Investigation on the Formation of Herringbone Structure in TypeⅡSolar Radio Bursts

We report detailed observation of the“herringbone”of a Type II solar radio burst that occurred on 2010 November 3rd.Data from the Space Weather Prediction Center,National Oceanic and Atmospheric Administration,e-CALLISTO,and Nan?ay Radio Heliograph are analyzed.We determine the brightness temperature and degree of circular polarization of the“herringbone”burst.Correlations between the physical parameters and the“herringbone”are examined.Based on the relationship,this is the first study that suggested this“herringbone”was generated through fundamental plasma.

A review of solar type Ⅲ radio bursts

Solar type III radio bursts are an important diagnostic tool in the understanding of solar accelerated electron beams. They are a signature of propagating beams of nonthermal electrons in the solar atmosphere and the solar system. Consequently, they provide information on electron acceleration and transport, and the conditions of the background ambient plasma they travel through. We review the observational properties of type III bursts with an emphasis on recent results and how each property can help identify attributes of electron beams and the ambient background plasma. We also review some of the theoretical aspects of type III radio bursts and cover a number of numerical efforts that simulate electron beam transport through the solar corona and the heliosphere.

Recent results of zebra patterns in solar radio bursts

This review covers the most recent experimental results and theoretical research on zebra patterns （ZPs） in solar radio bursts. The basic attention is given to events with new peculiar elements of zebra patterns received over the last few years. All new properties are considered in light of both what was known earlier and new theoretical models. Large-scale ZPs consisting of small-scale fiber bursts could be explained by simultaneous inclusion of two mechanisms when whistler waves ＂high- light＂ the levels of double plasma resonance （DPR）. A unique fine structure was observed in the event on 2006 December 13： spikes in absorption formed dark ZP stripes against the absorptive type Ⅲ-like bursts. The spikes in absorption can appear in accordance with well known mechanisms of absorptive bursts. The additional injection of fast particles filled the loss-cone （breaking the loss-cone distribution）, and the generation of the continuum was quenched at these moments. The maximum absorptive effect occurs at the DPR levels. The parameters of millisecond spikes are determined by small dimensions of the particle beams and local scale heights in the radio source. Thus, the DPR model helps to understand severai aspects of unusual elements of ZPs. However, the simultaneous existence of several tens of the DPR levels in the corona is impossible for any realistic profile of the plasma density and magnetic field. Three new theories of ZPs are examined. The formation of eigenmodes of transparency and opac- ity during the propagation of radio waves through regular coronal inhomogeneities is the most natural and promising mechanism. Two other models （nonlinear periodic space - charge waves and scattering of fast protons on ion-sound harmonics） could happen in large radio bursts.

Historical Dataset Reconstruction and a Prediction Method of Solar 10.7cm Radio Flux

We reconstruct the developing history of solar 10.7 cm radio flux （F10.7） since 1848, based on the yearly sunspot number and the variations. A relationship between the maximum and the linear regression slope of the first 3 years starting from minimum of the solar cycle is considered. We put forward a method of predicting the maximum of F10.7 by means of the slope-maximum relationship. Running tests for cycles 19 to 23 indicate that the method can properly predict the peak of F10.7.

BoxCox multi-output linear regression for 10.7 cm solar radio flux prediction

We consider the problem of predicting the mid-term daily 10.7 cm solar radio flux(F10.7),a widely-used solar activity index.A novel approach is proposed for this task,in which BoxCox transformation with a proper parameter is first applied to make the data satisfy the property of homoscedasticity that is a basic assumption of regression models,and then a multi-output linear regression model is used to predict future F10.7 values.The experiment shows that the BoxCox transformation significantly improves the predictive performance and our new approach works substantially better than the prediction from the US Airforce and other alternative methods like Auto-regressive Model,Multi-layer Perceptron,and Support Vector Regression.

