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PARTICLES and FIELDS NEAR JUPITER by James W

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PARTICLES and FIELDS NEAR JUPITER by James W NASA CONTRACTOR REPORT PARTICLES AND FIELDS NEAR JUPITER by James W. Wmwick Prepared by JETPROPULSION LABORATORY California Institute ofTechnology Pasadena, Calif. 3 1 103 for NATIONALAERONAUTICS AND SPACE ADMINISTRATION WASHINGTON,D. C. OCTOBER 1970 c- TECH LIBRARY KAFB, NY 1. Report No. 2. Govornmmnt Accassion No. 3. Recipimnt'sCatolog No. NASA CR-1685 I 4. Title and Subtitle 1.. PARTICLES AND FImS NEAR JUPITER 6. PerformingOrganization Code -~~~ -~ I 7. Author(s) 8. PerformingOrganization Rmport No. James W. Warwick . -. ~~ 9. P formingOrgonization Name and Address 10. Work Unit No. +et Propulsion Labor&y *&-if mnia-Ins"te -0f~oI~gy 11. Contract or Grant No. NAS Pasadena, Caktf'orrrrira-91-1XEL 7-100 13. Type of Report and Period Covermd 112; 'Sponsoring Agency Name and Address Contractor Report Nation&. Aer'onautics and Space Administration Wsohington, D . C . 20546 14. Sponsoring AgencyCode L~" . , . ~ .. .- ~ ." 15. Supplementary Notes ~ ~ ~ -~~~~. 16. Abstract Currentdata on particles and fields near Jupiter are based on interpre- tationsof Earth-based observations of radio emissions in the higher frequencyranges. Theemphasis inthis report is onelementary physical principles andon probable uncertainties in the results. Jupiter's magnetic field appears to be dipolar, with a moment of value 4.2 x 1030 c.g.s.,directed at 7O.7 tothe rotation axis. There is evidencefor a small quadrupole moment, about 0.06 RT in units of the dipole moment. Typicalenergies for the relativistic electron fluxes are estimated to be 10 MeV, andrepresent a rangeof about 3 to 30 MeV. Lower energy electrons are much more difficult to estimatelandproton fluxes are vir- tually unknown on thebasis of available data. However, sensitiveupper limitsto the thermal plasma density in themagnetosphere have been established empirically.'! 1' , j 17.' .icy Word.(SeGed by Author(s)) 18. Distribution Statement Jupiter;Magnetosphere; Magnetic Unclassified - Unlimited Field;Relativistic Electron Flux; Thermal Plasma Density . I ~~ ~~ ~ 1 Classif. (of this rmport) 20. Securitypage)PagesthisClassif. (of 22. Price* Unclassified Unclassified $3.00 *For sale by the Clearinghouse for Federal Scientific and Technical Information Springfield, Virginia 22151 d ACKNOWLEDGMENT Extensive parts of this paper were prepared while the author was a consultant in the Project Engineering Divisionat the Jet Propulsion Laboratory. Remaining parts were prepared at the University of Colorado and were supported in part by the National Science Foundation. The author is grateful for their support and for the help of Neil Divine of the Environmental Requirements Section, JPL . .. .L :!. 7 .J .... .... TABLE OFCONTENTS Abstract ............................ 1 1. StaticMagnetic Field ..................... 2 1.1 Estimates ofJupiter's Magnetic Moment .......... 2 1.2 Shapeofthe Magnetic Field ............... 9 1.3 Locationof Current SourcesWithin Jupiter ........ 36 1.4 The Magnitude of theMagnetic Dipole Moment ....... 49 1.5 Summary Table ...................... 50 2 . Particles ........................... 52 2.1 RelativisticElectrons .................. 52 2.2 Low EnergyElectrons ................... 68 2.3Energetic Protons .................... 86 2.4Relaxation Times ..................... 87 2.5 Summary Table ...................... 90 3 . Plasmas ............................ 92 3.1Plasmasphere ....................... 92 3.2Ionosphere ........................ 97 3.3 Summary Table ...................... 100 4 . Electromagnetic Wave Fields ..................101 4.1 Microwave Fields ..................... 101 4.2 VLF.LF. and HF Wave Fields ...............102 5 . Hydromagnetic Wave Fields ...................106 Appendix ............................ 115 References ........................... 118 FIGURES 1. The dipoleline of force and emission locus ....... 15 2 . Emissionloci and their occultation ........... 17 3 . Polarizationposition angle ............... 19 4 . Variationof occultation of far-side emission arc .... 22 5 . Magneticfield distortion by themotion of Io ......111 V PARTICLES AND FIELDS NEAR JUPITER by James W. Warwick ABSTRACT Ground-based observations of radio emissions in the high- frequencyand ultra-high-frequency ranges provide the only current data on particlesand fields near Jupiter. Their interpretation is largely phenomenologicalbut none-the-less definitive of major features of Jupiter's spatial environment. Jupiter's magnetic field appears to be dipolar, with a moment of value 4. 