The magnetic fields of Jupiter and Saturn

The magnetic fields of Jupiter and Saturn

Adv. Space Res. Vol.1, pp.17l—176. 0273—1177/81/0301—0171$05.OO/0 ©COSPAR, 1981. Printed in Great Britain. THE MAGNETIC FIELDS OF JUPITER AND SATUR...

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Adv. Space Res. Vol.1, pp.17l—176.

0273—1177/81/0301—0171$05.OO/0

©COSPAR, 1981. Printed in Great Britain.

THE MAGNETIC FIELDS OF JUPITER AND SATURN Norman F. Ness NASA-Goddard Space Flight Center, Greenbelt, Maryland, USA

SUMMARY Jupiter and Saturn are two of the more “exotic” planets in our solar system. The former possesses its own system with 15 satellites in orbit about the parent planet. Saturn has a uniquely well developed and distinctive ring system of particulate matter and also at least 11 satellites, including the largest one amongst all the planets, Titan, with a radius of 2900 km + 100 km. In the decade of the 70’s, the USA launched ~ unmanned spacecraft to probe these giant planets in—situ with a suite of highly advanced instrumentation. Four separate encounters have occurred at Jupiter: 1. Pioneer 10 in December 1973 2. Pioneer 11 in December l97l~ 3. Voyager 1 in March 1979 ~. Voyager 2 in July 1979 The characteristics of these trajectories is shown in Table I. Thus far, only a single encounter of Saturn has occurred, that by Pioneer 11 in September 1979. Future encounters of Saturn by Voyager spacecraft will occur in mid—November 1980 and late—August 1981. It is the purpose of this talk to summarize what is presently known about the magnetic fields of these planets and the characteristics of their magnetospheres, which are formed by interaction with the solar wind. Jupiter has been known to be a strong source of radio emissions for more than two decades. Shortly after the discovery of the terrestrial radiation belts in 1959, it was suggested that the observed non—thermal emission from Jupiter was due to trapped energetic particles in a Jovian radiation belt. Careful analyses of these decimetric and decmnetric radio emissions by a number of dedicated ground based observers deduced quantitative characteristics of the planetary field. These were subsequently verified in greater detail by in—situ spacecraft observations. Table II summarizes the pre— and post Pioneer estimates of the Jovian planetary magnetic field characteristics. Within 6 RT, the planetary field is not well represented by a dipole, due to the presence of ‘arge quadrupole and octupole moments. It had originally been hoped that the Voyager spacecraft would contribute to the study of the main planetary field, especially any possible secular variation. However, the existence of a global, azimuthal equatorial current sheet at B > 5 RJ, described as a magnetodisk by the Pioneer investigators, has thus far limited attempts to estimate a possible secular change in the planetary field with

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N.F. Ness

any degree of accuracy. Estimates of the planetary field made thus far are partially contaminated by those magnetic field sources external to the planet: mamely, the magnetodisk, the tail current sheet and the magnetopause currents. The most distinctive characteristic of the Jovian magnetosphere, in comparison with that of Earth’s, is that near the equator there exists an annular, disk shaped region of concentrated plasma and particle flux and reduced magnetic field intensity. This forms the magnetodisk and the magnetic field of the current sheet leads to an enlarged magnetosphere, when compared with that which is to be expected from only an internal planetary magnetic field. Near the subsolar point, the magnetopause has been observed at radial distances ranging over 50 to 100 RJ, implying arelatively compressible obstacle to solar wind flow. The study of the Jovian magnetosphere has been primarily of its equatorial region by the Pioneer 10 and Voyagers 1 and 2 spacecraft. Only Pioneer 11, on a retrograde orbit at high inclination, has provided observations of the high latitude and that only in the dayside magnetosphere. Data from Voyagers 1 and 2 give evidence of the development of an enormous magnetic tail trailing behind the Jovian magnetosphere. Across the middle of the tail is an embedded current sheet, which appears to merge smoothly with the inner magnetosphere nagnetodisk current sheet. This current sheet is very thin, approximately a few R~.by comparison with the size of the Jovian magnetosphere. Its composition appears to be influenced in the inner most regions by the volcanic eruptions and emissions from the inner most Galilean satellite lo (at 5.95 Rd). The presence of the Galilean satellites embedded deep within the Jovian magnetosphere provides a unique sink for the energetic particles trapped within the Jovian radiation belts. Distinct absorption features are observed associated with the L—shells swept out by each of the satellites. The absorption becomes increasingly effective as one proceeds from the outer and more distorted magnetosphere of Jupiter to its inner region. One special feature of the Pioneer 11 observations was a complex structure of peaks and troughs near pen— apsis inside the orbit of Almathea (at 2.514 Rd). While Jupiter does have a complex planetary field with very large quadrupole and octupole harmonic moments, this alone could not explain the troughs in the particles fluxes. The troughs were interpreted as possibly suggesting the existence of an as yet undetected ring of particles (like Saturn) or a satellite at approximately 1.8 R~. Subsequent Voyager 1 and 2 imaging observations showed that the outer edge of the Jovian particle ring lies in just that position. One of the more puzzling features of the Jovian radio emissions has been the modulation of their occurrence frequency by the satellite Ia. A strong electro— dynamic interaction was proposed which would gemerate an electrical current system connecting Ia with the Jovian ionosphere along the magnetic flux tube of 10. The magnetic field of this current system was detected by the Voyager 1 spacecraft during its close flyby at a 20,500 km miss from lo. The 6 amps with distance a total power dissipation of total current ~floV is approximately 2.8xl0equal to the power dissipated internally approximately watts. This is nearly by tidal heatimg, which has been postulated as the source of volcanic activity on Ia. Two of the major controversies associated with the study of the Jovian magnetic field and magnetosphere has been the role of the large azimuthal asymmetry of the planetary field and also the orientation and configuration of the magnetodisk current sheet. The concept of an “active” hemisphere was proposed to explain some features of the Pioneer data and to predict Voyager—observations. The Voyager spacecraft data have not confirmed those predictions. The orientation and configuratiom of the current sheet has been argued to be primarily a plane parallel to the magnetic equatorial plane or a warped surface which is parallel to the magnetic equatorial plane close to the planet but further away parallels

