Passage to a Ringed World - NASA's History Office

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field reverses direction across the tail’s plasma sheet (a thin sheet of plasma located approximately in the planet’s equatorial plane, where currents flow and particles are accelerated. The process of reconnection and opening of field lines on the sunward side of the magnetosphere is thus balanced by reconnection that closes field lines in the magnetotail. The newly closed field lines contract back toward the planet, pulling the plasma along and driving a circulation pattern, as shown in the figure below. The process of reconnection on the Sunward side of the magnetosphere is thus closely coupled to processes that occur in the magnetotail. These processes are known to be strongly SOLAR WIND CIRCULATION Large-scale circulation driven by the solar wind as it occurs at Earth. An analogous process occurs at Saturn. The orientations of magnetic field lines and plasma flows are shown. When the interplanetary magnetic field is oriented southward, as shown, field lines reconnect at the nose of the magnetopause and then again in the magnetotail, driving the flows described in the chapter text. affected by the changing conditions in the solar wind. At Earth, reconnection processes can give rise to large, erratic changes in the global configuration of the magnetosphere referred to as geomagnetic storms. Cassini’s MAPS instruments will investigate to see if similar magnetospheric storms occur at Saturn. Voyager 1 made the first direct measurement of Saturn’s magnetotail, finding it to resemble its terrestrial and Jupiter counterparts. The magnetotail was detected to be roughly 40 R in diameter at a distance 25 R S S downstream; it may extend hundreds of Saturn radii in the downstream solar wind. Understanding the processes that occur in the magnetotail is fundamental to understanding overall Solar Wind Reconnection magnetospheric dynamics; coordinated measurements by the MAPS instruments during the deep tail orbits planned for the Cassini tour will contribute to that understanding. In turn, by understanding overall magnetospheric dynamics, scientists will gain insight into how Saturn’s magnetosphere harnesses energy from the solar wind. Current Magnetospheric Systems Various large-scale current systems exist in Saturn’s magnetosphere due to the collective motions of charged particles. Cross-tail currents flow from dusk to dawn in the plasma sheet located near the center of the magnetotail. An equatorial ring current distorts the magnetic field from its di- Magnetopause Tailward Flow Reconnection A SPHERE OF INFLUENCE 71
72 PASSAGE TO A RINGED WORLD polar configuration, particularly in the outer magnetosphere where it stretches the magnetic field lines in the equatorial plane. This ring current, caused by electrons and ions drifting around the planet in opposite directions, is probably primarily due to the energetic particles discussed later in this chapter. The effect of this ring current is moderate when compared with Jupiter, however. Another major contribution to Saturn’s total magnetic field comes from currents flowing in the magnetopause, which result from interaction with the solar wind. Cassini’s Dual Technique Magnetometer, measuring the magnetic field, and the Cassini Plasma Spectrometer, measuring the currents, will help map the current systems. These measurements, together with those taken by the other Cassini plasma instruments, will allow scientists to make a global model of Saturn’s magnetic field throughout the magnetosphere. Major Magnetospheric Flows There are two primary sources of energy driving magnetospheric processes: the planet’s rotation and the solar wind. Correspondingly, there are two types of large-scale plasma flow within the magnetosphere — corotation and convection. The nature of the large-scale circulation of particles in the magnetosphere depends on which source is dominant. At Earth, the energy is derived primarily from the solar wind; at Jupiter it is derived from the planet’s rapid rotation rate. Saturn’s magnetosphere is especially interesting because it is somewhere in between: both energy sources should play an important role. Saturn’s ionosphere is a thin layer of partially ionized gas at the top of the sunlit atmosphere. Collisions between particles in the atmosphere and the ionosphere create a frictional drag that causes the ionosphere to rotate together with Saturn and its atmosphere. The ionosphere, which extends from 1500 kilometers above the surface (defined as the visible cloud layer) to about 5000 kilometers, has a maximum density of about 10,000 electrons per cubic centimeter at about 2000–3000 kilometers. The rotation of Saturn’s magnetic field with the planet creates a large electric field that extends into the magnetosphere. The electromagnetic forces due to the combination of this electric field and Saturn’s magnetic field cause the charged magnetospheric plasma particles to “corotate” (rotate together with Saturn and its internal magnetic field) as far out as Rhea’s orbit (about nine R ). S Convection, the other large-scale flow, is caused by solar wind pulling the magnetic field lines toward the tail. This leads to a plasma flow from day side to night side on open field lines and to a return flow from night side to day side on closed field lines (particularly near the equatorial plane). On the dawn side, the corotation and convective flows will be in the same direction, but on the dusk side, they are opposing flows. The interaction of these flows may be responsible for some of the large variability observed in the outer magnetosphere. While at present we can only speculate about the consequences of these plasma flow patterns, we may expect some answers from investigations by Cassini’s plasma instruments (especially the Cassini Plasma Spectrometer and the Magnetospheric Imaging Instrument). Magnetospheric Plasma Regions Saturn’s magnetosphere can be broadly divided in two parts: a fairly quiet inner magnetosphere extending to about 12 R (beyond all moons S except Titan), and an extremely variable hot outer magnetosphere. In both regions, the plasma particles are concentrated in a disk near the equatorial plane, where most plasma particle sources are located. In their brief passages through Saturn’s inner and outer magnetospheres, Pioneer and both Voyagers passed through several different plasma regions. The spacecraft observed a systematic increase in electron temperature with distance from Saturn, ranging from one electron volt (equivalent to a temperature of 11,600 kelvins) at four R in the in- S ner magnetosphere and increasing to over 500 electron volts in the outer magnetosphere. The thickness of the plasma disk increases with distance from Saturn. Inside about four R a dense (about S 100 per cubic centimeter) popula-
