Plasma Science Advancing Knowledge in the National Interest (2007) / Chapter Skim
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5 Space and Astrophysical Plasmas
5 Space and Astrophysical Plasmas
Pages 152-183
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From page 152... ...
What is the nature of our own solar system and its planetary plasma environment? And what is the nature of the extreme plasma environment around black holes?
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From page 153... ...
But while plasma physics can be considered a tool for space physics and astrophysics, the relationship is increas ingly a two-way street. Space and astrophysical observations uncover dramatic and exotic new plasma physics regimes for study and provide detailed data to illuminate fundamental plasma processes.
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From page 154... ...
On astrophysical scales, the range of physical conditions is even more extreme, with temperatures reaching ~109 K around some black holes and neutron stars and gas densities as small as ~10–4 particles per cubic centimeter in the space between
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From page 155... ...
. Magnetic field strengths range from ~1015 G for the most strongly magnetized neutron stars (a billion times greater than the largest sustained laboratory magnetic fields)
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From page 156... ...
The origin of magnetic fields in such objects (galaxies and moons) is one of the central puzzles in plasma astrophysics and space physics.
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From page 157... ...
Given the complexities of this problem, most research to date has ignored the magnetic field and the fact that most of the matter in the universe is an ionized plasma. If, however, the field was formed early in the evolution of the universe (either primordially or by the first generation of stars and black holes)
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From page 158... ...
Plasma and magnetic fields can also be ejected from the galaxy to form a galactic corona, analogous to the solar corona. Dense magnetized clouds of weakly ionized plasma in the ISM are often the sites of intense star formation, as clumps of gas collapse under their own gravita tional pull.
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From page 159... ...
Plasmas and Magnetic Fields in Accretion Disks The inflow (accretion) of matter toward a central gravitating object is one of the most ubiquitous processes in astrophysics and is responsible for forming much of the structure in the universe.
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From page 160... ...
Under certain conditions, the plasma flowing onto a black hole or a neutron star can be so hot and tenuous that the collisional mean free path greatly exceeds the size of the system, much like the solar wind. Initial progress has been made on FIGURE 5.4 The inner regions of an accretion disk around a black hole, as calculated in a general relativistic MHD numerical simulation.
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From page 161... ...
Surrounding the star is a magnetized plasma environment -- for example, the Sun has a hot plasma corona and farther out the solar wind. Loops of magnetic field emerge from the Sun's surface (Figure 5.1)
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From page 162... ...
Thus understanding how magnetic fields are generated, transported, and dissipated is a fundamental problem in basic plasma science and of great impor tance for describing magnetospheres. Three questions dominate current research: magnetic reconnection at boundaries, Alfvénic coupling and transport across magnetospheric regions, and planetary dynamos.
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From page 163... ...
minimum Wind ˆ ˆ ˆ (9 x, -321 y, 16 z) Jet FIGURE 5.5 Studying magnetic reconnection with spacecraft.
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From page 164... ...
Without con tinuing cooperation between laboratory and space plasma scientists it is doubtful that this problem can be solved. Alfvénic Coupling and Transport. Magnetic field lines emanating from Earth's core pass through the neutral atmosphere to the ionosphere (a partially ionized plasma layer)
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From page 165... ...
(a) Measured instantaneous shear Alfvén wave magnetic field pattern (colored surface)
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From page 166... ...
How Are Particles Accelerated Throughout the Universe? It is a remarkable observational fact that most astrophysical and space plasmas contain a significant population of highly energetic particles (particles with ener gies well above the typical thermal energy of the system)
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From page 167... ...
The field lines are drawn out to two Earth radii. Simulations such as this one have successfully produced spontaneous reversals of a dipole magnetic field similar to those inferred from Earth's paleomagnetic record.
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From page 168... ...
The latter particles, dubbed ultra-high-energy cos mic rays, have energies similar to that of a baseball and thus pack quite a punch! FIGURE 5.8 The spectrum of cosmic rays as detected on Earth (number of cosmic rays of a given energy reaching Earth as a function of energy)
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From page 169... ...
