Turbulent Scatter Research Articles

Deriving the Ionospheric Electric Field From the Bulk Motion of Radar Aurora in the E‐Region

AbstractIn the auroral E‐region strong electric fields can create an environment characterized by fast plasma drifts. These fields lead to strong Hall currents which trigger small‐scale plasma instabilities that evolve into turbulence. Radio waves transmitted by radars are scattered off of this turbulence, giving rise to the ‘radar aurora’. However, the Doppler shift from the scattered signal does not describe the F‐region plasma flow, the drift imposed by the magnetosphere. Instead, the radar aurora Doppler shift is typically limited by nonlinear processes to not exceed the local ion‐acoustic speed of the E‐region. This being stated, recent advances in radar interferometry enable the tracking of the bulk motion of the radar aurora, which can be quite different and is typically larger than the motion inferred from the Doppler shift retrieved from turbulence scatter. We argue that the bulk motion inferred from the radar aurora tracks the motion of turbulent source regions (provided by auroras). This allows us to retrieve the electric field responsible for the motion of field tubes involved in auroral particle precipitation, since the precipitating electrons must drift. Through a number of case studies, as well as a statistical analysis, we demonstrate that, as a result, the radar aurora bulk motion is closely associated with the high‐latitude convection electric field. We conclude that, while still in need of further refinement, the method of tracking structures in the radar aurora has the potential to provide reliable estimates of the ionospheric electric field that are consistent with nature.

Probing Turbulent Scattering Effects on Suprathermal Electrons in the Solar Wind: Modeling, Observations, and Implications

This study explores the impact of a turbulent scattering mechanism, akin to those influencing solar and galactic cosmic rays propagating in the interplanetary medium, on the population of suprathermal electrons in the solar wind. We employ a Fokker–Planck equation to model the radial evolution of electron pitch angle distributions under the action of magnetic focusing, which moves the electrons away from isotropy, and of a diffusion process that tends to bring them back to it. We compare the steady-state solutions of this Fokker–Planck equation with data obtained from the Solar Orbiter and Parker Solar Probe missions and find a remarkable agreement, varying the turbulent mean free path as the sole free parameter in our model. The obtained mean free paths are of the order of the astronomical unit, and display weak dependence on electron energy within the 100 eV–1 keV range. This value is notably lower than Coulomb collision estimates but aligns well with observed mean free paths of low-rigidity solar energetic particle events. The strong agreement between our model and observations leads us to conclude that the hypothesis of turbulent scattering at work on electrons at all heliospheric distances is justified. We discuss several implications, notably the existence of a low Knudsen number region at large distances from the Sun, which offers a natural explanation for the presence of an isotropic “halo” component at all distances from the Sun—electrons being isotropized in this distant region before traveling back into the inner part of the interplanetary medium.

Gravity Wave Ducting Observed in the Mesosphere Over Jicamarca, Peru

AbstractShort‐period gravity waves are ubiquitous in the mesosphere, but the vertical structures of their perturbations are difficult to observe. The Jicamarca 50‐MHz very high frequency radar allows observations of winds and turbulent scatter with high temporal and vertical resolution. We present a case of a quasi‐monochromatic gravity wave with period 520 (±40) s that is likely ducted below a southward wind jet between 68 and 74 km. Above this layer of evanescence, a northward wind enables it to emerge into a more stable layer, where it is refracted to a short vertical wavelength of 2.2 (±0.2) km; data show evidence of weak nonlinearity, and possible overturning or partial reflection from higher altitudes, above the observable region, in the form of a standing wave structure in vertical velocity at approximately 75 km. Based on the dispersion relation, and with help of a two‐dimensional model, we determine that most likely the wave is propagating northward and is being ducted below and tunneling through the regions of evanescence created by the wind flow and typical mesospheric thermal structure. This is the first time that such an event has been identified in the Jicamarca mesospheric echoes, and it is distinct from Kelvin‐Helmholtz billows also commonly seen with this sensitive radar—instead apparently revealing tunneling of the gravity wave through ambient winds.

