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A Post-Voyager View of Saturn's Environment

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A Post-Voyager View of Saturn's Environment STAMA TIOS M. KRIMIGIS A POST-VOYAGER VIEW OF SATURN'S ENVIRONMENT Some of the important results obtained in the magnetosphere of the planet Saturn by the two Voy­ ager spacecraft and by Pioneer 11 are discussed. These include the absence of a significant tilt be­ tween the rotational and magnetic axes of the planet, and the discovery of a torus of very hot (about 6 x 108 K) dilute plasma associated with the orbits of the satellites Rhea and Dione. INTRODUCTION The environment of Saturn has been investigated Mimas by three spacecraft in the course of two years, begin­ Enceladus ning with the encounter of Pioneer 11 in September Tethys 1979, and followed by Voyager 1 and Voyager 2 in Dione November 1980 and August 1981, respectively. "En­ Rhea vironment," as used in this paper, may be defined as that part of the space surrounding the planet that is controlled by the planet's own magnetic field, i.e. , its magnetosphere. * This definition excludes the planet itself and its atmosphere, the rings, and the satellites, but it includes energetic particles (consisting of ions Figure 1 - A schematic representation of the Saturn ian and electrons), neutral particles, plasmas, and wave system, showing the seven major inner satellites of Saturn. fields that may be present within the magnetospheric Titan, the largest Saturnian satellite, is about 20 Rs from cavity, and the interaction of these particles and plas­ the planet. Water ice is believed to form the outermost layer mas with the satellites and the rings. Even though the of most satellites. subject of this paper does not specifically include the satellites and the rings, a proper discussion and un­ ments have revealed the rings to be an extensive and derstanding of the planet's environment cannot be complex system of particulate matter, extending given without some knowledge of the solid bodies much farther out than is visible from ground-based and of the particulate matter that populate the mag­ telescopes or observed by Pioneer 11 instruments. netosphere. They extend to distances perhaps as far as 8 Rs (Rs, Figure 1 is a schematic view of most of the Satur­ the Saturn radius, is 60,330 kilometers). nian system, showing the major inner satellites and The inferred average shape of the Saturnian mag­ the rings. The Saturnian system, outside of Saturn netosphere is shown in Fig. 2, together with the tra­ itself, is dominated by Titan, the largest Saturnian jectories of the two Voyager spacecraft through the satellite and the only satellite in the solar system with system. The cutaway section shows that, on the aver­ an atmosphere. Outside Titan is located the small sat­ age, the orbit of Titan is generally inside the magne­ ellite Hyperion, while inside we see in sequence the tosphere, although at times Titan finds itself in the satellites Rhea, Dione, Tethys, Enceladus, and magnetosheath or even in the solar wind. 2 Both Voy­ Mimas. The interaction of these satellites with the ager spacecraft approached the magnetosphere from magnetospheric plasma, energetic particles, plasma the direction of the sun (local noon), with Voyager 1 waves, and radio emissions has been found to be encountering Titan on the inbound leg, dipping quite varied and complex. under the equatorial plane of Saturn, and then cross­ Inward of Mimas the visible rings have a profound ing into the northern hemisphere and leaving in the influence on energetic particles, as observed original­ general antisolar direction. Voyager 2, on the other ly by the Pioneer investigators. 1 The Voyager instru- hand, came in above the Saturnian equatorial plane, • A GLOSSARY of technical terms appears on page 186. crossed into the southern hemisphere, and left the 180 Johns Hopkins A PL Technical Digest ___________________________________________________________ SPECIALTOPICS 10-1 To- sun ~ 45'~'--""--'---'TT"11T"""Tr-r--r-r--r---'-1-"'1 -'I--,!.----rl -; ? l ! -45LF~! ==~~=:==n.i...1 uH~' l!i , ~I :ri:, i± I :::1 ~~l::::i:~pa::::::cec:;::±ri:::ft~la:;::t::::itu~~~e ~, ~dd Outbound Voyager 2 Figure 2 - Trajectories of the two Voyager spacecraft shown in the framework of an average-size magnetosphere of Saturn. On the average, the orbit of Titan lies inside the magnetopause on the sunward side. magnetosphere in the general vicinity of local morn­ ing on its way to an encounter with Uranus in 1986. Voyager 1 approached the planet to within 126,000 kilometers above Saturn's cloud tops, while Voyager 2 came closer at a distance of 100,800 kilometers. 