Magnetic Fields of Jupiter and Saturn

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the rotational equatorial plane. Thus far, the view emerging from a detailed analysis of Voyager data suggest that close to the planet the centrifugal equator is the correct location while beyond approximately 30 to 50 the magnetic equatorial plane becomes distorted by the interaction with the solar wind and the dominant direction is the Jupiter sun line. The close electrodynamic coupling of 10, its ionosphere and its heavy ion torus to the Jovian magnetosphere provides a unique example of the close relationship between planetology and space plasma physics which exists in the Jovian system. Future studies to elaborate on this connection will be possible utilizing not only spacecraft data obtained in-situ but observations from ground based or Earth orbit facilities focusing attention on the recently discovered features. Little was known about the Saturnian environment and its magnetic field prior to the Pioneer 11 encounter. Some estimates of the magnetic field were made prior to the in—situ observations but they proved to be in error by a substantial amount. (See Table III). That Saturn possesses a magnetic field had been earlier suggested on the basis of tentative identification of a very limited number of radio emission observations by spacecraft. There are two surprising features of Saturn’s magnetic field. One is that the planetary field is mainly dipolar with the dipole axis p~rallelto the rotation axis of the planet. Secondly, the magnetic field intensity is substantially less than expected

1 with an equatorial value less than that of the Earth, only 0.2 Gauss. A slight axial offset of the dipole, 0.05 B5, leads to asymmetric polar field intensities of approximately 0.6 and 0.14 Gauss in the north and south polar regions respectively. The magnetic fields of both Jupiter and Saturn are opposite in polarity to that of the Earth. The size of the Saturnian magnetosphere varies depending upon solar wind momentum flux, with an average stand—off distance of approximately 20 B5 at the subsolar point. This is close to the orbital distance of the largest solar system satellite, Titan. Thus, when near local noon, it may be immersed periodically in the magnetosheath or even the solar wind and unlike the Galilean satellites not embedded deep within the planetary magnetosphere. Pioneer 11 observations outbound at the dawn terminator suggest the development of a magnetic tail of the planet as the magnetic field is observed to be distorted in a fashion characteristic of the dawn region of the terrestrial magnetosphere. The axial symmetry of the inner Saturmian magnetosphere, within 10 B5, is attested to by not only the magnetic field observations but also the charged particle results. These show that the most rapid variations of the charged particle flux along the spacecraft trajectory are caused by absorption by the ring system of Saturn. Indeed, analysis of the charged particle absorption characteristics provides unique information about the physical properties of the absorbing particles and has led to the discovery of a new satellite. Since the observation of the Satunnian magnetosphere has only occurred recently, the full analysis of data obtained by the individual experiments has yet to be made. Progress continues to be made in understanding the Saturnian magnetosphere by using the Voyager 1 and 2 spacecraft. Radio emissions have recently and unambigously been identified as associated with the planet Saturn. The spectral range is similar to the terrestrial case (T}c~) and this is consistent with the estimated polar magnetic field intensities at Saturn derived from the magnetic field models. An exciting aspect of these radio emission observations has been the identification of the rotation period of the magnetic field and thereby the interior of the planet. This rotation rate of 10 hrs. 39.9 ±0.3 is

