Planetary magnetospheres - AGU Publications

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(possibly) Uranus occurred and provided a number of important new results. To review such an abundance of material with the imposed tight page restrictuion.
the

rotation

Pacific,

of

Neptune,

Publ.

Astron.

Murphy, R. E., and L. M. Trafton, Evidence for an internal heat source in Neptune, Astro?hys. J., 193,

253-255,

Rose, L. E.,

P. K.,

Klepczynski,

Anderson,

its

of Nereid and the mass of

J., 79, 489-490,

1974.

The mass of Neptune and the orbit

Trafton, L., The source of Neptune's internal heat and the value of Neptunes tidal dissipa-

REVIEWS

phenomena of Pluto

and

Amer. Astron. Soc., 10, of

D. P., C. B. Pilcher, and D. Pluto: evidence for methane frost,

Science, 194, 835-837, 1976.

OF GEOPHYSICS

PLANETARY

Bull.

Cruikshank, Morrison,

Astrophys. J., 193, 477-480, 1974.

NO. 7

Eclipse

rotation, Icarus, 20, 279-283, 1973. Christy, J. W., and R. S. Harrington, The satellite of Pluto, Astron. J., 83, 1005-1008, 1978.

R. L. Duncombe, and W. J.

of Uranus, Astron. J., 74, 776-778, 1969. tion factor,

L. E.,

satellite,

586, 1978. Anderson, L. E., and J. D. Fix, Pluto: new photometry and a determination of the axis

1974.

Orbit

Neptune, Astro. Seidelmann,

VOL. 17,

Pluto

Soc.

4__0,234-237, 1928.

AND

SPACE

OCTOBER

PHYSICS

1979

MAGNETO SPHERES

George L. Siscoeland James A. Slavin2 Department of Atmospheric Sciences 1 Department of Earth and SpaceSciences 2 The Institute of Geophysics and Planetary University of California Los Angeles, California 90024

INTRODUCTION

Physics

magnetosphere with magnetic flux densities as great as 400¾ (l¾=10-=Tesla)

(possibly)

Uranus occurred and provided

of important To

review

(1Rm=2439km) did not observe a bow shock or magnetopause,

an

meter experiment (Ness et al, 1974), electron plasma instrument (Ogilvie et al, 1974), and charged particle telescopes (Simpson et al, 1974).

a number

When scaled

abundance

of

material

the authors'

interests,

with

magnetosphere

course starts

Z=+3Re and goes to X=+0.5Re,

the

at X=-8Re,

Z=+10Re all

netospheric coordinates (Ogilvie et al, Similarly, the radius of Mercury scales

in mag-

1977). to •SRe

in relation to the terrestrial magnetosphere. The orientation of Mercury with respect to the sun

was

details

the of

same the

for

each

mission

views by Ness (1978; The determinations

encounter.

can

be

found

Additional in

the

re-

1979). of

the Hermaean magnetic

moment has been the subject

of many papers (Ness

et al, 1974; Ness et al, 1975; Ness et al, 1976; Whang, 1977; Jackson and Beard, 1977; Ng and Beard, 1979; Ness, 1978; Walker, 1979; Slavin and Holzer, 1979; Whang, 1979; Ness, 1979ab) and are considered in the accompanying review by Russell on planetary magnetism. Analyses of the Mariner 10 data have produced intrinsic fields

MERCURY

Mariner 10 passed near the nightside of the planet Mercury on March 29, 19 74 and March 16, 1975 and encountered a small terrestrial-type

Paper number 9R1096. 0034-6853/79 / 009R-1096506.00

terrestrial

+3.4R e while the MIII

and we

1979 by the American Geophysical

to the

MI trajectory corresponds to a nightside entry at X=-13Re, Z=-7R e and an exit at X---6Re, Z=

apologize to the many unreferenced. The bibliography, however, should be complete, and is divided into sections devoted to each planet. Figure 1 illustrates the scope of the subject under review. The large range of planetary sizes and magnetospheric sizes, some of which engulf large satellites, indicates the rich var • ty of the phenomena that the subject covers.