ON THE INFLUENCE OF MHD TURBULENCE ON THE STRUCTURE OF THE RADIOGALAXY LOBES

There is considered the evolution of the shape for the radio galaxy lobes of FRI and FRII types from the point of view on changing the configuration of large-scale structure of magnetic field, and energy transport in the turbulent MHD waves. There have been studied the interaction and transformation of waves in the active regions of the lobes, and so studied the role of MHD waves and vortexes in media-mixing processes and in the amplification of the average magnetic field. The transport of low-energy e-CRs responsible for the radio emission in the MHz band (recorded at the UTR-2 and GURT telescopes) is analyzed for the sources like to the Cygnus A and M87. It is shown that the transport of e-CRs mainly corresponds to the diffusion of CRs on MHD and turbulence scatter, and the entrainment of CRs by quasi-regular post-jet flows inside to the lobe. So, the MHz radio emission that observed emphasizes the peculiarities in the lobe which arising when the magnetic field is in reorganization.

Reduction of Thermal Conductive Flux by Non-local Effects in the Presence of Turbulent Scattering.

The heat flux in a plasma is determined by the degree of anisotropy in the particle distribution function, which is in turn driven by gradients in the ambient density and temperature. When the mean free path at the thermal speed is substantially smaller than the scale length associated with the temperature variation, the heat flux simply depends on the local value of the temperature gradient. However, when the temperature scale length and mean free path are comparable, heat conduction becomes substantially non-local in character: the magnitude of the heat flux now depends on the overall temperature profile and is generally smaller than the locally determined value. In the presence of angular scattering associated with turbulence, the mean free path (and its velocity dependence) can be significantly smaller than its collisional value; this makes the expression for the heat flux more local in character, but also results in a heat flux that is lower than that obtained through a purely collisional analysis. Therefore, whether or not turbulence is present, the heat flux is generally smaller than the value obtained from a local collisional analysis. We here present an analytic expression for the conductive heat flux in terms of a convolution of the local heat flux with a non-local kernel function that incorporates both Coulomb collisions and turbulent scattering. We comment on the need to include both non-local and turbulent scattering effects in the modeling of quasi-static active region loops and in the conductive cooling of post-flare loops.

Evaluation of momentum flux with radar

The statistics of gravity wave momentum flux estimation are investigated using data from the MU radar at Shigariki, Japan (136°E, 35°N). The radar has been operating during campaign periods since 1986. The first part of the paper focuses on a multi-day campaign during October 13–31, 1986. The second part of the paper investigates data after 2006 when the radar was operated in a meteor scatter mode. Momentum fluxes are derived from both the turbulent scatter and the meteor scatter measurements, but the techniques are quite different. Probability Distribution Functions are formed using turbulent scatter data. These show that wave packets sometimes have momentum flux magnitudes in excess of 100m2s−2. The technique for meteor radars, introduced by Hocking (2005), has been widely adopted by the radar community in recent years. The momentum flux estimated using this technique is found to be anti-correlated with the background tidal winds. A validation investigation is carried out for periods with a high meteor echo data rate. The conclusion was that the method can be used to calculate the sign of momentum flux, but does not accurately specify the magnitude.

Measurement and simulation of surface roughness noise using phased microphone arrays

A turbulent boundary-layer flow over a rough wall generates a dipole sound field as the near-field hydrodynamic disturbances in the turbulent boundary-layer scatter into radiated sound at small surface irregularities. In this paper, phased microphone arrays are applied to the measurement and simulation of surface roughness noise. The radiated sound from two rough plates and one smooth plate in an open jet is measured at three streamwise locations, and the beamforming source maps demonstrate the dipole directivity. Higher source strengths can be observed on the rough plates which also enhance the trailing-edge noise. A prediction scheme in previous theoretical work is used to describe the strength of a distribution of incoherent dipoles and to simulate the sound detected by the microphone array. Source maps of measurement and simulation exhibit satisfactory similarities in both source pattern and source strength, which confirms the dipole nature and the predicted magnitude of roughness noise. However, the simulations underestimate the streamwise gradient of the source strengths and overestimate the source strengths at the highest frequency.

