arXiv subjects

Kristopher G. Klein

Publications and source records attributed to Kristopher G. Klein.

76 records · Page 5Linked to original sources

arXiv2015

Predicted Impacts of Proton Temperature Anisotropy on Solar Wind Turbulence

Particle velocity distributions measured in the weakly collisional solar wind are frequently found to be non-Maxwellian, but how these non-Maxwellian distributions impact the physics of plasma turbulence in the solar wind remains unanswered. Using numerical solutions of the linear dispersion relation for a collisionless plasma with a bi-Maxwellian proton velocity distribution, we present a unified framework for the four proton temperature anisotropy instabilities, identifying the associated stable eigenmodes, highlighting the unstable region of wavevector space, and presenting the properties of the growing eigenfunctions. Based on physical intuition gained from this framework, we address how the proton temperature anisotropy impacts the nonlinear dynamics of the \Alfvenic fluctuations underlying the dominant cascade of energy from large to small scales and how the fluctuations driven by proton temperature anisotropy instabilities interact nonlinearly with each other and with the fluctuations of the large-scale cascade. We find that the nonlinear dynamics of the large-scale cascade is insensitive to the proton temperature anisotropy, and that the instability-driven fluctuations are unlikely to cause significant nonlinear evolution of either the instability-driven fluctuations or the turbulent fluctuations of the large-scale cascade.

physics.space-ph↗

arXiv2015

A Modified Version of Taylor's Hypothesis for Solar Probe Plus Observations

The Solar Probe Plus (SPP) spacecraft will explore the near-Sun environment, reaching heliocentric distances less than $10 R_{\odot}$. Near Earth, spacecraft measurements of fluctuating velocities and magnetic fields taken in the time domain are translated into information about the spatial structure of the solar wind via Taylor's "frozen turbulence" hypothesis. Near the perihelion of SPP, however, the solar-wind speed is comparable to the Alfvén speed, and Taylor's hypothesis in its usual form does not apply. In this paper, we show that, under certain assumptions, a modified version of Taylor's hypothesis can be recovered in the near-Sun region. We consider only the transverse, non-compressive component of the fluctuations at length scales exceeding the proton gyroradius, and we describe these fluctuations using an approximate theoretical framework developed by Heinemann and Olbert. We show that fluctuations propagating away from the Sun in the plasma frame obey a relation analogous to Taylor's hypothesis when $V_{\rm sc,\perp} \gg z^-$ and $z^+ \gg z^-$, where $V_{\rm sc,\perp}$ is the component of the spacecraft velocity perpendicular to the mean magnetic field and $\bm{z}^+$ ($\bm{z}^-$) is the Elsasser variable corresponding to transverse, non-compressive fluctuations propagating away from (towards) the Sun in the plasma frame. Observations and simulations suggest that, in the near-Sun solar wind, the above inequalities are satisfied and $\bm{z}^+$ fluctuations account for most of the fluctuation energy. The modified form of Taylor's hypothesis that we derive may thus make it possible to characterize the spatial structure of the energetically dominant component of the turbulence encountered by SPP.

physics.space-ph↗

arXiv2014

The Quasilinear Premise for the Modeling of Plasma Turbulence

The quasilinear premise is a hypothesis for the modeling of plasma turbulence in which the turbulent fluctuations are represented by a superposition of randomly-phased linear wave modes, and energy is transferred among these wave modes via nonlinear interactions. We define specifically what constitutes the quasilinear premise, and present a range of theoretical arguments in support of the relevance of linear wave properties even in a strongly turbulent plasma. We review evidence both in support of and in conflict with the quasilinear premise from numerical simulations and measurements of plasma turbulence in the solar wind. Although the question of the validity of the quasilinear premise remains to be settled, we suggest that the evidence largely supports the value of the quasilinear premise in modeling plasma turbulence and that its usefulness may also be judged by the insights gained from such an approach, with the ultimate goal to develop the capability to predict the evolution of any turbulent plasma system, including the spectrum of turbulent fluctuations, their dissipation, and the resulting plasma heating.

astro-ph.SR↗

arXiv2014

Physical Interpretation of the Angle Dependent Magnetic Helicity Spectrum in the Solar Wind: The Nature of Turbulent Fluctuations near the Proton Gyroradius Scale

Motivated by recent observations of distinct parallel and perpendicular signatures in magnetic helicity measurements segregated by wave period and angle between the local magnetic field and the solar wind velocity, this paper undertakes a comparison of three intervals of \emph{Ulysses} data with synthetic time series generated from a physically motivated turbulence model. From these comparisons, it is hypothesized that the observed signatures result from a perpendicular cascade of \Alfvenic fluctuations and a local, non-turbulent population of ion cyclotron or whistler waves generated by temperature anisotropy instabilities. By constraining the model's free parameters through comparison to \emph{in situ} data, it is found that, on average, $ \sim 95\%$ of the power near dissipative scales is contained in a perpendicular \Alfvenic cascade and that the parallel fluctuations are propagating nearly unidirectionally. The effects of aliasing on magnetic helicity measurements are considered and shown to be significant near the Nyquist frequency.

physics.plasm-ph↗