arXiv subjects

Robert T. Wicks

Publications and source records attributed to Robert T. Wicks.

24 records · Page 2Linked to original sources

arXiv2017

On Kinetic Slow Modes, Fluid Slow Modes, and Pressure-balanced Structures in the Solar Wind

Observations in the solar wind suggest that the compressive component of inertial-range solar-wind turbulence is dominated by slow modes. The low collisionality of the solar wind allows for non-thermal features to survive, which suggests the requirement of a kinetic plasma description. The least-damped kinetic slow mode is associated with the ion-acoustic (IA) wave and a non-propagating (NP) mode. We derive analytical expressions for the IA-wave dispersion relation in an anisotropic plasma in the framework of gyrokinetics and then compare them to fully-kinetic numerical calculations, results from two-fluid theory, and MHD. This comparison shows major discrepancies in the predicted wave phase speeds from MHD and kinetic theory at moderate to high $β$. MHD and kinetic theory also dictate that all plasma normal modes exhibit a unique signature in terms of their polarization. We quantify the relative amplitude of fluctuations in the three lowest particle velocity moments associated with IA and NP modes in the gyrokinetic limit and compare these predictions with MHD results and in-situ observations of the solar-wind turbulence. The agreement between the observations of the wave polarization and our MHD predictions is better than the kinetic predictions, suggesting that the plasma behaves more like a fluid in the solar wind than expected.

physics.space-ph↗

arXiv2015

Spectral Anisotropy of Elsässer Variables in Two Dimensional Wave-vector Space as Observed in the Fast Solar Wind Turbulence

Intensive studies have been conducted to understand the anisotropy of solar wind turbulence. However, the anisotropy of Elsässer variables ($\textbf{Z}^\pm$) in 2D wave-vector space has yet to be investigated. Here we first verify the transformation based on the projection-slice theorem between the power spectral density PSD$_{2D}(k_\parallel,k_\perp )$ and the spatial correlation function CF$_{2D} (r_\parallel,r_\perp )$. Based on the application of the transformation to the magnetic field and the particle measurements from the WIND spacecraft, we investigate the spectral anisotropy of Elsässer variables ($\textbf{Z}^\pm$), and the distribution of residual energy E$_{R}$, Alfvén ratio R$_{A}$ and Elsässer ratio R$_{E}$ in the $(k_\parallel,k_\perp)$ space. The spectra PSD$_{2D}(k_\parallel,k_\perp )$ of $\textbf{B}$, $\textbf{V}$, and $\textbf{Z}_{major}$ (the larger of $\textbf{Z}^\pm$) show a similar pattern that PSD$_{2D}(k_\parallel,k_\perp )$ is mainly distributed along a ridge inclined toward the $k_\perp$ axis. This is probably the signature of the oblique Alfvénic fluctuations propagating outwardly. Unlike those of $\textbf{B}$, $\textbf{V}$, and $\textbf{Z}_{major}$, the spectrum PSD$_{2D}(k_\parallel,k_\perp )$ of $\textbf{Z}_{minor}$ is distributed mainly along the $k_\perp$ axis. Close to the $k_\perp$ axis, $\left| {E}_{R}\right|$ becomes larger while R$_{A}$ becomes smaller, suggesting that the dominance of magnetic energy over kinetic energy becomes more significant at small $k_\parallel$. R$_{E}$ is larger at small $k_\parallel$, implying that PSD$_{2D}(k_\parallel,k_\perp )$ of $\textbf{Z}_{minor}$ is more concentrated along the $k_\perp$ direction as compared to that of $\textbf{Z}_{major}$. The residual energy condensate at small $k_\parallel$ is consistent with simulation results in which E$_{R}$ is spontaneously generated by Alfvén wave interaction.

astro-ph.SR↗

arXiv2014

Permutation Entropy and Statistical Complexity Analysis of Turbulence in Laboratory Plasmas and the Solar Wind

