Forward Modeling of Reduced Power Spectra From Three-Dimensional K-Space

Papen, Michael von; Saur, Joachim

doi:10.1088/0004-637x/806/1/116

Cited by 11 publications

(13 citation statements)

References 61 publications

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“…We have not measured intermittency in our datasets, and it seem unlikely that the selection criterion limits intermittency in the selected intervals. Instead, our results are consistent with a small α resulting from a non negligible contribution of the slab component (von Papen & Saur 2015). Such component is mostly observed in fast wind streams with Alfvénic fluctuations, a population that is more abundant in the strongexpansion dataset, which actually displays a flatter parallel SF than for weak expansion.…”

Section: Summary and Discussionsupporting

confidence: 85%

Three-dimensional local anisotropy of velocity fluctuations in the solar wind

Verdini

Grappin

Alexandrova

et al. 2019

Monthly Notices of the Royal Astronomical Society

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We analyse velocity fluctuations in the solar wind at magneto-fluid scales in two datasets, extracted from Wind data in the period 2005-2015, that are characterised by strong or weak expansion. Expansion affects measurements of anisotropy because it breaks axisymmetry around the mean magnetic field. Indeed, the small-scale threedimensional local anisotropy of magnetic fluctuations (δB) as measured by structure functions (SF B ) is consistent with tube-like structures for strong expansion. When passing to weak expansion, structures become ribbon-like because of the flattening of SF B along one of the two perpendicular directions. The power-law index that is consistent with a spectral slope −5/3 for strong expansion now becomes closer to −3/2. This index is also characteristic of velocity fluctuations in the solar wind. We study velocity fluctuations (δV) to understand if the anisotropy of their structure functions (SF V ) also changes with the strength of expansion and if the difference with the magnetic spectral index is washed out once anisotropy is accounted for. We find that SF V is generally flatter than SF B . When expansion passes from strong to weak, a further flattening of the perpendicular SF V occurs and the small-scale anisotropy switches from tube-like to ribbon-like structures. These two types of anisotropy, common to SF V and SF B , are associated to distinct large-scale variance anisotropies of δB in the strong-and weak-expansion datasets. We conclude that SF V show anisotropic threedimensional scaling similar to SF B , with however systematic flatter scalings, reflecting the difference between global spectral slopes.

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Section: Summary and Discussionsupporting

confidence: 85%

Three-dimensional local anisotropy of velocity fluctuations in the solar wind

Verdini

Grappin

Alexandrova

et al. 2019

Monthly Notices of the Royal Astronomical Society

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“…Modeling results by von Papen and Saur [] have shown that the spectral characteristics of magnetic fluctuations in Saturn's magnetosphere are consistent with a strong turbulent cascade of kinetic Alfvén waves (KAW) [ Schekochihin et al , ]. Note that strong turbulence does not refer to the amplitude of the magnetic field fluctuations but to the concept that the characteristic timescale for nonlinear interactions of eddies is the same as for linear wave‐wave interactions of KAW.…”

Section: Resultsmentioning

confidence: 99%

“…Turbulent magnetic fluctuations are omnipresent in the solar wind [see, e.g., Alexandrova et al , , and references therein] and are also observed in planetary magnetospheres such as Earth (see, e.g., reviews by Borovsky and Funsten [] and Zimbardo et al []), Jupiter [ Saur et al , , ; Tao et al , ], Saturn [ von Papen et al , ; von Papen and Saur , ], and Mercury [ Uritsky et al , ]. Turbulence can play an important role for the energy budget of the planetary magnetospheres as it forms a turbulent cascade, which transports energy through nonlinear interactions from large to small scales, where it is dissipated.…”

Section: Introductionmentioning

confidence: 99%

Longitudinal and local time asymmetries of magnetospheric turbulence in Saturn's plasma sheet

