Copyright © National Academy of Sciences. All rights reserved.
Solar and Space Physics: A Science for a Technological Society
SOLAR AND SPACE PHYSICS: RECENT DISCOVERIES, FUTURE FRONTIERS
61
Since the feedback of the ionosphere and thermosphere as a source of plasma and dissipation for the
magnetosphere has such profound effects, the evolution of the ionosphere and magnetosphere must be
studied as a globally coupled system. Thus, a key challenge is as follows: SWMI-3. Determine how coupling and feedback between the magnetosphere, ionosphere, and thermosphere govern the dynamics of
the coupled system in its response to the variable solar wind.
Earth’s magnetosphere is a prototype of a universal plasma system: an object with a global magnetic
field that is subjected to an externally flowing plasma and forms a magnetosphere. Five other planets in
Earth’s solar system have magnetospheres: Mercury, Jupiter, Saturn, Uranus, and Neptune. Ganymede, one
of Jupiter’s satellites, also has its own tiny magnetosphere embedded within Jupiter’s giant one. Although
planetary systems exhibit analogous structures, the contrasting dynamics, boundary conditions, and magnetic fields make their detailed study of unique importance for testing theories and models.
Jupiter’s moon Io, deep within the enormous Jovian magnetosphere, is a copious source of neutral gas,
which, upon ionization, is a dominant drag force on the rapidly co-rotating magnetic field of the planet.
Similarly, the moons of Saturn, particularly Titan and Enceladus, are major sources of plasma that affects
the dynamics of Saturn’s magnetosphere. A key enigma of the Saturnian system is the source of the regular, 10-hour 46-minute periodicity in Saturn’s radio emissions, which differs from its rotation period by 6
minutes. This difference, discovered in data from the Cassini and Voyager spacecraft, remains unexplained.
The magnetospheres of Uranus and Neptune are largely unexplored but present unique cases that will
likely further challenge scientific understanding. Finally, the tiny magnetosphere of Mercury is an extreme
example of a magnetospheric system because it possesses no ionosphere. In such a situation the coupling
processes that operate are radically different. Thus, these other systems present a suite of vastly different configurations. The opportunity to test current theories and models on these widely varying systems
motivates a fourth challenge: SWMI-4. Critically advance the physical understanding of magnetospheres
and their coupling to ionospheres and thermospheres by comparing models against observations from
different magnetospheric systems.
Challenges Related to Atmosphere-Ionosphere-Magnetosphere Interactions
Understanding ionosphere-thermosphere interactions is a major area of inquiry, especially during
geomagnetic storms. The intense energy input from the magnetosphere, reaching up to terawatts, typically
occurs in regions spanning less than 10 degrees in latitude but during storms is redistributed throughout the
polar regions and down to middle latitudes over timescales from tens of minutes to hours. High-latitude
heating (mainly below 200-km altitude) causes N 2 -rich air to upwell. Strong winds driven by this heating
transport N 2 equatorward. The mixing with ambient atomic oxygen produces dramatic changes in the
ratio between atomic oxygen and molecular nitrogen. This global response was first discovered more than
a decade ago, but researchers still cannot explain why it takes several hours for the global thermosphere
to “inflate” after the high-latitude heating begins.
The ionospheric plasma also experiences major reconfigurations during storms as magnetospheric
convection drives the mixing of low- and high-density regions of the ionosphere. Figure 3.3 shows an
example of a plasma plume extending over thousands of kilometers that formed during the main phase of
a geomagnetic storm. Redistributions of plasma by large-scale electric fields also occur in the middle and
lower latitudes. At the onset of a storm, electric fields penetrate from the polar region and lift the equatorial ionosphere, depleting the equatorial density and producing anomalously high ionospheric densities on
field lines that connect the high-altitude equator with ionospheric latitudes north and south of the equator.
Convection in the polar regions also drives large-scale thermospheric winds that in turn carry ionospheric
plasma across the polar regions to lower latitudes.
