Solar Radio Spectrometer Research Articles

Development of a 90–600 MHz Meter-wave Solar Radio Spectrometer

Radio observation is important for understanding coronal mass ejections (CMEs), coronal shock waves, and high-energy electron acceleration. Here, we developed a new Chashan broadband solar radio spectrometer at a meter wavelength for observing the (super)fine structure of the solar radio burst spectrum. In the signal-receiving unit, we adopt an antenna system consisting of a 12 m large-aperture parabolic reflector and dual-line polarized logarithmic periodic feed source, as well as a high-precision Sun-tracking turntable system, all of which ensure the high-precision acquisition of solar radiation signals. For the digital receiver, we use a high-speed analog-to-digital converter with a sampling rate of 1.25 GSPS to directly sample the signal amplified and filtered by the analog receiver, simplifying the structure of the analog receiver, and design a 16k-point fast Fourier transform algorithm in the field programmable gate array to perform time–frequency transformation on the sampled signals. The default frequency and temporal resolution of the system are 76.294 kHz and 0.839 ms (up to 0.21 ms), respectively. The noise coefficient of the system is less than 1 dB, the dynamic range is more than 60 dB, and the sensitivity is as high as 1 sfu. We have observed a large number of radio bursts, including type I radio storms, hundreds of type III, ∼20 type II, and ∼15 type IV bursts in the past year. These high-quality data are useful in the further study of CMEs and associated particle acceleration and the origins of solar radio bursts.

Predicting the Arrival Time of an Interplanetary Shock Based on DSRT Spectrum Observations for the Corresponding Type II Radio Burst and a Blast Wave Theory

Since fast head-on coronal mass ejections and their associated shocks represent potential hazards to the space environment of the Earth and even other planets, forecasting the arrival time of the corresponding interplanetary shock is a priority in space weather research and prediction. Based on the radio spectrum observations of the 16-element array of the Daocheng Solar Radio Telescope (DSRT), the flagship instrument of the Meridian Project of China, during its construction, this study determines the initial shock speed of a type II solar radio burst on 2022 April 17 from its drifting speed in the spectrum. Assuming that the shock travels at a steady speed during the piston-driven phase (determined from the X-ray flux of the associated flare) and then propagates through interplanetary space as a blast wave, we estimate the propagation and arrival time of the corresponding shock at the orbit of the Solar Terrestrial Relations Observatory-A (STEREO-A). The prediction shows that the shock will reach STEREO-A at 14:31:57 UT on 2022 April 19. The STEREO-A satellite detected an interplanetary shock at 13:52:12 UT on the same day. The discrepancy between the predicted and observed arrival time of the shock is only 0.66 hr. The purpose of this paper is to establish a general method for predicting the shock’s propagation and arrival time from this example, which will be utilized to predict more events in the future based on the observations of ground-based solar radio spectrometers or telescopes like DSRT.

The Calibration of the 35–40 GHz Solar Radio Spectrometer with the New Moon and a Noise Source

Calibrating solar radio flux has always been a concern in the solar community. Previously, fluxes were calibrated by matching load or the new Moon for relative calibration, and at times with the assistance of other stations’ data. Moreover, the frequency coverage seldom exceeded 26 GHz. This paper reports the upgraded and calibrated Chashan Broadband Solar millimeter spectrometer (CBS) working from 35 to 40 GHz at the Chashan Solar Observatory (CSO). Initially, the calibration of the solar radiation brightness temperature is accomplished using the new Moon as the definitive source. Subsequently, the 35–40 GHz standard flux is achieved by establishing the correlation between the solar radio flux, brightness temperature, and frequency. Finally, the calibration of the solar radio flux is implemented by utilizing a constant temperature-controlled noise source as a reference. The calibration in 2023 February and March reveals that the solar brightness temperature is 11,636 K at 37.25 GHz with a standard deviation (STD) of 652 K. The solar radio flux’s intensity is ∼3000–4000 solar flux units (SFU) in the range of 35–40 GHz with a consistency bias of ±5.3%. The system sensitivity is about ∼5–8 SFU by a rough evaluation, a noise factor of about 200 K, and the coefficient of variation of the system transmission slope of 6.5% @ 12 hr at 37.25 GHz. It is expected that the upgraded CBS will capture more activity during the upcoming solar cycle.

