Nikolay A. Zabotin

Role of meteoric dust in sprite formation

We show that ubiquitous small conducting particles of meteoric origin in the mesosphere and stratosphere may explain some features of sprite occurrence and fine structure. The main processes involved are: electrostatic field amplification by microspires on the dust surface, “cathode-like instability” in their vicinity, autoemission and explosive emission of electrons. Additional observations, theoretical development and perhaps laboratory experiments are warranted.

Interferometry of infragravity waves off New Zealand

Wave interferometry is a remote sensing technique, which is increasingly employed in helioseismology, seismology, and acoustics to retrieve parameters of the propagation medium from two-point cross-correlation functions of random wavefields. Here we apply interferometry to yearlong records of seafloor pressure at 28 locations off New Zealands South Island to investigate propagation and directivity properties of infragravity waves away from shore. A compressed cross-correlation function technique is proposed to make the interferometry of dispersive waves more robust, decrease the necessary noise averaging time, and simplify retrieval of quantitative information from noise cross correlations. The emergence of deterministic wave arrivals from cross correlations of random wavefields is observed up to the maximum range of 692 km between the pressure sensors in the array. Free, linear waves with a strongly anisotropic distribution of power flux density are found to be dominant in the infragravity wavefield. Lowest-frequency components of the infragravity wavefield are largely isotropic. The anisotropy has its maximum in the middle of the spectral band and decreases at the high-frequency end of the spectrum. Highest anisotropy peaks correspond to waves coming from portions of the New Zealands shoreline. Significant contributions are also observed from waves propagating along the coastline and probably coming from powerful sources in the northeast Pacific. Infragravity wave directivity is markedly different to the east and to the west of the South Island. The northwest coast of the South Island is found to be a net source of the infragravity wave energy.

Acoustic-gravity waves in the atmosphere generated by infragravity waves in the ocean

Infragravity waves are surface gravity waves in the ocean with periods longer than approximately 30 s. Infragravity waves propagate transoceanic distances and, because of their long wavelengths, provide a mechanism for coupling wave processes in the ocean, atmosphere, and the solid Earth. Here, we present a strict physical justification for the hypothesis that background ocean waves may generate waves in the upper atmosphere. We show that, at frequencies below a certain transition frequency of about 3 mHz, infragravity waves continuously radiate their energy into the upper atmosphere in the form of acoustic-gravity waves. Based on ionospheric observations and estimates of the fluxes of the mechanical energy and momentum from the deep ocean, we conclude that acoustic-gravity waves of oceanic origin may have an observable impact on the upper atmosphere. We anticipate our work to be a starting point for a detailed analysis of global manifestations of the ocean-generated background acoustic-gravity waves.

RADIATIVE TRANSFER IN A LAYER OF MAGNETIZED PLASMA WITH RANDOM IRREGULARITIES

Abstract The problem of radio wave reflection from an optically thick plane uniform layer of magnetized plasma is considered in the present work. The plasma electron density irregularities are described by a spatial spectrum of arbitrary form. The small-angle scattering approximation in invariant ray coordinates is proposed as a technique for the analytical investigation of the radiation transfer equation. The approximate solution describing the spatial and angular distribution of radiation reflected from a plasma layer is obtained. The solution obtained is investigated numerically for the case of ionospheric radio wave propagation. Two effects occur as a consequence of multiple scattering: a change in the reflected signal intensity and an anomalous refraction.The problem of radio wave reflection from an optically thick plane monotonous layer of magnetized plasma is considered at present work. The plasma electron density irregularities are described by spatial spectrum of an arbitrary form. The small-angle scattering approximation in the invariant ray coordinates is suggested for analytical investigation of the radiation transfer equation. The approximated solution describing spatial-and-angular distribution of radiation reflected from a plasma layer has been obtained. The obtained solution has been investigated numerically for the case of the ionospheric radio wave propagation. Two effects are the consequence of multiple scattering: change of the reflected signal intensity and anomalous refraction.

Acoustic Green's function extraction from ambient noise in a coastal ocean environment

We report on the results of an underwater acoustic field experiment conducted in December 2012 on the continental shelf off the Florida Keys in water of approximately 100 m depth. Ambient noise was recorded concurrently on three moored near-bottom instruments with horizontal separations of approximately 5 km, 10 km, and 15 km. We focus in this letter on instrument pairs with 5 km and 10 km separations. Consistent with theoretical predictions, coherent sums of many realizations of cross correlations of ambient noise records at two measurement locations are shown to yield approximations to deterministic acoustic Greens functions that describe propagation between those two locations. The results presented demonstrate the feasibility of extracting information suitable for use in tomographic inversions from measurements of underwater acoustic ambient noise.

