The IUE and Voyager spacecraft observations of Jupiter and Saturn were combined to obtain a consistent measurement of temporal variation of the equatorial subsolar hydrogen emission. The outer planets appear to have rather independent behavior over time scales of the order of 10 yr, particularly in emission from the H Ly alpha line. The time interval from 1978 to the present shows variation of mean equatorial H Ly alpha brightness of 2 at Jupiter and 5 at Saturn. The relative magnitudes of the variations is sufficiently different to suggest that response to input from the Sun is at least nonlinear. The brightness of H2 band emission appears to be relatively more stable than H Ly alpha. There is evidence in IUE observations of a moderate increase in H2 band brightness with increasing time at Jupiter, in opposition to the variation in H Ly alpha

Hall, D. T.

Holberg, J. B.

Shemansky, D. E.

English

NASA Technical Reports Server

N89 - 1069 493
LARGE TIME SCALE VARIATION IN IlYDROGEN EMISSION FROM JUPITER AND SATURN
D. E. Shemansky, D. T. Hail, and ,I. B. Holberg
Lunar and Planetary Laboratory, University of Arizona
Tucson, AZ 85721
ABSTRACT
IUE and Voyager spacecraft observations of Jupiter and Sat-
urn have been combined to obtain a consistent measurement of
temporal variation of the equatorial subsolar hydrogen em,s-
sion. The outer planets appear to have rather independent
behavior over time scales of the order of 10 years, particu-
larly in emission from the H Lya line The tllne interval from
1978 to the present shows variation of mean equatorial l[ Lye
brightness of _ 2 at Jupiter and _ 5 at Saturn The relative
magnitudes of the variations is sufficiently different to suggest
that response to input from the sun is at least nonhnear. The
brightness of H2 band emission appears to be relatively more
stable than H Lye. There is evidence in IUE observations of a
moderate increase in H_ band brightness with increasing time
at Jupiter, in opposition to the variation in H Lyc_
Keywords/S,_lar system/atmospheres
1. INTRODUCTION
Emission from atomic and molecular hydrogen is an
understandably dominant feature in the EUV spectra of the
outer planets, particularly on the sunlit hemispheres. A current
question of high interest involves the fundamental issue of
exactly what process produces electronically excited states in
the gas to generate the observed emission. The auroral
emissions are certainly produced by particle precipitation, but
the prominent emissions from the sunlit equatorial regions are
spatially diffuse and the interpretation of how the gas is
excited is less obvious. The strong emission of the H 2 Rydberg
band systems has been interpreted as primarily collisionally
induced by electrons (see Ref. l). This conclusion was based
on the fact that an electron excited model accurately matched
the spectrum and on the estimation that fluorescence of solar
radiation should be very weak. However, more recently (Ref.
2) _t has been suggested that fluorescence may play a more
dominant role in the excitation process through application of a
model using a brighter solar EUV source and converting most
of the flux into fluorescence. The difference between these
conclusions has significant implications for the energetics of
the atmosphere. If the source is dominantly electron
excitation, the relatively high temperatures at the top of the
thermospheres of Jupiter, Saturn and Uranus could be
explained by heating produced as an intrinsic part of the
process (Ref. 3). Some other explanation of the thermal
structures would have to be found if the fluorescence process
dominated.
We describe below an analysis of observations obtained at the
Voyager spacecraft encounters in combination with the long
time line of measurements provided by the IUE satellite. We
find that although the interpretation of the H Lyc_ emission in
terms of atmospheric excitation processes is more difficult, the
observed temporal variations are very distinctive, and planet to
planet variations are also evident. The results appear not to be
compatible with an atmosphere responding only to the
deposition of solar radiation.
2. ANALYSIS OF OBSERVATIONAL DATA
Several factors affect the data reduction process particularly in
the extraction of the planetary emission rate of H Lycz (1216A)
from the observed spectrum. The measured signal at 1216/k
includes H Lyct emission in varying amounts from the local
geocorona, emission from the local interstellar medmm (I_,ISM)
along the line of sight to the planet, and in the case of Saturn
a portion of the SWP large aperture includes emission along the
line of sight beyond the location of the planet. The effect of
extinction of the H Lyc_ signal from the planet is also
significant for Saturn in particular. All of these effects were
accounted for in the analysis through the use of a geocoronal
model developed in this program, and a model of the LISM
(Ref. 4). Most of the IUE SWP spectra of Saturn were
reduced in a line by line analysis of the image in order to
obtain uncontaminated measurements of the equatorial region.