Wavelet Cleaning of Solar Dynamic Radio Spectrograms

By applying the state-of-the-art mathematical apparatus, the wavelet transformation, we explore the possibility of a dynamic cleaning of raw data ob- tained with the Chinese solar radio spectrographs over a wide wavelength range (from 0.7 to 7.6 GHz). We consider the problem of eliminating the interference caused by combination rates of data sampling (10-20 ins), and the low-frequency interference (4-30 s) caused by the receiving equipment changing its characteristics with time. It is shown that the best choice to reconstruct a signal suffering from amplitude, frequency and phase instabilities, is by means of wavelet transformation at both high and low frequencies. We analysed observational data which contained interferences of nonsolar origin such as instrumental effects and other man-made signals. A subsequent comparison of the reference data obtained with the acousto- optical receiver of the Siberian Solar Radio Telescope (SSRT) with the 'cleaned' spectra confirms the correctness of this approach.

Diagnostic Functions of Solar Coronal Magnetic Fields from Radio Observations

In solar physics,it is a big challenge to measure the magnetic fields directly from observations in the upper solar atmosphere,including the chromosphere and corona.Radio observations are regarded as the most feasible approach to diagnose the magnetic field in solar chromosphere and corona.However,because of the complexity and diversity of the emission mechanisms,the previous studies have only presented the implicit diagnostic functions of the magnetic field for specific mechanism from solar radio observations.This work collected and sorted out all methods for diagnosing coronal magnetic field from solar radio observations,which are expressed as a set of explicit diagnostic functions.In particular,this work supplemented some important diagnostic methods missed in other reviews.This set of diagnostic functions can completely cover all regions of the solar chromosphere and corona,including the quiet region,active region and flaring source regions.At the same time,it also includes incoherent radiation such as bremsstrahlung emission of thermal plasma above the quiet region,cyclotron and gyro-synchrotron emissions of magnetized hot plasma and mildly relativistic nonthermal electrons above the active regions,as well as coherently plasma emission around flaring source regions.Using this set of diagnostic functions and the related broadband spectral solar radio imaging observations,we can derive the magnetic fields of almost all regions in the solar atmosphere,which may help us to make full use of the spectral imaging observations of the new generation solar radio telescopes(such as MUSER,EVOSA and the future FASR,etc.) to study the solar activities,and provide a reliable basis for the prediction of disastrous space weather events.

Synthesising solar radio images from Atmospheric Imaging Assembly extreme-ultraviolet data

During non-flaring times,the radio flux of the Sun at wavelengths of a few centimeters to several tens of centimeters mostly originates from thermal bremsstrahlung emission,very similar to extremeultraviolet(EUV) radiation.Owing to such a proximity,it is feasible to investigate the relationship between the EUV emission and radio emission in a quantitative way.In this paper,we reconstruct the radio images of the Sun through the differential emission measure obtained from multi-wavelength EUV images of the Atmospheric Imaging Assembly on board Solar Dynamics Observatory(SDO).Through comparing the synthetic radio images at 6 GHz with those observed by the Siberian Radioheliograph,we find that the predicted radio flux is qualitatively consistent with the observed value,confirming thermal origin of the coronal radio emission during non-flaring times.The results further show that the predicted radio flux is closer to the observations in the case that includes the contribution of plasma with temperatures above 3 MK than in the case of only involving low temperature plasma,as was usually done in the pre-SDO era.We also discuss applications of the method and uncertainties of the results.

Solar Radio Bursts in the Period Oct. 22-Nov. 4, 2003

A series of solar radio bursts were observed in AR NOAA 10486 with the SolarBroadband (1.1-7.6 GHz) Radio Spectrometers (SBRS of China). Here we analyze four significant eventsassociated with CME events and strong X-ray flares that occurred on 2003 October 22, 26, 27, 29.The Oct. 26 event is a long duration event (LDE) with drift pulsation structure (DPS), narrowbanddm-burst (DCIM), and more than seven types of Fine Structures (FSs); its time of the maximum flux(07:30 UT) is about half an hour later than the X-flare (06:54 UT).

