Plasma Physics of the Local Cosmos (2004) / Chapter Skim
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3 Formation of Structures and Transients
3 Formation of Structures and Transients
Pages 28-45
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From page 28... ...
The stresses imposed by the interactions also engender the formation of current sheets within the plasma regimes, leading to a cellular structure Such internal and boundary current sheets imply the existence of high concentrations of energy density and shear stress, and the system responds to dissipate and redistribute the energy and the stress, for example, through magnetic reconnection. These redistributions engender structuring within current sheets, with features such as magnenc flux ropes.
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From page 29... ...
Significant advances in our knowledge of Earth's bow shock and in our theoretical understanding of collisionless shocks were achieved during the 1980s in particular, with the demonstration of the importance of the magnetic field and plasma kinetic effects in shock dissipation.: An important development during this period was the discovery of a population of reflected ions at quasi-perpendicular shocks—that is, shocks where the angle between the solar wind magnetic field and the shock normal is greater than 4s degrees (cf. Figure 3.1)
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From page 30... ...
, 43 1 9 43 24,198 1 Copyrig ht 1 98 1, American Geophysical Union, the shock to satisfy the Rankine-Hugoniot conditions (the mathematical conditions that describe the changes in plasma and magnetic field parameters across discontinuities) , and thus help establish the structure of the shocks.
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From page 31... ...
, Outstanding Questions About Collisionless Shocks . How do strong particle acceleration and associated energetic particles modify shock structurei · How do pickup and other neutral gas interactions modify shock structure and particle acceleration at comets and heliospheric boundary shocksi · How do researchers extrapolate knowledge of shock structure derived from studies of solar system shocks to the strong shocks that prevail in astrophysical systemsi
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From page 32... ...
PLASMA PHYSICS OF THE LOCAL COSMOS ,,;q '"1 COMPARATIVE MAGNETOSPHERES CELLULAR STRUCTURES AND CURRENT SHEETS Cellular Structures Magnetized plasmas in space tend to form cells enclosed by current sheets. Owe find that space occupied by plasmas tends to be divided in 'compartments,' often separated by thin current sheets.
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From page 33... ...
Because of uncertainty with respect to the role of the intergalactic magnetic field and very energetic particles in the LISM, it is currently not known whether the LISM flow is supersonic or subsonic. If supersonic, then, like the solar wind deceleration at a planetary shock, the LISM also decelerates via a shock, and the system is referred to as a two-shock model (i.e., bow shock plus termination shock)
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From page 34... ...
In the Ganymede schematic,the thick line separating dashed (magnetic field lines unconnected to Ganymede) and solid (Ganymede-connected)
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From page 35... ...
. Whether a bow shockforms upstream of the nose of the heliosphere depends on whether the velocity of the LISM relative to the heliosphere is supersonic or subsonic which is not known.The magnetized interstellar plasma is excluded from the heliospheric cavity and flows around it.
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From page 36... ...
The current in current sheets may be oriented either perpendicular to the magnetic field or parallel to it, and the distinction between perpendicular and parallel current sheets is not always a clean one. Three distinct processes produce sheets of perpendicular current, in which the current density I is largely perpendicular to the magnetic field B
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From page 37... ...
Examples of boundary layers formed by structuring are Earth's low-latitude boundary layer and high-latitude mantle. Such boundary layers often have sheared plasma flows, magnetic field-aligned particle streaming, and strong particle pressure gradients extending substantial distances from the current sheet proper Current sheet structuring can also cause some current sheets to periodically or sporadically disappear altogether.
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From page 38... ...
, have recently been observed in the solar wind. These flux ropes differ from magnetic clouds in several respects, and they probably have an origin different from that of magnetic clouds, More than 20 years ago flux ropes were discovered in the ionosphere of Venus Y These were observed as a series of large magnetic field strength enhancements, many with a ratio of maximum to minimum field strength of the order of so.
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From page 39... ...
Roussev et al, A threedimensional flux rope model for coronal mass ejections based on a loss of eq uilibrium, AstrophysicclJourncl 588, L4 5148, 2003. Copyright 2003, American Astronomical Society · Under what conditions are flux ropes stable and unstablei .
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From page 40... ...
It is often difficult to understand the microscopic plasma processes because they result from complicated boundary conditions imposed by the macroscopic system and because they are normally observed in a state of nonlinear saturation, which is difficult to treat theoretically Furthermore, the microscopic process has a controlling influence on the large-scale situation, so that the system must be treated self-consistently. Examples of coherent coupling include magnetic reconnection, the generation of monochromatic Kelvin-Helmholtz waves on the magnetopause, and the generation of quiet auroral arcs from large-scale plasma flows.
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From page 41... ...
. Turbulence has been studied most completely in the solar wind.'2 The characteristic spectra of turbulence are observed over a few decades of scales in solar wind magnetic fields, velocities, densities, and temperatures, and these spectra evolve with distance from the Sun.
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From page 42... ...
Astrophysical shocks are an example of this challenge, Earth's bow shock and a shock at a supernova remnant are distinctly different structures. However, understanding how interplanetary shocks differ from Earth's bow shock and how the heliospheric termination shock differs from interplanetary shocks leads to an understanding of how shock structure is modified when the pressure is dominated by a very energetic particle population.
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From page 43... ...
The x-ray emissions from the jet islands could also be caused by particles accelerated in magnetic reconnection events. Similar island formation occurs in plasma boundary layers in the terrestrial magnetosphere (Figure 3.10)
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From page 44... ...
Even so, common phenomena such as MAD instabilities, shocks, and magnetic reconnection have been invoked to explain the structures in both envi ronments, Despite the vast differences in parameter regimes and boundary conditions that distinguish solar system and astrophysical plasma structures, the underlying plasma physical processes that give rise to and power these structures are the same. U Itimately, a fuller understanding of these processes in a general sense should be obtained with contributions from both space physics and plasma astrophysics.
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From page 45... ...
Priest, and L C Lee, eds , American Geophysical Union, Washington, D C, 1990 9 C T Russell and R C Elyhic, Observation of magnetic flus tones in the Venus ionosphere, Nature 279, 616-61 B
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