2. I Jupiter
The H Lycz emission from Jupiter's equatorial region shows two
separable temporal characteristics. The bulge phenomenon
associated with a broad magnetic longitude region (N125 o
FWHM) with enhanced emission centered on 110 ° AIII. The
results obtained in the current observational program and
earlier analyses show that the phenomenon is always present
and has persisted with approximately the same amplitude since
1979 (Refs. 1, 5). Figure 1 shows data from August 1984,
April 1985, and November 1986 as a function of central
meridian longitude (CML) obtained from the current
observational program. The amplitude and general shape and
location of the CML modulation is essentially unchanged from
results derived from earlier observations (Refs. 6, 7, 1, 5,).
The data in Figure 1 shows an emission peak near 105 o CML
and a minimum near 2900 CML. The consistency in location
shows that the phenomenon is defined in an absolute sense by
)'I11 magnetic longitude. The magnitude of the modulation
according to the IUE data shown here and the results reported
by Ref. 5. suggests some variability The mean modulation
shown in Figure 1 is ~1.45 in peak to trough ratio. Values
reported by Ref. 5. vary from 1.7 to 105 with no clear long
A Decade of UV Astronomy with IUE, Proc. Celebratory Symposmm, GSFC, Greenbelt, USA, 12-15 Aprd 1988, ESA SP-281, Vol. 1 (June 1988)
https://ntrs.nasa.gov/search.jsp?R=19890001323 2020-03-20T05:58:00+00:00Z
94 D E SHEMANSKY, D T HALL & J B HOLBERG
ORIOiNAL PAGE m
Of POO 0UALrrY
TABL! I
Jupller sub-_lar brlght_ _lth no _rlectlon for
ISb_/IP_ e_mcuo.
Observation Date IOtzLy÷WR ) I(H Lyo) I{H Lya) 1(16OOA) lIIZ
(kR) (kg) I (H2Ly_R) (R/A CML
_ket • 1972/144 2 4 I 0 4 5O 100
31 I_ O5 74 leo
R_ket b 197t/335 13 IO6
1978/335 19 106
IUE c 1979/1_1 74
V2 d 1979/tt7 3 0 t6 _ 3 76 24O-33O
V2 d 1979/I87 3 0 22 7 2 65 60-150
IUE • 1979/120 15 1(30
-I_O
IUE © 19t0/120 12 I00
-IIO
rUE f 1915/99 4 4LI 6 Z 0 t $ 76*_6 leo
IUE f 19_5/99 4 4+] 6 5 S I 3 76_6 280
IUE f 19$6/Y24 82 I 6 I00
IUE f 1916/304 52.1 _ 6O F I *5*_6 28O
[UE f 1916/305 5 4.1 7 7 5 I 4 86.__ 28O
a) Judge _ ._leraala_ky (1915) Imal_u of Cities et al (1976) _ket ¢xpertrnent,
E-W H farads d=mtmUola tho_ _ limb darktnm| (Sl_mtmky, 1985)
b) Clarke tt I1 (1911O) reeker expenaznt
c) pnneat work. era=ace f_ SWP J448
d) Shemamky (1_5) _alym e¢ V2 data
Dote: t(_0A) - O43. Tab_ 3 ihvekt _ °^tmospher_ P._ I(_0A. Sot, e o
_I_A)
e) Clarke _ IL (l_la) _ .P_ by a factor of I 16 on 1953 oo,_o
ZO IUE ip_t_e _e
D_k Average
Calc.lated
Seb-tolar Po,.t
Dak A_mle
Sub-Solar Po,nt
Sub-Solar Po,nt
Sob-Solar Po,m
Sob-Solar Po,nt
Sub-Solar Po,.t
Sob-Soar Po,.t
Sub-Solar Pout
Sob-Solar Po,nt
Sob-Solar Po,.t
Sub-solar Po,nt
S.b-Solar Po,m
Obe. Date
Rocket 1972/144 1.5
Rocket 1978/355 19.
V2 1979/187 22.
IUE 1980/120 15.
/150
IUE 1980/120 12.
/15o
IUE 1985/99 8.