Copyright

some upstream

of the Hermaean magnetosphere consists of •30 minutes of magnetospheric data gathered during two fly-bys with the Mariner 10 fluxgate magneto-

the imposed tight page restrictuion requires writing a little about everything, or writing something more substantial about selected things. We have chosen the second option, with its apparent weakness of unintentionally but also unavoid-

ably favoring

but may have encountered

disturbances associated with the shock (Ogilvie et al, 1977). Thus the total set of observations

new results. such

at the closest

approach altitude of 327 km. In addition to these encounters, labelled MI and MIII, a dayside pass on September 21, 1974 at a distance of •20R m

Although the Jupiter encounters of Pioneers 10 and 11 occurred in the previous quadrennium, the flood of new information that they released launched a wave of publications that crested in the present quadrennium. For completeness all of the magnetospheric literature relating to the two encounters is included in the bibliography. The present quadrennium also recorded one Venus encounter and three Mercury encounters by Mariner 10, as well as Soviet missions to both Venus and Mars. Earth based optical and radio exploration of the magnetospheres of Jupiter, Saturn and

ranging

from predominantly 22

3

of 6+2x10 G-cm (Slavin dipole-quadrupole-octupole

Union. 1677

dipolar

with

a moment

and Holzer, 1979b) to fields with the dipole

1678

O Outer

Satellites. Phoebe

I0,000

*Nereid

,:[apetus

a

I,OOO

i

,Moon

i

z



I00

z

,Deimos

IO

,Phobos

Figure

1.

Dimensions of the radii

and planets of the orbits and

their

the

of the sun

(cross-hatched), of their

characteristic

the radii

satellites

(dots)

dimensions

of

magnetospheres (bars)--defined

as the planetocentric distance to the subsolar stagnation point of the solar wind. The magnetospheric dimensions for Mercury, Venus, Earth, Mars, and Jupiter are based on spaceprobe data. Those for Saturn, Uranus, and Neptune are based on scaling relations for which there are some supporting radio

emission data (From Siscoe,

contribution as low as 2.4x1022G-cm 3 (Whang, 1977).

In configuration smaller

and more

Mercury's magnetosphere is a

distorted

version

of

the

terres-

1979).

that the dynamic pressure during MI and MIII may have been much larger than average with the value at MIII twice that of MI (Slavin and Holzer, 1979a). Assuming terrestrial shapes for the bow

shock and magnetopause Slavin and Holzer (1979a)

trial case. By scaling solar wind dynamic pressure at 1 AU to the orbit of Mercury and assum-

have

ing a dipolar intrinsic field of 5.1x1022G-cm 3

from the Mariner

Siscoe and Christopher (1975) predicted mean solar wind stand-off altitudes of 0.7R m at perihelion, 0.31AU, and 1.0Rm at aphelion, 0,47 AU, as compared with •10R•• at the earth and •50Rq at J Jupiter. It was also estimated that the solar wind would compress the magnetopause to the surface only on very rare occasions of exceptionally high velocity and/or density. The observed state of compression of the field

of Mercury

suggests

calculated

solar

wind

stand-off

distances

10 boundary crossings

and scaled

themto a single typical pressureof 6x10-Sdynes/ cm2.

In this way meanaltitudes for this pres-

sureof

the nose of

and MIIi

obtained. Holzer the lesser height tion transferring tail

as

the magnetopause

during

of 0.5 and 0.9Rm, respectively,

is

They also

MI

were

and Slavin (1978) attributed during MI to dayside reconnecmagnetic flux to the magneto-

observed

suggested

to

that

often

occur

dayside

at

the

earth.

reconnection

1679

could possibly expose the front surface of the planet to direct interaction with solar wind plasma a significant portion of the time. How-

ever, Hood and Schubert (1979) and Suess and Goldstein (1979) noted that planetary induction currents may inhibit the compression of the dayside magnetopause down to the planetary surface, requiring pressures of 25 to 150 times the mean to push the magnetopause to within an ion gyroradius of the surface, depending upon the actual electrical conductivity profile of the planet and the time scales of the pressure enhancements. The greater distortion of the planetary field at Mercury relative to the earth is also evident in the large amount of magnetic flux contained in t'he magnetotail. From observations during MI inbound the tail field near the planet is 30-40¾ which is larger than that at the earth by approximately a factor of the square root of the mean solar wind densities at their respective orbits

site situation cannot account

at Mars. While steady convection for the energetic particles re-

ported by Simpson et al (1974), Tsurutani et al (1976) have suggested large amplitude sporadic cross-tail

electric

plosive"

fields

reconnection

associated

with

in the magnetotail

"ex-

as the

source of very energetic particles in both the magnetospheres of the earth and Mercury by analogy with the results of laboratory experiments.

VENUS

Prior to the start of the last quandrenium the particle and field environment of Venus had been investigated by the U.S. spacecraft Mariners 2, 5, and 10 and the Soviet probes Venera 4 and 6.

