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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Thermodynamics and collisionality in firehose-susceptible high- β plasmas

We study the evolution of collisionless plasmas that, due to their macroscopic evolution, are susceptible to the firehose instability, using both analytic theory and hybrid-kinetic particle-in-cell simulations. We establish that, depending on the relative magnitude of the plasma β, the characteristic time scale of macroscopic evolution and the ion-Larmor frequency, the saturation of the firehose instability in high-β plasmas can result in three qualitatively distinct thermodynamic (and electromagnetic) states. By contrast with the previously identified ‘ultra-high-beta’ and ‘Alfvén-inhibiting’ states, the newly identified ‘Alfvén-enabling’ state, which is realised when the macroscopic evolution time τ exceeds the ion-Larmor frequency by a β-dependent critical parameter, can support linear Alfvén waves and Alfvénic turbulence because the magnetic tension associated with the plasma’s macroscopic magnetic field is never completely negated by anisotropic pressure forces. We characterise these states in detail, including their saturated magnetic-energy spectra. The effective collision operator associated with the firehose fluctuations is also described; we find it to be well approximated in the Alfvén-enabling state by a simple quasi-linear pitch-angle scattering operator. The box-averaged collision frequency is ν eff ∼ β/τ, in agreement with previous results, but certain subpopulations of particles scatter at a much larger (or smaller) rate depending on their velocity in the direction parallel to the magnetic field. Our findings are essential for understanding low-collisionality astrophysical plasmas including the solar wind, the intracluster medium of galaxy clusters and black hole accretion flows. We show that all three of these plasmas are in the Alfvén-enabling regime of firehose saturation and discuss the implications of this result.

astrophysical plasmas↗
NASA NTRS1981

Linear theory of tearing in a high-beta plasma

The linear dispersion relation for a collisionless plasma in a sheared one-dimensional current sheet is calculated with reference to conditions in the daytime magnetopause. Calculations are extended to include plasmas with beta approximately equal to 1. It is found that the tearing mode eigenstructure and temporal growth rate are a sensitive function of the ratios l sub s/l sub G, l sub s/l sub T, and l sub s/l sub n, where l sub s is the shearing length of the magnetic field, l sub G is the gradient scale length, and l sub T is the temperature gradient scale length. In particular, if beta is approximately equal to 1, and l sub s is less than l sub G, l sub n, and l sub T, then the thickness of the layer over which particles are resonantly accelerated by the induction magnetic field is approximately rho, a thermal gyroradius. If the above conditions are not satisfied, plasma gradients may electrostatically stabilize the mode.

Quest, K. B.↗
NASA NTRS1997

Ponderomotive Force and Lower Hybrid Turbulence Effects in Space Plasmas Subjected to Large-Amplitude Low-Frequency Waves

In the auroral region, simultaneous occurrences of upward-flowing ions and field-aligned electrons have been observed by the Viking satellite. The occurrence is strongly correlated with large amplitude low frequency fluctuations of the electric field. Large-amplitude shear Alfven waves have also been observed by sounding rockets in the auroral ionosphere. When such LF waves are propagating in a plasma, a ponderomotive force and other types of waves are produced which may lead to significant effects on the plasma. This force is directed toward decreasing density, providing the electromagnetic lift of the background plasma and an increase of collisionless plasma expansion. We find that even for modest wave strengths, the influence on the outflowing oxygen ions can be dramatic, increasing the high-altitude density by orders of magnitude. It is also demonstrated that large-amplitude low-frequency waves (LFW) may generate lower hybrid waves (LHW) in the auroral zone. The excitation of LHW by a LF wave may lead to the appearance of an additional channel of energy transfer from, for example, Alfven or fast magnetosonic waves, to particles. This process then influences the formation of the plasma distribution function at the expense of acceleration in the tail of the distribution during the collapse of the LHW. The ion energization due to the LHW can be comparable with that produced by the ponderomotive force of the LFW. It is shown that the LH turbulence leads to equalization of the ponderomotive acceleration of the different ion species. The mechanism of LHW excitation due to the oxygen ion relative drift in a plasma subjected to low-frequency waves is used for analysis of Viking satellite data for events in the cusp/cleft region. It is found that, in some cases, such a mechanism leads to LHW energy densities and ion distribution functions close to those observed.

Khazanov, G. V.↗
NASA NTRS2023

Reconnection in a Pinch

A recently published analysis of current sheets has updated the classic Harris 1D static solution by considering multiple classes of charged particle trajectories in a generalized and dynamic current sheet. It uses a 1D PIC simulation to describe dynamic pinching and bifurcation of the current sheet. These 1D results strongly suggest that properties of the inflowing plasma, including the plasma beta, have an important effect on the equilibrium thickness of the pinched current sheet. Since 1D studies cannot describe magnetic reconnection, the time appears right to carry such 1D studies over to 2D or 3D simulations to explore current sheet thickness effects on reconnection. The Magnetospheric Multiscale Mission (MMS), with its well-resolved multipoint measurements of collisionless plasma and fields, has found that collisionless reconnection is accompanied by non-adiabatic motions of electrons that only occur in magnetic structures with a narrow scale comparable to electron inertial lengths (d e ). The recent 1D studies suggest that a plasma pinch to such scales may only occur for inflowing magnetized plasmas with relatively low plasma beta. We conclude that a parametric exploration of simulated and observed reconnection inflow conditions, particularly plasma beta, should shed light on the enablement of reconnection in collisionless plasmas.

reconnection↗
NASA NTRS1972

Approximations to the plasma dispersion function

Linear wave propagation in hot collisionless plasmas is described by the linearized Vlasov and Maxwell equations. In uniform media, the utilization of spatial and temporal transforms of those equations leads to the consideration of integrals of the Hilbert transform type. Analysis and comparison of two simple approximations are provided based on the utilization of resonance velocity distributions. Application is then made to the Landau and whistler waves, along with a discussion of the results, and commentary on possible improvements.

