is observed at

with Longitudea CML well separated from the nominal peak. This effectappears mainly in those scans taken when the aurora areThe integrated intensity of auroral emission in the long

wavelength H2 (1550–1620 A) bandpass for the north and significantly brighter than normal, and in most cases ismanifested as a shift of the entire profile in CML to eithersouth polar regions is plotted against CML(system III) in Fig.

7. Except for the difference in the spectral extraction tech- side of the average location. It is not clear to what extentthese shifted profiles represent a significant change in thenique and bandpass, our methodology is identical to that

employed in previous studies of IUE spectra (e.g., Liv- auroral characteristics or if they are an artifact of the poorsampling of the IUE. It is also unclear what relationshipengood and Moos 1990, Skinner et al. 1984). The clear

pattern of CML dependent brightening that is readily ap- may exist in the source mechanisms triggering differentbright events. The shift and intensity change was particu-parent in our results is consistent with the earlier findings.

Auroral H2 emission appears to peak between CML 180– larly prominent in the north auroral scan obtained on 21December, 1990, which contains the brightest jovian auro-2008 in the northern auroral zone and near CML 0–1008

in the south. The contrast in intensity with CML is more ral spectrum in the IUE data set used in this work. Thebrightest emission in this scan peaks at CML 2378 andpronounced in the north than in the south. This effect

is commonly attributed to differences in the location of remained much brighter than average at 08, a CML com-monly identified as a minimum in northern auroral inten-magnetic pole between the hemispheres that leads to


TABLE 2 brightening and offsets from the nominal CML peaks areBright Auroral Spectra Used for observed for both H and H2 emissions. This suggests that

Temporal Characterizationathe two emission features have the same distribution withinthe IUE aperture for both average and perturbed auro-ral activity.

To determine the correlation between H2 and H Ly-a emission intensities at both poles, emission from thedifferent bandpasses in each individual spectrum are com-pared and subjected to a linear correlation test. The corre-lation coefficients and the coefficients for the best linearfit to the data are shown in Table III and Fig. 8. We usedthe relationship between the two H2 bands as a control,since they contain emission produced by the same processin different bandpasses. The probability of obtaining thelarge derived coefficients, r 5 0.93 and 0.95 in the northand south, respectively, from a noncorrelated sample isinsignificant (Bevington 1964), and a linear relationship isindicated. When the same comparison is conducted usingH Ly-a vs both of the H2 bandpasses, large coefficients of0.80 , rlinear , 0.85 are also obtained, showing a link inthe spatial distribution of these emission features. Theextension of these comparisons to small scale structure islimited by poor spatial sampling and the broad psf of theIUE. However, the relative intensities of the H2 and HLy-a aurora are still clearly related, and considering thecorrelated response observed from short time scale vari-ability in bright auroral events, the argument for a coinci-dent spatial distribution is strengthened.

Comparison of Hydrocarbon Extinction of H Ly-a andH2 Emissions

The presence of CML dependent CH4 absorption inIUE spectra of auroral H2 emission has been extensivelystudied, and has led to the current limits on the averagealtitude where H2 emission is produced (Livengood et al.a The spectral sequences correspond to a series of Short Wavelength

Prime (SWP) spectra in the IUE archive. The CML ranges are in 1990, Gladstone and Skinner 1989). We have adopted aSystem III (1965) magnetic coordinates for the sub-Earth longitude. strategy similar to that of these earlier studies in our analy-

sis of the relative amount of H2 and H Ly-a attenuationand again have compared our results with the earlier reduc-tions as a control. CH4 attenuation is observable as anincrease in the ratio of flux in the two H2 bandpasses (Eq.sity. More recent observations now suggest that the distri-

bution of the emission along the oval is very different (2)). In IUE spectra this ratio has followed a consistent andrepeatable pattern over the history of auroral observationsduring these bright events (Gerard et al. 1994, Prange et

al. in preparation, and Ballester et al. in preparation), which characterized by values significantly higher than are ex-pected from laboratory measurements (Yung et al. 1982,might indicate that a morphological change in the auroral

process is associated with them. Ajello et al. 1984), and a clear CML dependent variabilitywhere the magnitude of the ratio is tied to increasing inten-When compared directly, the brightness of the single

auroral H Ly-a emission line is comparable to the inte- sity. The high CRH2 values have been attributed to auroralenergy deposition at an altitude near the CH4 homopausegrated emission in either H2 bandpass. A modulation with