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 Multichannel Parallel Real-time FFT Algorithm for a Solar Radio Observation System Based on FPGA

The real-time fast Fourier transform (FFT) is the essential algorithm for signal processing in a solar radio receiver. However, field-programmable gate array (FPGA) computation resources have become the limitation of real-time processing of signals with increasing time and spectral resolutions. It is necessary to design a real-time parallel FFT algorithm with reduced resource occupation in the development of future receiving systems. In this paper, we developed a multichannel parallel FFT algorithm named the multichannel parallel real-time fast Fourier transform (MPR-FFT), which can greatly reduce FPGA resource occupation while increasing the real-time processing speed. In this algorithm, the 4L simultaneous N-point FFTs are first converted into L simultaneous 4N-point FFTs. Fusion processing is then performed to obtain the 4 ∗ L ∗ N-point spectrum. This method has been used in developing a solar radio spectrometer, which works in the frequency range of 0.5–15 GHz in the Chashan Observatory. In this spectrometer, 16 channel MPR-FFT with 8k-point data is realized in a Xilinx UltraScale KU115 FPGA. The MPR-FFT algorithm reduced the computational resources to a large extent compared to the Cooley-Tukey-based parallel FFT method; for instance, the Look-Up-Table, Look-Up-Table RAM, Flip-Flop, and Digital Signal Process slices were reduced by 37%, 50%, 17%, and 2.48%, respectively. Although the MPR-FFT consumes 14 block RAM resources more than the Cooley-Tukey-based parallel FFT, the MPR-FFT algorithm presents an overall reduction in resource usage.

Decimetric Type-U Solar Radio Bursts and Associated EUV Phenomena on 2011 February 9

Abstract A GOES M1.9 flare took place in active region AR 11153 on 2011 February 9. With a resolution of 200 kHz and a time cadence of 80 ms, the reverse-drifting (RS) type-III bursts, intermittent sequence of type-U bursts, drifting pulsation structure (DPS), and fine structures were observed by the Yunnan Observatories Solar Radio Spectrometer (YNSRS). Combined information revealed by the multiwavelength data indicated that after the DPS was observed by YNSRS, the generation rate of type-U bursts suddenly increased to 5 times what it had been. In this event, the generation rate of type-U bursts may depend on the magnetic-reconnection rate. Our observations are consistent with previous numerical simulation results. After the first plasmoid produced (plasma instability occurred), the magnetic-reconnection rate suddenly increased by 5 to 8 times. Furthermore, after the DPS, the frequency range of the turnover frequency of type-U bursts was obviously broadened to thrice what it was before, which indicates a fluctuation amplitude of the density in the loop top. Our observations also support numerical simulations during the flare-impulsive phase. Turbulence occurs at the top of the flare loop and the plasmoids can trap nonthermal particles, causing density fluctuation at the loop top. The observations are generally consistent with the results of numerical simulations, helping us to better understand the characteristics of the whole physical process of eruption.