Brinca, A. L.↗
NASA NTRS1975

Laboratory plasma probe studies

Diagnostic experiments performed in a collisionless plasma using CO2 as the working gas are described. In particular, simultaneous measurements that have been performed by means of Langmuir- and RF-probes are presented. A resonance occurring above the parallel resonance in the frequency characteristic of a two electrode system is interpreted as being due to the resonant excitation of electroacoustic waves.

Heikkila, W. J.↗
NASA NTRS1992

Correlation function ratios and the identification of space plasma instabilities

Wave-particle transport in a collisionless plasma is due to particle scattering by enhanced fluctuations associated with the growth of instabilities. In particular, relatively short wave-length kinetic instabilities are frequently invoked to explain many different types of plasma transport in space. Although there is an extensive theoretical and simulation literature describing the potential applications of many such instabilities, there are only a few cases of clear-cut identification of kinetic modes in space. The research described in this paper uses linear Vlasov dispersion theory to study correlation functions and dimensionless correlation function ratios for fluctuations or instabilities in three space plasma regimes. This research shows that both the compressibility and the parallel compressibility are likely to be useful in distinguishing modes in the magnetosheath as well as in the plasma sheet boundary layer and that helicity remains a useful identifier of electromagnetic ion/ion instabilities in the foreshock.

Gary, S. P.↗
NASA NTRS1997

An Analysis of Closure Mechanisms in the Plasma Wake of the TSS-1R Satellite

Collisionless Plasma Expansion (CPE), also known as "plasma expansion into a vacuum," in its most fundamental form, is the process by which a plasma expands into a void, or region highly depleted of particles. In CPE, the expansion is driven by the highly mobile electron constituent of the plasma as it moves across the density gradient and into the void region. A bi-polar electric field is set up between this rapidly expanding electron front and the massive ions, which have a low thermal speed and lag behind in the region of the plasma-void interface. These ions are quickly accelerated by the expansion electric field and can, theoretically, approach the thermal speed of the electrons. CPE is a fundamental and a very robust process in that it only requires that a sharp density gradient exist in a plasma. It has already been observed to exist in various plasmas ranging over five orders of magnitude in density. The range of phenomena in which CPE has been observed includes the closure of plasma wakes created in simulated space plasmas in the laboratory; it was found to be the process controlling the refilling of the wake created by the Space Shuttle in the ionospheric plasma; and recent in situ data, obtained by the WIND spacecraft as it passed through the wake of the earth's moon last year, have shown CPE to be involved in the closure of the Lunar wake. CPE is also expected to influence the morphology and physics of the solar wind interactions with Mercury and Mars, and has been suggested as a potential factor in such wide ranging phenomena as the acceleration of plasma away from the coma of comets and the energization of ions that underlies the upward expansion of the polar wind into the earth's magnetosphere. The Tethered Satellite System (TSS)-1R data provides an opportunity to test this process in situ under semi-controlled conditions. The available data shows the intensity, drift energy and angle of inclination to the wake of the converging ion streams at a down stream distance of one satellite radius (80 CM).

Stone, N. H.↗
NASA NTRS1975

Plasma in the Jovian magnetosphere

It is shown that the plasma in Jupiter's ionosphere is collisionless above a certain level. In the outer magnetosphere, where the rotational force dominates the gravitational force, the collisionless plasma has a beam-like distribution and gives rise to a two-stream instability. This leads to trapping of plasma in the centrifugally dominated region of the magnetosphere. Plasma is lost by recombination. Equilibrium-trapped particle densities are calculated by requiring a balance between trapping by wave-particle interaction and loss by recombination. The results are compared with recent observations from Pioneer 10. It is suggested that the observations require an unexplained ion-heating mechanism. Some consequences of the model are discussed.

Goertz, C. K.↗
NASA NTRS1976

Plasma in the Jovian magnetosphere

The plasma in Jupiter's ionosphere is collisionless above a certain level. In the outer magnetosphere, where the rotational force dominates the gravitational force, the collisionless plasma has a beamlike distribution and gives rise to a two-stream instability. This leads to trapping of plasma in the centrifugally dominated region of the magnetosphere. Plasma is lost through recombination. The equilibrium concentration of trapped particles is calculated by assuming a balance between trapping by wave-particle interaction and loss by recombination. The results are compared with recent observations from Pioneer 10. The observations appear to require an unexplained ion-heating mechanism.

Goertz, C. K.↗