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Title: Numerical Study on the Validity of the Taylor Hypothesis in Space Plasmas

Abstract

In situ heliospheric measurements allow us to resolve fluctuations as a function of frequency. A crucial point is to describe the power spectral density as a function of the wavenumber, in order to understand the energy cascade through the scales in terms of plasma turbulence theories. The most favorable situation occurs when the average wind speed is much higher than the phase speed of the plasma modes, equivalent to the fact that the fluctuations’ dynamical times are much longer than their typical crossing period through the spacecraft (frozen-in Taylor approximation). Using driven compressible Hall-magneothydrodynamics simulations, in which an “imaginary” spacecraft flies across a time-evolving turbulence, here we explore the limitations of the frozen-in assumption. We find that the Taylor hypothesis is robust down to sub-proton scales, especially for flows with mean velocities typical of the fast solar wind. For slow mean flows (i.e., speeds of the order of the Alfvèn speed) power spectra are subject to an amplitude shift throughout the scales. At small scales, when dispersive decorrelation mechanisms become significant, the frozen-in assumption is generally violated, in particular for k -vectors almost parallel to the average magnetic field. A discussion in terms of the spacetime autocorrelation function is proposed.more » These results might be relevant for the interpretation of the observations, in particular for existing and future space missions devoted to very high-resolution measurements.« less

Authors:
; ;  [1];  [2]
  1. Dipartimento di Fisica, Università della Calabria, Via P. Bucci, I-87036 Rende (Italy)
  2. Swedish Institute of Space Physics, Uppsala (Sweden)
Publication Date:
OSTI Identifier:
22661111
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal, Supplement Series; Journal Volume: 231; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ALFVEN WAVES; AMPLITUDES; CORRELATIONS; FLUCTUATIONS; FREQUENCY DEPENDENCE; HALL EFFECT; HYPOTHESIS; MAGNETIC FIELDS; NUMERICAL ANALYSIS; PLASMA; PROTONS; RESOLUTION; SOLAR WIND; SPACE VEHICLES; SPACE-TIME; SPECTRAL DENSITY; TURBULENCE; VELOCITY

Citation Formats

Perri, Silvia, Servidio, Sergio, Valentini, Francesco, and Vaivads, Andris, E-mail: silvia.perri@fis.unical.it. Numerical Study on the Validity of the Taylor Hypothesis in Space Plasmas. United States: N. p., 2017. Web. doi:10.3847/1538-4365/AA755A.
Perri, Silvia, Servidio, Sergio, Valentini, Francesco, & Vaivads, Andris, E-mail: silvia.perri@fis.unical.it. Numerical Study on the Validity of the Taylor Hypothesis in Space Plasmas. United States. doi:10.3847/1538-4365/AA755A.
Perri, Silvia, Servidio, Sergio, Valentini, Francesco, and Vaivads, Andris, E-mail: silvia.perri@fis.unical.it. Sat . "Numerical Study on the Validity of the Taylor Hypothesis in Space Plasmas". United States. doi:10.3847/1538-4365/AA755A.
@article{osti_22661111,
title = {Numerical Study on the Validity of the Taylor Hypothesis in Space Plasmas},
author = {Perri, Silvia and Servidio, Sergio and Valentini, Francesco and Vaivads, Andris, E-mail: silvia.perri@fis.unical.it},
abstractNote = {In situ heliospheric measurements allow us to resolve fluctuations as a function of frequency. A crucial point is to describe the power spectral density as a function of the wavenumber, in order to understand the energy cascade through the scales in terms of plasma turbulence theories. The most favorable situation occurs when the average wind speed is much higher than the phase speed of the plasma modes, equivalent to the fact that the fluctuations’ dynamical times are much longer than their typical crossing period through the spacecraft (frozen-in Taylor approximation). Using driven compressible Hall-magneothydrodynamics simulations, in which an “imaginary” spacecraft flies across a time-evolving turbulence, here we explore the limitations of the frozen-in assumption. We find that the Taylor hypothesis is robust down to sub-proton scales, especially for flows with mean velocities typical of the fast solar wind. For slow mean flows (i.e., speeds of the order of the Alfvèn speed) power spectra are subject to an amplitude shift throughout the scales. At small scales, when dispersive decorrelation mechanisms become significant, the frozen-in assumption is generally violated, in particular for k -vectors almost parallel to the average magnetic field. A discussion in terms of the spacetime autocorrelation function is proposed. These results might be relevant for the interpretation of the observations, in particular for existing and future space missions devoted to very high-resolution measurements.},
doi = {10.3847/1538-4365/AA755A},
journal = {Astrophysical Journal, Supplement Series},
number = 1,
volume = 231,
place = {United States},
year = {Sat Jul 01 00:00:00 EDT 2017},
month = {Sat Jul 01 00:00:00 EDT 2017}
}
  • The interpretation of single-point spacecraft measurements of solar wind turbulence is complicated by the fact that the measurements are made in a frame of reference in relative motion with respect to the turbulent plasma. The Taylor hypothesis—that temporal fluctuations measured by a stationary probe in a rapidly flowing fluid are dominated by the advection of spatial structures in the fluid rest frame—is often assumed to simplify the analysis. But measurements of turbulence in upcoming missions, such as Solar Probe Plus, threaten to violate the Taylor hypothesis, either due to slow flow of the plasma with respect to the spacecraft ormore » to the dispersive nature of the plasma fluctuations at small scales. Assuming that the frequency of the turbulent fluctuations is characterized by the frequency of the linear waves supported by the plasma, we evaluate the validity of the Taylor hypothesis for the linear kinetic wave modes in the weakly collisional solar wind. The analysis predicts that a dissipation range of solar wind turbulence supported by whistler waves is likely to violate the Taylor hypothesis, while one supported by kinetic Alfvén waves is not.« less
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  • Magnetic Rayleigh-Taylor instability is addressed in compressible hydrostatic media. A full model is presented and compared to numerical results from a linear perturbation code. A perfect agreement between both approaches is obtained in a wide range of parameters. Compressibility effects are examined and substantial deviations from classical Chandrasekhar growth rates are obtained and confirmed by the model and the numerical calculations.