Anisotropic Density Fluctuations Research Articles

First determination of the angular dependence of rise and decay times of solar radio bursts using multi-spacecraft observations

A large arsenal of space-based and ground-based instruments is dedicated to the observation of radio emissions, whether they originate within our solar system or not. Radio photons interact with anisotropic density fluctuations in the heliosphere which can alter their trajectory and influence the properties that are deduced from observations. This is particularly evident in solar radio observations, where anisotropic scattering leads to highly directional radio emissions. Consequently, observers at varying locations will measure different properties, including different source sizes, source positions, and intensities. However, it is not known whether the measurements of the decay time of solar radio bursts are also affected by the observer’s position. Decay times are dominated by scattering effects, and so are frequently used as proxies of the level of density fluctuations in the heliosphere, making the identification of any location-related dependence crucial. We combine multi-vantage observations of interplanetary Type III bursts from four non-collinear, angularly separated spacecraft with simulations to investigate the dependence of the decay- and rise-time measurements on the separation of the observer from the source. We propose a function to characterise the entire time profile of radio signals, allowing for the simultaneous estimation of the peak flux, decay time, and rise time, while demonstrating that the rise phase of radio bursts is non-exponential, having a non-constant growth rate. We determine that the decay and rise times are independent of the observer’s position, identifying them as the only properties that remain unaffected and thus do not require corrections for the observer’s location. Moreover, we examine the ratio between the rise and decay times and find that it does not depend on the frequency. Therefore, we provide the first evidence that the rise phase is also significantly impacted by scattering effects, adding to our understanding of the plasma emission process.

Plasma Motions and Compressive Wave Energetics in the Solar Corona and Solar Wind from Radio Wave Scattering Observations

Radio signals propagating via the solar corona and solar wind are significantly affected by compressive waves, impacting the properties of solar bursts as well as sources viewed through the turbulent solar atmosphere. While static fluctuations scatter radio waves elastically, moving, turbulent, or oscillating density irregularities act to broaden the frequency of the scattered waves. Using a new anisotropic density fluctuation model from the kinetic scattering theory for solar radio bursts, we deduce the plasma velocities required to explain observations of spacecraft signal frequency broadening. The inferred velocities are consistent with motions that are dominated by the solar wind at distances ≳10 R ⊙, but the levels of frequency broadening for ≲10 R ⊙ require additional radial speeds ∼(100–300) km s−1 and/or transverse speeds ∼(20–70) km s−1. The inferred radial velocities also appear consistent with the sound or proton thermal speeds, while the speeds perpendicular to the radial direction are consistent with nonthermal motions measured via coronal Doppler-line broadening, interpreted as Alfvénic fluctuations. Landau damping of parallel propagating ion-sound (slow MHD) waves allows an estimate of the proton heating rate. The energy deposition rates due to ion-sound wave damping peak at a heliocentric distance of ∼(1–3) R ⊙ are comparable to the rates available from a turbulent cascade of Alfvénic waves at large scales, suggesting a coherent picture of energy transfer, via the cascade or/and parametric decay of Alfvén waves to the small scales where heating takes place.

Source positions of an interplanetary type III radio burst and anisotropic radio-wave scattering

Interplanetary solar radio type III bursts provide the means to remotely study and track energetic electrons propagating in the interplanetary medium. Due to the lack of direct radio source imaging, several methods have been developed to determine the source positions from space-based observations. Moreover, none of the methods consider the propagation effects of anisotropic radio-wave scattering, which would strongly distort the trajectory of radio waves, delay their arrival times, and affect their apparent characteristics. We investigate the source positions and directivity of an interplanetary type III burst simultaneously observed by Parker Solar Probe, Solar Orbiter, STEREO, and Wind and we compare the results of applying the intensity fit and timing methods with ray-tracing simulations of radio-wave propagation with anisotropic density fluctuations. The simulation calculates the trajectories of the rays, their time profiles at different viewing sites, and the apparent characteristics for various density fluctuation parameters. The results indicate that the observed source positions are displaced away from the locations where emission is produced, and their deduced radial distances are larger than expected from density models. This suggests that the apparent position is affected by anisotropic radio-wave scattering, which leads to an apparent position at a larger heliocentric distance from the Sun. The methods to determine the source positions may underestimate the apparent positions if they do not consider the path of radio-wave propagation and incomplete scattering at a viewing site close to the intrinsic source position.

Evolution of magnetic field in a weakly relativistic counterstreaming inhomogeneous e−/e+ plasmas

AbstractThe nonlinear evolution of electron Weibel instability in a symmetric, counterstream, unmagnetized electron–positron e−/e+ plasmas is studied by a 2D particle-in-cell (PIC) method. The magnetic field is produced and amplified by the Weibel instability, which extracts energy from the plasma anisotropy. A weakly relativistic drift velocity of 0.5c is considered for two counterstreaming e−/e+ plasma flows. Simulations show that in a homogeneous e−/e+ plasma distribution, the magnetic field amplifies exponentially in the linear regime and rapidly decays after saturation. However, in the case of inhomogeneous e−/e+ plasma distribution, the magnetic field re-amplifies at post-saturation. We also find that the amount of magnetic field amplification at post-saturation depends on the strength of the density inhomogeneity of the upstream plasma distribution. The temperature calculation shows that the finite thermal anisotropy exists in the case of an inhomogeneous plasma distribution which leads to the second-stage magnetic field amplification after the first saturation. Such density inhomogeneities are present in a variety of astrophysical sources: for example, in supernova remnants and gamma-ray bursts. Therefore, the present analysis is very useful in understanding these astrophysical sources, where anisotropic density fluctuations are very common in the downstream region of the relativistic shocks and the widely distributed magnetic field.