Observational Characteristics of Radio Emission Related to Multi-polar Magnetic Configuration

We present a large complex radio burst and its associated fast time structures observed on 2001 April 10 in the frequency range of 0.65-7.6 GHz. The NoRH radio image observation shows very complex radio source structures which include preexisting, newly emerging, submerging/cancelling polarities and a bipolar, a tripolar （a ＇bipolar ＋ remote unipolar＇）, and a quadrupolar structure. This suggests that the radio burst is generated from a very complicated loop structure. According to the spectral and image observations, we assume that the beginning of this flare was caused by a single bipolar loop configuration with a ‘Y-type＇ re- connection structure. A composite of radio continuum and fast time structures is contained in this flare. The various fast radio emission phenomena include normal and reverse drifting type Ⅲ bursts, and slowly drifting and no-drift structures. The tripolar configurations may form a double-loop with a ＇three-legged＇ struc- ture, which is an important source of the various types of fast time structures. The two-loop reconnection model can lead simultaneously to electron acceleration and corona heating. We have also analyzed the behaviors of coronal magnetic polarities and the emission processes of different types radio emission qualitatively. Interactions of a bipolar or multi-polar loop are consistent with our observational results. Our observations favor the magnetic reconnection configurations of the ‘inverted Y-type＇ （bipolar） and the ‘three-legged＇ structures （tripolar or quadrupo- lar）.

A New Solar Radio Spectrometer at 1.10-2.06 GHz and First Observational Results

An improved Solar Radio Spectrometer working at 1.10-2.06 GHz with much improved spectral and temporal resolution, has been accomplished by the National Astronomical Observatories and Hebei Semiconductor Research Institute,based on an old spectrometer at 1 2 GHz. The new spectrometer has a spectral resolution of 4 MHz and a temporal resolution of 5 ms, with an instantaneous detectable range from 0.02 to 10 times of the quiet Sun flux. It can measure both left and right circular polarization with an accuracy of 10% in degree of polarization. Some results of preliminary observations that could not be recorded by the old spectrometer at 1-2 GHz are presented.

A solar radio dynamic spectrograph with flexible temporal-spectral resolution

Observation and research on solar radio emission have unique scientific values in solar and space physics and related space weather forecasting applications, since the observed spectral structures may carry important information about energetic electrons and underlying physical mechanisms. In this study, we present the design of a novel dynamic spectrograph that has been installed at the Chashan Solar Radio Observatory operated by the Laboratory for Radio Technologies, Institute of Space Sciences at Shandong University. The spectrograph is characterized by real-time storage of digitized radio intensity data in the time domain and its capability to perform off-line spectral analysis of the radio spectra. The analog signals received via antennas and amplified with a low-noise amplifier are converted into digital data at a speed reaching up to 32 k data points per millisecond. The digital data are then saved into a high- speed electronic disk for further off-line spectral analysis. Using different word lengths （1-32k） and time cadences （5 ms-10 s） for off-line fast Fourier transform analysis, we can obtain the dynamic spectrum of a radio burst with different （user-defined） temporal （5 ms-10 s） and spectral （3 kHz-320kHz） resolutions. This enables great flexibility and convenience in data analysis of solar radio bursts, especially when some specific fine spectral structures are under study.

A 96-antenna radioheliograph

Here we briefly present some design approaches for a multifrequency 96-antenna radioheliograph. The configuration of the array antenna, transmission lines and digital receivers is the main focus of this work. The radioheliograph is a T-shaped centrally condensed radiointerferometer operating in the frequency range 4–8 GHz.The justification for the choice of such a configuration is discussed. The signals from antennas are transmitted to a workroom by analog optical links. The dynamic range and phase errors of the microwave-over-optical signal are considered. The signals after downconverting are processed by digital receivers for delay tracking and fringe stopping. The required step of delay tracking and data rates are considered. Two 3-bit data streams（I and Q） are transmitted to a correlator with the transceivers embedded in Field Programmed Gate Array chips and with PCI Express cables.

Are Homologous Radio Bursts Driven by Solar Post-Flare Loops?

Three particularly complex radio bursts （2001 October 19, 2001 April 10 and 2003 October 26） obtained with the spectrometers （0.65-7.6GHz） at the National Astronomical Observatories, Chinese Academy of Sciences （NAOC, Beijing and Yunnan） and other instruments （NoRH, TRACE and SXT） are presented. They each have two groups of peaks occurring in different frequency ranges （broad-band microwave and narrow-band decimeter wavelengths）. We stress that the second group of burst peaks that occurred in the late phase of the flares and associated with post-flare loops may be homologous radio bursts. We think that they are driven by the post-flare loops. In contrast to the time profiles of the radio bursts and the images of coronal magnetic polarities, we are able to find that the three events are caused by the active regions including main single-bipole magnetic structures, which are associated with multipole magnetic structures during the flare evolutions. In particular, we point out that the later decimetric radio bursts are possibly the radio counterparts of the homologous flares （called ＂homologous radio bursts＂ by us）, which are also driven by the single-bipole mag- netic structures. By examining the evolutions of the magnetic polarities of sources （17 GHz）, we could presume that the drivers of the homologous radio bursts are new and/or recurring appearances/disappearances of the magnetic polarities of radio sources, and that the triggers are the magnetic reconnections of single-bipole configurations.

