scholarly journals Turbulence in nearly incompressible fluids: density spectrum, flows, correlations and implication to the interstellar medium

2005 ◽  
Vol 12 (1) ◽  
pp. 139-148 ◽  
Author(s):  
S. Dastgeer ◽  
G. P. Zank
Keyword(s):  
Interstellar Medium ◽  
Pressure Fluctuations ◽  
Thermal Fluctuation ◽  
Passive Scalar ◽  
Density Fluctuations ◽  
Density Spectrum ◽  
Electron Density Fluctuations ◽  
Nonlinear Fluid ◽  
Solar Wind Electron

Abstract. Interstellar scintillation and angular radio wave broadening measurements show that interstellar and solar wind (electron) density fluctuations exhibit a Kolmogorov-like k-5/3 power spectrum extending over many decades in wavenumber space. The ubiquity of the Kolmogorov-like interstellar medium (ISM) density spectrum led to an explanation based on coupling incompressible magnetohydrodynamic (MHD) fluctuations to density fluctuations through a "pseudosound" relation within the context of "nearly incompressible" (NI) hydrodynamics (HD) and MHD models. The NI theory provides a fundamentally different explanation for the observed ISM density spectrum in that the density fluctuations can be a consequence of passive scalar convection due to background incompressible fluctuations. The theory further predicts generation of long-scale structures and various correlations between the density, temperature and the (magneto) acoustic as well as convective pressure fluctuations in the compressible ISM fluids in different thermal regimes that are determined purely by the thermal fluctuation level. In this paper, we present the results of our two dimensional nonlinear fluid simulations, exploring various nonlinear aspects that lead to inertial range ISM turbulence within the context of a NI hydrodymanics model. In qualitative agreement with the NI predictions and the in-situ observations, we find that i) the density fluctuations exhibit a Kolmogorov-like spectrum via a passive convection in the field of the background incompressible fluctuations, ii) the compressible ISM fluctuations form long scale flows and structures, and iii) the density and the temperature fluctuations are anti-correlated.

Pulsars and Interstellar Scintillations

2000 ◽  
Vol 177 ◽  
pp. 539-544
Author(s):  
Y. Gupta
Keyword(s):  
Interstellar Medium ◽  
Electron Density ◽  
Density Fluctuations ◽  
Current Understanding ◽  
Pulsar Emission ◽  
The Galaxy ◽  
Electron Density Fluctuations

AbstractIn this paper, I review our current understanding of interstellar scintillations (ISS) of pulsars. The emphasis is on new results that have appeared during the last five years. The topics covered include (i) review of the understanding of refractive ISS (ii) the shape of the spectrum of electron density fluctuations in the interstellar medium (iii) the distribution of scattering plasma in the Galaxy (iv) resolving pulsar emission regions using ISS and (v) ISS and pulsar velocities.

scholarly journals Pulsar Scintillation: Overview and Some Recent Results

2001 ◽  
Vol 182 ◽  
pp. 25-30
Author(s):  
Yashwant Gupta
Keyword(s):  
Interstellar Medium ◽  
Electron Density ◽  
Density Fluctuations ◽  
Radio Signals ◽  
Basic Concepts ◽  
Electron Density Fluctuations

AbstractRadio signals from pulsars are significantly affected by scattering in the interstellar medium. A review of this phenomenon of pulsar scintillation forms the main objective of this paper. The basic concepts are described and some new results related to the following aspects are presented: (i) understanding of refractive scintillation effects and (ii) constraining the spectrum of electron density fluctuations in the interstellar medium.

scholarly journals Ionospheric irregularities and scintillations: a direct comparison of in situ density observations with ground-based L-band receivers

2020 ◽  
Vol 72 (1) ◽  
Author(s):  
Sharon Aol ◽  
Stephan Buchert ◽  
Edward Jurua
Keyword(s):  
Electron Density ◽  
Satellite Communication ◽  
Density Fluctuations ◽  
Ionospheric Irregularities ◽  
Langmuir Probes ◽  
Amplitude Scintillation ◽  
Swarm Satellites ◽  
L Band ◽  
Electron Density Fluctuations

