Copyright © National Academy of Sciences. All rights reserved.
Solar and Space Physics: A Science for a Technological Society
REPORT OF THE PANEL ON SOLAR WIND-MAGNETOSPHERE INTERACTIONS
235
simultaneous information about upstream solar wind conditions. In recognition of the relatively immature
current understanding of other planetary magnetospheres, the corresponding critical science goal for the
coming decade is SWMI science goal 8.
9.4.4.1 SWMI Science Goal 8. Identify the Structures, Dynamics, and Linkages in Other Planetary
Magnetospheric Systems
Digging Deeper
Six planets in our solar system have strong internal magnetic fields and associated magnetospheres:
Mercury, Earth, Jupiter, Saturn, Uranus, and Neptune. The sizes of these magnetospheres vary considerably
(from ~1.5 to ~100 planetary radii), and their plasma and field structures are quite different, for reasons
scientists have just begun to discover and explore.
At Earth the main plasma sources are solar wind capture and ionization of the upper atmosphere,
and magnetospheric dynamics are driven largely by interactions with the solar wind. In other planetary
magnetospheres there is considerable variety in both plasma sources and drivers. With no atmosphere
or moons, Mercury’s small magnetosphere has no significant internal plasma source (aside from a small
population of sputtered material from Mercury’s surface), and interaction with the solar wind is probably
the primary source of energization and structure of the magnetospheric plasma. Giant-planet magnetospheres such as those of Jupiter and Saturn have dramatically different plasma sources and dynamics.
Neutral gases produced by volcanic activity on Jupiter’s moon Io and water geysers on Saturn’s moon
Enceladus provide the main internal plasma sources. The rapidly rotating planetary magnetic fields pick
up and accelerate new plasma to form dense, spinning plasma disks whose outer edges are flung outward
by centrifugal force and replaced by hotter, more tenuous bubbles of plasma that are drawn inward to
fill the void. In these rotationally dominated giant magnetospheres, major, fundamental mysteries remain
concerning time-variable magnetospheric rotation rates, poorly understood periodicities, plasma transport,
and magnetosphere-ionosphere coupling.
Similar effects are expected to occur at Uranus and Neptune, although currently very little is known
about the internal structure of the magnetospheres of these planets because of the limited data from the
Voyager 2 flybys. Though their small icy moons are not believed to be primary plasma sources, thorough
exploration of these magnetospheres will undoubtedly yield major new discoveries. The magnetospheres
of Uranus and Neptune are fundamentally different from others in our solar system, in that their magnetic
dipole axes are tilted by 59º and 47º relative to their rotational axes—with Uranus’s rotation axis lying
uniquely very close to the orbital plane of the planet. These large tilts are believed to cause enormous
rotationally induced variations and create complicated and unusually dynamic internal plasma structures—
which may change profoundly in less than 1 planetary day. So far, the only existing measurements near
these planets, from the single flyby of Voyager 2, were too limited to adequately resolve these complex
rotational variations.
The space environments of other planets offer natural laboratories for exploration of a wider range of
structures and dynamic linkages than are found at Earth. Scientific investigation of other planets enriches
understanding of how fundamental processes manifest themselves in real space environments. Discoveries
of unique features of other planetary magnetospheres—such as moons with volcanoes and geysers, and
drastically tilted magnetic and rotational axes—hint at the variety of new phenomena and challenges to
be found as human society expands into the larger universe. These discoveries are universal, applicable to
all four decadal survey key science goals, but in particular, comparative magnetospheric studies document
the variety of ways that the Sun interacts with the solar system (3) and enable exploration of fundamental
processes over a broad range of conditions inaccessible at Earth (4).
Solar and Space Physics: A Science for a Technological Society
REPORT OF THE PANEL ON SOLAR WIND-MAGNETOSPHERE INTERACTIONS
235
simultaneous information about upstream solar wind conditions. In recognition of the relatively immature
current understanding of other planetary magnetospheres, the corresponding critical science goal for the
coming decade is SWMI science goal 8.
9.4.4.1 SWMI Science Goal 8. Identify the Structures, Dynamics, and Linkages in Other Planetary
Magnetospheric Systems
Digging Deeper
Six planets in our solar system have strong internal magnetic fields and associated magnetospheres:
Mercury, Earth, Jupiter, Saturn, Uranus, and Neptune. The sizes of these magnetospheres vary considerably
(from ~1.5 to ~100 planetary radii), and their plasma and field structures are quite different, for reasons
scientists have just begun to discover and explore.
At Earth the main plasma sources are solar wind capture and ionization of the upper atmosphere,
and magnetospheric dynamics are driven largely by interactions with the solar wind. In other planetary
magnetospheres there is considerable variety in both plasma sources and drivers. With no atmosphere
or moons, Mercury’s small magnetosphere has no significant internal plasma source (aside from a small
population of sputtered material from Mercury’s surface), and interaction with the solar wind is probably
the primary source of energization and structure of the magnetospheric plasma. Giant-planet magnetospheres such as those of Jupiter and Saturn have dramatically different plasma sources and dynamics.
Neutral gases produced by volcanic activity on Jupiter’s moon Io and water geysers on Saturn’s moon
Enceladus provide the main internal plasma sources. The rapidly rotating planetary magnetic fields pick
up and accelerate new plasma to form dense, spinning plasma disks whose outer edges are flung outward
by centrifugal force and replaced by hotter, more tenuous bubbles of plasma that are drawn inward to
fill the void. In these rotationally dominated giant magnetospheres, major, fundamental mysteries remain
concerning time-variable magnetospheric rotation rates, poorly understood periodicities, plasma transport,
and magnetosphere-ionosphere coupling.
Similar effects are expected to occur at Uranus and Neptune, although currently very little is known
about the internal structure of the magnetospheres of these planets because of the limited data from the
Voyager 2 flybys. Though their small icy moons are not believed to be primary plasma sources, thorough
exploration of these magnetospheres will undoubtedly yield major new discoveries. The magnetospheres
of Uranus and Neptune are fundamentally different from others in our solar system, in that their magnetic
dipole axes are tilted by 59º and 47º relative to their rotational axes—with Uranus’s rotation axis lying
uniquely very close to the orbital plane of the planet. These large tilts are believed to cause enormous
rotationally induced variations and create complicated and unusually dynamic internal plasma structures—
which may change profoundly in less than 1 planetary day. So far, the only existing measurements near
these planets, from the single flyby of Voyager 2, were too limited to adequately resolve these complex
rotational variations.
The space environments of other planets offer natural laboratories for exploration of a wider range of
structures and dynamic linkages than are found at Earth. Scientific investigation of other planets enriches
understanding of how fundamental processes manifest themselves in real space environments. Discoveries
of unique features of other planetary magnetospheres—such as moons with volcanoes and geysers, and
drastically tilted magnetic and rotational axes—hint at the variety of new phenomena and challenges to
be found as human society expands into the larger universe. These discoveries are universal, applicable to
all four decadal survey key science goals, but in particular, comparative magnetospheric studies document
the variety of ways that the Sun interacts with the solar system (3) and enable exploration of fundamental
processes over a broad range of conditions inaccessible at Earth (4).