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substantially different than that which had been earlier estimated on the basis of visual features in the cloud tops (10 hrs. 114 mm). During the next two years while these radio observations by the Voyager spacecraft continue, it can be expected that the rotation period of the planet and the characteristics and implications from these radio emissions will be much more fully elaborated upon. That the radio emissions show a periodicity suggests strongly that the magnetic field of Saturn is not axially symmetric in the polar regions and that there are anomalies due to either local sources or to global sources whose presence was obscured because of the Pioneer 11 trajectory. The origins of the magnetic fields of both Jupiter and Saturn are clearly to be associated with a dynamo process deep in the planetary interior. The elaboration of the significance of these results relative to planetary interiors is the subject of a separate symposium being held at COSPAB and so will not be discussed in detail in this report. It has been sugge~tedthat the Jovian magnetosphere is quite possibly analogous to that of a pulsar magnetosphere because of the massive central body, rapid rotation and an associated intense magnetic field. The Saturnian magnetosphere appears to be much more Earth—like in its topology and characteristics except for the presence of the absorbing rings of Saturn. Readers interested in more detailed discussions of these topics are referenced to the articles and review papers listed. (114—18) TABLE I Spacecraft

In-Situ Studies of Jovian Magnetosphere Date

SPS Angle (Outbound at 100 B

Periapsis 3)

Pioneer 10

14 Dec 1973

Pioneer 11

3 Dec 19714

Voyager 1 Voyager 2

1010

2.83



1.62

5 March 1979

111°

14.89

9

1141°

July 1979

10.1

Magnetic Fields of Jupiter and Saturn TABLE II

175

Comparison of the Dipole Terms Representing Jupiter’s Magnetic Field

P—b PRE PIONEER

D 1 Smith et2 al. Hide 1976 & Stannard (19714)

P—li Van Allen3 (1976)

Smith SEA (nl) et al. ~ Acuna 014 (nl) & Ness5 (1976) (1976)

Moment 3

3

-

15

14.000

10° ±2~

Tilt Longitude

230° ~

Offset

0.1 to 0.1

(Rd)

Polar Field (Gauss)

%

Quadrupole

%

Octupole

TABLE III

10

6



10.62° 222.l~

30

14.19

14.28

±•5~

9.9°

9.6°

230°~ 3°

2214°

229°

0

0

0

---

9~50

0.110

9

30

±1 ———



11.5

10.8

———

10.14



———

0

20

214

———

0

15

21

114.0

Saturn Main Field (Dipole) Estimates Equatorial Field (G)

Source

Momen~ (G CM )

Kennel (1973)6

14

8

Scarf

1

2.2 x

io29

Kaiser & Stone (1975)8 Dolgmnov (l978)~

1 0.1143

2.2 x 3 x

29 1o io28

Russell

0.5

1.2 x

io29

(l973)~

(1979)10

Acuna & Ness (1979)11 Smith et al.

114.14



(1979)12

x 1029

Rationale Magnetic Bode’s Law Radio Noise Considerations Radio Noise Scaling Precession Dynamo Scaling Busse and Jacobs Dynamo Scaling

O.20+.Ol

14.3

x ~o28

P11 Observations

0.20

14.3

x 1028

P11 Observations

176

N.F. Ness REFERENCES

1. 2.

3.

14.

5. 6.

7. 8. 9. 10. 11. 12. 13. 114. 15. 16. 17.

B. Hide and D. Stanmard in Jupiter, ed. by T. Gehrebs (pp.767-787) Univ. of Arizona Press, Tucson, 1976. E. J. Smith, L. Davis, Jr., D. E. Jones, P. J. Coleman, Jr., D. S. Colburn, P. Dyal, C. P. Sonett and A. Framdson, J. Geophys. Res., 79, 3501-3513 (19714). 6O) Univ. of Arizona J. S. Van Allen in Jupiter, ed. by T. Gehrels (pp.928—9 Press, Tucson, 1976. E. J. Smith, L. Davis, Jr., and D. E. Jones, in Jupiter, ed. by T. Gehrels (pp.788—829) Univ. of Arizona Press, Tucson, 1976. M. H. Acuna and N. F. Ness in Jupiter, ed. by T. Gehrels (pp.83O—8147) 1976. C. F. Kennel, Space Sci. Revs. 114, 511 (1973). F. Scarf, Cosmic Electrodynamics, 3, 1437 (1973). M. L. Kaiser and B. G. Stone, Science, 189, 285 (1975). Sh. Sb. Dolgmmov, private communication, 1978. C. T. Russell, Nature, 281, 552, (1979). M. H. Acuna and N. F. Ness, Science, 207, 141414 (1980). E. J. Smith, L. Davis, Jr., D. E. Jones, P. J. Coleman, Jr., D. S. Colburn, P. Dyal and C. P. Sonett, Science, 207, 1407 (1980). M. L. Kaiser, N. Desch, J. Warwick and G. Pearce, Science (in press) 1980. J. E. P. Connerney, M. H. Acuna and N. F. Ness, J. Geophys. Res., 86, (to appear) 1981. T. Gebrels, (ed). Jupiter, Univ. of Arizona Press, Tucson (1976). C. K. Goertz and N. F. Thomsen, Revs. Geophys. Sp. Phys., 17, 731 (1979). C. K. Goertz, Space Sci. Revs., 23, 319 (1979); J. Geophys. Res., 81, 3373,

(1976). 18. M. Schulz, Space Sci. Revs., 23, 277 (1979).