Copyright © National Academy of Sciences. All rights reserved.
Solar and Space Physics: A Science for a Technological Society
ENABLING DISCOVERY IN SOLAR AND SPACE PHYSICS
29
Underlying the extraordinarily complex and dynamic space environment are identifiable fundamental
processes that can sometimes be explored as independent problems. These fundamental processes can also
play a role in other astrophysical settings. In that sense, the Sun, the heliosphere, and Earth’s magnetosphere
and ionosphere serve as cosmic laboratories for studying universal plasma phenomena, with applications
to laboratory plasma physics, fusion research, and plasma astrophysics. 7 Discoveries from these fields, of
course, also contribute to the scientific progress in solar and space physics.
There are numerous examples of the universal processes that control the dynamics of the space
environment:
• Dynamos. Turbulence in the convection zone of the solar interior twists and transports magnetic
fields and ultimately determines the large-scale solar dipolar magnetic field, which reverses polarity on
average every 11 years. Similar dynamos produce magnetic fields in stars, magnetars, and even in galaxies,
black holes, and other compact objects. Earth’s ionosphere exhibits several neutral wind dynamo processes
that at high latitudes generate large-scale electric fields that affect the magnetosphere, and at low latitudes
control the growth of plasma densities generated by solar radiation.
• Solar and planetary winds. The heating and subsequent outward expansion of the solar atmosphere
create the solar wind. Produced by a variety of mechanisms, winds are features of essentially all stars.
Winds from the poles of Earth—the polar wind—can similarly fill the magnetosphere with ionospheric
plasma.
• Magnetic reconnection. Magnetic fields in regions of opposing field direction can annihilate each
other to convert magnetic energy into high-speed flows, heated plasma, and energetic particles. This
explosive release of energy drives flares on the Sun and other stars and possibly the magnetospheres of
galactic accretion disks and astrophysical jets. Reconnection in Earth’s magnetosphere leads to the erosion
of Earth’s protective magnetic shield during storms and is the driver of magnetospheric substorms.
• Collisionless shocks. Shock waves appear throughout the heliosphere where they facilitate the transition from supersonic to subsonic flow, heat the plasma, and act as accelerators of energetic particles. Shocks
are widely observed in astrophysical systems in the form of supernova shocks, which are a predicted source
of galactic cosmic rays, at the termination of astrophysical jets, and more generally during collisions and
mergers of galaxies.
• Turbulence. Plasma turbulence is ubiquitous in the space environment and throughout the broader
universe. It carries energy from the interior of the Sun to its surface and drives the solar dynamo. It is also
one of the proposed mechanisms for heating the ambient corona and accelerating energetic particles in
flares. Turbulence drives the transport of particles, and energy in the magnetosphere—the region of space
dominated by Earth’s magnetic field and radiation belt—heats electrons and ions in the auroral region
and is ubiquitous in the charged upper layers of Earth’s atmosphere (the ionosphere). The recognition that
turbulence may facilitate accretion has transformed understanding of the environments of compact astrophysical objects and even of the mechanisms of star and planetary formation.
• Plasma-neutral interactions. The interaction of the ionized plasma in Earth’s magnetosphere and
neutral particles in the ionosphere/thermosphere lead to ionization, outflows into the magnetosphere,
and the generation of neutral winds whose rich dynamics have only recently been appreciated. Similar
interactions between neutral and ionized particles take place at the Sun and in the solar wind. In the
broader universe, plasmas are often only partially ionized, so that plasma motions are often constrained
by mass loading due to neutrals. The observed structuring of molecular clouds is believed to result from
the ionization dynamics of radiation and plasma-neutral interaction.
7 See National Research Council, Plasma Physics of the Local Cosmos, The National Academies Press, Washington, D.C., 2004.
Solar and Space Physics: A Science for a Technological Society
ENABLING DISCOVERY IN SOLAR AND SPACE PHYSICS
29
Underlying the extraordinarily complex and dynamic space environment are identifiable fundamental
processes that can sometimes be explored as independent problems. These fundamental processes can also
play a role in other astrophysical settings. In that sense, the Sun, the heliosphere, and Earth’s magnetosphere
and ionosphere serve as cosmic laboratories for studying universal plasma phenomena, with applications
to laboratory plasma physics, fusion research, and plasma astrophysics. 7 Discoveries from these fields, of
course, also contribute to the scientific progress in solar and space physics.
There are numerous examples of the universal processes that control the dynamics of the space
environment:
• Dynamos. Turbulence in the convection zone of the solar interior twists and transports magnetic
fields and ultimately determines the large-scale solar dipolar magnetic field, which reverses polarity on
average every 11 years. Similar dynamos produce magnetic fields in stars, magnetars, and even in galaxies,
black holes, and other compact objects. Earth’s ionosphere exhibits several neutral wind dynamo processes
that at high latitudes generate large-scale electric fields that affect the magnetosphere, and at low latitudes
control the growth of plasma densities generated by solar radiation.
• Solar and planetary winds. The heating and subsequent outward expansion of the solar atmosphere
create the solar wind. Produced by a variety of mechanisms, winds are features of essentially all stars.
Winds from the poles of Earth—the polar wind—can similarly fill the magnetosphere with ionospheric
plasma.
• Magnetic reconnection. Magnetic fields in regions of opposing field direction can annihilate each
other to convert magnetic energy into high-speed flows, heated plasma, and energetic particles. This
explosive release of energy drives flares on the Sun and other stars and possibly the magnetospheres of
galactic accretion disks and astrophysical jets. Reconnection in Earth’s magnetosphere leads to the erosion
of Earth’s protective magnetic shield during storms and is the driver of magnetospheric substorms.
• Collisionless shocks. Shock waves appear throughout the heliosphere where they facilitate the transition from supersonic to subsonic flow, heat the plasma, and act as accelerators of energetic particles. Shocks
are widely observed in astrophysical systems in the form of supernova shocks, which are a predicted source
of galactic cosmic rays, at the termination of astrophysical jets, and more generally during collisions and
mergers of galaxies.
• Turbulence. Plasma turbulence is ubiquitous in the space environment and throughout the broader
universe. It carries energy from the interior of the Sun to its surface and drives the solar dynamo. It is also
one of the proposed mechanisms for heating the ambient corona and accelerating energetic particles in
flares. Turbulence drives the transport of particles, and energy in the magnetosphere—the region of space
dominated by Earth’s magnetic field and radiation belt—heats electrons and ions in the auroral region
and is ubiquitous in the charged upper layers of Earth’s atmosphere (the ionosphere). The recognition that
turbulence may facilitate accretion has transformed understanding of the environments of compact astrophysical objects and even of the mechanisms of star and planetary formation.
• Plasma-neutral interactions. The interaction of the ionized plasma in Earth’s magnetosphere and
neutral particles in the ionosphere/thermosphere lead to ionization, outflows into the magnetosphere,
and the generation of neutral winds whose rich dynamics have only recently been appreciated. Similar
interactions between neutral and ionized particles take place at the Sun and in the solar wind. In the
broader universe, plasmas are often only partially ionized, so that plasma motions are often constrained
by mass loading due to neutrals. The observed structuring of molecular clouds is believed to result from
the ionization dynamics of radiation and plasma-neutral interaction.
7 See National Research Council, Plasma Physics of the Local Cosmos, The National Academies Press, Washington, D.C., 2004.