Circular Scanning of the Solar Limb Zone from Observations with the RT-32 Radio Telescope of IAA RAS. Method and First Results

I. A. Rakhimov, T. S. Andreeva, A. G. Mikhailov, N. A. Topchilo, N. G. Peterova

Transactions of IAA RAS, issue 73, 3–10 (2025)

DOI: 10.32876/ApplAstron.73.3-10

Keywords: radio telescope, radio map, active region, prominence, coronal mass ejection, radio radius of the Sun, polarization of radio emission, magnetic field

Abstract

Observations of the solar limb zone are of particular importance for studying the Sun's atmosphere as they provide information about its height structure. However, investigating observed non-stationary three-dimensional structures in this zone encounters significant difficulties due to the presence of a strong overall brightness height gradient. The raster mapping method commonly used in radio observations leads to substantial errors when identifying sources at the solar limb. The aim of this work is to demonstrate the possibility of improving the accuracy of limb observations on existing large fully-steerable antennas, in particular the RT-32 at IAA RAS, and to select effectively observable objects and phenomena within its operating frequency range. To do that, we propose using an observation method called circular scanning, where the telescope's beam pattern carries out circular movements relative to the center of the solar disk. This method was previously developed for solar observations using radio telescopes operating in the millimeter wavelength range with the beam size of several arcminutes, to eliminate errors caused by telescope motion defects and atmospheric instability. At this stage of the work on RT-32, quasi-circular scanning was used as the discrete version of circular scanning that is simpler to implement but less effective. In this mode, the telescope sequentially tracks a series of points uniformly distributed along a circle relative to the center of the solar disk. The paper presents the first results of test observations with RT-32 at the Svetloe observatory at a wavelength of 3.5 cm during the period from April 2024 to February 2025. It is shown that, with RT-32's beamwidth of 4 arcminutes, individual details of active regions are easily identifiable across the entire limb zone, which is highly problematic, for example, for RATAN-600 type radio telescopes. The RT-32 radio telescope also successfully detects weak but sufficiently large-scale sources above the solar limb (such as prominences and loop structures). Using the observation from April 15, 2025, as an example, it is demonstrated that future implementation of standard circular scanning will significantly speed up the mapping process. Implementing the methodology at all three IAA RAS observatories will enable practically continuous monitoring of solar activity in the limb zone.

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