CML similar that seen in H2 emission is also apparent for H and the variability to an asymmetry in either this altitudeor in the CH4 column abundance above it. Viewing geome-Ly-a (Fig. 7). This relationship also extends to anomalous

events such as the December 1990 aurora, where the same try has not previously been considered as a source for the

360 HARRIS ET AL.

FIG. 7. A graphical representation of the results of a linear correlation test for H2 and H Ly-a auroral emissions in the north and south aurorae.The ordinate and abscissae terms are shown in the top left of each plot, while the correlation coefficients are displayed in the top right. For allcases the correlations are statistically significant given the coefficients and the number of samples (see Table III).

variability, because the greatest attenuation is observed the contribution of the line of sight should be reexamined(Prange et al. 1996, Ballester et al. 1996). Because H2 emis-over a CML range where the auroral oval is tilted farthest

toward the equator and the line of sight to the emission is sion is subject only to CH4 attenuation there is ambiguityabout the location of the observed emissions, and it hasa minimum. Recent modeling of auroral emission observed

with the FOC and convolved with the IUE psf now suggests proven difficult to determine which of the possible mecha-nisms contributes most strongly to the CRH2 variability insome ambiguity about what regions of the auroral oval

dominate the observed emission in this orientation and that IUE spectra.

TABLE 3Results from Linear Correlation Testing of Auroral H2 and H Ly-aa

a The table gives the linear correlation coefficient for each flux comparison at either pole(rcorr), the equation for the best linear fit to the data (mx 1 b), and the uncertainty in they intercept value (sb). All intensities are in Rayleighs.


depth of t p 1. Similar slopes of 0.37 and 0.28 are ob-tained from the best linear fits to north and south auroralH2(1550–1620 A) vs H Ly-a emissions, while comparison withH2(1230–1300 A) yields much steeper slopes of 0.96 and 0.72.This combination also gives results consistent with unscat-tered H Ly-a attenuation within a moderately thick columnof CH4 similar to or larger than for the H2 emission.

Limitations on the Contribution of High Altitude AuroralH Ly-a Processes

The common CML variability of CH4 attenuation ofauroral H Ly-a and H2 emissions suggests a limit to thecontribution of higher altitude processes such as enhancedresonant scattering in a superthermal H population or pro-ton charge exchange to the observed H Ly-a emission,since these would occur far above the CH4 homopausewhere extinction is negligible. We estimate this limit byexamining the changes in the CRa with CML. The CRa1

FIG. 8. The integrated intensity of auroral H2 and H Ly-a emissions and CRH2 ratios both change by t p 1 between the areasare displayed with CML. The clear peak in the H2 intensity near CML of greatest and least CH4 attenuation for most spectral1808 is consistent with earlier analysis of IUE spectra (e.g., Skinner et al.

sequences. For H2 emission this difference is entirely due1984, Livengood and Moos 1990). The pattern of variability with CMLto CH4 attenuation, and the change in the line of sightdisplayed by auroral H Ly-a is peaked at the same longitudes with an

overall distribution most similar to that of the 1230- to 1300-A H2 column of absorbers can be determined directly. For Hbandpass. Ly-a emission, however, this change represents the combi-

nation of absorption of emission from below the CH4 ho-

The CML distributions for CRa1 and CRa2 (Eqs. (2)and (3)) ratios are shown for each pole in Fig. 9. An asym-metry in the CRa1 ratio very similar to that of the CRH2

is readily apparent at each pole, and, as discussed above,is consistent with enhanced absorption of unscattered HLy-a emission relative to the unattenuated bandpass of H2