Anomalous attenuation of extraordinary waves in ionosphere heating experiments

Multiple scattering from artificial random irregularities HF-induced in the ionosphere F region causes significant attenuation of both ordinary and extraordinary radio waves together with the conventional anomalous absorption of ordinary waves due to their conversion into the plasma waves. To study in detail features of this effect, purposeful measurements of the attenuation of weak probing waves of the extraordinary polarization have been performed at the Sura heating facility. Characteristic scale lengths of the involved irregularities are ~0.1-1 km across the geomagnetic field lines. To determine the spectral characteristics of these irregularities from the extraordinary probing wave attenuation measurements, a simple procedure of the inverse problem solving has been implemented and some conclusions about the artificial irregularity features have been drawn. Theory and details of experiments have been stated earlier. This paper reports results of two experimental campaigns carried out in August 2000 and June 2001 under support of Russian Foundation for Basic Research (grants No. 99-02-17525 and No. 01-02-31008). Particularity of these experiments consisted in using of lower heating power (20-80 MW ERP). Regular character of the multiple scattering effects has been confirmed.Multiple scattering of radio waves by artificial random irregularities HF-induced in the ionosphere F region may cause significant attenuation of both ordinary and extraordinary waves together with common anomalous absorption of ordinary waves due to their non-linear conversion into plasma waves. To demonstrate existence and strength of this effect, direct measurements of attenuation of both powerful pump wave and weak probing waves of extraordinary polarization have been carried out during an experimental campaign on September 6, 7 and 9, 1999 at the Sura heating facility. The attenuation magnitude of extraordinary waves reaches of 1-10 dB over a background attenuation caused by natural irregularities. It is interpreted in the paper on the base of the theory of multiple scattering from the artificial random irregularities with characteristic scale lengths of 0.1-1 km. Simple procedure for determining of irregularity spectrum parameters from the measured attenuation of extraordinary waves has been implemented and some conclusions about the artificial irregularity formation have been obtained.

Ionospheric irregularity diagnostics from the phase structure functions of MF/HF radio echoes

We present a new approach to investigating ionospheric irregularities, using the temporal structure function of totally reflected radio echo phase variations. Modern digital ionosondes (e.g., the dynasonde) measure the echo phase with very high resolution and precision, at closely spaced antennas, frequencies, and times. A “stringing” procedure gives continuous and unambiguous phase variation data for time intervals of any desired length. Quasi-periods of tens of seconds up through several minutes are caused by large-scale movements of the ionospheric plasma, while shorter-period phase variations result from the interaction of the sounding signal with small-scale irregularities. The relevant irregularity spatial domain extends from decameter radio wavelengths to the first Fresnel scale, a few kilometers. We obtain a theoretical relation between structure functions of the temporal phase variations and spatial irregularities with a simple model of frozen horizontal drift. The relation permits solutions of both the direct and inverse problems. Although long-period phase measurements are practicable and essential to exploring larger irregularity scales, they require observing modes dedicated to multiple fixed-frequency time series, and this undesirably limits the number of altitudes that can be monitored simultaneously. An alternative “rudimentary structure function” is obtainable from standard dynasonde “B-mode” ionograms; it offers good altitude and time resolution for irregularity studies while permitting other established diagnostics (electron density profiles, vector velocities, critical frequencies, etc.) with the same data. We show some example analyses by these methods as applied to auroral and magnetic-equatorial dynasonde observations. We find irregularity amplitudes in the range 0.001 < ΔN/N < 0.1 (for a nominal scale of 1 km) and spectral indices in the range 2 < v < 4, with evidence of diurnal variation in both quantities at both locations.

Response of topside radio sounding signals to small-scale field-aligned ionospheric irregularities

Abstract Small-scale ionospheric irregularities produce numerous observed effects during high-frequency radio wave sounding from a satellite. Scattering of z-mode waves radiated by an ionosonde, which results in the occurrence of characteristic traces on ionograms, are among the most complex of them. In the present paper the factors determining the power characteristics of such signals are considered. These are the size and shape of the surface of orthogonality for a family of z-mode waves radiated from a satellite, features of the radiating and propagating radio waves in a non-uniform magnetized plasma, and the details of the scattering process. Simple but adequate physical models for each of these factors are presented and a numerical calculation algorithm of the power of the z-mode scattered signals received by the satellite sounder receiver has been constructed. Values for this calculated quantity are presented as a function of time delay, sounding frequency, and magnetic latitude. A synthesis of topside ionograms showing z-mode scattered traces has been performed. The results support new diagnostic methods of detecting small-scale ionospheric irregularities using topside-sounder z-mode signal returns and the concept that the sounder radiation can, under certain conditions, stimulate artificial plasma turbulence. In addition, it was shown that z-mode scattering may contribute to the resonance observed at the plasma frequency on topside ionograms.

Simple numerical model of radio wave multiple scattering effects in the ionospheric plasma layer

Abstract Numerical simulation of radio energy transfer in the ionospheric layer with random small-scale irregularities for the case of a point ground-based source and total reflection has been carried out using a specially designed algorithm based on Monte Carlo method ideas. The model demonstrates a redistribution of the reflected radiation power at the Earths surface caused by multiple scattering. The data obtained can serve as confirmation of the effect of the anomalous attenuation of the signal power in the vicinity of the sounder. This result is in agreement with a more rigorous theory of the anomalous attenuation effect based on the solution of the radiative transfer equation.Computer simulations of radio energy transfer in the ionospheric layer with random small-scale irregularities for the case of a point ground-based source and total reflection have been carried out using specially designed algorithm based on Monte Carlo method ideas. The model demonstrates redistribution of the reflected radiation power on the Earths surface caused by multiple scattering process. The obtained data can serve as illustration of the effect of anomalous attenuation of the signal power in the sounder vicinity. This result is in agreement with more rigorous theory of the anomalous attenuation effect based on the solving of the radiative transfer equation.

Group propagation time of quasi-monochromatic radiation in a plane layer of plasma with random irregularities

The influence of multiple scattering in a plasma layer with random irregularities on the propagation time of a totally reflected pulse signal has been considered. Two approaches have been implemented. The first one is based on an analytical solution of the radiative transfer equation. The second one is based on a numerical simulation in the spirit of the Monte Carlo technique. The general conclusion is that, subject to relative position of a transmitter and a receiver and the irregularity shape, one can observe both a decrease and an increase of the propagation time in comparison with the reflection from the same layer deprived of random irregularities. The numerical estimates made in this paper regard the case of radio sounding of the ionospheric plasma. It has been shown that the magnitude of the considered effect is of the order of the characteristic pulse length applied in the ionosphere vertical sounding.