Statistical analysis of solar radio fiber bursts and relations with flares

Fiber bursts are a type of fine structure that frequently occurs in solar flares. Although observations and theory of fiber bursts have been studied for decades, their microphysical process, emission mechanism, and especially the physical links with the flaring process still remain unclear. We performed a detailed statistical study of fiber bursts observed by the Chinese Solar Broadband Radio Spectrometers in Huairou with high spectral-temporal resolutions in the frequency ranges of 1.10−2.06 GHz and 2.60−3.80 GHz during 2000−2006. We identify more than 900 individual fiber bursts in 82 fiber events associated with 48 solar flares. From the soft X-ray observations of the Geostationary Operational Environmental Satellite, we found that more than 40% of fiber events occurred in the preflare and rising phases of the associated solar flares. Most fiber events are temporally associated with hard X-ray bursts observed by RHESSI or microwave bursts observed by the Nobeyama Radio Polarimaters, which implies that they are closely related to the nonthermal energetic electrons. The results indicate that most fiber bursts have a close temporal relation with energetic electrons. Most fiber bursts are strongly polarized, and their average duration, relative bandwidth, and relative frequency-drift rate are about 1.22 s, 6.31%, and −0.069 s−1. The average duration and relative bandwidth of fiber bursts increase with solar flare class. The fiber bursts associated with X-class flares have a significantly lower mean relative frequency-drift rate. The average durations in the postflare phase are clearly longer than the duration in the preflare and rising phases. The relative drift rate in the rising phase is clearly higher than that in preflare and postflare phases. The hyperbola correlation of the average duration and the relative drift rate of the fiber bursts is very interesting. These characteristics are very important for understanding the formation of solar radio fiber bursts and for revealing the nonthermal processes of the related solar flares.

Study of the truncation strategy in the FPGA of a solar radio digital receiver

Abstract Computation resource is the limiting factor in higher operational accuracy of field-programmable gate arrays (FPGAs) in solar radio digital receivers. The data truncation strategy which determines the accuracy of data is then the essential technology in the design of a receiving system. Based on the solar radio spectrometer (dual channel, 14 bit, 1.25 gigasamples per second) at the Chashan Solar Radio Observatory (CSO), this paper presents a data truncation strategy which can realize real-time solar radio observation (35–40 GHz) with high time and frequency resolution as well as a large dynamic range, and at the same time saves the computation resource to a large extent. Simulations of truncations during signal processing are carried out in MATLAB, and the best truncation mechanisms are deduced for windowing and fast Fourier transform (FFT). Using the simulation results, the best truncation strategies have been implemented in the solar radio receiver at CSO with the result that the best truncation bits for the windowing operation are [27 : 14], with an error of 2.5 × 10−4, and the best truncation bits for the FFT output are [20 : 5] with an error of 1.5 × 10−3. Compared with the processing of full-precision data, occupation of the computation resources in the FPGA can be reduced significantly. For instance, the lookup table, lookup table RAM, flip flop, and digital signal processing slices are reduced by 7.36%, 14.65%, 8.38%, and 24.94%, respectively, which guarantees broad-band real-time solar radio observations (35–40 GHz).

A broadband digital receiving system with large dynamic range for solar radio observation

Solar radio spectra and their temporal evolution provide important clues to understand the energy release and electron acceleration process in the corona, and are commonly used to diagnose critical parameters such as the magnetic field strength. However, previous solar radio telescopes cannot provide high-quality data with complete frequency coverage. Aiming to develop a generalized solar radio observing system, in this study, we designed a digital receiving system that could capture solar radio bursts with a broad bandwidth and a large dynamic range. A dual-channel analog-to-digital converter (ADC) printed circuit board assembly (PCBA) with a sampling rate of 14-bit, 1.25 Giga samples per second (GSPS) cooperates with the field-programmable-gate-array (FPGA) chip XC7K410T in the design. This receiver could realize the real-time acquisition and preprocessing of high-speed data of up to 5 GB s−1, which ensures high time and spectral resolutions in observations. This receiver has been used in the solar radio spectrometer working in the frequency range of 35 to 40 GHz in Chashan Solar Observatory (CSO) established by Shandong University, and will be further developed and used in the solar radio interferometers. The full-power bandwidth of the PCBA in this receiving system could reach up to 1.5 GHz, and the performance parameters (DC–1.5 GHz) are obtained as follows: spur free dynamic range (SFDR) of 64.7–78.4 dB, signal-to-noise and distortion (SINAD) of 49.1–57.2 dB, and effective number of bits (ENOB) of > 7.86 bit. Based on the receiver that we designed, real-time solar microwave dynamic spectra have been acquired and more solar microwave bursts with fine spectral structures are hopeful to be detected in the coming solar maximum.

