Decametric Solar Radio Spectrometer Based on 4-element Beamforming Array and Initial Observational Results

The dynamic spectral observation at decametric wavelength is important to study solar radio physics and space weather.However,the observing system is difficult to observe with high sensitivity at this band due to the fact that the system temperature is dominated by the sky background noise and the antenna is difficult to design with high gain.An effective solution to improve the sensitivity is constructing an antenna array based on the beamforming method.Accordingly,we develop a decametric solar radio spectrometer system based on a 4-element beamforming array.The system consists of four antennas,an 8-channel analog receiver and a digital receiver.We use the true time delay to implement the beamformer and the classical FFT method to perform spectrum analysis in the digital receiver.Operating at a frequency range of 25–65 MHz with dual-circular polarizations,the system provides high resolution dynamic spectrum with spectral resolution of~12 kHz and temporal resolution of~5.3 ms(typical).Tens of solar radio bursts have been observed successfully during the period of the trial observation,demonstrating the system's ability to detect fine structures with high spectral and temporal resolution.In this article,we present the design,implementation,and initial observational results of the decametric solar radio spectrometer system in detail.

A New Position Calibration Method for MUSER Images

The Mingantu Spectral Radioheliograph(MUSER),a new generation of solar dedicated radio imagingspectroscopic telescope,has realized high-time,high-angular,and high-frequency resolution imaging of the Sun over an ultra-broadband frequency range.Each pair of MUSER antennas measures the complex visibility in the aperture plane for each integration time and frequency channel.The corresponding radio image for each integration time and frequency channel is then obtained by inverse Fourier transformation of the visibility data.However,the phase of the complex visibility is severely corrupted by instrumental and propagation effects.Therefore,robust calibration procedures are vital in order to obtain high-fidelity radio images.While there are many calibration techniques available—e.g.,using redundant baselines,observing standard cosmic sources,or fitting the solar disk—to correct the visibility data for the above-mentioned phase errors,MUSER is configured with non-redundant baselines and the solar disk structure cannot always be exploited.Therefore it is desirable to develop alternative calibration methods in addition to these available techniques whenever appropriate for MUSER to obtain reliable radio images.In the case where a point-like calibration source contains an unknown position error,we have for the first time derived a mathematical model to describe the problem and proposed an optimization method to calibrate this unknown error by studying the offset of the positions of radio images over a certain period of the time interval.Simulation experiments and actual observational data analyses indicate that this method is valid and feasible.For MUSER’s practical data the calibrated position errors are within the spatial angular resolution of the instrument.This calibration method can also be used in other situations for radio aperture synthesis observations.