Mariner

2 passed within

to observe in

the

6.6R v of Venus and failed

a bow shock or any other

solar

wind

that

could

be

perturbation

associated

with

(Ogilvie et al., 1977). Such tail fields require a polar cap of 18-26 ø degrees colatitude as com-

the planet (Smith et al, 1963). With a close approach distance of 1.7R v Mariner 5 crossed the

pared with a quiescent value around 10ø for the

bow shock of Venus both inbound and outbound, but did not enter into any regions of magnetic field

earth

(Ness et al,

1975b).

neutral sheet and plasma fied by the magnetometer

(Ness et al,

A terrestrial-type sheet were also identiand plasma experiments

1975b; Ogilvie

et al,

1977).

Plasma

electron observations showed energies very similar to the earth's plasma sheet, but with higher number densities (Ogilvie et al, 1974; 1977). In considering particle orbits within the magnetosphere Ogilvie et al. (1977) have excluded the possibility of a stably trapped plasmasphere or particle belts as found at the earth due to the large size of Mercury relative to its magnetospheric cavity and its slow rate of rotation. In addition due to the expected dominance of azimuthal drifts Ogilvie et al concluded that the plasma sheet particles will approach no closer

than 1.4Rm to the planet in the equatorial plane.

The region of precipitation on the surface was estimated to be a ring similar, but wider than, the

terrestrial

auroral

oval

with

colatitudes

of 30-40 ø in the dayside meridian and 55-65 ø at midnight. During the outbound portion of MI large amplitude variations in the magnitude field were observed and the plasma experiment indicated the

presence of a "hot" plasma sheet by comparison with MI inbound and all of MIII (Ness et al, 1975b; Ogilvie et al., 1977). In addition, the energetic

large

particle

fluxes

experiment

(Simpson et al,

recorded

1974),

sporadic

which indi-

cate the presence of magnetospheric particle acceleration. Finally, the interplanetary magnetic field was northward in the magnetosheath prior to MI, but southward upon exiting the magnetosphere. Siscoe et al (1975) have found the above

observations

expected

properties

to

be

consistent

of terrestrial

with

the

sub storms

that appeared intrinsic to the planet (Bridge et al, 1967). On its way to the already discussed encounters with Mercury Mariner 10 spent 47 days within a cone of 8.5 ø semiangle centered on the

sun-Venus

line,

prior

to a close

approach

of •2R v and an outbound bow shock encounter, but did not enter into the wake itself (Ness et al, 1974; Ogilvie et al, 1974). Venera 4 arrived at Venus a day before Mariner 5 and crossed the bow shock once before entry into the Venusian atmosphere during which magnetic field observations

were

made

down

to

an

altitude

of

200km

and an upper limit on theplanetary moment of . 10 22 G-cm3 determined (Dolginov et al, 1969). Venera

6 also

recorded

an

inbound

bow

shock

be-

fore entering the atmosphere (Gringauz et al, 1970). A comprehensive discussion of both the U.S. and U.S.S.R. missions and their impact on the determination of the planetary field at Venus can be found in a recent paper by Russell (1979) and in the review of planetary magnetism accompanying this report. The average position and shape of the bow shock has been the subject of several studies with the purpose of gaining information on the nature of the magnetosheath flow and the obstacle at Venus as well as at i•fercury and Mars. Russell

(1977a) fitted

the 6 shock crossings then avail-

able with a conic specifying the planetary center as one foci and found the least squares eccentricity to be 0.92+0.03 (where a value of 1 cor-

responds to a parabola), and 0.2+0.07Rv as the altitude of the nose of the shock. In that study it was also concluded that the shape of the shocks

at

Mars

other while

and

that

Venus

were

similar

to

of Mercury was different

each

and

scaled to the Hermaean magnetosphere. In particular, they showed that the substorm time scale for conditions at Mercury to be on the order of 1 minute as compared with 1 hour at the earth and that the amplitude of the observed fluctuations were indeed reasonable on energy grounds.

nearly the same as the terrestrial case. The finding of blunter shock shapes at Venus and Mars as compared with the earth and Mercury was attributed to differences in shape between ionospheric and magnetic obstacles. In a second

Hill

of the shock observed by Mariner

et al (1976) have concluded that the mag-

netosphere of Mercury can support steady state convection capable of accelerating particles up to •13keV due to a relative lack of line-typing effects at Mercury as contrasted with the oppo-

paper Russell

(1977b) argues that the position 10 is

too close

to the planet to be consistent with a non-absorbing ionosphere. It as suggested that for this apparently anamalous event about one third of the

incident

flow was being

absorped by the iono-

1680

sphere and resulted in the shock becomimg "attached" to the obstacle. Ness (1977) has expressed reservations concerning both the method employed by Russell and the statistical validity of his conclusion. Preliminary analysis of the Pioneer-Venus orbiter bow shock crossings during the first 24 orbits by Russell et al (1979) has produced a best fit conic to the average shape of eccentricity 0.92+0.06 with a nose altitude

of 0.23+0.05R v in agreement with earlier

findings

(Russell, 1977a) o In addition it has been found that the position of the Venusian bow shock is more variable than would be expected for a "hard" spherical object from studies of the position of the terrestrial shock. This conclusion by Russell et al suggests that influence of solar wind conditions on magnetosheath flow at Venus is more by means other than simple changing of obstacle height than is the case at the earth. On such mechanism is mass loading of the magnetosheath flow by the pick up of photoions as proposed by Cloutier et al. (1974). This process will be dependent on-VxB in the magnetosheath in that

more

mass

will

be

transferred

to

regions where the electric field opposed to into the ionosphere.