Polar mesosphere summer echoes (PMSE) studied at Bragg wavelengths of 2.8m, 67cm, and 16cm

We present observations of radar volume reflectivities under conditions of polar mesosphere summer echoes (PMSE) at three frequencies, i.e., 53.5, 224, and 930 MHz corresponding to Bragg wavelengths of 2.8, 0.67, and 0.16 m. These measurements were made with the ALWIN radar in Andenes and the EISCAT VHF and UHF radars in Tromsø. Contributions to the signal at 930 MHz by incoherent scatter are used to estimate electron number densities and their gradient at PMSE altitudes, and spectral width measurements of Doppler spectra recorded at 224 MHz are used to estimate the turbulent energy dissipation rate. We further derive a theoretical expression for the radar volume reflectivity for the case of turbulent scatter aided by a large Schmidt number (i.e., the current standard theory of PMSE) and show that our observations quantitatively agree with this theory if Schmidt numbers between 2500 and 5000 are assumed. We then show that these Schmidt numbers correspond to ice particles with radii in the range 20–30 nm which should frequently occur in the polar summer mesopause region. In addition, we show that for the short period when PMSE was observed at UHF frequencies the volume reflectivity is proportional to a factor determined by the turbulent energy dissipation rate, electron number density, and the electron number density gradient in agreement with theory. We consider our findings as strong support that PMSE at all considered frequencies is indeed created by turbulent scatter in the presence of a large Schmidt number. We finally highlight that ultimate proof of this concept will require the direct measurement of ice particle sizes in a PMSE environment probed by radars covering frequencies between 50 MHz and 1 GHz.

Radar measurements of turbulence, electron densities, and absolute reflectivities during polar mesosphere winter echoes (PMWE)

On January 21, 2005, strong radar echoes known as ‘polar mesosphere winter echoes’ (PMWE) were observed by the ALWIN VHF radar at 69°N. Peak reflectivites of ∼10 −14/m were observed at approximately 13 UT. At the same time the Saura MF radar which is only ∼15 km away from ALWIN measured electron densities ( N e) and turbulence energy dissipation rates ( ϵ). This combination is rather seldom because PMWE are rare (occurrence rate is 1–2%) and electron densities are critical: too low values do not give a PMWE, and too large values prohibit to derive N e due to signal absorption. At PMWE altitudes (∼62 km) typical energy dissipation rates before, during, and after the PMWE are ϵ∼100 mW/kg. The electron density during the PMWE is typically 10 9/m 3 and much smaller prior to PMWE. We have applied a theoretical model based on turbulence theory to derive absolute reflectivities for these ϵ and N e values and arrive at ∼2 × 10 −15/m during PMWE (in nice agreement with measurements) and ∼3 × 10 −17/m outside PMWE. The latter value is indeed below the detection limit of the ALWIN radar. The nice quantitative agreement between measured and calculated absolute volume reflectivities confirms earlier conclusions that neutral air turbulence is the main cause for PMWE. Furthermore, we have analyzed the autocorrelation function of the EISCAT 224 MHz PMWE observations from November 10, 2004, and find that the spectral form is different inside the PMWE compared to outside. The shape is Gaussian inside PMWE (compatible with turbulent scatter), whereas it is Lorentzian above the PMWE, indicating non-turbulent, incoherent scatter.

Monte Carlo simulation of propagation of a short light beam through turbulent oceanic flow

We use Monte Carlo time-dependent simulations of light pulse propagation through turbulent water laden with particles to investigate the application of Multiple Field Of View (MFOV) lidar to detect and characterize oceanic turbulence. Inhomogeneities in the refractive index induced by temperature fluctuations in turbulent ocean flows scatter light in near-forward angles, thus affecting the near-forward part of oceanic water scattering phase function. Our results show that the oceanic turbulent signal can be detected by analyzing the returns from a MFOV lidar, after re-scaling the particulate back scattering phase function.

