Anisotropy and Alfvénicity of hourly fluctuations in the fast polar solar wind

Neugebauer, M.

doi:10.1029/2003ja009947

Cited by 17 publications

(8 citation statements)

References 26 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…When determined according to this rule, the overall distribution of sunward and antisunward propagation in our data set is about 50-50, a result that is in disagreement with reports in the literature (e.g., Neugebauer, 2004) to the effect that propagation is usually away from the sun. In none of our events is the propagation sense, determined in this manner, unambiguously the same for all four spacecraft.…”

Section: Discussioncontrasting

confidence: 55%

More about arc-polarized structures in the solar wind

2012

View full text Add to dashboard Cite

Abstract.We report results from a Cluster-based study of the properties of 28 arc-polarized magnetic structures (also called rotational discontinuities) in the solar wind. These Alfvénic events were selected from the database created and analyzed by Knetter (2005) by use of criteria chosen to eliminate ambiguous cases. His studies showed that standard, four-spacecraft timing analysis in most cases lacks sufficient accuracy to identify the small normal magnetic field components expected to accompany such structures, leaving unanswered the question of their existence. Our study aims to break this impasse. By careful application of minimum variance analysis of the magnetic field (MVAB) from each individual spacecraft, we show that, in most cases, a small but significantly non-zero magnetic field component was present in the direction perpendicular to the discontinuity. In the very few cases where this component was found to be large, examination revealed that MVAB had produced an unusual and unexplained orientation of the normal vector. On the whole, MVAB shows that many verifiable rotational discontinuities (B n = 0) exist in the solar wind and that their eigenvalue ratio (EVR = intermediate/minimum variance) can be extremely large (up to EVR = 400). Each of our events comprises four individual spacecraft crossings. The events include 17 ionpolarized cases and 11 electron-polarized ones. Fifteen of the ion events have widths ranging from 9 to 21 ion inertial lengths, with two outliers at 46 and 54. The electronpolarized events are generally thicker: nine cases fall in the range 20-71 ion inertial lengths, with two outliers at 9 and 13. In agreement with theoretical predictions from a onedimensional, ideal, Hall-MHD description (Sonnerup et al., 2010), the ion-polarized events show a small depression in field magnitude, while the electron-polarized ones tend to show a small enhancement. This effect was also predicted by Wu and Lee (2000). Judging only from the sense of the plasma flow across our DDs, their propagation appears to be sunward as often as anti-sunward. However, we argue that this result can be misleading as a consequence of the possible presence of magnetic islands within the DDs. How the rotational discontinuities come into existence, how they evolve with time, and what roles they play in the solar wind remain open questions.

show abstract

Section: Discussioncontrasting

confidence: 55%

More about arc-polarized structures in the solar wind

2012

View full text Add to dashboard Cite

show abstract

“…Axisymmetry about the mean magnetic field direction is assumed in this analysis. This is a common assumption in studies of spectral anisotropy in solar wind turbulence [e.g., Matthaeus et al , 1990; Bieber et al , 1996; Dasso et al , 2005], and is motivated physically by variance anisotropy [ Belcher and Davis , 1971; Bavassano et al , 1982; Klein et al , 1991, 1993; Horbury et al , 1995; Neugebauer , 2004]. Therefore, the spatial autocorrelation function values can be projected onto a two‐dimensional plane spanned by the field parallel r ∦ and perpendicular r ⊥ spatial separations.…”

Section: Resultsmentioning

confidence: 99%

Quantitative estimates of the slab and 2‐D power in solar wind turbulence using multispacecraft data

Osman

Horbury

2009

J. Geophys. Res.

View full text Add to dashboard Cite

[1] A quantitative estimate of the field-aligned anisotropy of magnetohydrodynamic inertial range turbulence is obtained by comparing multispacecraft data with a fully threedimensional turbulent magnetic field numerical model. The simulated turbulence is a superposition of slab fluctuations parallel to the mean field and 2-D fluctuations perpendicular to it. This model is fitted to spatial autocorrelation functions, computed from measurements made by the four Cluster spacecraft in the solar wind, by varying the slab-2-D fraction. The mean power in the 2-D fluctuations, 81 ± 3%, is consistent with solar wind fluctuations being anisotropic with energy mainly in wave vectors perpendicular to the large-scale mean magnetic field. Eight intervals of multipoint magnetic field data are analyzed, and a large degree of variation in the anisotropy estimates about the mean is observed. The origin of this variability is unclear. However, variations in the anisotropy estimates are correlated between different field components.Citation: Osman, K. T., and T. S. Horsbury (2009), Quantitative estimates of the slab and 2-D power in solar wind turbulence using multispacecraft data,

show abstract

“…Note (Figure 6) that the spacecraft crosses the ecliptic near perihelion and aphelion. We can see a very clear decrease of the Alfvénicity with distance in the outer heliosphere, as noted previously [e.g., Roberts et al ., 1987; Neugebauer , 2004; B. E. Goldstein et al , 1995]. In the “noisy” regions where the value of σ c is rapidly fluctuating, there will be some contribution from sector crossings, but, more significantly, the nonlinear evolution of the system produces sign reversals that are sometimes interpreted as the “production of inward propagating waves,” although this is not the only possibility, and it may be that other modes or static structures may be involved [see Bruno and Carbone , 2005, section 3.2].…”

Section: Example Spectra From 03 To 5 Aumentioning

confidence: 99%

Evolution of the spectrum of solar wind velocity fluctuations from 0.3 to 5 AU

Roberts

2010

J. Geophys. Res.

View full text Add to dashboard Cite

[1] Recent work has shown that at 1 AU from the Sun, the power spectrum of the solar wind magnetic field has the −5/3 spectral slope expected for Kolmogorov turbulence but that the velocity has closer to a −3/2 spectrum. This paper traces the changes in solar wind velocity spectra from 0.3 to 5 AU using data from the Helios and Ulysses spacecraft to show that this is a transient stage in solar wind evolution. The spectrum of the velocity is found to be flatter than that of the magnetic field for the higher frequencies examined for all cases until the slopes become equal (at −5/3) well past 1 AU when the wind is relatively non-Alfvénic. In some respects, in particular in the evolution of the frequency at which the spectrum changes from flatter at larger scales to a "turbulent" spectrum at smaller scales, the velocity field evolves more rapidly than the magnetic, and this is associated with the dominance of the magnetic energy over the kinetic at "inertial range" scales. The speed of the flow is argued to be largely unrelated to the spectral slopes, consistent with previous work, whereas high Alfvénicity appears to slow the spectral evolution, as expected from theory. This study shows that, for the solar wind, the idea of a simple "inertial range" with uniform spectral properties is not realistic, and new phenomenologies will be needed to capture the true situation. It is also noted that a flattening of the velocity spectrum often occurs at small scales.

show abstract

Anisotropy and Alfvénicity of hourly fluctuations in the fast polar solar wind

Cited by 17 publications

References 26 publications

More about arc-polarized structures in the solar wind

More about arc-polarized structures in the solar wind

Quantitative estimates of the slab and 2‐D power in solar wind turbulence using multispacecraft data

Evolution of the spectrum of solar wind velocity fluctuations from 0.3 to 5 AU

Contact Info

Product

Resources

About