OUTER SATELLITE ATMOSPHERES: THEIR

EXTENDED NATURE AND PLANETARY INTERACTIONS

William H. Smyth

and

Michael R. Combi

Atmospheric and Environmental Research, Inc.

840 Memorial Drive

Cambridge, Massachusetts 02139

December 1983

Interim Report for Period

September 1, 1983 to November 30, 1983

Prepared for

NASA Headquarters
TECHNICAL REPORT STANDARD TITLE PAGE

1. Report No. 2 Government Accession No.

4 Title and Subtitle

Outer Satellite Atmospheres: Their Extended

Nature and Planetary Interactions

Wilfiam H. Smyth and Michael R. Combi

9. Performing Orgomxation Nome and Address

Atmospheric and Environmental Research, Inc.

840 Memorial Drive

Cambrdige, Massachusetts 02139

12. Sponsoring Agency Nome end Address

NASA Headquarters

Headquarters Contract Division

Washington, DC 20546

IS. Supplementary Notes

16. Abstract

Model calculations for the spatial morpho]

sodium cloud are presented that include both tl

torus sink and solar radiation pressure in the

culations exhibit the qualitative behavior obs«

cloud and have diagnostic capabilities to probt

ejection characteristics of the sodium source,

in specifying the plasma properties of Saturn's

use in the Titan hydrogen torus model is report

the AER comet model to calculations of the spat

the H atoms in cometary atmospheres are also d:

3. Recipient's Catolog No.

5. Report Dote

December 1983

6 Performing Organisation Code

8. Performing Organisation Report No

10. Word Unit No.

11. Contract or Grant No.

NASW-3387

13. Type of Report and Period Covered

Interim Report

September-November 1983

i4. Sponsoring Agency Code

HW-2

Logy of the Region B

le oscillating plasma

D-lines. These calirved in the sodium

i more deeply the atom

Significant progress

5 magnetosphere for

:ed. Efforts to apply

:ial distribution of

.scussed.

17. Key Words (Selected by Autkor(s)) 18. Distribution Statement

satellite atmospheres

planetary magnetospheres

comets

19. Security Clossif. (of this report) 20. Security dossil, (of this poge)

Unclassified Unclassified

21. No. of Poo** 22. Price*

12

•For salr by ihr Clraringhoutr for Frdrr.l Scientific and Technical Information. Springfield. Virginia 22151-
I. SUMMARY OF RESEARCH PERFORMED IN THE SECOND QUARTER

Research activities in the past quarter have been primarily focused upon

(1) further modeling studies of the spatial morphology of the region B sodium

cloud, (2) implementation of the Saturn magnetospheric plasma data in our

Titan hydrogen torus model, and (3) application of our earlier comet model to

analyze the spatial distribution of cometary hydrogen.

1. Spatial Morphology of the Sodium Cloud

Model calculations for the spatial morphology of the Region B sodium

cloud have been performed this quarter where effects of both the oscillating

plasma torus sink and the acceleration of cloud atoms by solar radiation pressure in the D-lines have been included. Model calculations illustrating these

effects for lo at both eastern and western elongation are shown projected onto

the sky plane in Figure 1 and projected normal to the satellite plane in

Figure 2. These calculations assume radial and isotropic ejection of sodium

from lo. The more elongated sodium cloud at western elongation results directly from the combined effects of orbit perburtions by solar radiation pressure and the spatially non-uniform lifetime of sodium in the plasma torus.

This preferential elongation was anticipated by Smyth (1983) and has been

observed for the sodium cloud (Goldberg et al., 1980; Goldberg, 1983).