Increase in Interference Levels in the 45 – 870 MHz Band at the Spanish e-CALLISTO Sites over the Years 2012 and 2019

Two sets of radio-frequency interference (RFI) measurements in the 45 – 870 MHz band are compared. The first set was taken in 2012 at various sites in the province of Guadalajara (Spain) as part of a worldwide site-testing campaign for the deployment of an international network of solar radio-spectrometers, the Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Observatory (e-CALLISTO) array. Peralejos de las Truchas was found to be an ideal location, even for high-sensitivity non-solar observations, with the lowest interference levels ever measured in the framework of e-CALLISTO. The same set of measurements have been repeated seven years later using the same experimental setup at the same locations. The results presented in this article show that the RFI levels after seven years have notably increased, at some places by a factor of two, thereby placing at risk broadband spectroscopic radio-astronomy studies from the ground.

LSTM neural network for solar radio spectrum classification

A solar radio spectrometer records solar radio radiation in the radio waveband. Such solar radio radiation spanning multiple frequency channels and over a short time period could provide a solar radio spectrum which is a two dimensional image. The vertical axis of a spectrum represents frequency channel and the horizontal axis signifies time. Intrinsically, time dependence exists between neighboring columns of a spectrum since solar radio radiation varies continuously over time. Thus, a spectrum can be treated as a time series consisting of all columns of a spectrum, while treating it as a general image would lose its time series property. A recurrent neural network (RNN) is designed for time series analysis. It can explore the correlation and interaction between neighboring inputs of a time series by augmenting a loop in a network. This papermakes the first attempt to utilize an RNN, specifically long short-termmemory (LSTM), for solar radio spectrum classification. LSTM can mine well the context of a time series to acquire more information beyond a non-time series model. As such, as demonstrated by our experimental results, LSTM can learn a better representation of a spectrum, and thus contribute better classification.

A self-adaptive method for the determination of solar bursts for high-resolution solar radio spectrometer

The solar radio spectrum analyzer is the main means of observing solar radio bursts. Although increasing the temporal resolution allows the fine structure of the solar radio spectrum to be observed in more detail, it also increases exponentially the amount of data received, thereby creating difficulties for the receiving system. To reduce the amount of data handled by the high-time-resolved spectrometer when the Sun is quiet, this paper proposes a self-adaptive method for determining solar bursts by selecting 20 frequencies for radio flux profiles. A solar burst is assumed to be occurring if the radio flux increases by a factor of 200 in 500 ms at one or more frequencies, whereupon the data are stored with a time resolution of up to 1 ms. The end of the burst is determined when the real-time flux drops below the average flux over this 500 ms period. Finally, the data observed every 100 ms during the quiet Sun are averaged and stored, thereby reducing the amount of data storage. The method has been commissioned at the Chashan Solar Observatory in China: during solar flares and coronal mass ejections, the fine structure in the corresponding radio spectra is recorded with high time resolution to understand better the associated dynamic processes, whereas when the Sun is quiet the data are saved at low time resolution, thereby reducing the amount of stored data by a factor of 100 (can be set as needed). All of this helps make the system suitable for long-term observation. As for (i) the time resolutions of the data stored in the solar-burst and quiet periods and (ii) the parameters used to judge the threshold of a radio burst (e.g., 500 ms, 200 times, etc.), these can be set and changed as required. The data-processing method used in this paper can act as a reference for similar high-speed big-data systems.