Ocean acoustic backscattering: When you can ignore acoustic scatter from turbulence and when you can’t

While models predicting measurable levels of acoustic backscattering from oceanic turbulence have been around for decades, they have proven notoriously hard to confirm. This is, in part, because potential turbulent scattering layers often coincide with zooplankton layers. Therefore, as zooplankton are known to cause measurable acoustic scatter, the source of scatter is obscured. Furthermore, estimates of zooplankton abundance from acoustic scattering measurements have been shown to agree with independent measures under a number of circumstances. This gives circumstantial evidence to fuel the belief that scatter from turbulence is negligible. In addition, even if the turbulent scattering theory is correct, it predicts that over most of the ocean the turbulent intensities and/or stratifications are too weak to give turbulent scatter of a similar magnitude as is observed from zooplankton layers. Yet, despite all this, here data are presented that show circumstances when scatter from turbulence is significant. The areas of the ocean (mostly coastal) that one might have to be concerned about turbulent scatter are discussed, as well as the intriguing idea of using technology already developed for zooplankton observation to measure turbulence. This technology could facilitate the in situ study of biophysical interactions between zooplankton and turbulence.

The role of charged ice particles for the creation of PMSE: A review of recent developments

We review the current understanding of the role of charged ice particles for the creation of PMSE. We briefly describe the historical background leading to the Cho et al. [1992]-theory of PMSE which tried to explain the radar echoes as a turbulent scatter where the scattering electrons have a high Schmidt number due to the fact that somewhat more than 50% of the overall negative charge is bound to heavy aerosol particles of low mobility. We then discuss recent experimental results showing that 1. neutral air turbulence is regularly found only in the upper part of a PMSE layer and 2. that PMSE exist even if only a minor fraction of the overall negative charge is bound to aerosol particles. We then present the solution of this contradiction between experiment and theory by discussing the recent updated electron diffusion theory by Rapp and Lübken [2003]. Their main result is that electron diffusivity is reduced due to the presence of charged aerosol particles almost independently of the ratio between the aerosol charge number density and the electron number density. Furthermore, the diffusive lifetime of electron irregularities can be hours depending on the aerosol radius such that it appears likely that at low altitudes, where we expect the presence of large particles, irregularities exist even though neutral air turbulence might have ceased a considerable time ago. Thus the question for the spatial and temporal variability of mesospheric turbulence is identified as the most pressing task for future research in order to finally understand PMSE.

Evidence for viscosity, thermal conduction and diffusion waves in the Earth’s atmosphere (invited)

Over the last 40 years, radar studies of atmospheric backscatter have shown tantalizing evidence for flat, mirror-like partial reflectors embedded in the air. These have been observed at all heights, but especially in the stratosphere (above ∼10 km) and mesosphere (especially in the region 60–90 km altitude), and at a variety of frequencies, from 2 to at least 50 MHz. These structures appear to have vertical depths of only a few meters or less. They often seem independent of atmospheric turbulence scatter. Various explanations have been advanced to account for these unusual structures, but a strong candidate is the possibility that they are due to diffusion waves generated by reflection and/or critical level interactions of gravity waves or infrasound. In this review, we will examine the evidence for these structures, and expand on the possibility that they are indeed organized structures which develop due to the diffusive characteristics of the medium.