this

mechanism and the greater

of flow with B parallel

than

this

distance

an asymmetry in

in

found by Romanov et al the

Venera

the

the Venusian

9 and

10

flow

in

compressibility

to V, y--5/3,

as com-

pared with B perpendicular to V, ¾ = (1976) predicted as asymmetry in the shock with the distance to the shock of the terminator perpendicular to B er

the

is out of as On the basis of

direciton

2, Cloutier Venusian in the plane being greatof

B.

Such

bow shock was later

(1977) during analysis

orbiter

observations.

of

Perez

de Tejada and Dryer (1976) have extended with work by formulating a one fluid viscous interaction model of the Venusian magnetosheath and found signatures of viscous interaction with the ionopause in the Venera 6 data that are consistent with model predictions. In addition, Mariner 10 magnetic field and electron plasma observations made during its extended pass through the Venusian magnetosheath also found evidence for planetary ion pick up and a viscous solar wind interaction with the ionosphere

(Yeates

et al.

Preliminary Pioneer

1978; Lepping and Behannon, 1978).

findings

Venus

orbiter

of experiments have

shown

that

onboard the the

iono-

pause structure and height to be variable and dependent upon interplanetary conditions with dawn sector ionopause altitudes ranging from

about 200 to 2000 km (e.g. Brace et al, 1979). In addition, the magnetometer experiment has confirmed that the average ionospheric magnetic field that

is the

small on obstacle

the order of to the solar

Venusian ionosphere

(Russell

lnT and hence wind flow is the

et al,

1979).

How-

ever, bundles of twisted magnetic flux 10 to 100 km in diameter with magnetic flux densities of up to 90nT have been observed on numerous orbits down to the altitude of periapsis (Russell and

in the magnetotail, or plasma wake, of Venus are Mariner 5, Venera 9, and 10. Russell (1976) has conducted a detailed Mariner 5 observations

examination on the and found evidence for

a

boundary layer and terrestrial-type magnetotail with the flux in the northern lobe pointing in the

antisolar

direction•

In

addition

Goldstein

and Wolff (1978) have found that the Soviet observations show more magnetic flux in the Venusian magnetotail than can be accounted for by induction models such as that of Daniell and Cloutlet (19 77). MARS

With the exception

of a distant

crossing by Mariner 4 (Smith et al,

bow shock

1965) and

ionospheric particle data from the Viking mission (Hanson et al, 1976) the only source of experimental observations on the particle and magnetic field

environment

of

Mars

continues

to

be

the

Soviet orbiter missions Mars 2, 3, and 5 which were conducted during the previous quadrenium, 1971-1974 (Gringauz, 1976; Dolginov, 197•; Vaisberg, 1976). While the Soviet experinents revealed a strong interaction between Mar• and the solar wind with a permanent bow shock, it is not yet possible to state conclusively whether the magnetosphere of Mars is associated wi•h an induced or intrinsic magnetic field• This uncertainty is due principally to the orbital parameters of the Soviet satellites whose periapses apparently did not go below an altitude of 1100km and did not provide observations of the region directly behind the planet (Russell, 1979a). By comparison the orbit of the Pioneer Venus satellite and the two low altitude nightside passes by Mercury of Mariner 10 are near optimum for initial exploration of an induced or small intrinsic magnetosphere. A comprehensive review of the Soviet findings can be found in a recent

article

by Russell

(1979b),

If the solar wind at Mars is stood off predominantly by the ionosphere, then the Martian magnetosphere is the second example of such an interaction with the solar wind which may then be studied in comparison with the induced magnetosphere of Venus (Cloutier, 1976), In this eventuality the differences between the interactions

at

Venus

and

Mars

associated

with

their

respective ionospheric scale heights, ionospheric compositions, exospheres, and solar wind environments will be of major interest. A significant intrinsic magnetic field at Mars would provide a limiting case of a small terrestrial-type magnetosphere similar in size to Mercury, but with a relatively high contact conductivity due to the expected low magnetic field in the ionosphere

