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Mar 2013

Volume 20, Issue 3, Articles (03xxxx)

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Phys. Plasmas 20, 032106 (2013); http://dx.doi.org/10.1063/1.4794320 (10 pages)

M. Raghunathan and R. Ganesh
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back to top Ionospheric, Solar-System, and Astrophysical Plasmas

Theory and experiments characterizing hypervelocity impact plasmas on biased spacecraft materials

Nicolas Lee, Sigrid Close, Ashish Goel, David Lauben, Ivan Linscott, Theresa Johnson, David Strauss, Sebastian Bugiel, Anna Mocker, and Ralf Srama

Phys. Plasmas 20, 032901 (2013); http://dx.doi.org/10.1063/1.4794331 (9 pages)

Online Publication Date: 4 March 2013

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Space weather including solar activity and background plasma sets up spacecraft conditions that can magnify the threat from hypervelocity impacts. Hypervelocity impactors include both meteoroids, traveling between 11 and 72 km/s, and orbital debris, with typical impact speeds of 10 km/s. When an impactor encounters a spacecraft, its kinetic energy is converted over a very short timescale into energy of vaporization and ionization, resulting in a small, dense plasma. This plasma can produce radio frequency (RF) emission, causing electrical anomalies within the spacecraft. In order to study this phenomenon, we conducted ground-based experiments to study hypervelocity impact plasmas using a Van de Graaff dust accelerator. Iron projectiles ranging from 10−16 g to 10−11 g were fired at speeds of up to 70 km/s into a variety of target materials under a range of surface charging conditions representative of space weather effects. Impact plasmas associated with bare metal targets as well as spacecraft materials were studied. Plasma expansion models were developed to determine the composition and temperature of the impact plasma, shedding light on the plasma dynamics that can lead to spacecraft electrical anomalies. The dependence of these plasma properties on target material, impact speed, and surface charge was analyzed. Our work includes three major results. First, the initial temperature of the impact plasma is at least an order of magnitude lower than previously reported, providing conditions more favorable for sustained RF emission. Second, the composition of impact plasmas from glass targets, unlike that of impact plasmas from tungsten, has low dependence on impact speed, indicating a charge production mechanism that is significant down to orbital debris speeds. Finally, negative ion formation has a strong dependence on target material. These new results can inform the design and operation of spacecraft in order to mitigate future impact-related space weather anomalies and failures.
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52.70.Gw Radio-frequency and microwave measurements
52.25.Tx Emission, absorption, and scattering of particles
52.50.Qt Plasma heating by radio-frequency fields; ICR, ICP, helicons
52.65.-y Plasma simulation

Effects of ion-neutral collisions on Alfvén waves: The presence of forbidden zone and heavy damping zone

C. J. Weng, L. C. Lee, C. L. Kuo, and C. B. Wang

Phys. Plasmas 20, 032902 (2013); http://dx.doi.org/10.1063/1.4796043 (10 pages)

Online Publication Date: 21 March 2013

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Alfvén waves are low-frequency transverse waves propagating in a magnetized plasma. We define the Alfvén frequency ω0 as ω0 = kVAcosθ, where k is the wave number, VA is the Alfvén speed, and θ is the angle between the wave vector and the ambient magnetic field. There are partially ionized plasmas in laboratory, space, and astrophysical plasma systems, such as in the solar chromosphere, interstellar clouds, and the earth ionosphere. The presence of neutral particles may modify the wave frequency and cause damping of Alfvén waves. The effects on Alfvén waves depend on two parameters: (1) α = nn/ni, the ratio of neutral density (nn), and ion density (ni); (2) β = νni/ω0, the ratio of neutral collisional frequency by ions νni to the Alfvén frequency ω0. Most of the previous studies examined only the limiting case with a relatively large neutral collisional frequency or β≫1. In the present paper, the dispersion relation for Alfvén waves is solved for all values of α and β. Approximate solutions in the limit β≫1 as well as β≪1 are obtained. It is found for the first time that there is a “forbidden zone (FZ)” in the α−β parameter space, where the real frequency of Alfvén waves becomes zero. We also solve the wavenumber k from the dispersion equation for a fixed frequency and find the existence of a “heavy damping zone (HDZ).” We then examine the presence of FZ and HDZ for Alfvén waves in the ionosphere and in the solar chromosphere.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.25.Fi Transport properties
52.25.Ya Neutrals in plasmas
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.72.+v Laboratory studies of space- and astrophysical-plasma processes
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Model of magnetic reconnection in space and astrophysical plasmas

Allen H. Boozer

Phys. Plasmas 20, 032903 (2013); http://dx.doi.org/10.1063/1.4796051 (12 pages)

Online Publication Date: 26 March 2013

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Maxwell's equations imply that exponentially smaller non-ideal effects than commonly assumed can give rapid magnetic reconnection in space and astrophysical plasmas. In an ideal evolution, magnetic field lines act as stretchable strings, which can become ever more entangled but cannot be cut. High entanglement makes the lines exponentially sensitive to small non-ideal changes in the magnetic field. The cause is well known in popular culture as the butterfly effect and in the theory of deterministic dynamical systems as a sensitive dependence on initial conditions, but the importance to magnetic reconnection is not generally recognized. Two-coordinate models are too constrained geometrically for the required entanglement, but otherwise the effect is general and can be studied in simple models. A simple model is introduced, which is periodic in the x and y Cartesian coordinates and bounded by perfectly conducting planes in z. Starting from a constant magnetic field in the z direction, reconnection is driven by a spatially smooth, bounded force. The model is complete and could be used to study the impulsive transfer of energy between the magnetic field and the ions and electrons using a kinetic plasma model.
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52.35.Vd Magnetic reconnection
52.25.Dg Plasma kinetic equations
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
95.30.Qd Magnetohydrodynamics and plasmas
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