(1550–1620 A). By contrast, there is no evidence of amaximum near the CML of greatest auroral emission inthe CRa2; indeed the CRa2 is generally lowest over thisCML range. This combination of results implies that theobserved auroral H Ly-a emission is optically thin to scat-tering, and is subject to the same or greater CH4 attenua-tion than H2 , with a similar dependence on CML. Whilethe CRa color ratios cannot quantify the actual opticaldepth of the Ly-a emission to CH4 absorption, they areconsistent with production of Ly-a emission within a mod-erately thick effective column of CH4 , just as is the casefor H2 .

Additional evidence for attenuation in auroral H2

and Ly-a emissions is found from the linear fits to theintegrated bandpass intensity comparisons (Fig. 8; Ta- FIG. 9. The H2 color ratio variability with CML is compared withble II). In laboratory spectra of 100-eV electrons on H2 ratios showing the relative attenuation of auroral H Ly-a and H2 emissions

in the 1230- to 1300-A and 1550- to 1620-A bandpasses. A clear similarity(Yung et al. 1982) the unattenuated ratio H2(1550–1620 A) :is seen between the H2 ratio and the ratio of H2(1550–1620 A) and H Ly-a,H2(1230–1300 A) is 1.19, which corresponds to a constantsuggesting that H Ly-a and H2(1230–1300 A) emissions are subject to a similarlinear slope with increasing H2 intensity of 0.84. The bestamount of hydrocarbon extinction. The ratio of H Ly-a to H2(1230–1300 A)linear fit to auroral H2 archival bandpass intensities has a emission shows a less sharply peaked or inverted distribution that may

slope of 0.35 in the north and 0.39 south, which is equiva- imply a CML distribution favoring increased H Ly-a attenuation wherethe aurora is brightest when viewed from the Earth.lent to average CRH2 values of 2.5–2.7 or a CH4 optical

362 HARRIS ET AL.

mopause and a contribution from an unknown amount of Ly-a emissions are produced, because the weakest auroralemission corresponds to CMLs where the viewing angle isassociated optically thin emission from higher altitudes. A

conservative maximum for the high altitude component most extreme.Understanding the source of this excess is importantcan be derived for the case where the optical depth for

low altitude H Ly-a emission increases much more dramat- because its contribution has a strong effect on the CRa2at CMLs away from the auroral intensity peak. In Fig. 10ically than its effective value. For this case the integrated

H Ly-a intensity is composed of two components such that the region of peak emission is the local minimum in theCRa2 ratio, which for correlated emission implies an asym-metry with CML where auroral H Ly-a suffers greaterICML(total) 3 e2teff 5 ICML(low) 3 e2tl 1 ICML(high),relative extinction than H2 . However, if the excess is sub-(5a)tracted from the auroral H Ly-a intensity, the CRa2 CMLdistribution becomes uniform, which is consistent with nowhere ICML(high/low) are the CML dependent contribu-additional attenuation of H Ly-a relative to H2 (Fig. 9).tions at different altitudes, teff is the effective CH4 opticalSome possible sources for this excess include aurora relateddepth, tl is the optical depth of the low altitude emission,emission features and reduction artifacts such as the under-and the high altitude component is assumed to be optic-subtraction of substantial high latitude H Ly-a limb bright-ally thin to CH4 . For H2 emission teff(max) 2 teff(min) p 1,ening that was discussed above in Section 3.and the Cra ratios suggest that for H Ly-a teff(max) 2