(Dessler, 1976; Bauer, 1976). As first suggested by Rassbach et al (1974) the presence of a high Pedersen conductivity at the foot of magnetic field lines may be expected to lead to a strong coupling between the ionosphere and magnetosphere•

Elphic, 1979). The continued study of the occurrence probability and physical characteris-

This work has been extended by Hill

et al (1976)

as

Hermaean

tics

magnetosphere. They concluded that this situation might lead to merging and convection rates slow enough to create a magnetosphere with a nightside magnetotail and a predominantly ionospheric obstacle to the solar wind flow on the dayside. In addition, Hill et al offer large contact conduc-

of these "flux

ropes" as a function

of posi-

tion in the ionosphere and solar wind conditions may provide important information on conditions at the ionopause where they are expected to originate. Presently, the only sources of observations

discussed

in

connection

with

the

1681

tivities

as the ultimate

explanation

paucity of energetic particles

for

the

at Mars (Vaisberg,

1976) as compared with the earth or Mercury. JUPITER

In the four years since the last quadrennial report, literature on Jupiter's magnetosphere has been dominated largely by the profusion of new observations provided by the very successful Pioneer 10 and 11 encounters with Jupiter; and by the many attempts to understand these mostly unanticipated results in terms of definite physical models. In addition, ground based spectroscopic observations of emissions from magnetospheric clouds associated with Io have given important new information and raised intriguing questions about the composition and dynamics of the inner magnetosphere. The reader interested in learning about the Pioneer observations and their implications is fortunate. Many excellent, comprehensive collections of articles have appeared beginning with the first reports in Science and Journal of

Geophysical Research and including

the published

proceedings of symposia held in Frascati in 1974 (Formisano, 1975) and in Tucson in 1975 (Gehrels, 1976). For convenience in identifying these general sources, they are listed separately in the bibliography. The excellent review articles by Kennel and Coroniti (1975, 1977a), Goertz (1976a), and Schulz (1978) should be mentioned here also as sources providing broad coverage of Jovian magnetospheric topics with emphasis on interpretations of the observations. At Jupiter the Pioneers encountered a magnetosphere markedly larger and more variable than predicted by scaling the terrestrial example to Jovian dimensions. The increased variability results in part, but apparently not totally, because the variability of the solar wind ram pressure, which confines the magnetosphere, increases

dramatically

between Earth's

orbit

and Jupiter's

orbit, owing to the progressive amplification with heliocentric distance of compression and rarefaction regions in the solar wind associated with interacting solar wind streams (Smith et al,

1978). In addition there may be a component to the variability driven by some process within the magnetosphere, which manifests itself in a roughly 10 hour periodicity seemingly locked to the

planet' s rotation

(Dessler,

1978).

Although particle and field measurements of the bow shock and in the magnetosheath yielded no surprises, the interior of the magnetosphere was found to be rich in particle populations, some of ø which were directly observed while others were inferred, for example, to account for a discovered disc-like distortion of the interior magnetic

field (e,g., Van Allen et al, 1974a), The particles that produce the disc-like distortion might

analytically

the magnetic

configuration

of the

disc (Beard and Jackson, 1976; Barish and Smith, 1975 - Pioneer 10 inbound; Goertz et al, 1976 Pioneer 10 outbound). Other investigations were directed at determining the forces producing the disc. The data appear to favor a model in which the magnetic distortion, viewed as a magnetic stress, acts to balance a pressure stress, rather like the situa-

tion

characterizing

the plasma sheet of Earth's

magnetotail, in contrast to pre-encounter expectations that the particle stress would be dominated by the centrifugal force of corotating plasma. In the second instance the disc is predicted to extend parallel to the rotational equator and in the first instance to the magnetic equator, which conforms more closely to the ob-

servations

(Goertz,

1976b).

Evidence was indeed

presented that the disc is formed by a high 8 plasma sheet with the particle pressure supplied primarily by ions with energies exceeding 60 keV

(Walker et al,

1978).

The particle-free

magnetic

field outside of the disc is very possibly open (Goertz et al, 1976). Self-consistent high 8 models reproducing some of the observed features have been presented by Goldstein (1977) and Vickers (1977). A parallel development has occurred for disc models in which a centrifugal force balances the magnetic tension. The general structure of such a disc in the case of equatorially confined particles was explored in a semi-empirical

and analytical (1976) and its current

sheet

analysis by Gleeson and Axford similarities to an azimuthal demonstrated.

A more

self-

consistent calculation of this type in which equatorial confinement results naturally from near equatorial injection of the ions at the Jovian satellites was subsequently performed by

Hill

and Carbary (1979).