The Application Design of Solar Radio Spectrometer Based on FPGA

The Solar radio spectrometer is the key instrument to observe solar radio. By programing the computer software, we control the AD signal acquisition card which is based on FPGA to get a mass of data. The data are transferred by using PCI-E port. This program has realized the function of timing data collection, finding data in specific time and controlling acquisition meter in real time. It can also map the solar radio power intensity graph. By doing the experiment, we verify the reliability of solar radio spectrum instrument, in the meanwhile, the instrument simplifies the operation in observing the sun.

Calibration of Solar Radio Spectrometer of the Purple Mountain Observatory

Calibration is a basic and important job in solar radio spectral observations. It not only deduces the solar radio flux as an important physical quantity for solar observations, but also deducts the flat field of the radio spectrometer to display the radio spectrogram clearly. In this paper, we first introduce the basic method of calibration based on the data of the solar radio spectrometer of Purple Mountain Observatory. We then analyze the variation of the calibration coefficients, and give the calibrated results for a few flares. These results are compared with those of the Nobeyama solar radio polarimeter and the hard X-ray observations of the RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager) satellite, it is shown that these results are consistent with the characteristics of typical solar flare light curves. In particular, the analysis on the correlation between the variation of radio flux and the variation of hard X-ray flux in the pulsing phase of a flare indicates that these observations can be used to study the relevant radiation mechanism, as well as the related energy release and particle acceleration processes.

Spatially resolved observations of a coronal type II radio burst with multiple lanes

Relative dynamics of the radio sources of the metric type II burst with three emission lanes and coronal mass ejection (CME) occurred in the lower corona (r≲1.5R⊙) during the SOL2011-02-16T14:19 event is studied. The observational data of the Nancay Radioheliograph (NRH) and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) are used. These observations are also supplemented by the data sets obtained with the STEREO-A and -B, RHESSI, and GOES spacecraft, as well as with the ground-based solar radio spectrometers. It is found that the sources of the radio burst were located ahead of the expanding CME and had a complex spatial structure. The first and the second lanes were both emitted from the “magnetic funnel” — a bundle of open magnetic field lines separated the south and north systems of magnetic loops of the active region. Due to the projection effect and limited angular resolution of the NRH it is not possible to determine, whether the spatial locations of the radio sources of the two first emission lanes differed or not. It is argued that the observations support the hypothesis that the radio sources of the first and second lanes could be emitted respectively ahead of and behind a front of the same weak (the Alfvén Mach number MA≈1.1–1.2), fast mode, quasi-parallel piston MHD shock wave. However, the third lane of the burst was definitely emitted from a different place. Its radio sources were situated ahead of the north-west part of the CME propagated through the north system of magnetic loops. This indicates clearly that different emission lanes of the same type II burst can be a result of propagation of different parts of a single CME through regions with different physical conditions (geometries and plasma densities) in the lower corona.

Decimetric and metric digital solar radio spectrometers of the Yunnan Astronomical Observatories and the first-light results

Based on an old decimetric solar radio spectrometer working in the frequency range of 625–1500MHz of the Yunnan Astronomical Observatories (YNAO) during the last solar cycle, we designed a fully digital Fast Fourier Transform (FFT) spectrometer to upgrade the old one. The new digital spectrometer has the spectral resolution of 200kHz, much higher than the old one (about 1.3MHz). In addition, we also established a new metric solar radio telescope working in the frequency range of 70–700MHz located at the Fuxian Solar Observatory of YNAO, deploying the same type of the digital FFT spectrometer. The two instruments have begun to operate in a daily survey mode since September 2009 and March 2012, respectively, and many solar radio bursts have been observed. In these events, various types of decimetric and metric fine structures with fairly meticulous spectral features were recognized. These features were never resolved in previous observation and studies. We have introduced these two instruments with their detailed technological components, as well as a set of observational data obtained during the first-light of the instruments. The information revealed by these data can improve our knowledge and understanding of the physics of the energy conversion, particle acceleration and transportation during the solar eruption.