If the excess is an auroral emission feature, then it couldteff(min) $ 1. However, in the limiting case where teff(max)result from a high altitude process that is decoupled from(ICML(low), H Ly-a) @ 1, the H Ly-a component will bethe H2 excitation. An example of such a process would becomposed entirely of high altitude emission, so thatheating of the thermosphere above the auroral zones givingrise to a super thermal population of H atoms that wouldIH Ly-a(total) 3 e2(1) 5 IH Ly-a(high) 5 0.37 IH Ly-a(total)locally scatter the solar H Ly-a line with greater efficiency.(5b)This mechanism has been suggested as a possible sourcefor the observed excess of H Ly-a near the equator (Clarke

at the peak of auroral intensity and CH4 attenuation. Since et al. 1991, Ben Jaffel et al. 1993), and more extreme condi-the distribution, surface brightness, and peak optical depth tions should arise in the auroral atmosphere where theof auroral emission is observed to vary significantly in energy deposition is much higher. The affected regionsHST images and in particular during bright auroral events of the atmosphere would not necessarily have the same(Gerard et al. 1994, Clarke et al. 1980, Prange et al. 1996),it seems unlikely that this limiting case could describe theextinction of auroral emission for the different lines ofsight and energy deposition rates observed in the sequencesreduced here, while still demonstrating a positive correla-tion with H2 in both intensity and effective optical depth.It is, however, consistent with the observed H Ly-a behav-ior and so cannot be discounted on the basis of IUE spec-tra alone.

Auroral Ly-a Emission Excess

The linear correlative analysis shown in Fig. 8 reveals apossible secondary feature in the H Ly-a emission CMLdistribution relative to H2 . The best line fits for the inten-sity of H2(1230–1300 A) vs H2(1550–1620 A) emissions have inter-cepts within 1.5 kR from the plot origin at both poles.Larger intercepts of p3.5 kR in the north and p7.0 kR inthe south (Table III) are seen in both of the H2 vs HLy-a emission fits, with the excess composed of H Ly-aemission in all cases. This excess and the polar asymmetry FIG. 10. A demonstration of how a residual nonauroral component

in H Ly-a emission can affect the CRa2 ratio modulation with CML. (acould reflect the approximation inherent in the linear fit-and c) The CML distribution is a minimum where auroral emission isting technique, or in the reduction method used for Hbrightest, which is consistent with enhanced attenuation of H Ly-a whenLy-a; however, its consistent appearance in all intensitythe aurora is brighter. (b and d) Following the subtraction of the excess

comparisons with H2 at either pole bears some investiga- suggested from the linear fits shown in Fig. 7, the pattern flattens outtion. An excess of H Ly-a is inconsistent with emission with CML, and the tendency toward enhanced relative attenuation is no

longer seen.from the same altitudes where the majority of H2 and H


longitude structure as the H2 auroral zone and could be The model results indicate that any emission producedwithin 670 mA of line center will escape only after multiplebrighter than the observable aurora at CMLs where it

is weakest. scatterings to an optically thin wavelength, creating astrongly self-absorbed feature. This means that informa-Implications of Resonant Scattering on the Interpretationtion about the low altitude auroral H Ly-a emission processof Auroral H Ly-awill be accessible only from the optically thin wings of very

An important factor in the escape process for auroral broad nonthermal features or from asymmetric featuresH Ly-a emission produced at the altitude of the CH4 homo- such as the tail of a proton auroral emission line (Regopause is the contribution of resonant scattering. Current 1994) or emission associated with field aligned currentsatmospheric models suggest that, at these altitudes, the (Clarke et al. 1989) that are preserved as an asymmetry invertical scattering optical depth will be very large (105 , the emergent line (Wallace and Yelle 1989).t , 106) at line center due to the concentration of H atoms While it affects our ability to obtain useful informationat and above this altitude (e.g., Trafton et al. 1994). Figure from the emergent H Ly-a line profile, the contribution11 shows the effect of scattering with wavelength on the of resonant scattering does have the potential to constrainoptical depth of H Ly-a produced by H2 dissociative excita- the ambiguity about the altitude distribution of both Htion for a typical H concentration. Dissociative excitation Ly-a and H2 auroral emissions and the source for theirof H2 produces excited H atoms with an energy branching longitude variability in optical depth. Once the redistrib-ratio of 87% at 0.6 eV and 13% at 6 eV (Waite et al. 1983, uted H Ly-a photons become optically thin, they escapeAjello et al. 1991). The H atoms return to the ground the CH4 absorbing layer subject only to the same amountstate by generating a broad emission line with an intensity of extinction experienced by H2 emission. The caveat isrelative to H2 band emission similar to what is observed that radiative escape via this method requires pt scatteringin jovian auroral emission (Y. J. Kim, personal communica- events (Gladstone 1985), and some fraction of the photonstion). As can be seen in Fig. 11, over 98% of this broad will be attenuated before becoming optically thin if thefeature is optically thick to resonant scattering. local atmosphere contains any absorbers. At the altitudes

