Decametre Waveband Research Articles

Application of techniques for separating anomalous samples during the processing of SW LFM signal

The paper deals with the processing of linear frequency modulated signal of decameter waveband based on statistical approach of separating anomalous samples in experimental observations. Techniques for the rejection of narrow-band interferences and separation of desired signal have been developed on the basis of model proposed by the authors for a mixture of distributions. The specified techniques were simulated and experimentally evaluated.

A low-noise, high-dynamic-range, digital receiver for radio astronomy applications: an efficient solution for observing radio-bursts from Jupiter, the Sun, pulsars, and other astrophysical plasmas below 30 MHz

A new two-channel digital receiver that can be used for observing both stationary and sporadic radio sources in the decameter wave band is presented. Current implementation of the device operating at the sampling frequency of 66 MHz is described in detail, including the regimes of waveform capture, spectrogram analysis, and coherence analysis (cross covariance between the two inputs). Various issues pertaining to observational methods in the decameter waveband affected significantly by man-made interferences have been taken into account in the receiver design, as well as in the architecture of the interactive software that controls the receiver parameters in real time. Two examples of using the receiver with the UTR-2 array (Ukraine) are reported: S-bursts from Jupiter and low-frequency wide-band single pulses from the pulsar PSR0809+74

Solar sporadic radio emission in the decametre waveband

The first results of observations of solar sporadic radio emission with the UTR-2 radio telescope in 2001–2003 are reported in this paper. Using new back-end facilities, the digital spectro-polarimeter and 60-channel spectrometer allowed us to obtain data with a time resolution up to 2 ms and a frequency resolution of 12 kHz in a continuous frequency band of 12 MHz. The usual type III bursts, type IIIb bursts, U and J bursts, spikes and unusual bursts in the decametre wavelength band are discussed in this paper. Special attention is paid to the detection and analysis of type II bursts and their properties, the fine structure of type III bursts, the new observational features of drift pair bursts, ‘absorption’ bursts and the statistical analysis of solar radio bursts.

The influence of meteorological conditions on quality of radio-astronomical observations in the decameter wave band

The influence of meteorological conditions on quality of radio-astronomical observations in the decameter wave band

Midlatitude E region coherent backscatter observed simultaneously at two HF radar frequencies

We present results from a dual‐frequency ionospheric backscatter experiment carried out with the Valensole HF radar facility in the south of France. The radar was operated in a mode that allowed detailed Doppler spectrum recordings of coherent echoes from midlatitude E region irregularities simultaneously at 9.0 MHz and 12.4 MHz, corresponding to aspect‐sensitive backscatter from plasma waves with 16.7‐m and 12.1‐m wavelength, respectively. In this decameter wave band and in the presence of intense nighttime sporadic Es layers that contribute strong destabilizing density gradient components perpendicular to B, the E region is susceptible to direct plasma wave generation by the gradient drift instability even if electric fields are only a few mV/m. We observe that 9.0‐MHz echoes are stronger, more frequent, and more spatially extended than the 12.4‐MHz ones, which is in line with the threshold requirements of the gradient drift instability. Further, the data supports the notion of primary and secondary waves with spectral shapes similar to those seen, for example, at the equator. In one case, a situation analogous to the equatorial electrojet was clearly seen, in which the Doppler spectrum changed from a negatively shifted narrow peak (type I‐like) to a broad type II near zero shift and then to a positively shifted narrow type I‐like component, in the radar's azimuthal sector span from 22° east to 38° west of geomagnetic north. We have studied the ratio of the observed 12.4‐MHz versus 9.0‐MHz Doppler velocities in an attempt to test the widely used hypothesis that primary gradient drift waves have their phase velocity limited at instability threshold values. For simultaneous velocities in the range from 40 to 120 m/s, the V12.1 m/V16.7 m ratio was found on the average close to 1.1 and thus quite small compared to the anticipated value of 1.6 if the waves were to propagate at instability threshold values. Instead, the evidence is more supportive of the theory's dispersion relationship which requires that the wave phase velocity matches the relative electron‐ion drift along the direction of propagation.

High‐latitude ionospheric phenomena diagnostics by high‐frequency radio wave propagation observations

With the help of a system of radio paths in the decameter wave band the problem of diagnostics of high‐latitude ionospheric phenomena is solved experimentally. The latter are understood to be auroral substorms, main ionization trough, and daytime polar cusp. On the term “cusp” we suggest cusp/cleft region. As a source of information on a phenomenon, the use is made of either the statistical parameters of radio signals at radio channel output or the radio wave propagation parameter by the data of the oblique sounding of ionosphere. The first approach is taken in diagnostics of auroral substorms and main ionization trough. The oblique sounding is used in daytime polar cusp diagnostics. The experimental observations were carried out by two systems of paths: one is situated north of Eastern Siberia and the other encompasses the area north of the European part of the USSR. The range of corrected geomagnetic latitudes covered is Φ′ = 55°–75°. The main conclusion of the paper is that this system of radio paths allows us to conduct qualitative short‐time prediction of the said ionospheric phenomena.

Structure investigations of 3C 196 and 3C 280 in the decameter waveband

Observational results are presented concerning the structure of the quasar 3C 196 and the radio galaxy 3C 280 at 20 and 25 MHz, obtained by the scintillation method with the URAN-1 interferometer. Angular dimensions of the scintillating components and extended regions of the sources have been evaluated. In the case of the quasar 3C 196, the effective angular size of the scintillating component equals 2±1 . ″ 5 and that of the extended region 18×25″.The contribution of the compact component into the total radiation flux is 0.46±0.20. Spectra of the structural formations in 3C 196 have been obtained in the range 20–5000 MHz.

Scintillations of cosmic radio sources in the decametre waveband

The effect of fluctuations of the interplanetary plasma and the ionosphere upon the scintillation spectra of radio sources at decametre waves is considered with due regard for the finite antenna aperture, fluctuation anisotropy, and the direction of their drift in space. It has been shown that scintillation due to interplanetary plasma (IPP), can be reliably separated from the ionospheric scintillation background at decametre wavelengths.

Scintillations of cosmic radio sources in the decametre waveband

Scintillations of cosmic radio sources in the decametre waveband

Neutral 14N in the interstellar medium

The transition frequencies between the hyperfine structure sublevels of atomic nitrogen lie in the decametre waveband. As Shklovsky1 predicted, these transitions can be observed in the form of narrow spectral lines. The 14N ground state (4S3/2, nucleus spin I = 1) is split into three sublevels F = 5/2, 3/2 and 1/2 with frequencies and transition probabilities ν = 26.127 MHz, A = 4.32 × 10−20 s−1 for F = 5/2 → 3/2 and ν = 15.676 MHz, A = 1.29 × 10−20 s−1 for F = 3/2 → 1/2 The Einstein coefficients given here have been calculated from the improved values of line intensities2. We describe here an attempt made between 1978 and 1979 at the Institute of Radiophysics and Electronics, Academy of Science of the Ukrainian SSR, to detect the 14N absorption line for some point and extended cosmic objects. This effort has been successful for Cassiopeia A.