Observations of Low Frequency Solar Radio Bursts from the Rosse Solar-Terrestrial Observatory

The Rosse Solar-Terrestrial Observatory (RSTO; ) was established at Birr Castle, Co. Offaly, Ireland (53°05′38.9″, 7°55′12.7″) in 2010 to study solar radio bursts and the response of the Earth’s ionosphere and geomagnetic field. To date, three Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy in Transportable Observatory (CALLISTO) spectrometers have been installed, with the capability of observing in the frequency range of 10 – 870 MHz. The receivers are fed simultaneously by biconical and log-periodic antennas. Nominally, frequency spectra in the range of 10 – 400 MHz are obtained with four sweeps per second over 600 channels. Here, we describe the RSTO solar radio spectrometer set-up, and present dynamic spectra of samples of type II, III and IV radio bursts. In particular, we describe the fine-scale structure observed in type II bursts, including band splitting and rapidly varying herringbone features.

Imaging interplanetary CMEs at radio frequency from solar polar orbit

Coronal mass ejections (CMEs) represent a great concentration of mass and energy input into the lower corona. They have come to be recognized as the major driver of physical conditions change in the Sun–Earth system. Consequently, observations of CMEs are important for understanding and ultimately predicting space weather conditions. This paper discusses a proposed mission, the Solar Polar Orbit Radio Telescope (SPORT) mission, which will observe the propagation of interplanetary CMEs to distances of near 0.35 AU from the Sun. The orbit of SPORT is an elliptical solar polar orbit. The inclination angle between the orbit and ecliptic plane should be about 90°. The main payload on board SPORT will be an imaging radiometer working at the meter wavelength band (radio telescope), which can follow the propagation of interplanetary CMEs. The images that are obtained by the radio telescope embody the brightness temperature of the objectives. Due to the very large size required for the antenna aperture of the radio telescope, we adopt interferometric imaging technology to reduce it. Interferometric imaging technology is based on indirect spatial frequency domain measurements plus Fourier transformation. The SPORT spacecraft will also be equipped with a set of optical and in situ measurement instruments such as a EUV solar telescope, a solar wind ion instrument, an energetic particle detector, a magnetometer, a wave detector and a solar radio burst spectrometer.

Calibration of the solar radio spectrometer

This paper shows some improvements and new results of calibration of Chinese solar radio spectrometer by analyzing the daily calibration data recorded in the period of 1997–2007. First, the calibration coefficient is fitted for three bands (1.0–2.0 GHz, 2.6–3.8 GHz, 5.2–7.6 GHz) of the spectrometer by using the moving-average method confined by the property of the daily calibration data. By this calibration coefficient, the standard deviation of the calibration result was less than 10 sfu for 95% frequencies of 2.6–3.8 GHz band in 2003. This result is better than that calibrated with the constant coefficient. Second, the calibration coefficient is found in good correlation with local air temperature for most frequencies of 2.6–3.8 GHz band. Moreover, these results are helpful in the research of the quiet solar radio emission.

CONSTRUCTION OF AN E-CALLISTO STATION IN KOREA

The e-CALLISTO is a global network of frequency-agile solar radio spectrometers that was constructed in a collaboration between Swiss Federal Institute of Technology Zurich (ETH Zurich) and local host institutes. It is intended to monitor solar radio bursts 24 hours a day in frequency range between 45 MHz and 870 MHz. One of e-CALLISTO spectrometer was installed at Korea Astronomy and Space Science Institute (KASI) in 2007 October. The spectrometer gets signals from a horizontally polarized log-periodic antenna mounted on an automatic Sun-tracking system. Tracking status and data are monitored in Space Weather Monitoring Laboratory (SWML) of KASI in real time, and flare time data are transferred to ETH Zurich data archive daily. Using this spectrometer we obtained a couple of type II solar radio bursts on 2007 December 31, and found that these bursts are associated with a CME which occurred on the east limb.