Statistical properties of radio flux densities of solar flares

Short timescale flux variations are closely related to the energy release process of magnetic reconnection during solar flares.Radio light curves at 1,2,3.75,9.4,and 17 GHz of 209 flares observed by the Nobeyama Radio Polarimeter from 2000 to 2010 are analyzed with a running smooth technique.We find that the impulsive component(with a variation timescale shorter than 1 s)of 1 GHz emission of most flares peaks at a few tens of solar flux unit and lasts for about 1 minute and the impulsive component of 2 GHz emission lasts a shorter period and peaks at a lower flux level,while at the three high frequency channels the occurrence frequency of flares increases with the decrease of the flux density up to the noise level of the corresponding background.However,the gradual components of these emissions have similar duration and peak flux density distributions.We also derive the power spectrum on different timescales and a normalized wavelet analysis is used to confirm features on short timescales.At a time resolution of 0.1 s,more than^60%of these radio light curves show significant flux variation on 1 s or shorter time scales.This fraction increases with the decrease of frequency and reaches^100%at 1 GHz,implying that short timescale processes are universal in solar flares.We also study the correlation between the impulsive radio flux densities and soft X-ray fluxes obtained with the GOES satellites and find that more than 65%of the flares with an impulsive component have their impulsive radio emission reach a peak value ahead of the soft X-ray fluxes and this fraction increases with the radio frequency.

Flare evolution and polarization changes in fine structures of solar radio emission in the 2013 April 11 event

The measurement of positions and sizes of radio sources in observations is important for un- derstanding of the flare evolution. For the first time, solar radio spectral fine structures in an M6.5 flare that occurred on 2013 April 11 were observed simultaneously by several radio instruments at four different observatories： Chinese Solar Broadband Radio Spectrometer at Huairou （SBRS/Huairou）, Ondrejov Radio Spectrograph in the Czech Republic （ORSC/Ondrejov）, Badary Broadband Microwave Spectropolarimeter （BMS/Irkutsk）, and spectrograph/IZMIRAN （Moscow, Troitsk）. The fine structures included microwave zebra patterns （ZPs）, fast pulsations and fiber bursts. They were observed during the flare brightening lo- cated at the tops of a loop arcade as shown in images taken by the extreme ultraviolet （EUV） telescope onboard NASA＇s satellite Solar Dynamics Observatory （SDO）. The flare occurred at 06：58-07：26 UT in solar active region NOAA 11719 located close to the solar disk center. ZPs appeared near high frequency boundaries of the pulsations, and their spectra observed in Huairou and Ondrejov agreed with each other in terms of details. At the beginning of the flare＇s impulsive phase, a strong narrowband ZP burst occurred with a moderate left-handed circular polarization. Then a series of pulsations and ZPs were observed in almost unpolarized emission. After 07：00 UT a ZP appeared with a moderate right-handed polarization. In the flare decay phase （at about 07：25 UT）, ZPs and fiber bursts become strongly right-hand polarized. BMS/Irkutsk spectral observations indicated that the background emission showed a left-handed circular polarization （similar to SBRS/Huairou spectra around 3 GHz）. However, the fine structure appeared in the right-handed polarization. The dynamics of the polarization was associated with the motion of the flare ex- citer, which was observed in EUV images at 171 A and 131 A by the SDO Atmospheric Imaging Assembly （AIA）. Combining magnetograms observed by the SDO Helioseismic and Magnetic Imager （HMI） with the homologous assumption of EUV flare brightenings and ZP bursts, we deduced that the observed ZPs correspond to the ordinary radio emission mode. However, future analysis needs to verify the assumption that zebra radio sources are really related to a closed magnetic loop, and are located at lower heights in the solar atmosphere than the source of pulsations.