This led to the predic-

tion of a cusp-like structure which the authors interpreted to be the theoretical counterpart of the magnetodisc inferred from observations. A similar type of self-consistent calculation for the case of a Jovian ionospheric source of the ions failed to produce a disc-like structure

(Carbary and Hill, 1979). Spacecraft encounters with the postulated disc were announced by large increases in the observed particle

intensities

and by a decrease

magnetic field strength 1974). Such encounters

in the

(McKibben and Simpson, recurred with a roughly

10 hr periodicity approximately with Jupiter's rotation period.

commensurate The disc model

'interprets the observed recurrence in terms of the misalignment of the plane of the disc with the rotational equator of Jupiter. Planetary rotation then sweeps the disc past the spacecraft

once

each

rotation.

1976).

All observers noted a progressive lag in the disc-encounter signatures with distance from the planet on the Pioneer 10 outbound leg compared to encouner times expected for a rigidly rotating tilted plane. Such behavior might be explained

The evident disc-like distortion of the magnetosphere, which was most clearly defined and most

by slippage relative to Jupiter of the high latitude field lines that connect to the distant part

stably present when Pioneer 10 penetrated the mag-

of the disc (Northrup et al.,

netosphere while moving outward through its dawn flank, provoked many attempts to model theoretically such a feature. One class of investigations produced semi-empirical models to describe

ly, the disc may not be a rotating tilted plane, but a surface warped such that its topography changes with distance from Jupiter in a way to produce different encounter times while rigidly

also be able

to inflate

observed overly

large

the magneto sphere to its

size

(Beard and Jackson,

1974).

Alternative-

1682

rotating. The warping expected to result from the finite outward propagation speed of the signal carrying the information that the underlying tilted dipole is rotating with the planet can account for the observed lag if the propagation speed is 43 Jovian radii per hour (Kivelson et al• 1978).

Chenette et al. (1974) and Simpson and McKibben (1976) suggested that rather than being the result of the rotation of a purely spatial feature, the quasi-10 hour variations in particle intensities could be a purely temporal phenomenor•

something like

the beats of a Jovian magnetospher-

ic clock. They show that this hypothesis accords better with some of the data, especially data from Pioneer 11, which display marked 10 hour variations even at high latitudes where the influence

of

the

disc

should

be minimal.

Neither

the purely •.spatial nor the purely temporal hypothesis as presently conceived seems capable of ordering the full set of observations pertaining to the 10 hour periodicity. That there are 10 hr changes in the magnetospheric configuration, that is, temporal changes,

is strongly indicated by a 10 hr periodicity

in

the energetic particle that are released into the solar wind. Energetic electrons of Jovian origin observed in the interplanetary medium show minimum intensity and softest energy spectrum when a certain Jovian longitude region, the so-called active sector, faces away from the tail, which is

the exit

portal

for the electrons

earth.

However, Davis and Smith (1976) note that

a disc-like

magnetostatic

geometry

is

also

more

compressible than that of Earth's. Kennel and Coroniti (1977b) resurrected the possibility discarded by Brice and Ioannidis (1970) that merging with the interplanetary magnetic field can cause important magneto spheric variations

with

a time

scale

associated

with

sector boundary passages, or once again roughly a week. A third possibility for producing longer term variations of the Jovian magnetosphere emerged from the ground based observation of signifi-

cant long term changes in the sulfur associated sulfur

with

ions

can

importantly

Io,

and the

diffuse

to the

outward

forces

ion cloud

speculation and

controlling

of the outer magnetosphere (Hill Eviatar et al., 1978).

that the

state

and Michel,

1976;

The question of the sources of the particles that populate the interior plasma structures of the Jovian magnetosphere received much attention. Low energy ion sources include the ionosphere of Jupiter, the satellites, and the interstellar

medium (e.g., Goertz, 1976c; Neugebauer and Eviatar, 1976; Hill and Michel, 1976; Siscoe and Chen, 1977; Siscoe, 1978). Pioneer 10 discovered a low energy

plasma

feature

sphere (Frank et al.,

in the

inner

magneto-

1976) which may or may not

be associated with Io, but if so, then it is most likely composed primarily of energetic, possibly heavy, ions derived from neutral particle clouds

(see above plus Goertz and Thomsen, 1979).

(Vasyliunas,

these

contribute

In

1975). A mechanism has been suggested by which a longitudinal asymmetry in the magnetic field near the planet leads to differential mass loading of magnetic flux tubes which communicates the asymmetry to the outer magnetosphere and

addition, a sulfur ion nebula associated with has been discovered with ground based spectro-

might thus account for the longitude-related, 10 hour variations there (Dessler and Hill, 1975; Hill and Dessler, 1976).

nebula (Brown, 1976, 1979; Meckler and Eviatar, 1978) which may be resolved by the Voyager fly-

Pioneer 10 and 11 experimenters agree that the outer magnetosphere is highly variable, even apart from the 10 hour variations. The observed change in character of the particle and field observations

between

the

inbound

and

o•utbound

scopic observations

(Kupo et al.,

1976).