Solar and Space Physics: A Science for a Technological Society
SOLAR AND SPACE PHYSICS: RECENT DISCOVERIES, FUTURE FRONTIERS
61
Since the feedback of the ionosphere and thermosphere as a source of plasma and dissipation for the
magnetosphere has such profound effects, the evolution of the ionosphere and magnetosphere must be
studied as a globally coupled system. Thus, a key challenge is as follows: SWMI-3. Determine how coupling and feedback between the magnetosphere, ionosphere, and thermosphere govern the dynamics of
the coupled system in its response to the variable solar wind.
Earth’s magnetosphere is a prototype of a universal plasma system: an object with a global magnetic
field that is subjected to an externally flowing plasma and forms a magnetosphere. Five other planets in
Earth’s solar system have magnetospheres: Mercury, Jupiter, Saturn, Uranus, and Neptune. Ganymede, one
of Jupiter’s satellites, also has its own tiny magnetosphere embedded within Jupiter’s giant one. Although
planetary systems exhibit analogous structures, the contrasting dynamics, boundary conditions, and magnetic fields make their detailed study of unique importance for testing theories and models.
Jupiter’s moon Io, deep within the enormous Jovian magnetosphere, is a copious source of neutral gas,
which, upon ionization, is a dominant drag force on the rapidly co-rotating magnetic field of the planet.
Similarly, the moons of Saturn, particularly Titan and Enceladus, are major sources of plasma that affects
the dynamics of Saturn’s magnetosphere. A key enigma of the Saturnian system is the source of the regular, 10-hour 46-minute periodicity in Saturn’s radio emissions, which differs from its rotation period by 6
minutes. This difference, discovered in data from the Cassini and Voyager spacecraft, remains unexplained.
The magnetospheres of Uranus and Neptune are largely unexplored but present unique cases that will
likely further challenge scientific understanding. Finally, the tiny magnetosphere of Mercury is an extreme
example of a magnetospheric system because it possesses no ionosphere. In such a situation the coupling
processes that operate are radically different. Thus, these other systems present a suite of vastly different configurations. The opportunity to test current theories and models on these widely varying systems
motivates a fourth challenge: SWMI-4. Critically advance the physical understanding of magnetospheres
and their coupling to ionospheres and thermospheres by comparing models against observations from
different magnetospheric systems.
Challenges Related to Atmosphere-Ionosphere-Magnetosphere Interactions
Understanding ionosphere-thermosphere interactions is a major area of inquiry, especially during
geomagnetic storms. The intense energy input from the magnetosphere, reaching up to terawatts, typically
occurs in regions spanning less than 10 degrees in latitude but during storms is redistributed throughout the
polar regions and down to middle latitudes over timescales from tens of minutes to hours. High-latitude
heating (mainly below 200-km altitude) causes N 2 -rich air to upwell. Strong winds driven by this heating
transport N 2 equatorward. The mixing with ambient atomic oxygen produces dramatic changes in the
ratio between atomic oxygen and molecular nitrogen. This global response was first discovered more than
a decade ago, but researchers still cannot explain why it takes several hours for the global thermosphere
to “inflate” after the high-latitude heating begins.
The ionospheric plasma also experiences major reconfigurations during storms as magnetospheric
convection drives the mixing of low- and high-density regions of the ionosphere. Figure 3.3 shows an
example of a plasma plume extending over thousands of kilometers that formed during the main phase of
a geomagnetic storm. Redistributions of plasma by large-scale electric fields also occur in the middle and
lower latitudes. At the onset of a storm, electric fields penetrate from the polar region and lift the equatorial ionosphere, depleting the equatorial density and producing anomalously high ionospheric densities on
field lines that connect the high-altitude equator with ionospheric latitudes north and south of the equator.
Convection in the polar regions also drives large-scale thermospheric winds that in turn carry ionospheric
plasma across the polar regions to lower latitudes.