Structure and Phase Transitions of Poly(heptamethylene p,p‘-bibenzoate) As Studied by DSC and Real-Time SAXS/MAXS Employing Synchrotron Radiation

The structure and phase transitions of liquid-crystalline poly(heptamethylene p,p'bibenzoate) have been studied by DSC and variable-temperature real-time SAXS/MAXS employing synchrotron radiation. It was suggested that three-dimensional crystal order appeared within the smectic domains during cooling the sample because the crystal correlation length is nearly the same as that of the smectic domains. These estimated domain sizes are of the order of the average macromolecular chain length, so that it seems that no chain folding exists in the present case. It was found that SAXS intensity in Porod's region obeys the power law, and it deviates significantly from Porod's law. These deviations were explained by an assumption of anisotropic density fluctuations within the phases. It was supposed that the melting process is ruled by an increasing quantity of anisotropic fluctuations within the crystallites but not by surface melting.

Electronic Raman scattering in single domain YBa 2Cu 3O 7−δ, k-space anisotropy and comparison to LDA Fermi surface masses

Measurements of the absolute electronic Raman cross sections are presented for a single domain Y-123 crystal for different polarization geometries. They are compared to the corresponding inverse effective mass fluctuations evaluated from the LMTO full-potential band structure of O.K. Andersen et al. The agreement of the relative intensities for the xx- yy anisotropy and between the crossed polarizations parallel and at 45° to the crystal ab axes, i.e. B 2 g and B 1 g symmetries, with the LDA single particle masses is good and suggests that isotropic and anisotropic density fluctuations involving various sheets and regions of the Fermi surface are responsible for the electronic Raman continuum. Resonance effects and screening are taken into account.

Influence of higher order density correlation in a fluid on the linewidths of rotational transitions of a dissolved molecule

Time dependent perturbation theory gives the relation between the dipole correlation function of a rotating dipole molecule and the anisotropic density fluctuations in a fluid around that molecule. Second order terms give the width of the absorption line associated with a rotational transition as a linear combination of terms containing the values of the power spectra of the density correlations taken at the frequencies of virtual transitions from initial and final levels. Fourth order terms account for the phase relations that might exist between some of these second order terms and constitute negative contributions to the linewidths. It is shown how this opens the possibility for linewidth measurements to give information about the power spectra H(ω,ω,ω) of the three time eight point density correlation functions. The complete expressions for the calculation of the coefficients is given and relative importance of the terms of different order is estimated.

Four time density correlations around a dissolved HCl molecule in dense argon, krypton, and xenon as determined from linewidth data

The far infrared pure rotational spectrum of HCl molecules in a medium of xenon at T = Tc and ρ = 1.6ρc has been measured. The result expressed in terms of linewidths is compared with earlier results obtained in krypton and in argon, also at Tc and 1.6ρc. Coefficients that connect linewidths with the power spectrum of the fourth order anisotropic density fluctuations were calculated and are given together with existing coefficients for the second order terms. A scaling procedure is used to connect the data obtained in argon, krypton, and xenon with molecular dynamics data on the second order anisotropic density fluctuations. This leads to fourth order density fluctuations for the three noble gases of which the power spectra do indeed obey the same scaling laws. The physical meaning of the power spectrum, H(ω,0,ω) of the four time anisotropic density correlation function is discussed.

Growth morphology and defects in plasma-deposited a-Si:H films

Structural studies of plasma-deposited a-Si:H are presented that show that a major class of defect is an anisotropic density fluctuation. Studies of the hydrogen environment suggest that an inhomogeneous hydrogen distribution is associated with these fluctuations. From considerations of the deposition chemistry and nucleation theory a model is proposed to describe the film growth process and its relationship to defects.

Anisotropic density fluctuations around a probe molecule dissolved in fluid argon and krypton

The rotational far infrared spectra of HCl perturbed by krypton are presented. Krypton densities varied from the dilute gas to higher than the critical density, all along the critical isotherm. The linewidths are related to the anisotropic density fluctuations by the perturbing potential. A comparison with earlier results on the HCI-argon system shows that a second-order treatment (gaussian approximation) is not sufficient. An appreciable contribution of the fourth-power terms mixes high-frequency components in the detected power-spectrum of the density fluctuations.

Anisotropic density fluctuations in argon at different densities: Far infrared measurements and molecular dynamic calculations

From measurements of the pure rotational lines of a linear probe molecule dissolved in a simple fluid one can deduce the power spectra of the anisotropic density fluctuations of l=1 and l=2 symmetry of the fluid. The power spectra thus obtained for argon at various densities are compared with the results of molecular dynamics calculations and with the approximate power spectra which can be calculated starting from neutron-scattering data. The density dependence of the high frequency components of the l=1 power spectrum and of the zero frequency component of the l=2 power spectrum give a strong characteristic of the dense fluid.

Rotational relaxation of solute molecules in dense noble gases and the relation with local anisotropy fluctuations

Many molecular relaxation processes in fluids are sensitive to the time-dependence of local, anisotropic density fluctuations. The role played by anisotropic density fluctuations in the rotational relaxation of a linear, quantized rotor will be discussed in some detail. An expression for the dipolecorrelation function of a probe molecule, dissolved in a simple fluid, will be derived along these lines. This expression will be shown to account for the experimentally observed density-dependence of the rotational linewidths of HCl in argon. Some simple models for anisotropic density fluctuations or, more precisely, for the four-point density-correlation functions, will be discussed.