Abstract Ionospheric irregularities can affect satellite communication and navigation by causing scintillations of radio signals. The scintillations are routinely measured using ground-based networks of receivers. This study presents observations of ionospheric irregularities by Langmuir probes on the Swarm satellites. They are compared with amplitude scintillation events recorded by the Global Positioning System-Scintillation Network and Decision Aid (GPS-SCINDA) receiver installed in Mbarara (Lat: $$0.6^{\circ }\hbox {S}$$ 0 . 6 ∘ S , Lon: $$30.8^{\circ }\hbox {E}$$ 30 . 8 ∘ E , Mag. lat: $$10.2^{\circ }\hbox {S}$$ 10 . 2 ∘ S ). The study covers the years from 2014 to 2018 when both data sets were available. It was found that the ground-based amplitude scintillations were enhanced when Swarm registered ionospheric irregularities for a large number of passes. The number of matching observations was greater for Swarm A and C which orbited at lower altitudes compared to Swarm B. However, some counterexamples, i.e., cases when in situ electron density fluctuations were not associated with any observed L-band amplitude scintillation and vice versa, were also found. Therefore, mismatches between observed irregularity structures and scintillations can occur just over a few minutes and within distances of a few tens of kilometers. The amplitude scintillation strength, characterized by the S4 index was estimated from the electron density data using the well-known phase screen model for weak scattering. The derived amplitude scintillation was on average lower for Swarm B than for A and C and less in accordance with the observed range. Irregularities at an altitude of about 450 km contribute strongly to scintillations in the L-band, while irregularities at about 510-km altitude contribute significantly less. We infer that in situ density fluctuations observed on passes over or near Mbarara may be used to indicate the risk that ionospheric radio wave scintillations occur at that site.

scholarly journals Interstellar Scattering

1984 ◽  
Vol 110 ◽  
pp. 303-307
Author(s):  
James M. Cordes
Keyword(s):  
Interstellar Medium ◽  
Electron Density ◽  
Angular Size ◽  
Density Fluctuations ◽  
Length Scale ◽  
Radio Sources ◽  
Fine Scale ◽  
Interstellar Scattering ◽  
Electron Density Fluctuations ◽  
Temporal Broadening

Fine scale electron density fluctuations in the interstellar medium (ISM) are manifest as scintillations and temporal broadening of pulsar signals and as angular broadening of galactic and extragalactic sources. Although scattering off the fluctuations is often a nuisance for conventional studies of radio sources, analysis or searches for interstellar scintillations (ISS) can lead to information about the ISM or radio sources that otherwise would not be obtainable. For example, the length scale probed by ISS in the ISM is typically 1011 cm and the characteristic angular size for quenching ISS is less than 1 μarc sec.

scholarly journals Pulsars and the ISM

2002 ◽  
Vol 199 ◽  
pp. 363-368
Author(s):  
Yashwant Gupta
Keyword(s):  
Interstellar Medium ◽  
Electron Density ◽  
Faraday Rotation ◽  
Density Fluctuations ◽  
Pulsar Emission ◽  
Main Emphasis ◽  
Propagation Effects ◽  
Recent Developments ◽  
Radio Signals ◽  
Electron Density Fluctuations

Radio signals from pulsars are significantly affected by propagation effects such as dispersion, faraday rotation and scintillations in the interstellar medium (ISM). In this paper, I review some aspects of our understanding about pulsars and interstellar scintillations (ISS). The study of pulsar scintillation has dual benefits in that it allows us to learn about the properties of the ISM using pulsars as probes, as well as to infer some properties about pulsars, using the ISM as a tool. Both these aspects are addressed in this paper. The main emphasis is on recent developments in the following topics : (i) the shape of the spectrum of electron density fluctuations in the interstellar medium (ii) the distribution of scattering plasma in the local ISM and (iii) resolving pulsar emission regions using ISS.

scholarly journals Looking for a proxy of the ionospheric turbulence with Swarm data

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Paola De Michelis ◽  
Giuseppe Consolini ◽  
Alessio Pignalberi ◽  
Roberta Tozzi ◽  
Igino Coco ◽  
...  
Keyword(s):  
Electron Density ◽  
Geomagnetic Activity ◽  
Density Fluctuations ◽  
Rate Of Change ◽  
Topside Ionosphere ◽  
Joint Probability Distribution ◽  
Activity Levels ◽  
Scaling Exponents ◽  
Electron Density Fluctuations ◽  
Scaling Features

AbstractThe present work focuses on the analysis of the scaling features of electron density fluctuations in the mid- and high-latitude topside ionosphere under different conditions of geomagnetic activity. The aim is to understand whether it is possible to identify a proxy that may provide information on the properties of electron density fluctuations and on the possible physical mechanisms at their origin, as for instance, turbulence phenomena. So, we selected about 4 years (April 2014–February 2018) of 1 Hz electron density measurements recorded on-board ESA Swarm A satellite. Using the Auroral Electrojet (AE) index, we identified two different geomagnetic conditions: quiet (AE < 50 nT) and active (AE > 300 nT). For both datasets, we evaluated the first- and second-order scaling exponents and an intermittency coefficient associated with the electron density fluctuations. Then, the joint probability distribution between each of these quantities and the rate of change of electron density index was also evaluated. We identified two families of plasma density fluctuations characterized by different mean values of both the scaling exponents and the considered ionospheric index, suggesting that different mechanisms (instabilities/turbulent processes) can be responsible for the observed scaling features. Furthermore, a clear different localization of the two families in the magnetic latitude—magnetic local time plane is found and its dependence on geomagnetic activity levels is analyzed. These results may well have a bearing about the capability of recognizing the turbulent character of irregularities using a typical ionospheric plasma irregularity index as a proxy.

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