Io

There

is at present a controversy about the densities and relative composition of the Io-related ion

bys.

Radial

transport

of these

ions is believed

to be by diffusion (see above plus Machida and Nishida, 1978; Nishida, 1978) and to result in a thin, flat disc distribution (Hill and Michel, 1976; Siscoe, 1977). Although the pre-Pioneer encounter predictions

legs of the Pioneer 10 encounters might reflect a true temporal change in the global state of the magneto sphere or merely a local time variation. However, the inbound passes of the two

of energetic particle intensities in the inner and middle magnetosphere based on physical princi-

Pioneers although relative sphere. idea that

also differed markedly in character, they were nearly spatially coincident to the configuration of the magneto-

es of theoretical space physics (Coroniti, 1975; Goertz, 1976a; Scarf, 1976), it is not clear whether the particles originate in the solar wind,

Such long term variability leads to the there may be more than one state o• the

as is believed

magnetosphere (Kennel and Coroniti, an example, model

in

it

which

1977a).

was noted that

a radial

the

was

anced by a planetary

solar

wind

met

As

outflow and

bal-

wind could have two states

that depended on whether the planetary wind was super-Alfvenic or sub-Alfvenic. Which state existed at any given time could depend on the ram pressure of the solar wind, which controls the fetch of the planetary wind and hence its

access to the super-Alfvenic regime (Coroniti and Kennel, 1977). Thus variations related to solar

wind

streams

could

oscillate

the

state

of

the Jovian magnetosphere on a time scale of rough-

ly one week.

The existence of hydrodynamic rath-

er than magnetostatic balance with the solar wind might account for the larger range of magnetopause distances compared to the situation at

ples developed in studies sphere

are

acknowledged

some internal

of the earth's

magneto-

to be one of the success-

to be the case for

source (e.g.,

Earth,

Van Allen,

or from

1976).

Energetic particles in the Jovian magnetosphere exhibit some distinctly non-terrestrial aspects such as satellite absorption features which were expected and which were reported by all particle

detector

teams.

In addition,

anomalous (dumbbell)

pitch angle distribution and field-aligned streaming in the middle magnetosphere were observed

(Van Allen,

1976; McDonald and Trainor:

Sentman and Van Allen, 1976). concepts proposed in response

1976;

New theoretical to these observa-

tions included recyling of the particles (Nishida, 1976; Carbary et al., 1976; Sentman et al, 1975) and a novel form of non-adiabatic, magnetic pumping (Goertz, 1978). Many attempts were made to use the satelliteabsorption signatures in the data to determine the radial diffusion coefficient for the particles

1683

(Mogro-Campero and Fillius, 1974, 1976; MogroCampero, 1976; Thomsenet al., 1977a,b)• It was found to be generally

consistent

driven by winds in Jupiter's

with

diffusion

upper atmosphere,

larities in magnetospheres. Important differences, however, may result from the different arrangements of satellites and rings in the two magnetospheres. Whereas Io is a source of ions for

than by the solar wind as is the case for however its exact form could not be determined because of uncertainties in the absorption efficiencies of the satellites and the presence

Jupiter's inner magnetosphere, Titan may be a copious supplier of ions to Saturn's outer mag-

of other non-adiabatic loss processes (e.g., Schulz and Eviatar, 1977; Goertz et al., 1979)

Titan's

rather earth,

The problems of the electrodynamic coupling between Io and Jupiter and of Io-related radio emissions (see R.A. Smith, 1976) continued to be studied. At present there is a controversy whether the magnetic flux tube is frozen to Io as proposed in early theories, for example, by Pi-dington and Drake (1968) and Goldreich and

Lynden-Bell (1969) or whether Io slips through the Jovian magnetic field, being decoupled from it by strong electric fields parallel to the magnetic field as might result from the presence of plasma sheaths around Io (Gurnett, 1972; Shawhan et al., 1975; Shawhan, 1976) or potential

netosphere through ionization of the neutral particle torus fed by the escape of hydrogen

atmosphere (McDonoughand Brice,

from

1973a,b;

Hunten, 1977). Also, whereas the Jovian satellites do not completely absorb the inward difusing energetic particles, Saturn's rings should, thus extending the inner boundary of the trapped radiation domain out into the magnetosphere to a distance of more than one planetary radius beyond the planet's atmosphere. Cheng and Lanzerotti (1978) note that the sputtering of water ice products off of the rings, which will accompany the absorption of the energetic particle radiation, can produce a much denser ring "atmosphere"