The preferential elongation of the sodium cloud in the west may be

enhanced, however, by suitably modifying the ejection conditions of sodium

from lo. This is illustrated in Figure 3, where sodium is ejected radially

and uniformly only from the band region (defined in the Figure caption). This

band ejection region is geometrically very similar to that shown by Pilcher et

al. (1983) as the source of high-velocity sodium ( 20 km sec"1) which can

explain the space-time variations of the sodium directional features. The

initial directions and speeds of sodium escaping lo that form the directional

features were noted (Pilcher et al. 1983) to be understood in terms of a magnetospheric-wind-driven escape mechanism. Careful modeling of data describing

this east-west orbital asymmetry in the sodium cloud will allow a more refined

description of the ejection conditions to be deduced. This information will

then allow us to understand better the gas escape mechanism at lo as well as

the properties of the local satellite atmosphere.
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2. Titan's Hydrogen Torus

Information describing in two-dimensions (radial and vertical) the density and temperature of the ions and electrons in Saturn's magnetosphere was

obtained this quarter from Sittler (1983). This information, derived from the

Voyager PLS data, is currently being studied. It will later be used to calculate the spatially non-uniform lifetime of H atoms in the Saturn environment, which is required as input information in our Titan torus model. Calculations using this model are to be compared with the spatial distribution of

H atoms deduced from data recorded by the UVS instrument of Voyager 1 and

Voyager 2, in order to understand better the role of Titan in the production

of hydrogen in the Saturn system.

3. Comet Modeling

Cometary research at AER represents a broadening and natural extension of

our previous modeling of extended satellite atmospheres, that was initially

undertaken for NASA in 1979 and 1980. A continuation of this effort was

scheduled and consolidated within this, the third year of our current program. Because of the expertise developed at AER in modeling extended satellite atmospheres, which are in a collisionless regime but remain in a complex

gravitation field, we have addressed that area of cometary atmospheres where

similar conditions are present. The three dimensional particle trajectory

model developed at AER earlier is being applied to the extended neutral atomic

gas clouds observed in comets (e.g. H, C and 0). In particular we have begun

a modeling effort to understand the observed spatial distribution of cometary

hydrogen. The AER three dimensional particle trajectory model is ideally

suited to modeling cometary H atoms which travel under the influence of the

solar gravitation field and a variable solar radiation pressure field owing to

resonance scattering of solar Lyraan a photons.

The current state of extended cometary gas cloud models is one in which

sets of simplifying assumptions are made to tailor the models to one particular aspect of observational data. No completely general model yet exists.

The two major and somewhat complementary modeling efforts to date have been

put forth by Keller and his co-workers (Keller and Thomas, 1975, Keller and

Meier, 1976) and Festou et al. (1979). Keller and co-workers have developed
models for the extended H cloud (>10" km) that approximate the source region

as a point. These models however use Maxwell-Boltzmann velocity distributions

and various approximation schemes to calculate atom trajectories under the

influence of the solar gravitational field and solar radiation pressure.

Festou et al. (1979) have used the vectorial model (Festou 1981 a,b) to

describe more realistically the source region of H in their analysis of the

inner coma (~1CP km) Lyman a profiles from Comet Kobayashi-Berger-Milon (1975

IX). Their model assumed a steady state production rate and neglected

(appropriately for this particular observation) the effects of the solar

gravity field and solar radiation pressure.

The new hydrogen cloud model currently being developed at AER correctly

characterizes both the time variation and velocity vector distribution of the

source region, as well as the correct atom trajectories in three dimensions.

Our first model application is to re-analyze the observed Lyman a distribution

of Meier et al. (1976) in Comet Kohoutek (1973 XII). The basic physical features being incorporated into the model are as follows:

1) water production rate which varies according to the brightness curve

compiled by Delsemme (1975) which indicated that some constituent

(CC>2 was suggested) more volatile than 1^0 at least influenced the

vaporization of Comet Kohoutek;

2) production of hydrogen by the photodissociation of 1^0 and OH (itself

produced by ^0);

3) a speed dispersion for hydrogen shown in Table 1 from the direct

photodissociation of 1^0, as well as an 8 km sec"1- component from

the photodissociation of OH;

4) a heliocentric velocity dependent lifetime for the OH source

according to the recent calculations of Schleicher and A'Hearn

(1983); and

5) radiation pressure acceleration on H atoms by resonance scattering of

solar Lyman a photons as computed from the solar disk averaged

Lyman a profile of Lemaire et al. (1978).

