Copyright © National Academy of Sciences. All rights reserved.
Solar and Space Physics: A Science for a Technological Society
232
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
to Earth’s magnetosphere, and coupling plasma in the heliosphere to that in the interstellar medium. Magnetic reconnection is a multiscale process linking global physics with microphysics, and as such is a grand
challenge problem. Geospace provides a unique opportunity to study magnetic reconnection because it is
one of the few locations allowing in situ measurements of the reconnection process. These measurements
facilitated great progress in the past decade, but critical questions remain unsolved.
How is magnetic reconnection triggered? A fundamental question remains as to exactly how the explosion is allowed to happen in magnetic reconnection. This occurs at very tiny scales and has eluded direct
measurement. What turns on reconnection or suppresses it? On the dayside magnetosphere, magnetic
reconnection often happens gradually over long time periods, whereas on the nightside, it suddenly turns
on, explosively releasing large amounts of energy during a substorm. Why do these two regions behave
so differently? Does inflowing turbulence inhibit or actually encourage reconnection? Questions similar to
these were called out as one of the highest-priority science objectives of the 2003 decadal survey and are
the focus of the upcoming Magnetospheric Multiscale Mission (MMS), now in development. The present
survey finds that these questions continue to be of very high importance, and the SWMI panel anticipates
major progress from MMS.
In addition to the microphysics questions to be addressed by MMS, there is a second fundamental
issue regarding the reconnection process: How is magnetic reconnection modulated? A critical unsolved
aspect of reconnection modulation is the relative role of global conditions (boundary conditions) versus
microscale physics in determining the properties of reconnection. Addressing this question will require
observations at both microscale and macroscales simultaneously. Specific questions regarding modulation
are: What determines the temporal characteristics of reconnection? Does reconnection spontaneously create bubbles of magnetic field periodically? Or are these bubbles due to changing inflow conditions? How
do changes in boundary conditions modify reconnection? How does shear flow affect the occurrence and
efficiency of reconnection? How do different asymmetric inflow or outflow conditions affect the structure
and dynamics of reconnection? How do multiple ion species affect reconnection? How does inflowing
turbulence modulate magnetic reconnection?
Finally, the third frontier in reconnection physics is understanding its behavior in more realistic geometries. Current understanding of magnetic reconnection is limited primarily to a simplistic two-dimensional
picture where magnetic field lines break at simple lines called x-lines. However, the solar wind flow around
the tear-drop-shaped magnetosphere can break these simple x-lines into points called null points joined by
magnetic separator lines. Does magnetic reconnection happen primarily at null points or separator lines?
How fast does reconnection happen in these more complex three-dimensional geometries?
The questions outlined above, while specifically addressing decadal survey key science goal 4, also
contribute to decadal survey key science goal 2.
The third SWMI science goal relating to fundamental physical processes is goal 6.
9.4.2.3 SWMI Science Goal 6. Understand the Origins and Effects of Turbulence and Wave-Particle
Interactions
Digging Deeper
Turbulence has been detected in the collisionless plasma of Earth’s magnetotail, but the nature of the
turbulence and its impacts on the dynamics of the magnetosphere are not known. Three major outstanding questions about the nature of the turbulence focus on its dynamics (What is the nature of the turbulent
fluctuations and how do they interact to transfer energy?), its driving (What process supplies power to the
turbulence?), and its dissipation (What physical processes extract energy from the turbulence and where
does the power go?). Three outstanding questions about the impacts deal with three universal properties
Solar and Space Physics: A Science for a Technological Society
232
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
to Earth’s magnetosphere, and coupling plasma in the heliosphere to that in the interstellar medium. Magnetic reconnection is a multiscale process linking global physics with microphysics, and as such is a grand
challenge problem. Geospace provides a unique opportunity to study magnetic reconnection because it is
one of the few locations allowing in situ measurements of the reconnection process. These measurements
facilitated great progress in the past decade, but critical questions remain unsolved.
How is magnetic reconnection triggered? A fundamental question remains as to exactly how the explosion is allowed to happen in magnetic reconnection. This occurs at very tiny scales and has eluded direct
measurement. What turns on reconnection or suppresses it? On the dayside magnetosphere, magnetic
reconnection often happens gradually over long time periods, whereas on the nightside, it suddenly turns
on, explosively releasing large amounts of energy during a substorm. Why do these two regions behave
so differently? Does inflowing turbulence inhibit or actually encourage reconnection? Questions similar to
these were called out as one of the highest-priority science objectives of the 2003 decadal survey and are
the focus of the upcoming Magnetospheric Multiscale Mission (MMS), now in development. The present
survey finds that these questions continue to be of very high importance, and the SWMI panel anticipates
major progress from MMS.
In addition to the microphysics questions to be addressed by MMS, there is a second fundamental
issue regarding the reconnection process: How is magnetic reconnection modulated? A critical unsolved
aspect of reconnection modulation is the relative role of global conditions (boundary conditions) versus
microscale physics in determining the properties of reconnection. Addressing this question will require
observations at both microscale and macroscales simultaneously. Specific questions regarding modulation
are: What determines the temporal characteristics of reconnection? Does reconnection spontaneously create bubbles of magnetic field periodically? Or are these bubbles due to changing inflow conditions? How
do changes in boundary conditions modify reconnection? How does shear flow affect the occurrence and
efficiency of reconnection? How do different asymmetric inflow or outflow conditions affect the structure
and dynamics of reconnection? How do multiple ion species affect reconnection? How does inflowing
turbulence modulate magnetic reconnection?
Finally, the third frontier in reconnection physics is understanding its behavior in more realistic geometries. Current understanding of magnetic reconnection is limited primarily to a simplistic two-dimensional
picture where magnetic field lines break at simple lines called x-lines. However, the solar wind flow around
the tear-drop-shaped magnetosphere can break these simple x-lines into points called null points joined by
magnetic separator lines. Does magnetic reconnection happen primarily at null points or separator lines?
How fast does reconnection happen in these more complex three-dimensional geometries?
The questions outlined above, while specifically addressing decadal survey key science goal 4, also
contribute to decadal survey key science goal 2.
The third SWMI science goal relating to fundamental physical processes is goal 6.
9.4.2.3 SWMI Science Goal 6. Understand the Origins and Effects of Turbulence and Wave-Particle
Interactions
Digging Deeper
Turbulence has been detected in the collisionless plasma of Earth’s magnetotail, but the nature of the
turbulence and its impacts on the dynamics of the magnetosphere are not known. Three major outstanding questions about the nature of the turbulence focus on its dynamics (What is the nature of the turbulent
fluctuations and how do they interact to transfer energy?), its driving (What process supplies power to the
turbulence?), and its dissipation (What physical processes extract energy from the turbulence and where
does the power go?). Three outstanding questions about the impacts deal with three universal properties