Distribution Of Electron Density Fluctuations Research Articles

SCATTER BROADENING OF PULSARS AND IMPLICATIONS ON THE INTERSTELLAR MEDIUM TURBULENCE

ABSTRACT Observations reveal a uniform Kolmogorov turbulence throughout the diffuse ionized interstellar medium (ISM) and supersonic turbulence preferentially located in the Galactic plane. Correspondingly, we consider the Galactic distribution of electron density fluctuations consisting of not only a Kolmogorov density spectrum but also a short-wave-dominated density spectrum with the density structure formed at small scales due to shocks. The resulting dependence of the scatter broadening time on the dispersion measure (DM) naturally interprets the existing observational data for both low- and high-DM pulsars. According to the criteria that we derive for a quantitative determination of scattering regimes over wide ranges of DMs and frequencies ν, we find that the pulsars with low DMs are primarily scattered by the Kolmogorov turbulence, while those at low Galactic latitudes with high DMs undergo more enhanced scattering dominated by the supersonic turbulence, where the corresponding density spectrum has a spectral index of . Furthermore, by considering a volume filling factor of the density structures with the dependence on ν as in the supersonic turbulence, our model can also explain the observed shallower ν scaling of the scattering time than the Kolmogorov scaling for the pulsars with relatively large DMs. The comparison between our analytical results and the scattering measurements of pulsars in turn makes a useful probe of the properties of the large-scale ISM turbulence, e.g., an injection scale of ∼100 pc, and also characteristics of small-scale density structures.

Investigation of a high spatial resolution method based on polar coordinate maximum entropy method for analyzing electron density fluctuation data measured by laser phase contrast

Laser phase contrast is a powerful diagnostic method to determine the spatial distribution of electron density fluctuations in magnetically confined plasmas, although its applicability depends on magnetic field configurations. The spatial resolution of fluctuations is linked with the resolution of the propagation direction that is derived from the two-dimensional spectral analysis of the wavenumber for the fluctuations. The method was applied to fluctuation measurements in a compact helical system. In order to improve the resolution of the propagation direction with a relatively small number of data points, the maximum entropy method with polar coordinates was employed. A spatial resolution of the order of 1 cm was obtained, which is satisfactory in a plasma with a 20 cm minor radius.

Improving pulsar distances by modelling interstellar scattering

We present here a method to study the distribution of electron density fluctuations in pulsar directions as well as to estimate pulsar distances. The method, based on a simple two-component model of the scattering medium discussed by Gwinn et al. (1993), uses scintillation & proper motion data in addition to the measurements of angular broadening & temporal broadening to solve for the model parameters, namely, the fractional distance to a discrete scatterer and the ascociated relative scattering strength. We show how this method can be used to estimate pulsar distances reliably, when the location of a discrete scatterer (e.g. an HII region), if any, is known. Considering the specific example of PSR B0736-40, we illustrate how a simple characterization of the Gum nebula region (using the data on the Vela pulsar) is possible and can be used along with the temporal broadening measurements to estimate pulsar distances.

Pulsar Scintillation and the Local Bubble

We present the results from an extensive scintillation study of 20 pulsars in the dispersion measure range 3-35 pc cm-3, carried out using the Ooty Radio Telescope at 327 MHz, to investigate the distribution of ionized material in the local interstellar medium (LISM). Observations were made during the period 1993 January-1995 August, in which the dynamic scintillation spectra of these pulsars were regularly monitored over 10-90 epochs spanning ~100 days. Reliable and accurate estimates of strengths of scattering have been deduced from the scintillation parameters, averaged out for their long-term fluctuations arising from refractive scintillation effects. Our analysis reveals several anomalies in the scattering strength, which suggest that the distribution of scattering material in the solar neighborhood is not uniform. We have modeled these anomalous scattering effects in terms of inhomogeneities in the distribution of electron density fluctuations in the LISM. Our model suggests the presence of a low-density bubble surrounded by a shell of much higher density fluctuations. We are able to put some constraints on geometrical and scattering properties of such a structure and find it to be morphologically similar to the Local Bubble known from other studies.

Towards an Understanding of the Galactic Distribution of Electron-Density Fluctuations

AbstractVLBA observations of 32 compact sources within 30° ≤ ᶩ ≤ 75°, | b | < 3° are being used to test whether the galactic electron-density fluctuations contain a spiral component.

Improvement of the Laser Phase Contrast Method for Measuring the Spatial Distribution of Electron Density Fluctuations in Heliotron E

The laser phase contrast method is suitable for measuring long wavelength fluctuations propagating perpendicular to the laser beam axis. The obtainable data, which have so far been integrated along line-of-sights, are improved to be spatially resolved along the beam axis by selecting a particular direction of the sheared magnetic field of a toroidal confinement system utilizing a slit attached to a phase plate. First, the spatial resolution was formulated, and its validity was examined using an ultrasonic wave which simulated density fluctuations in plasmas. Then, the method was applied to plasma discharges in the Heliotron E device, and wavenumber (K)-frequency (f=Ω/2π) spectra, S(K, f), shows that the spatial regions were surely resolved.

Small-scale electron density turbulence in the interstellar medium

view Abstract Citations (321) References (74) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Small-scale electron density turbulence in the interstellar medium. Cordes, J. M. ; Weisberg, J. M. ; Boriakoff, V. Abstract Diffraction phenomena associated with the scattering of pulsar radiation are used to constrain the power spectrum and galactic distribution of electron density fluctuations. The authors combine new measurements of interstellar scintillation bandwidths and pulse broadening times with those from the literature to yield information on 76 lines of sight. For individual objects, the scaling of scintillation bandwidth with frequency is in agreement with a Kolmogorov spectrum. The study of all objects, however, indicates strong disagreement with a model in which turbulence is uniformly distributed. The data suggest a two-component model for electron density turbulence: (1) a clumped medium with scale height H ⪉ 100 pc; (2) a nearly uniform medium with H ⪆ 0.5 kpc. Scattering material is ubiquitous but is preferentially located near regions of large energy input into the interstellar medium. Publication: The Astrophysical Journal Pub Date: January 1985 DOI: 10.1086/162784 Bibcode: 1985ApJ...288..221C Keywords: Electron Density (Concentration); Interstellar Matter; Pulsars; Turbulence; Cross Correlation; Line Of Sight; Radiative Transfer; Stellar Radiation; Stellar Spectra; Wave Scattering; Astrophysics full text sources ADS | data products SIMBAD (75)

The effect of impurities on the spectrum of laser light scattered by a plasma

The expression for the frequency distribution of electron density fluctuations in a plasma containing more than one ionic species is used as the basis for numerical computation which investigates the effect of varying the charge, mass and abundance of impurity ions on the ion feature of the scattered light spectrum. The calculation is done for various values of the electron/ion temperature ratio and for correlation parameter alpha =1 and = infinity . The effect of a temperature difference between the two ionic species in the plasma is also studied.