Evidence for an inner scale to the density turbulence in the interstellar medium

Spangler, S. R.; Gwinn, C. R.

doi:10.1086/185700

Cited by 135 publications

(144 citation statements)

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“…It is interesting that a similar curved spectrum is expected in the Interstellar Medium turbulence, but at ion scales (Spangler and Gwinn 1990;Haverkorn and Spangler 2013).…”

Section: Ion Scale Instabilities Driven By Solar Wind Expansion and Cmentioning

confidence: 67%

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Solar Wind Turbulence and the Role of Ion Instabilities

et al. 2013

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Solar wind is probably the best laboratory to study turbulence in astrophysical plasmas. In addition to the presence of magnetic field, the differences with neutral fluid isotropic turbulence are: (i) weakness of collisional dissipation and (ii) presence of several characteristic space and time scales. In this paper we discuss observational properties of solar wind turbulence in a large range from the MHD to the electron scales. At MHD scales, within the inertial range, turbulence cascade of magnetic fluctuations develops mostly in the plane perpendicular to the mean field, with the Kolmogorov scaling k −5/3 ⊥ for the perpendicular cascade and k −2 for the parallel one. Solar wind turbulence is compressible in nature: density fluctuations at MHD scales have the Kolmogorov spectrum. Velocity fluctuations do not follow magnetic field ones: their spectrum is a power-law with a −3/2 spectral index. Probability distribution functions of different plasma parameters are not Gaussian, indicating presence of intermittency. At the moment there is no global model taking into account all these observed properties of the inertial range. At ion scales, turbulent spectra have a break, compressibility increases and the density fluctuation spectrum has a local flattening. Around ion scales, magnetic spectra are variable and ion instabilities occur as a function of the local plasma parameters. Between ion and electron scales, a small scale turbulent cascade seems to be established. It is characterized by a well defined power-law spectrum in magnetic and density fluctuations with a spectral index close to −2.8. Approaching electron scales, the fluctuations are no more self-similar: an exponential cut-off is usually observed (for time intervals without quasi-parallel whistlers) indicating an onset of dissipation. The small scale inertial range between ion and electron scales and the electron dissipation range can be together described by ∼ k −α ⊥ exp(−k ⊥ d ), with α 8/3 and the dissipation scale d close to the electron Larmor radius d ρ e . The nature of this small scale cascade and a possible dissipation mechanism are still under debate.

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“…It is interesting that a similar curved spectrum is expected in the Interstellar Medium turbulence, but at ion scales (Spangler and Gwinn 1990;Haverkorn and Spangler 2013).…”

Section: Ion Scale Instabilities Driven By Solar Wind Expansion and Cmentioning

confidence: 67%

“…when λ i > ρ i . 11 It is quite natural that the largest characteristic scale (or the smallest characteristic wave number) affects the spectrum first (Spangler and Gwinn 1990). It will be interesting to verify these results for slow solar wind streams and high β i regimes.…”

Section: Turbulence Around Ion Scalesmentioning

confidence: 99%

Solar Wind Turbulence and the Role of Ion Instabilities

et al. 2013

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“…This part of PSDs often exhibits a substantial steepening with the increasing frequency that can be fitted with an exponential function. The exponential decrease of PSDs within the ion scale in the interstellar space was described by Spangler & Gwinn (1990). They interpreted these observations as evidence for an inner scale that limits the wavenumbers of possible kinetic structures in ion turbulence.…”

Section: Discussionmentioning

confidence: 92%

“…Alexandrova et al (2012) analyzed PSDs of magnetic field fluctuations and found that an exponential fit would be appropriate for a characterization of the spectral shape in the range of frequencies that were attributed to the electron dissipation range. Moreover, Spangler & Gwinn (1990) interpreted observations of interstellar scintillations and argued that the spectrum of interstellar turbulence exhibits an exponential decay at the ion kinetic scale. We followed this approach and fitted the downstream spectra in the frequency range that starts with the spectral flattening at the end of the MHD range and ends at 8 Hz with a function g(f):…”

