RADIO PHYSICS AND RADIO ASTRONOMY

PACS number: 95.75.Kk, 95.75.-z

 Purpose: The effect of the Earth’s ionosphere and interplanetary plasma on the radio astronomy observations, in particular, interferometric ones, at decameter waves has been investigated. The amplitude and phase fluctuations of an incident electromagnetic wave field are observed upon diffraction by a plasma layer with inhomogeneities of electron density. In interferometric observations, the phase fluctuations of an output signal cause a decrease in coherence time and an increase in the interferometer response spectrum width that limits the time of coherent averaging and reduces the instrument sensitivity. The purpose of the study is to assess the degree of ionospheric and interplanetary scintillations influence on the sensitivity of interferometric observations and to determine the possibility of sensitivity increasing that is needed in observations of angular structure of weak compact radio sources.

Design/methodology/approach: To determine the contribution of ionospheric scintillations to the broadening of an interferometer output signal spectrum, the radio source with such an angular size, which prevents its scintillations on the solar wind inhomogeneities, but makes it possible to observe the source using the URAN-2 interferometer with the baseline of 153 km, was used. Processing a large amount of observational data made it possible to determine the signal spectrum width at the output of thе interferometer under the condition of amplitude and phase scintillations on ionospheric inhomogeneities. Then, the interferometer was used to observe a compact radio source, whose signal fluctuations are affected by both the ionosphere and the interplanetary medium. The preliminary obtained spectral widths of the ionosphere scintillations made it possible to single out the effect of just interplanetary inhomogeneities on the interferometer response.

Findings: The constraints on the time of coherent averaging imposed by the influence of interplanetary plasma have been determined. Some suggestions as for changing the method of observation with the URAN interferometers at decameter wavelengths are made for more correct and efficient study of weak compact objects. The influence of large-scale features of the solar wind structure, related to the form of lines of force of the interplanetary magnetic field and to the form of spatial spectrum of inhomogeneities on the sensitivity of low-frequency interferometric observations was found. An increase in turbulence of the interplanetary medium beyond the Earth’s orbit lasting up to several days, conjugate with manifestations of the sporadic solar activity, was also detected.

Conclusions: The study allows to determine the interplanetary medium influence on the limitation of sensitivity in the interferometric observations and find optimal elongations for studying the angular structure of weak radio sources. An account of solar activity events can be used for a short-term forecast when determining the strategy of highly sensitive observations. The proposed method for detecting the solar wind turbulence can be used in studying the process of interaction of the solar events with the interplanetary medium.

Key words: ionosphere, interplanetary medium, interferometer, scintillation spectrum, sensitivity

Manuscript submitted  12.03.2020

Radio phys. radio astron. 2020, 25(2): 87-99

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