This idea is at the heart of two of the primary models for particle acceleration in space and astrophysical plasmas: dif fusive shock acceleration and acceleration by plasma turbulence. It is generally believed that galactic cosmic rays between 1016 and 1018 eV originate in supernova shocks in the ISM.
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From page 170... ...
Particle Acceleration by Reconnection As discussed in Chapter 1, magnetic reconnection converts magnetic energy at large spatial scales to fast plasma flows and energetic electrons and ions. Satel lite measurements during solar flares have provided a wealth of evidence that a substantial fraction of the released energy is channeled into energetic electrons and ions.
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From page 171... ...
(a) Electron temperature during magnetic reconnection in a configuration with two adjacent current layers and an initial ambient out-of-plane magnetic field.
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From page 172... ...
Auroral Acceleration Earth's aurora provides a nearby natural plasma physics laboratory for the study of parallel electric field formation, with applications to other magnetized planets such as Jupiter or to any object with strongly convergent magnetic fields, such as pulsar magnetospheres or astrophysical jets from active galactic nuclei. The plasma processes responsible for and caused by these parallel electric fields proceed on microscopic scales far below the mean free path and many orders of magnitude below any resolvable astronomical scales.
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From page 173... ...
(a) Energetic electron distribution function contours perpendicular and parallel to the magnetic field from FAST.
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From page 174... ...
In the last decade, direct simulation techniques have been ap plied to the relativistic shock problem, for shocks both with and without upstream magnetic fields (see, for example, Figure 5.11)
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From page 175... ...
The majority of this cold gas is neutral atomic or molecular material that only indirectly feels the effects of the ambient electric and magnetic fields, via collisions with the comparatively rare ionized matter. One specific context in which these plasma physics issues have been extensively studied is the accretion disks present in sites of star and planet forma tion.
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From page 176... ...
The accretion disks out of which planets form are believed to be only weakly ionized (Figure 5.12)
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From page 177... ...
Grain fluxes in the outer heliosphere are an order of magnitude higher than in the inner heliosphere. Some of the unsolved problems regarding the interaction between interstellar dust and the heliosphere are the following: • Understand the charging, filtration, and deflection of small charged grains as the grains cross the bow shock in the outer heliosheath regions and enter the heliosphere.
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From page 178... ...
These issues are of great importance in understanding possible changes in our atmosphere. Indeed, predictive modeling of the mesosphere requires a better understanding of the plasma science.
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From page 179... ...
As an indication of the importance of plasma science to space and astrophysics, note that many of the highly recommended ground-based and
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From page 180... ...
study of the magnetism and instabilities of the Sun Solar Dynamics Observatory Solar magnetic fields, dynamo, variability Interstellar Boundary Explorer (IBEX) Exploring boundary with ISM Magnetospheric Multiscale (MMS)
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From page 181... ...
Indeed, some compelling research questions in plasmas physics will be best answered by research in space and astrophysical contexts. This chapter presents examples of where space and astrophysical observations have led to new a understanding of basic plasma physics processes, including fast reconnection, dusty plasma interactions, and high-energy particle acceleration.
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From page 182... ...
Recommendation: Agency coordination mechanisms such as the Physics of the Universe Interagency Working Group and the Astronomy and Astro physics Advisory Committee should explicitly include plasma physics when they coordinate research in laboratory, space, and astrophysical plasma sci ence. Such coordination would be greatly facilitated by improved steward ship of laboratory plasma science by DOE's Office of Science.
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From page 183... ...
On the other hand it would not be desirable to separate space physics and astrophysics plasma research from their broader context in space and astrophysics. Although the committee was not charged with conducting a comprehensive review of the federal solar and space physics research portfolio, it is important to note that the above recommendation has significant overlap with the recommen dations of NRC's Solar and Space Physics Survey Committee for its 2003 report The Sun to the Earth -- and Beyond: A Decadal Research Strategy in Solar and Space Physics. In other words, the traditional space and astrophysics communities and the traditional plasma science community have identified enhanced federal coor dination as a key action item.
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