236 242 Closest approach occurred at a south latitude of 0 about 37 in the case of Voyager 1, but just north of Figure 3 - Fifteen-minute averages of selected channels 0 the equatorial plane (8 ) for Voyager 2. of the APL experiment on Voyager 2. Bow shock (BS) and magnetopause (MP) encounters are noted. Vertical dashed ENERGETIC PARTICLE MEASUREMENTS lines indicate crossings of outer satellite dipole L-shelis. Among the most important questions about Saturn prior to the spacecraft encounters was whether Sa­ The lower panel shows the intensity profile of ener­ turn possessed a magnetic field and trapped radia­ getic ions over the range from about 28 kiloelectron­ tion, i.e., the equivalent of Van Allen Belts. These volts to 2.1 megaelectronvolts. It is evident that the questions were answered in the affirmative following profile of the lowest-energy ions (top curve) is sub­ the Pioneer encounter in September 1979. The full stantially different from that at the highest energy extent of the magnetosphere of Saturn and its ions (bottom curve). In particular, fluxes of low­ trapped particles is depicted in Fig. 3, which shows energy ions increase rapidly inside the orbit of Dione, observations obtained by the APL Low Energy while higher-energy ions appear to be depleted in this Charged Particle (LECP) experiment on the Voyager region. The particle intensity profile suggests that the 2 spacecraft. The top panel shows the intensity pro­ satellites of Saturn play an important role in shaping file of low (22 kiloelectronvolt) and intermediate (250 the spatial distribution of ions within the radiation kiloelectronvolt) energy electrons and of high-energy belts of the planet. A most noticeable feature of the protons as the spacecraft approached and then reced­ overall energetic particle profile is the obvious asym­ ed from the planet. The data show the magneto­ metry between the dayside of the planet and the early sphere to be populated largely by low-energy (soft) morning region, where the Voyager 2 spacecraft left electrons in the outer regions, while more energetic the magnetosphere. The magnetopause crossing oc­ electrons appear to dominate closer in. Inside the or­ curred at a distance of 18 .5 Rs inbound, while the bits of Enceladus and Mimas there appear substantial equivalent crossing outbound occurred at a distance fluxes of high-energy (~80 megaelectronvolt) pro­ of about 50 Rs. It appears that the physical state of tons, forming the hard core of the radiation belt. the magnetosphere of Saturn changed substantially These protons originate from the interaction of cos­ while the spacecraft was inside. mic rays with the rings of the planet, as pointed out The intensity profiles for the various energy chan­ by the Pioneer 11 investigators. 3 nels in Fig. 3 suggest that the electron and ion energy Vo lume 3, Number 2, 1982 181 • Saturn outbound • Saturn inbound 238: 0345 - 238: 1930 237: 0800 - 238: 0345 105~~~~~~~~~~1~~~~~1~~~~~ 43-80 keV ions - Voyager 2 "C C 8 .... ' t., .• t. 1 •• CD 3 ". '0' ",: " .: •• : •••••••• ": ••••••~ ~.... 10 r- c .. :;..... ::J o en CD ctI '" > c CD I ". (J 102 r- ..r:. 0 ..r:. 5' a:: - CD ~ 0 '. •• t '. '.' ••••• ~ :: 10° 101~~-J---'-~---'-~----'--L_L-1 ~L-~~1~-J~-J~ Maxwellian./ N 75 ,· ~~~~~~~~~1~~~~-~1~~~~ E distribution ~ Maxwellian temperature .~c;; c $ - .5 Day 237 c 'II : .2 1930-1945 UT . !. ..' L = 10 Rs ',. .... , :" ~ 10-2 c kT = 55 keV .: -.•. f . ...•....... N = 8.5 x 10- 4 cm-3 •• ' •• ' . : .. : ". ell • o~ cO~~-J---'-~---'-~----'--L_L-IL-~~L-I~~~~~ "C~1~-.-.~~~~~or-,~~~.-,-,-.-.-.-. l!;l .; f- I I.. - :: CD f- Standard deViation - ctI"C 10-4L------'---L--L-L....l...-_..L..----'-----'---'-L------L+--=---'---L-L..l E"C r- - 1 10 102 103 104 o ~ I- <h •• •••• • :: ..... ' ••::1 ' :: ". :;:,::,: ':'\' " ::1:.;:,' .,-!.. Z"CO~~-J---'-~~~~~~~~~~~~U=~~ Energy (keV) ~ 0 5 10 15 20 v. Dipole L (Rs) Figure 4 - Observed differential energy spectrum at the Figure 5 - The ion temperature, derived from fits similar indicated radial distance, and a fit by a Maxwellian distribu­ to that shown in Fig. 4, as a function of radial distance from tion (solid line) for the lower-energy channels. Note that the the planet expressed in terms of the magnetic dipole transition from thermal to nonthermal spectrum occurs at L-shell parameter. The top panel shows the characteristic an ion energy of about 500 kiloelectronvolts. This is the hot­ intensity profile for one of the energy channels used in the test plasma ever measured in the solar system, with the fit; the bottom panel indicates the normalized goodness of characteristic temperature, kT, about 55 kiloelectronvolts. fit to the Maxwellian distribution. spectra undergo considerable change as a function of tween the orbits of Rhea and Tethys, both inbound distance from the planet. An example of a typical ion and outbound. spectrum in the outer part of the magnetosphere is A schematic representation of the high-tempera­ shown in Fig. 4. The spectrum seems to be described ture region observed by the Voyager 2 spacecraft is well by a Maxwellian distribution at the lowest ener­ shown in Fig. 6. The asymmetry between the inbound gies of the form j (E) = KE exp ( - E/ kT), while at and outbound trajectories is indicated, as is the ex­ higher energies it is described well by a spectrum of pectation that the density of the hot plasma increases the form E - const.
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