Plasma Physics of the Local Cosmos (2004) / Chapter Skim
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2 Creation and Annihilation of Magnetic Fields
2 Creation and Annihilation of Magnetic Fields
Pages 11-27
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From page 11... ...
Solar and stellar flares and magnetospheric substorms are examples of such explosive phenomena. Magnetic reconnection, in which oppositely directed magnetic field components rapidly merge to release the stored magnetic energy, has been identified as the dominant mechanism for dissipating magnetic energy.
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From page 12... ...
What, therefore, are the processes that control magnetic reconnection in environments where dynamo action is important (e.g., the convection zone in the Sun or in the interior of planetary bodies) i in a self-consistent dynamo model, all these questions are related and so must be studied together Kinematic dynamo theory studies the generation of a magnetic field by a given flow.
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From page 13... ...
radial component The field line is stretched in longitudinal directions by (zonal) differential rotations in the fluid core (the so-called 03-effect in dynamo theory)
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From page 14... ...
in the fluid momentum equation. CREATION OF MAGNETIC FIELDS INTHE SUN Solar magnetic energy is continually being created, annihilated, and ejected.
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From page 15... ...
For example, the inferred Turbulent diffusion of the magnetic field in the convective zone, which is essential in establishing the proper scale and period of the solar magnetic field, is not understood. In addition, the magnetic fields extending through the visible surface of the Sun actually consist of unresolved, widely spaced, very intense (1500 gauss)
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From page 16... ...
The giant planets approach more closely the solar case, with the convection zone extending to the planetary surface. Observational and theoretical studies of planetary magnetic fields began with the study of the geomagnetic field.
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From page 17... ...
The recent numerical modeling of planetary dynamos has been very successful and is rapidly becoming the main tool for studying in detail the nonlinear dynamics of dynamo action. Although the mathematical models are very simple compared to the actual planetary cores, they can produce solutions that agree qualitatively with observations.
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From page 18... ...
of planetary magnetic fieldsi . What are the turbulent flow structures in planetary coresi MAGNETIC FIELD ANNIHILATION: RECONNECTION THEORY A variety of phenomena in the universe are powered by the sudden release of magnetic energy and its conversion into heat and high-velocity plasma flows.
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From page 19... ...
In panel (a) oppositely directed magnetic fields ~reconnect~to form a magnetic x-line config u ration The bent fields to the left and rig ht of the x-line act like oppositely directed 'sllngshots"that expand outward to release their energy, driving the high-speed outflows shown in panel (b)
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From page 20... ...
This time scale is shorter than the measured energy release times. The rate of magnetic reconnection depends ultimately on the mechanism by which oppositely directed field lines reconnect.
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From page 21... ...
Intrinsically three-dimensional reconnection is therefore a topic of great importance, but one of which current understanding remains limited. MAGNETIC RECONNECTION IN THE SUN'S CORONA Magnetic field annihilation in the solar atmosphere typically proceeds in an explosive manner, producing flare energy releases over a broad range from 1032 to 1033 ergs down to the threshold for detection at about 1 o24 ergs.
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From page 22... ...
Magnetic reconnection is the principal mechanism by which energy, mass, and momentum are transferred from the solar wind to the magnetosphere and by which magnetic energy stored in the magnetotail is released in explosive events known as magnetospheric substorms. It thus plays a prominent role in the dynamics of Earth's magnetosphere.
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From page 23... ...
orientation of the interplanetary field relative to the geomagnetic field. In the simplest picture of magnetopause reconnection, the IMF is strongly southward—that is, it has an out-of-the-ecliptic component that is anti-parallel to Earth's northward-directed field at the subsolar magnetopause and merges with the geomagnetic field across an extended portion of the dayside magnetopause, producing open field lines that are swept back into the magnetotail by the solar wind flow as shown in Figure 2.4.
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From page 24... ...
Because the IMF is generally not oriented directly southward but has a finite east-west component, the notion of oppositely directed field lines reconnecting at the subsolar magnetopause is an oversimplification. The location of magnetic reconnection at the magnetopause varies, depending on the direction of the IMF identifying the location of reconnection and understanding the physical processes that determine where reconnection takes place on the magnetopause continue to spark intense discussion in the scientific literature.
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From page 25... ...
The transport and pileup of magnetic flux earthward of the x-line lead to a reconfiguration of the tail magnetic field and therefore the release of the magnetic stress associated with the stretching of the field lines by the solar wind flow. Anti-sunward of the reconnection region, the one or more reconnection sites combine to create plasmoids, large-scale traveling plasma structures entrained in magnetic flux ropes.
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From page 26... ...
An experiment that selfgenerates a seed magnetic field as a result of externally supplied flows would provide a wealth of data for benchmarking theoretical models. CONCLUDING REMARKS The generation of magnetic fields and their subsequent conversion into plasma kinetic energy have abundant examples throughout the universe.
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From page 27... ...
magnetic reconnection challenge, loumal of Geophysical Research 106, 371 5, 2001
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