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Passage to a Ringed World The Cassi
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On the Cover: A collage of images s
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FOREWORD This book is for you. You
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ACKNOWLEDGMENTS This book would not
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Three separate images, taken by Voy
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descent to the surface. The Huygens
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address the challenges of the missi
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Participant Nations* * First flag i
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PROBING A MOON OF MYSTERY Huygens
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TOUR T18-3 TARGETED SATELLITE FLYBY
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SATURN CASSINI-HUYGENS MISSION SCIE
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Cassini-Huygens designed to determi
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In this diagram from his book, Syst
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This Voyager 1 image shows Saturn
Page 30 and 31: An Extended Atmosphere From Liquid
Page 32 and 33: Haze layers are also observed above
Page 34 and 35: IN THE EYE OF THE BEHOLDER Taken fr
Page 36 and 37: This spectacular view of the edge o
Page 38 and 39: the interaction was described as a
Page 40 and 41: have a signature of a tone decreasi
Page 42 and 43: CONSTITUENTS OF THE TITAN ATMOSPHER
Page 44 and 45: The processes of plate tectonics an
Page 46 and 47: tan was accreting, it is possible t
Page 48 and 49: Surface Science. The highest resolu
Page 50 and 51: This enhanced Voyager 2 image shows
Page 52 and 53: Voyager 1 captured this striking im
Page 54 and 55: Prometheus and Pandora, two tiny sa
Page 56 and 57: In the stellar occultation experime
Page 58 and 59: scattering than are larger particle
Page 60 and 61: In fact, Saturn’s rings — as we
Page 62 and 63: The satellites of Saturn comprise a
Page 64 and 65: Before the advent of spacecraft exp
Page 66 and 67: The bright surface of Saturn’s mo
Page 68 and 69: Galilean satellites. Most of what i
Page 70 and 71: duced that one hemisphere is compos
Page 72 and 73: patches on the surface. Although it
Page 74 and 75: Artist’s conception of Ithaca Cha
Page 76 and 77: An artist’s rendition of Saturn
Page 78 and 79: The MAPS Instruments Coordinated ob
Page 82 and 83: SATURN’S MAGNETIC FIELD Saturn’
Page 84 and 85: Solar Wind Magnetosheath Titan neut
Page 86 and 87: per atmosphere. Energetic particles
Page 88 and 89: and plasma wave emissions from narr
Page 90 and 91: CHAPTER 7 Space mission design aims
Page 92 and 93: (SOI) propulsive maneuver to initia
Page 94 and 95: the tour cannot continue. Of course
Page 96 and 97: crease inclination to this value. T
Page 98 and 99: This illustration shows the main de
Page 100 and 101: discussed earlier) and data handlin
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Page 104 and 105: • Opening or shutting thermal lou
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Page 110 and 111: The Cassini-Huygens spacecraft stan
Page 112 and 113: The Imaging Science Subsystem (ISS)
Page 114 and 115: • Map the composition and thermal
Page 116 and 117: cise distance between the surface f
Page 118 and 119: • Study the interaction of the ma
Page 120 and 121: • Study the erosional and electro
Page 122 and 123: midway out on the magnetometer boom
Page 124 and 125: The RPWS instrument will be used to
Page 126 and 127: altitudes down to 40 kilometers abo
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CHAPTER 10 Mission operations are t
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PLANETARY MISSION OPERATIONS Functi
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two low-gain antennas 1 and will pr
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During much of Cassini’s orbital
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articulation control subsystem’s
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AFTERWORD It has been said that in
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Developments tied to the Cassini-Hu
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APPENDIX A Command. A data transact
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APPENDIX A 138 PASSAGE TO A RINGED
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APPENDIX A Plasma wave. A wave char
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APPENDIX A 142 PASSAGE TO A RINGED
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APPENDIX B H O 2 + Water molecule w
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APPENDIX C 146 PASSAGE TO A RINGED
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APPENDIX D 148 PASSAGE TO A RINGED
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APPENDIX D 150 PASSAGE TO A RINGED
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APPENDIX F 152 PASSAGE TO A RINGED
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APPENDIX F 154 PASSAGE TO A RINGED
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APPENDIX F Peralta, F. and S. Flana
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National Aeronautics and Space Admi
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