DSC based Dual-Resunet for radio frequency interference identification

Radio frequency interference(RFI)will pollute the weak astronomical signals received by radio telescopes,which in return will seriously affect the time-domain astronomical observation and research.In this paper,we use a deep learning method to identify RFI in frequency spectrum data,and propose a neural network based on Unet that combines the principles of depthwise separable convolution and residual,named DSC Based Dual-Resunet.Compared with the existing Unet network,DSC Based Dual-Resunet performs better in terms of accuracy,F1 score,and MIoU,and is also better in terms of computation cost where the model size and parameter amount are 12.5%of Unet and the amount of computation is 38%of Unet.The experimental results show that the proposed network is a high-performance and lightweight network,and it is hopeful to be applied to RFI identification of radio telescopes on a large scale.

Solar radio filtering algorithm based on improved long short-term memory

The effective observation of burst events in solar radio research has been impeded by various interference signals,especially interference signals with a wide frequency range and high intensity,as they can partially or completely obscure the observation of burst events.Image processing methods that directly remove the interference signal channels and subtract the average of the interference signal channel are not suitable for processing all types of interference signals.This paper proposes the use of a specific kind of recurrent neural networks,called long short-term memory networks,to predict the value of the radio frequency interference signals with high intensity of the burst event in the solar radio spectrum.The predicted interference can then be removed in accordance with the principle that signals can be linearly added.Therefore,predicted value is subtracted from the data containing the burst event signals and the RFI signals(The radio frequency interference signals to be processed in this article refer to the signal of the broadcast signal that can be received in the frequency range,the signal transmitted by the mobile phone,and the signal transmitted by the sea vessel,and the like)to remove the interference.Then,in order to reduce the error caused by the stepwise prediction in the network and further improve the prediction accuracy,this paper analyzes the characteristics of the value of the radio interference and applies the digital mapping method to convert the prediction problem into the classification problem in the time series.The experimental results show that the proposed method can effectively remove the radio interference in the solar spectrum and clearly show the burst events.

Generating a radioheliograph image from SDO/AIA data with the machine learning method

Radioheliograph images are essential for the study of solar short term activities and long term variations, while the continuity and granularity of radioheliograph data are not so ideal, due to the short visible time of the Sun and the complex electron-magnetic environment near the ground-based radio telescope. In this work, we develop a multi-channel input single-channel output neural network, which can generate radioheliograph image in microwave band from the Extreme Ultra-violet(EUV) observation of the Atmospheric Imaging Assembly(AIA) on board the Solar Dynamic Observatory(SDO). The neural network is trained with nearly 8 years of data of Nobeyama Radioheliograph(No RH) at 17 GHz and SDO/AIA from January 2011 to September 2018. The generated radioheliograph image is in good consistency with the well-calibrated No RH observation. SDO/AIA provides solar atmosphere images in multiple EUV wavelengths every 12 seconds from space, so the present model can fill the vacancy of limited observation time of microwave radioheliograph, and support further study of the relationship between the microwave and EUV emission.

CALLISTO facilities in Peru:spectrometer commissioning and observations of typeⅢsolar radio bursts

The Astrophysics Directorate of CONIDA has installed two radio spectrometer stations belonging to the e-CALLISTO network in Lima,Peru.Given their strategic location near the Equator,it is possible to observe the Sun evenly throughout the whole year.The receiver located at Pucusana,nearby the capital city of Lima,took data from October 2014 until August 2016 in the metric and decimetric bands looking for radio bursts.During this period,this e-CALLISTO detector was unique in its time-zone coverage.To asses the suitability of the sites and the performance of the antennas,we analyzed the radio ambient background and measured their radiation pattern and beamwidth.To demonstrate the capabilities of the facilities for studying solar dynamics in these radio frequencies,we have selected and analyzed typeⅢSolar Radio Bursts.The study of this kind of burst helps to understand the electron beams traversing the solar corona and the solar atmospheric density.We have characterized the most common radio bursts with the following mean values:a negative drift rate of-25.8±3.7 MHz s^(-1),a duration of 2.6±0.3 s and 35 MHz bandwidth in the frequency range of 114 to 174 MHz.In addition,for some events,it was possible to calculate a global frequency drift which on average was 0.4±0.1 MHz s^(-1).