Table 3
Rough ¢mtimate of extinction by the
ISM/IPM for obeervations of H Ly a
l(H Ly a)" r b k(H Lyj) c Ic(H Ly _) AtH
(kR) (kR) I(H Ly+WR) CML
0.03 1.6 O.S 1 100
0.11 21.2 100
0.00 22.0 7.2 60-150
0.07 16.0 100
0.21 14.8 1(30
0.15 9 3 2.2 100
01_. Date I(H Ly a)" _ Ic(H Lyj)c
(kR} (kR)
IU_ t98o/1_,13o 1.as t.l_ s.o_
IUE d 1980/822,323 1.60 0.95 4 18
Vl 1980/316 4.90 0.00 4.90
v2 1981/2_ 3.00 o.oo 3oo
IUE 19861252,254 0.84 0.17 0 99
IUE 1987/114 1.21 0.10 1.33
IUE 1987/204,205 0.87 0.13 0.99
I{H Lv+WR)
5.3
2.8
a) Brightncm before correction for extinction
b) Calculated optical tkicknem (see' text)
c) Brightn¢_ aftercorrectionfor extinction
d_ Rd. t0 correctedupward by factord 1.1eon basia of 1083 correctionto IUE
aperture sise. Saturn hs known to have uo limb darlmning effect in H Ly a and
therefore no limb darkening correctiou_ are applied to the measured data.
TABLE 2
1986 DOY 252 and 254 SATURN IUE OBSERVATIONS OF it LYa
SWP Exposure Scattering ILy-n TLy-a ILy-a c iLy_a b
Duration Background Observed Geocorona ISM Saturn Emission
(Min) ((FN/S) (kR) (kR) (kR) (kR)
29170 a 30 --- 0.917 0.295 0.622 ....
29172 80 2,83 2.862 1,651 0,508 0,976
29173 30 2.65 3,137 2,019 0.508 0.849
29174 a 15 -- 2.276 1,593 0,622 --
29175 15 2.25 2.295 1,220 0.508 0,788
29176 25 2.83 1,979 0.920 0.508 0,765
29177 25 2.50 1.747 0.686 0.508 0.768
29189 a 30 -- 2.166 1,604 0,622 ---
29190 30 2.39 2,220 1,124 0,508 0,817
29191 30 2.27 1,903 0.803 0,508 0,822
29192 15 2.37 1.819 0,625 0.508 0,953
_,IGH'/"_D MEAN 0.88_0.2
a - Sky Backs_omad Observation (60" N of Sstuxn)
b - Implied Satc.:n E=la.ion = (IoB s -Iis M - 16EOCORONA)I.72
c - I(Ly-a)iS M - [I(LYa)iSN_]*.28 + [I(LYa)ISWSAT],0.72
I(Lya)iSMm = 0,622 kit
I(LYo)ISMSAT " 0,463 kR
_" '°'_1 t,
VARIATION IN HYDROGEN EMISSION FROM JUPlTER & SATURN 95
term trend.
The long term variation of the Jupiter H Lya emission is
shown in Figure 2, which combines results from Voyager and
IUE for the outer planets. The variation of the anti-bulge
emission rate over the time period 1979-1987 is approximately
a factor of 2, in agreement with the results recently reported
by Ref. 5. The IUE measurements at Jupiter are corrected for
extinction by the LISM; the extinction effect for Jupiter H
Lye is expected to be small, but may account for the slightly
larger Voyager values. Extinction of the H Lyc_ line has also
been corrected in the analysis of the Saturn and Uranus data
as, described below.
The IUE SWP system is the only experimental system currently
capable of providing measurements of the H z Lyman and
Werner Rydberg bands in the equatorial region. The total
brightness of the H2 bands (Lyman + Werner , I(H z Ly+Wr)) is
estimated by comparison with model calculations (see Ref, 1).
Brightness values for various observations, including rocket and
Voyager data, are compiled in Table 1. The values for I(H z
Ly+Wr) obtained from the IUE data tend to be larger than the
earlier Voyager and rocket data (Table 1, see Ref 7).
However, uncertainty in the measured quantities overlap the
earlier results The results derived here from the more recent
IUE data, particularly near the end of 1986 (Table 1), indicate
that the H z Lyman and Werner bands were as bright or
brighter at solar mimmum as they were near solar maximum m
1979.
Table 1 includes the measured differential brightness at 1600/k,
which represents the reflection of solar continuum radiation
near the homopause This region of the solar spectrum ts
essentially constant in absolute flux compared to the temporal
variability of the shorter wavelength radiation• The various
measurements of this quantity obtained from 1972-1987 (Table
1) show that within measurement error the equatorial subsolar
flux at 1600/k is constant (see Ref. 7). The comparison of the
results from the different experiments at 1600/k provides
confidence in the absolute calibrations.