The most efficient mechanism of escape for very opti- where H2 emission is produced, the local CH4 density cor-cally thick resonant radiation is frequency redistribution, responds to an effective optical depth for H Ly-a of t $whereby each scattering event leads to a new frequency 1, a part of the curve of growth where the amount ofdefined by the velocity of the scattering center (e.g., Hum- extinction changes rapidly with increasing concentrationsmer, 1962). The larger the number of scatterings, the of CH4 . A simple multiple scattering calculation using thegreater the probability that the frequency will shift to one single scattering albedo,that is optically thin. We examined the effect of resonantscattering on the shape of the emergent H2 dissociative go 5 n(s)s(s)/[n(s)s(s) 1 n(a)s(a)], (6)excitation H Ly-a line profile for the case of a local temper-ature of 600 K and a line center optical depth of 105 using where n(s) and s(s) are the number density and cross section

for scatterers, respectively, and n(a) and s(a) are the numberradiative transfer code provided by D. Rego (Rego 1994).

FIG. 11. The effects of resonant scattering on emission produced from H2 dissociative excitation are shown for the case of a local temperatureof 600 K and a line center optical depth of 105. Despite the nonthermal line shape generated by H2 dissociative excitation, all but p2% of the lineis optically thick to scattering.

364 HARRIS ET AL.

density and cross section for absorbers, respectively, sug- to the local ratio of CH4/H. This may prove a useful diag-nostic for limiting the altitude extent of the emitting layergests that correlated H Ly-a and H2 emission and optical

depth variability will occur only for cases where the local for correlated H Ly-a and H2 band emissions.average ratio of absorbers to scatterers (n(a)/n(s)) remainsa constant. For other cases the relative optical depth of ACKNOWLEDGMENTSH2 and H Ly-a emissions will differ as the amount of

The authors thank Randy Gladstone, Alex Dessler, Bill Kuhn, Hunteremission lost in the scattering process changes with theWaite, Tim Livengood, and in particular Daniel Rego, who also suppliedn(a)/n(s) ratio. Since the ratio of n(CH4) : n(H) in model auroralsupporting model calculations, for many useful discussions about theatmospheres changes rapidly near the CH4 turbopause, thenature of resonance line radiative transfer and the interpretation of the

effect of vertical movement on the emergent intensity from IUE archival data set, as well as the long set of observers who relentlesslythe auroral H Ly-a emitting layer will be different than pursued Jupiter’s aurora with IUE over the years. The precision of their

work has made our study possible. We would also like to acknowledgefor correlated H2 emission; a variability that is not sup-Lotfi Ben Jaffel for his contributions and calculations on the subject ofported by our results. It is important to note that sinceLy-a limb brightening, Yoo-Jea Kim for her contribution of a model H2scattering to an optically thin wavelength occurs within aspectrum, and an anonymous referee for useful commentary on time

narrow altitude range, the amount of absorption is depen- dependence and auroral activity. This research was funded under NASAdent on the local n(a)/n(s) ratio rather than the line of sight Grants NAG5–1030 and NAGW-1766 to the University of Michigan.columns of the two species. This implies that the patternof correlated optical depth variability would not be affected REFERENCESby changing viewing geometry or by an upwelled columnof CH4 , but that it would be sensitive to a change in the AJELLO, J. M., D. E. SHEMANSKY, AND G. K. JAMES 1991. Cross sections

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Documents

Analysis of Jovian Auroral H Ly-α Emission (1981–1991)