Dear Editor-in Chief, 1. This year in issue No. 2 of Vol. 43 of the journal “Radiophysics and Quantum Electronics,” the paper ”On some features of decameter radio astronomy” by V.A.Alimov and A.V.Rakhlin was published (pp. 95–105) in which the influence of the ionosphere on the reception of cosmic radio emission was taken into account theoretically. Upon reading this paper, one can easily arrive at the conclusion that, using groundbased instruments, it is impossible to perform regular radio astronomical observations in the decameter band. To be thuthful, we should mention some remarks scattered in the text of the paper that point out that sometimes things are not so bad. At the same time, observations of cosmic decameter radio emission in the mentioned band are performed extensively in the USA, Ukraine, and other countries within the framework of various astronomical programs including, e.g., measurement of coordinates and fluxes, reception of pulsar and solar radio emission, radio spectroscopy, radio interferometric measurements of angular sizes and model distributions of radio brightnes of cosmic sources, etc. These experiments lead to the conclusion that, despite the fact that the effect of the Earth’s ionosphere is strong, regular radio astronomical observations during nighttime in autumn or winter of the years off the solar maximum are still possible in the short-wavelength part of the decameter band at frequencies ν = 10− 30 MHz or lower. Of course, the technique of such measurements is drastically different compared to those used at higher frequencies. In particular, statistical methods of data processing must be applied. In this case, absolute fluxes of radio emission from cosmic sources can be determined as weighted mean values of large data sets, and the corresponding accuracy is plausible for practical purposes. For example, using the wideband UTR-2 radio telescope of the Radio Astronomical Institute of the National Academy of Sciences of Ukraine, we have performed for many years a regular survey of the northen-hemisphere sky at frequencies 25, 20, 16.7, 14.7, 12.6, and 10 MHz, aimed at compiling the Grakovo catalog of coordinates and decameter spectra of radio sources. In the course of these observations, we have obtained information on a few thousand different objects, such as radio galaxies, nonidentified objects, supernova remnants, and absorption regions in the cosmic medium. Of course, as the authors of the paper correctly mentioned, the errors of some measurements can amount to many tens or even hundreds of percent, and one can achieve a plausible measurement accuracy (e.g., about 10–15% at frequencies of 20–25 MHz for a set of a few tens of measurements) for a large data set by excluding the cases most affected by the ionosphere. If the amount of the experimental data is large enough, the main resulting error appears to be related to systematic errors due to incomplete and not absolutely correct data on the antenna parameters, losses in the soil, as well as calibration-system errors, rather than due to the random dispersion of counts resulting from ionospheric effects. Evidently, at lower frequencies ν < 20−30 MHz random errors due to ionospheric effects increase, and only occasional measurements of radio-emission fluxes from cosmic sources can be performed at ν ≤ 8− 10 MHz under the most favorable ionospheric conditions. We should also stress that, according to experimental experience, the method of selection of the individual records disturbed weakly by intense small-scale ionospheric irregularities proposed in the paper and referred to as the only way of accurate measurement of radiation fluxes is unapplicable, since the fluxes determined using such records prove to be significantly different from the actual values due to the influence of large-scale electron-density irregularities
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Jul 25, 2023
C L Van Eck + 13
Open Access