Activities during the past quarter have concentrated on data gathering

for the basic physical source region description and implementing on the AER

computer both the original particle trajectory code of 1979 and 1980 as well

as the Monte Carlo particle trajectory model code of Combi and Delsemme

(1980). This Monte Carlo code will be incorporated into the new AER model to

generate a realistic source region for the distributions of ^0 and OH.
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II. PROGRAM FOR THE NEXT QUARTER

Research activities in the next quarter will involve (1) further model

studies of the spatial morphology of the sodium cloud, (2) continued efforts

to implement the Saturn magnetospheric plasma data in our Titan torus model,

and (3) incorporation of the Lyman a profile in the comet particle trajectory

model for computing radiation pressure as well as additional efforts to properly generate the source region in our new hydrogen cometary cloud model.

10
REFERENCES

Combi, M.R. and Delsemme, A.H. (1980) Ap. _J. 237, 633.

Delserame, A.H. (1975) "Comet Kohoutek," edited by G.A. Gary, NASA SP No. 393,

p. 711.

Festou, M.C. (1981a) "The Density Distribution of Neutral Compounds in

Cometary Atmospheres. I. Models and Equations. Astron. Astrophys. 95,

69

Festou, M.C. (1981b) "The Density Distribution of Neutral Compounds in

Cometary Atmospheres. II. Production Rate and Lifetime of OH Radicals in

Comet Kobayashi-Berger-Milon (1975IX)." Astron. Astrophys. 96, 52.

Festou, M., Jenkins, E.B., Keller, H.U. Barker, E.S. Bertaux, J.L., Drake,

J.F., and Upson, W.L., II (1979) Ap. J. 232, 318.

Goldberg, B.A. (1983) Private Communication.

Goldberg, B.A., Mekler, Yu, Carlson, R.W., Johnson, T.V. and Matson, D.L.

(1980) lo's sodium emission cloud and the Voyager 1 encounter. Icarus

44, 305.

Keller, H.U. Meier, R.R. (1976) Astron. Astrophys. 52, 273.

Keller, H.U. and Thomas, G.E. (1975) Astron. Astrophys. 39, 7.

Lemaire, P., Charra, J., Jouchoux, A., Vidal-Madjar A., Antzner, G.E., Vidal,

J.C., Bonnet, R.M., and Skumanich, A. (1978) Ap. J_. 223, L55.

Meier, R. R., Opal, C.R., Keller, H.U., Page, T.L., and Carruthers, G.R.

(1976) Astron. Astrophys. 52, 283.

Pilcher, C.B., Smyth, W.H., Combi, M.R., and Fertel, J.H. (1983) lo's sodium

directional features: Direct evidence for a magnetospheric driven gas

escape mechanism. BAAS. 15, 810.

Schleicher, P.G., and A'Hearn, M.F. (1983) BAAS. 15, 806.

Sittler, E.G. (1983) Private Communication.

Smyth, W.H. (1983) lo's sodium cloud: Explanation of the east-west assymetries. II. Ap. J. 264, 708.

11
REFERENCES FOR TABLE 1

1. Stief, I.J., Payne, W.A. and Klemm, R.B. 1975. j;. Chem.

Phys. _36_, 605.

2. Welge, K.H. and Stuhl, F. 1967. J[- Chem. Phys. 16, 2440.

3. Carrington, T. 1964. J_. Chem. Phys. 41, 2012.

4. McNesby, J.R., Tanaka, I., and Okabe, H. 1962. J_. Chem. Phys.

J36, 605.

5. Festou, M.C. 1981. Astron. Astrophys. 96, 52.

6. Slanger, T.G. 1982. Private communications.

7. Lee, L.C. 1980. J_. Chem. Phys. _72> 4334.

8. Oppenheimer, M. and Downey, C.J. 1980. Ap. J_. Lett. 241, L123.

9. Hudson, R.D. 1971. Critical review of UV photoabsorption cross

sections for molecules of astrophysical and aeronomic

interest. Natl. Stand. Ref. Data Ser., Natl. Bur. Stand.

(U.S.) 38, 50-52.

10. Okabe, H. 1978. Photochemistry of Small Molecules. John Wiley

& Sons, Inc., New York, p. 201-203.

12