2.2 Saturn
Analysis of the equatorial emission spectra of Saturn is
restricted to tile determination of H Lyc_ brightness and the
solar reflection continuum at 1600_. The factors affecting the
determination of the H Lyc_ brightness as discussed above, are
more critical at Saturn than Jupiter because of the greater
distance to the planet combined with weaker source rates.
Table 2 shows the sequence of observations with the measured
H Lye intensity in column 4 Interspersed with the Saturn
observations are background data obtained 60" north or" the
planet• These background data may contain contributions from
the Saturn corona (Ref. 3) but we have not attempted to
remove this component from the analys_s due to lack of a
reference point This problem will be corrected in subsequent
observations. Background is composed of geocoronal and
LISM components as estimated in column 5 and 6 of Table 2.
The data also contain a foreground component from the Saturn
corona, but none of the reported observations including the
Voyager results (Refs. 8, 2, 3,) are corrected for this
contribution. Although the observed emission brightness varied
from - 3.1 kR to - 1.8 kR during the sequence, most of the
variation appears to be caused by the geocoronal component,
and the derived Saturn emission brightness is basically constant
during the sequence with an estimated mean value of 1 H Lyc_
= 088_+.2kR (Table 2). The geometry for the 1986 DOY
252,254, observations is ideal in the sense that the planet is
essentially directly upstream from the earth relative to the bulk
flow of the LISM, so that extinction of the planetary signal is
at a minimum. However, the observing geometry at the time
of the Ref. 9 observations indicates a substantially larger
extinction factor. Estimates of the extinction coefficient are
given in Table 3. The coefficient has been normalized by
analyzing the 1UE data obtained near the time of Voyager
encounters in 1980 and 1981. As shown in Figure 2, the H
Lye line according to the combined Voyager and IUE data,
declined significantly by a factor of about 5, between
November 1980 and August 1986. The Voyager data above
indicates a reduction by a factor of 1.7 in the - I year interval
between encounters, a period in which the solar H Ly_ line
showed no significant long term variation.
3. DISCUSSION AND CONCLUSIONS
The rather independent behavior of the three outer planets in
relation to each other and to the major solar cycle indicated in
the results compiled here for H Lyc_ subsolar emission rates
suggests that the responses of the atmospheres to solar radiative
energy deposition is substantially decoupled or at least very
nonlinear. Jupiter shows a long term trend from solar
maximum to solar minimum of a factor of about 2 comparable
to the variation of the solar H Lye flux. In the same period,
the Saturn emission rate declined by a factor of 5. Uranus,
according to the Ref. 9 results shows no particular trend from
1982 although there is variability. The observations of Jupiter
in the present IUE program are in agreement with the recent
results reported by Ref. 5, in respect to both the H Lya bulge
phenomenon and temporal trend on the time scale of the I1
year solar cycle. Although th_s variation corresponds well to
the trend in solar H Lyc_ flux over the same period, two facts
tend to argue against a direct correlation First, results
obtained by Ref. 7 indicate that H Lye brightness in 1974 was
an order of magnitude below the value in 1979 showing a poor
correspondence with the magnitude of the solar line variation.
Second, the H Lya bulge phenomenon persists with the same
magnitude in mean modulation during the years 1979-1987.
The H Lyc_ bulge is obviously unrelated to solar flux
variability. The process appears to be impossible to explain in
terms of response of the atmosphere to the deposition of solar
radiation.
The H Lye em,ssion of Saturn shows a variation of a factor of
1.7 between November 1980 and August 1981, while emission
from Jupiter as well as solar flux remained basically constant
over the same period• Overall, the variation of Saturn H Ly_
emission _s a factor of 5 from November 1980 to August 1986
(Figure 2)
The difficulty in determining and differentiating the processes
controlling the emission of H Ly_ mainly stems from the fact
that atomic hydrogen is a minor atmospheric constituent. The
abundance of HI in the atmosphere is therefore subject to a
number of source and sink processes, and given the extent of
available information it is difficult to determine whether
variation in observed H Lyc_ em,ssion rate is caused by
variation in the excitation xate, in the abundance of the gas or
m a combination of the two effects The problem is
complicated by the fact that some reactions producing H Lyc_
emission are also directly related to the production of atomic
hydrogen• One of the puzzling aspects of the observed
atmospheric behavior on both Jupiter and Saturn is the fact
that emission in the H z Rydberg bands, which serve as at least
one indicator of H_ dissociation rate, do not correlate well with
the observed variation of H Lye. the H Lyc_ transition is
excited by both electrons and resonance scattering.