Section: Psds In the Downstream Regionmentioning

confidence: 99%

Density Fluctuations Upstream and Downstream of Interplanetary Shocks

Pitňa

Šafránková

Němeček

et al. 2016

ApJ

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Interplanetary (IP) shocks as typical large-scale disturbances arising fromprocesses such asstream-stream interactions or Interplanetary Coronal Mass Ejection (ICME) launching play a significant role in the energy redistribution, dissipation, particle heating, acceleration, etc. They can change the properties of the turbulent cascade on shorter scales. We focus on changes of the level and spectral properties of ion flux fluctuations upstream and downstream of fastforward oblique shocks. Although the fluctuation level increases by an order of magnitude across the shock, the spectral slope in the magnetohydrodynamic range is conserved. The frequency spectra upstream of IP shocks are the same as those in the solar wind (if not spoiled by foreshock waves). The spectral slopes downstream are roughly proportional to the corresponding slopes upstream, suggesting that the properties of the turbulent cascade are conserved across the shock;thus, the shock does not destroy the shape of the spectrum as turbulence passes through it. Frequency spectra downstream of IP shocks often exhibit "an exponential decay" in the ion kinetic range that was earlier reported at electron scales in the solar wind or at ion scales in the interstellar medium. We suggest that the exponential shape of ion flux spectra in this range is caused by stronger damping of the fluctuations in the downstream region.

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“…While this value is reminiscent of Kolmogorov turbulence in neutral Ñuids, turbulence in an ionized medium is an active area of theoretical research and values for b ranging from 3.5 to 4.0 have been proposed (Kraichnan 1965 ;Goldreich & Sridhar 1995 ;Terry, Fernandez, & Ware 1998). The inner scale 1/q i is not known, but proposed values range from 300 to 106 kilometers (Coles et al 1987 ;Spangler & Gwinn 1990 ;Mutel & Lestrade 1990 ;Gupta, Rickett, & Coles 1993 ;Wilkinson et al 1994 ;Molnar et al 1995). The outer scale is also not known, but proposed values range from a 1/q o few AU to tens of pc (Anantharamaiah & Narayan 1988 ;Desai, Gwinn, & Diamond 1994 ;Wilkinson et al 1994).…”

Section: Turbulence and Interstellar Scatteringmentioning

confidence: 99%

Anisotropic Interstellar Scattering toward the Cygnus Region

Desai¹,

Fey²

2001

ASTROPHYS J SUPPL S

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We were unable to reliably determine scattering disk parameters for 2021]317 at frequencies from 8.5 GHz down to 1.67 GHz because of its complex intrinsic source structure. The scattering disks are elliptical with axial ratios of about 0.75 : 1 with little measurable variation between these sources. We interpret our measurements as due to the e †ects of anisotropic interstellar turbulence. The anisotropy parameters, axial ratio and position angle, for those sources for which we have data at multiple frequencies appear to be frequency-independent making refractive distortions of isotropic turbulence an unlikely explanation. Our estimates of b, the power-law index of the power spectrum of electron density Ñuctuations, based upon direct model Ðts to the data on a source-by-source and frequency-by-frequency basis, are consistent with b \ 4 for most of the data but are strongly a †ected by our estimate of the intrinsic structure of the sources. Two of these sources, 2005]403 and 2021]317, exhibit signiÐcant intrinsic source structure at frequencies as low as 1.67 GHz, while the other two, 2008]332 and 2048]312, lack discernible source structure at frequencies lower then 2.3 GHz. The frequency scalings of the scattered angular sizes are also consistent with b \ 4. We discuss the implications of our measurements for the inner and outer scales of the turbulence.

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Evidence for an inner scale to the density turbulence in the interstellar medium

Cited by 135 publications

References 0 publications

Solar Wind Turbulence and the Role of Ion Instabilities

Solar Wind Turbulence and the Role of Ion Instabilities

Density Fluctuations Upstream and Downstream of Interplanetary Shocks

Anisotropic Interstellar Scattering toward the Cygnus Region

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