(unbound) than had been predicted previsouly lecting this process. drogen in the vicinity

neg-

The amount of neutral hyof the rings produced this

double layers (R.A. Smith and Goertz, 1978), (For

way can account for the "anomalous" Lymane in-

a discussion

see Piddington,

tensities

of a decoupled Io

the rings (Weiser et al.,

1977).

of the controversy

Someof the predictions

observed in apparent

association

with

1977).

model have been verified by the Pioneer encounters, in particular the presence of a spike of energetic, i.e., accelerated, electrons at the orbit of Io (Fillius and McIlwain, 1974) and absorption signa-

With regard to Uranus and Neptune, radio signals from Uranus were tentatively identified (Brown, 1976; but see also Kaiser, 1977). The general question of whether radio signals of the

tures implying that energetic deflected around the Io flux

magnetospheric burst variety are able to be observed at Earth from those planets was raised

be

if

the

flux

tube

were

particles are not tube as they would

frozen

to

Io.

The

and answered in the affirmative

en-

counter of Voyager I with the Io flux tube may decide the question. The radio-occultation experiment on Pioneer 10

determined that Io has an ionosphere (Kliore 1974, 1975). It presented a marked entrance

et al limb-

(Kennel and Maggs,

1976). PALEO-EARTH

exit limb asymmetry, which provoked the interesting suggestion that the interaction between Io and the Jupiter-corotating magnetospheric wind could have aspects similar to the interaction between the solar wind and comets, including a long

The magnetic field of Earth is known to have changed its strength, direction, and its relative amounts of dipolar and non-dipolar components over geological ages. The changes in direction involve both a migration of the dipole axis about the rotational axis with a typical deviation of

ionic

about 10ø between the two axes, and full

tail,

and this

could be a significant

source of ions to the magnetosphere (Cloutier

et

al., 1978; Axford: comments in panel discussion, Chapman Conference on Jovian magnetospheresatellite interactions, UCLA, 1978). SATURN, URANUS,.NEPTUNE Saturn is inferred on the

basis

of

to possess a magnetic

received

radio

emissions

field

which

of the

direction

of

the

dipole

axis

reversals

relative

the rotational axis (magnetic reversals). dipole strength varies by about a factor

to

The of two

in both directions from an average value not much different than the present value, except during magnetic reversals when it might drop to 10 or 20

percent of its prereversal value (see, for example, the review by Kane, 1976). The magnetosphere of Earth than at present

has been different because of these

in the past long-term geo-

are interpreted as Saturnian analogs of the magnetospheric radio bursts generated at Earth and

magnetic variations

Jupiter (L.W. Brown, 1975; Kaiser and Stone, 1975•.

1976a).

Scaling based on the frequency of the sporadic radio bursts agrees with scaling based on an empirical relation between planetary magnetic dipoles and angular moments in assigning to Saturn a surface field strength oi about one gauss, a

Changes in the sizes and strengths of magnetospheric features and processes that correspond to changes in the strength of the dipole have been

value

also

consistent

with

the

observed

absence

of synchrotron radiation from Saturn (Kennel, 1973; Kaiser and Stone, 1975; Scarf, 1975). On the basis

of this

inferred

magnetic

field

and by analogy to the magnetospheres of Jupiter and Earth, attempts have been made to predict some of the properties of a Saturnian magneto-

sphere (Scarf, 1973, 1975; Siscoe, 1978). The similarity between the sizes and situations of Jupiter and Saturn suggests corresponding simi-

(see the review by Siscoe,

presented by Schulz (1975) (radiation belt intensities), Siscoe and Chen (1975) (sizes of magnetosphere,

plasmasphere,

magnetic activity)

polar

cap,

and the level

of

and Siscoe and Christopher

(1975) (size and extent of the auroral zone). The size of the magnetosphere and the distribution of auroral zones for the case of a purely non-dipolar geomagnetic field, perhaps representative of geo-

magnetic reversals, were illustrated (Siscoe et al, 1976; Siscoe and Crooker, 1976). However, Saito et al. (1978) note that even a dipole moment that

produces

a field

weaker

than that

of

1684

the non-dipole components at the earth's surface, where the paleomagnetic field is recorded, could dominate

the

altitudes

interaction

with

the

solar

of 2 or 3 earth radii.

wind

The •xis

at

of

such a residual dipole is a priori most likely to lie more toward the equator than toward the pole. As Saito et al (1978) describe, an equatorially

oriented dipole produces a new category of magnetosphere in the classification of magnetospheric types. References

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