Papen

Saur

2016

JGR Space Physics

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Based on earlier studies that have shown Saturn's middle magnetosphere to contain turbulent magnetic field fluctuations, we analyze the spatial and temporal variations of the magnetic fluctuations and turbulent heating rate as a function of local time and magnetic phase. The region of study is Saturn's plasma sheet at a distance of 6–20 Rs, where Rs is Saturn's equatorial radius. The data set consists of magnetic field data measured during 92 orbits (revolutions) from the equatorial phases of Cassini covering 9 years from 2004 to 2012. We find asymmetries in the magnetosphere with enhanced fluctuations around noon. With respect to longitude we find increased fluctuations at 65° southern and 250° northern magnetic phase. This leads to an increased turbulent heating rate in these regions and is consistent with regions of increased plasma density and maximum downward field‐aligned currents according to the quasi‐dipolar perturbation fields. Analysis of single orbits reveals that the heating rate of 79% of all analyzed inbound and outbound legs is significantly (statistical error less than 1%) sinusoidally modulated. The modulation of the turbulent heating rate is predominantly observed during times when Cassini is located between dusk and midnight and additionally at dawn.

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“…The only assumptions in this methodology are that the turbulence is Alfvénic and strong (in the critically balance sense), and that the sampling direction is sufficiently oblique that it can be considered nearly perpendicular to the field. Solar wind observations have been found to be largely consistent with a critically-balanced nonlinear cascade and its associated spectral anisotropy, see for instance (Horbury et al 2008;Chen et al 2011;von Papen & Saur 2015). Under these conditions, the TH hypothesis is expected to remain as a good approximation as long as 0.5 ).…”

Section: Discussionmentioning

confidence: 93%

Applicability of Taylor’s hypothesis during Parker Solar Probe perihelia

Perez

Bourouaine

Chen

et al. 2021

A&A

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We investigate the validity of Taylor’s hypothesis (TH) in the analysis of velocity and magnetic field fluctuations in Alfvénic solar wind streams measured by Parker Solar Probe (PSP) during the first four encounters. The analysis is based on a recent model of the spacetime correlation of magnetohydrodynamic (MHD) turbulence, which has been validated in high-resolution numerical simulations of strong reduced MHD turbulence. We use PSP velocity and magnetic field measurements from 24 h intervals selected from each of the first four encounters. The applicability of TH is investigated by measuring the parameter ϵ = δu0/√2V⊥, which quantifies the ratio between the typical speed of large-scale fluctuations, δu0, and the local perpendicular PSP speed in the solar wind frame, V⊥. TH is expected to be applicable for ϵ ≲ 0.5 when PSP is moving nearly perpendicular to the local magnetic field in the plasma frame, irrespective of the Alfvén Mach number MA = VSW∕VA, where VSW and VA are the local solar wind and Alfvén speed, respectively. For the four selected solar wind intervals, we find that between 10 and 60% of the time, the parameter ϵ is below 0.2 and the sampling angle (between the spacecraft velocity in the plasma frame and the local magnetic field) is greater than 30°. For angles above 30°, the sampling direction is sufficiently oblique to allow one to reconstruct the reduced energy spectrum E(k⊥) of magnetic fluctuations from its measured frequency spectra. The spectral indices determined from power-law fits of the measured frequency spectrum accurately represent the spectral indices associated with the underlying spatial spectrum of turbulent fluctuations in the plasma frame. Aside from a frequency broadening due to large-scale sweeping that requires careful consideration, the spatial spectrum can be recovered to obtain the distribution of fluctuation’s energy across scales in the plasma frame.

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Forward Modeling of Reduced Power Spectra From Three-Dimensional K-Space

Cited by 11 publications

References 61 publications

Three-dimensional local anisotropy of velocity fluctuations in the solar wind

Three-dimensional local anisotropy of velocity fluctuations in the solar wind

Longitudinal and local time asymmetries of magnetospheric turbulence in Saturn's plasma sheet

Applicability of Taylor’s hypothesis during Parker Solar Probe perihelia

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