The Bandt-Pompe permutation entropy and the Jensen-Shannon statistical complexity are used to analyze fluctuating time series of three different plasmas: the magnetohydrodynamic (MHD) turbulence in the plasma wind tunnel of the Swarthmore Spheromak Experiment (SSX), drift-wave turbulence of ion saturation current fluctuations in the edge of the Large Plasma Device (LAPD) and fully-developed turbulent magnetic fluctuations of the solar wind taken from the WIND spacecraft. The entropy and complexity values are presented as coordinates on the CH plane for comparison among the different plasma environments and other fluctuation models. The solar wind is found to have the highest permutation entropy and lowest statistical complexity of the three data sets analyzed. Both laboratory data sets have larger values of statistical complexity, suggesting these systems have fewer degrees of freedom in their fluctuations, with SSX magnetic fluctuations having slightly less complexity than the LAPD edge fluctuations. The CH plane coordinates are compared to the shape and distribution of a spectral decomposition of the waveforms. These results suggest that fully developed turbulence (solar wind) occupies the lower-right region of the CH plane, and that other plasma systems considered to be turbulent have less permutation entropy and more statistical complexity. This paper presents the first use of this statistical analysis tool on solar wind plasma, as well as on an MHD turbulent experimental plasma.

physics.plasm-ph↗

arXiv2013

Correlations at large scales and the onset of turbulence in the fast solar wind

We show that the scaling of structure functions of magnetic and velocity fields in a mostly highly Alfvénic fast solar wind stream depends strongly on the joint distribution of the dimensionless measures of cross helicity and residual energy. Already at very low frequencies, fluctuations that are both more balanced (cross helicity $\sim 0$) and equipartitioned (residual energy $\sim 0$) have steep structure functions reminiscent of "turbulent" scalings usually associated with the inertial range. Fluctuations that are magnetically dominated (residual energy $\sim -1$), and so have closely anti-aligned Elsasser-field vectors, or imbalanced (cross helicity $\sim 1$), and so have closely aligned magnetic and velocity vectors, have wide `$1/f$' ranges typical of fast solar wind. We conclude that the strength of nonlinear interactions of individual fluctuations within a stream, diagnosed by the degree of correlation in direction and magnitude of magnetic and velocity fluctuations, determines the extent of the $1/f$ region observed and thus the onset scale for the turbulent cascade.

physics.space-ph↗

arXiv2012

Alignment and scaling of large-scale fluctuations in the solar wind

We investigate the dependence of solar wind fluctuations measured by the Wind spacecraft on scale and on the degree of alignment between oppositely directed Elsasser fields. This alignment controls the strength of the non-linear interactions and, therefore, the turbulence. We find that at scales larger than the outer scale of the turbulence the Elsasser fluctuations become on average more anti-aligned as the outer scale is approached from above. Conditioning structure functions using the alignment angle reveals turbulent scaling of unaligned fluctuations at scales previously believed to lie outside the turbulent cascade in the `1/f range'. We argue that the 1/f range contains a mixture of non-interacting anti-aligned population of Alfvén waves and magnetic force-free structures plus a subdominant population of unaligned cascading turbulent fluctuations.

physics.space-ph↗

arXiv2011

Power Anisotropy in the Magnetic Field Power Spectral Tensor of Solar Wind Turbulence

We observe the anisotropy of the power spectral tensor of magnetic field fluctuations in the fast solar wind for the first time. In heliocentric RTN coordinates the power in each element of the tensor has a unique dependence on the angle between the magnetic field and velocity of the solar wind (θ) and the angle of the vector in the plane perpendicular to the velocity (ϕ). We derive the geometrical effect of the high speed flow of the solar wind past the spacecraft on the power spectrum in the frame of the plasma P(k) to arrive at the observed power spectrum P(f,θ,ϕ) based on a scalar field description of turbulence theory. This allows us to predict the variation in the ϕdirection and compare it to the data. We then transform the observations from RTN coordinates to magnetic-field-aligned coordinates. The observed reduced power spectral tensor matches the theoretical predictions we derive in both RTN and field-aligned coordinates which means that the local magnetic field we calculate with wavelet envelope functions is an accurate representation of the physical axis of symmetry for the turbulence and implies that on average the turbulence is axi-symmetric. We also show that we can separate the dominant toroidal component of the turbulence from the smaller but significant poloidal component and that these have different power anisotropy. We also conclude that the magnetic helicity is anisotropic and mostly two-dimensional, arising from wavevectors largely confined to the plane perpendicular to B.

physics.space-ph↗