Wavelet analysis of high‐resolution temperature data with applications to radio wave scattering

High‐resolution balloon‐borne temperature measurements have been made in the troposphere and stratosphere during late fall over the western plains of the United States. In one such experiment the data are of remarkable quality and quite suitable for investigating methods of separating organized and turbulent features from a geophysical data stream, exploring atmospheric dynamics, and estimating VHF radar backscatter. We are particularly interested in the mechanisms creating aspect sensitivity, i.e., a nonunity ratio of vertical to well off‐vertical radar backscatter. We find that very steep positive vertical temperature gradients, as high as 40.0 K/km, can be supported. On the other hand, negative gradients are limited to values near the marginal stability boundary. The structures are thus anisotropic and similar to ramp cliff features found in other fluids. We use wavelet analysis to isolate the organized components of the signal and, after subtraction, the residual signal is investigated to determine its character. The Fourier transform of the residual is Kolmogorov in nature, unlike that of the original data stream, and yields Cn2, the refractive index structure parameter, in good agreement with the higher‐order structure function approach; this supports the success of our partition. We calculate the Fresnel reflection coefficient using the wavelet coefficients; the turbulent scatter is found using Cn2. The ratio between the Fresnel scatter and the turbulent scatter, i.e., the aspect sensitivity, is ∼10 decibels in the troposphere and over 20 decibels in the stratosphere, in agreement with published observations and supporting our assumption that the organized features have horizontal extent exceeding the Fresnel scale.

Radiative Transfer of Sound Waves in a Random Flow: Turbulent Scattering, Straining, and Mode-Coupling

We study the sound wave propagation in a random flow, whose mean flow is large compared with its fluctuation, in the infinite three-dimensional space. We consider the intermediate regime, where the range of acoustic wave numbers overlaps with the range of turbulent wave numbers. We use the multiscale expansions for the Wigner distributions to derive the radiative transport equations that describe the evolution of acoustic correlation and the turbulent scattering, straining, and mode-coupling of sound waves. We show that, because of the flow-straining term, the flow-acoustic scattering becomes nonconservative and, depending on the propagation direction, a sound wave can gain or lose energy. We calculate the attenuation/amplification coefficients due to mode-coupling and/or turbulent scattering with flow-straining. These coefficients depict interesting dependence on the propagating direction and the wave length of sound wave. We demonstrate numerically that the attenuation/amplification coefficients are enhanced significantly when both the straining and the mode-coupling effects are present. We also obtain the diffusion equations on the physical space and, thus, further reduce the dimension of the flow-acoustic equations.

Confinement and Isotropization of Galactic Cosmic Rays by Molecular‐Cloud Magnetic Mirrors When Turbulent Scattering Is Weak

Theoretical studies of magnetohydrodynamic (MHD) turbulence and observations of solar wind —uc- tuations suggest that MHD turbulence in the interstellar medium is anisotropic at small scales, with smooth variations along the background magnetic —eld and sharp variations perpendicular to the back- ground —eld. Turbulence with this anisotropy is inefficient at scattering cosmic rays, and thus the scat- tering rate l may be smaller than has been traditionally assumed in diUusion models of Galactic cosmic-ray propagation, at least for cosmic-ray energies E above 1011¨1012 eV at which self-con—nement is not possible. In this paper, it is shown that Galactic cosmic rays can be eUectively con—ned through magnetic re—ection by molecular clouds, even when turbulent scattering is weak. Elmegreens quasi- fractal model of molecular-cloud structure is used to argue that a typical magnetic —eld line passes through a molecular cloud complex once every D300 pc. Once inside the complex, the —eld line will in most cases be focused into one or more dense clumps in which the magnetic —eld can be much stronger than the average —eld in the intercloud medium (ICM). Cosmic rays following —eld lines into cloud com- plexes are most often magnetically re—ected back into the ICM, since strong-—eld regions act as magnetic mirrors. For a broad range of cosmic-ray energies, a cosmic ray initially following some particular —eld line separates from that —eld line sufficiently slowly that the cosmic ray can be trapped between neigh- boring cloud complexes for long periods of time. The suppression of cosmic-ray diUusion due to mag- netic trapping is calculated in this paper with the use of phenomenological arguments, asymptotic analysis, and Monte Carlo particle simulations. Formulas for the coefficient of diUusion perpendicular to the Galactic disk are derived for several diUerent parameter regimes within the E-l plane. In one of these parameter regimes in which scattering is weak, it is shown that molecular-cloud magnetic mirrors strongly reduce cosmic-ray anisotropy in the ICM, and analytic formulas for the angular harmonics are derived. Subject headings: acceleration of particlescosmic raysISM: cloudsISM: magnetic —elds ¨ turbulence