Dissociative excitation of It Lye is only a small component of
the observed total.
A major factor contributing to the uncertainty in
understanding the behavior of H Ly_ lies with the processes
controlling the sink for atomic hydrogen in the atmosphere.
This problem has been discussed previously by Ref. 7. The
removal of atomic hydrogen from the atmosphere, apart from
the process of escape (Ref. 3), can only take place by transport
to the homopause with subsequent recombination. It is not
clear that the factors controlling rates for this process can show
substantial variability.
On the whole the evidence seems to indicate that a substantial
fraction of the observed emission must be electron excited.
96 D E SHEMANSKY, D T HALL & J B HOLBERG
The persistent H Lya bulge phenomenon on Jupiter and the
tendency for independent behavior between the planets are
very difficult to explain without the introduction of particle
energy deposition.
10
8
2L
7
?
m
A
0
0
i1,11
D
&
D
o O
I I 415 I I 910 I I I I I I I f I I I I I I r r P135 180 225 270 315 360
CML (degrees)
Figure 1. Jupiter subsolar H Lya brightness, calculated from
1UE measurcments in 1984-1986, as a function of CML Geo-
coronal and LISM components have been removed using model
calculations referenced to interspersed IUE background mea-
suremnts. [] [] symbols - August 1984; (D(D symbols - April
1985; /k_ symbols - November 1986.
4. REFERENCES
1. Shemansky, D E 1985, An explanation for the H Ly a longi-
tudinal asymmetry in the equatorial spectrum of Jupiter; An
outcrop of paradoxical energy deposition in the exosphere, J
Geophys Res vol 90, 2673
2. Yelle, R V, et al. 1987, The dependence of electroglow on the
solar flux, J Geophys Res vol 92, 15110.
3 Shemansky, D E and D T Hall 1987, Atomic hydrogen in the
magnetosphere of Saturn and Uranus, Bull AAS vol 19, 850.
4. Ajello, J Met al 1987, Solar cycle study of interplanetary ly-
man alpha variations; Pioneer Venus orbiter sky backgrtound
results, Ap J vol 317,964.
5. Skinner, T E et al 1988, Temporal variation of the Jovian HI
lyman-alpha emission (1979-1986), J Geophys Res vol 93, 29.
6. Sandel, B R, A L Broadfoot, and D F Strobel 1980, Discovery
of a longitudinal asymmetry in the lyman-alpha brightness of
Jupiter, Geophys Res Lett vol 7, 5.
7. Shemansky, D E and D L Judge 1988, Evidence for change in
particle excitation of Jupiter's atmosphere 1968-1979, J Geo-
phys Res vol 93, 21.
6. Shemansky, D E, and J M Ajello 1983, The Saturn spectrum
in the EUV electron excited hydrogen, J Geophys Res vol 88,
459.
9. Clarke, J T et al. 1986, Continued observations of the H Ly a
emission from Uranus, J Geophys Res vol 91, 8771-81.
10. Clarke, J T et al. 1981, IUE detection of bursts of H Lya
emission from Saturn, Nature vol 290, 226.
11. Clarke, J T et al. 1980, Spatial imaging of hydrogen Lyman a
emission from Jupiter. Ap J vol 240, 696
v
v
1978 1980 1982
0
5
4
3
2
1
0
20
15
10
5
0
1978
: : : { : : :
1984 1986
Uranien Equotorlol Emission
Saturnian Equatorial Emission
: : : ', : : : i : : : I : : : I : : :
} } Jovian Equatorial Emission
(Bulge Region)
1980 1982 1984 1986
Year
Figure 2. Long term variation of H Lya subsolar emission rate
for Jupiter, Saturn and Uranus. The data for Jupiter and Sat-
urn is obtained from Voyager, rocket and IUE measurements
The results for Uranus are averages from IUE measurements
by Ref 9. The IUE data has becn corrected for estimated
extinction by the LISM. The rocket data point (A) at Jupiter
was obtained 1978 DOY 335 by Ref. 10. The Voyager data are
indicated by square symbols, and IUE by circular symbols.
5. ACKNOWLEDGMENT
This work is supported by NASA grant No. 5-560.


Large time scale variation in hydrogen emission from Jupiter and Saturn

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