2x1030 c. g .s . , directed at 70 7 to the rotation axis. The zenographic north pole of the magnetic field lies in or parallels the central meridianlongitude plane AIII( 1957.0) = 193O (in1963; in 1968the value is 202O). Thispole is probablynorth seeking. There is evidence for a small quadrupole moment, about 0.06 RJ in units of the dipole moment. There is alsoevidence for strong north-south and also lesser east-west displacementsof the dipole away from the mass centroid of the planet. On thebasis of these conclusions on field strength taken fromobserved details of the UHF flux, .de cantoday accurately estimate relativisticelectron fluxes near Jupiter. Typical energies are 10 MeV, andrepresent a rangefrom about 3 to 30 MeV. Lower energyelectrons are much more difficult to estimate andinferences even of their mere presence depend entirely on interpre- tationsof the high-frequency emissions. Proton fluxes are virtually unknown on thebasis of current observational data. On theother hand, sensitive upper limits to the thermal plasma density throughout Jupiter's magnetosphere have been established empirically. Throughout , theemphasis is on el?mentary physical prin- ciples , rather thar! details of formulas, and on pr,obable uncwtainties in the results. 1. STATIC MAGNETIC FIELD 1.1 Estimates of Jupiter'smagnetic moment 1. 2.1 Moroz (1968)suggests that the ratio of theangular momenta of two planetsequals the ratio of theirmagnetic dipole moments. For Jupiter, (see Appendix) as we shall shortly see, this estimate is verynearly equal to a best estimate on otherbases. The angular momentum of Jupiter is notobserved directly, but depends on the distribution of mass andangular velocitywithin the planet. From Allen'stables (1963), we learnthat the 2 moment of inertiaof Jupiter, CJ = 0.241 MJRJ (equatorial);the angular velocity , RJ, of Jupiter's radio sources corresponds to a periodof about hm 9 55 , i.e., 0.413days. M = mass of Jupiter; R = its radius,subscript J J E refers tothe earth. Setting the angular momentum ofJupiter, LJ = CJRJ we .find 0m241 x 317.8 x (11.19)2 + 0.413 LJ = LE 0.3335 4 = 7.25 x 10 LE 2 where0.3335 = CE/MES , 317.8 = M /M and11.19 = R / . Sincethe earth's J E' 3 J% ~ magneticdipole moment is M = 8.07x10?5gauss cm ,. we estimate , alongwith E 30 3 Moroz, thatJupiter's magnetic dipole moment is M = 5.86~10 gauss cm J . A physical basis for this estimate is, of course, similarity of terrestrial and Jovian internal conductivity, fluid motion, and perhaps initial condition of magnetism at the time of the formation of the solar system.In particular, if we supposethat magnetic-active parts of the two planetsoccupy the same fraction of the total volume of each,and if the volumeand size of the individual rotors of each dynamo are the same, and if the angular velocities of the rotors vary as.the angular velocity of the surfaceof each planet, then we mightpredict Moroz' result. But dynamo theoryremains an extremely difficult subject today and we haveno great confidence in its ability to produce deductive theories of magnetism in rotatingbodies (see Gibsonand Roberts, 1968). 2 Small additional comfort may be derived from this technique of estimation on the basisof its results for thesun. The general field of thesun is of theorder of onegauss. From its angular momentum, 48 40 Ls = 1.7~10 c.g.s. compared with = 5.86~10 c.g.s., we derive LE 48 3 33 MS = (1.7~10/5.86~10~~~x8.07~10~~ gauss cm = 2.34~10c.g.s. The cor- 33 3 responding surface field2.34~10 /Rs = 6.7gauss, comparable to observedvalues. In fact if, unlikethe case for theearth, only non- central regions of the sun are involved in the field-sustaining processes, this estimate might be theoretically as well as observationally toc large. However, this estimate may break down badly in the context of other stars. 1.2.2 A range of valuesofthe magnetic moment canbederived from certair: aspectsof decimetric radiation from synchrotron emission from the trapped radiation belts of Jupiter (at 2.5 RJ from its center, which is a rough centroid distance for the emission oneach side of the planet). Basically the estimates dependon the stable properties in position and time .ofthe belts. So far as theintegrated flux across the belts is concerned, thereappears to have been no large change since the first observations in the late 1950's. The observationsof structural details by interferometers also present a picture of stable, well-trapped emission, very different from what might be expected on the basis of phenomena at relativistic energies n'ear theearth (Hess, 1968). Omnidirectionalflux variations over a factor of 100 occur in the outer belts in time scales of a few weeks. Observationally we therefore believe that the energydensity inthe field at mustexceed the relativistic particle energy 2.5 RJ density,and by a considerableamount, say lOO-fold, to account for the probability that protons also will be stably trapped,in the same spatial region. Furthermore, it seems clear thatvariations of the emission on a time scale shorterthan one year will, if they are
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