High‐resolution stratospheric dynamics measurements with the NASA/JPL Goldstone Solar System Radar

We have used, for the first time that we are aware of, the NASA/JPL Goldstone planetary radar to study the Earth's atmosphere. With its high bandwidth and power, we were able to achieve a height resolution of 20 m, which is significantly better than the usual 150‐m resolution for stratospheric radars. Here we discuss the observation of a very thin scattering layer that persisted over several hours at the same height just above the tropopause. We question the assumption of turbulent radar scatter based on the available evidence, and also investigate the two‐minute oscillation observed in the vertical velocity.

Further effects of charged aerosols on summer mesospheric radar scatter

In an earlier paper, we showed that charged aerosols play a crucial role in enhancing radar echoes from the summer polar mesosphere through reduced diffusion turbulent scatter and dressed aerosol scatter (Cho et al., 1992a). Here, we explore the effects of charged aerosols on radar scatter through ‘fossil’ turbulence and electron density depletion layers. We find that the former can produce radar scatter even after the decay of neutral gas turbulence, while the latter, which are probably produced by the scavenging of free electrons by ice particles, are a candidate for causing partial reflection or Fresnel scatter. Furthermore, we examine the mutual aerosol interaction restriction on dressed aerosol scatter more closely. We find that a high ambient electron density and low aerosol number density are needed for effective dressed aerosol scatter to occur. We then show that very small (less than 1 nm radii), negatively charged aerosols enhance electron diffusivity, and thus inhibit radar scatter. Also, ice aerosol sedimentation, in the light of the reduced diffusion theory, leads us to conclude that the statistical peak in Polar Mesospheric Summer Echoes (PMSE) power should be located between the mean mesopause and the average noctilucent cloud (NLC) height, which agrees with observations. Finally, we invoke time lags in the ice particle formation cycle to account for the observed non-correlation between PMSE and NLC occurrence.

Spaced antenna and interferometric velocity measurements with MF and VHF radars

The spaced antenna and interferometry techniques are applied to data collected on two MF radars (at Christchurch, New Zealand, and at Scott Base, in the Antarctic) and one VHF radar (the MU radar at Shigaraki, Japan). Velocity results obtained by the two techniques are compared as functions of characteristics such as the temporal scale and anisotropy of the scatter. In general, there was good agreement between the two techniques; however, when the temporal scale is small (indicating turbulent scatter) there was less agreement. The interferometry technique did not appear to be influenced greatly by anisotropy in the scatter. The interferometry technique was found to be affected by averaging of the Doppler spectra and by the use of an extra receiver. It is suggested that less averaging of the spectra results in increased random noise components in the spectra, which in the phase spectra are interpreted as scattering from a greater range of zenith angles. It was found that the use of three receivers rather than four also lead to scattering that appeared to originate from a greater range of zenith angles. These effects lead to lower interferometric velocity estimates that are more biased towards full correlation analysis (FCA) true velocities.

Heat transfer characteristics of the supercritical helium in a cable-in-conduit conductor

To study the thermal stability of a forced-cooled superconducting coil with a cable-in-conduit conductor (CICC), it is important to measure the steady-state heat transfer characteristics of the supercritical helium in the narrow channel like a CICC at various conditions. The measured heat transfer coefficient is compared with one calculated by the Giarratano correlation and other studies. The results obtained show that measured values for turbulent flow scatter widely and the values for laminar flow are 1.5-1.8 times larger than those calculated from Nu=4.36. The stability of a CICC is discussed with the relation between the Stekly's stability limit and the measured heat transfer coefficient. >