Radio Spectrum Observations and Studies of the Solar Broadband Radio Dynamic Spectrometer (SBRS)

Abstract

Solar radio spectral observation is one of the essential approaches for solar physics research, which helps us study the plasma dynamics in the solar atmosphere. The Solar Broadband Radio Dynamic Spectrometer (SBRS) started observing the Sun at Huairou Solar Observing Station in Beijing, China, in 1999. It has obtained a large amount of high-quality observation data of solar radio dynamic spectra in the centimeter–decimeter wavelengths (1.10–7.60 GHz). In particular, the observations with high-temporal resolution of millisecond and high-frequency resolution of MHz display plenty of superfine structures in the dynamic spectrum, which provide crucial information on the radiation process of various radio bursts. We review the past history of solar radio spectral observation and scientific results of SBRS. It is meaningful and will undoubtedly help us inspire new ideas for future research. The understanding of the basic plasma processes in solar plasma could also promote the development of solar physics, astrophysics, and space weather. To broaden the observation frequency range, we propose a new spectrometer at millimeter wavelengths (20–100 GHz) with ultra-wideband and high time–frequency resolution to study the physical processes in the solar transition region. This will open a new window for solar physics research and will provide crucial observational evidence for exploring a series of major issues in solar physics, including coronal heating, solar eruptions, and the origin of solar winds.

1. Introduction

Solar radio studies are essential aspects of solar physics research, and radio observation is an important part of multi-band observation of the Sun [1]. Although the energy radiated in radio wavelengths of the Sun is almost negligible, the radio signals have extremely significant responses to almost all the physical processes on the Sun. Solar radio observation can be obtained in three modes: the flux at single frequency by the radiometer, the spectrum in a frequency range by the spectrometer, and the image at single- or multi-frequency bands by the synthetic aperture array [2,3]. It covers an extremely wide wavelength range, i.e., submillimeter, millimeter, centimeter, decimeter, meter, decameter, kilometer wave, and so on. The radio signals include the almost unchanged component of the quiet Sun, the gradual component of the active regions, and the fast-changing and intensive component of various processes of solar activity at different altitudes of the solar atmosphere [4,5,6,7,8]. The observations provide us a good chance to investigate not only the long time evolution of the solar active regions and atmosphere but also the drastically changed processes of the solar activity. Radio bursts are always recorded in solar activities, which display very complex structures on the spectrum and in the imaging distribution. They are sensitive to nonthermal particle acceleration and propagation, plasma instabilities, magnetic fields and variations, magnetic reconnections, and various scales of plasma ejections and shock waves, etc. [9,10]. From the broadband dynamic spectral observations with high temporal–spectral resolutions, we can identify many spectral parameters, which may reflect different dynamic processes during the process of solar activity. Combined with the radio imaging observations, we can know the locations of the bursts, which helps us better understand the evolution of various processes of solar activity.

As a naturally upward extension of the optical magnetic field measurements of the solar photosphere and chromosphere, radio observations can be applied to diagnose the magnetic field in the huge, very hot, and diluted corona [11,12,13,14,15,16]. Other parameters, like plasma density, temperature, energetic electron density or energy, source size, and so on, can also be diagnosed from radio observations [17,18,19,20,21]. With these measurements, we may contribute to more extensive aspects of solar physics and space weather, including the mechanism of coronal heating, energetic particle acceleration and propagation, the origin of violent solar eruptions (like solar flares, coronal mass ejection (CME), jets, etc.) and their temporal and spatial evolutions, and provide crucial information for predicting disastrous space weather events [22]. Moreover, the solar atmosphere can be regarded as a natural plasma laboratory. The study of the radio radiation and propagation process in the solar atmosphere could promote the comprehensive investigation of the basic plasma process. It is useful for understanding the physical processes in other celestial environments. Therefore, solar radio astronomy has become a burgeoning field affecting almost all branches of solar physics, space weather, and even other branches of astrophysics.

With the development of related technology, many solar radio telescopes have been installed and operated, such as Nobeyama Radio Polarimeters (NoRP) [23], the Radio Solar Telescope Network (RSTN), Phoenix spectrometers [24,25], SBRS [1], RT5 of the spectrometers located at Ondřejov [26], Hiraiso Radio Spectrograph (Hiras) [27], Nobeyama radioheliograph (NoRH) [28], Nançay Radio Heliograph (NRH) [29], Siberian Solar Radio Telescope (SSRT) [30], Mingantu Spectral Radioheliograph (MUSER) [31], Expanded Owens Valley Solar Array (EOVSA) [32], Low-Frequency Array (LOFAR) [33], Daocheng Solar Radio Telescope (DART) [34] and so on. They have obtained plenty of high-quality radio data of flux, spectra, or images, and have driven many new discoveries and understandings in solar physics. Here, we do not intend to cover all the results based on the data available. On the occasion of celebrating the 40th anniversary of the Huairou Solar Observing Station, we would like to review the history of the observations and studies of SBRS at Huairou Solar Observing Station and look forward to our future research.

In the 1990s, the Chinese Solar Broadband Radio Spectrometer (SBRS) was successfully developed, which included three components, and they were located at the National Astronomical Observatories in Beijing, Purple Mountain Observatory in Nanjing, and Yunnan Observatory in Kunming. The SBRS in Beijing was first set up at Shahe Station and then transferred to Huairou Solar Observing Station in 1999 (Figure 1). We generally labeled the instruments as SBRS/Huairou [1]. Here, we mainly introduce the data and study based on the observations of SBRS/Huirou from 1999. The system of SBRS/Huairou covers decimeter–centimeter wavelengths and consists of three parts. Their observation frequency ranges are 1.10–2.06 GHz, 2.60–3.80 GHz, and 5.20–7.60 GHz, respectively. The main parameters of SBRS/Huairou are listed in

Table 1. Main parameters of the Solar Broadband Radio Spectrometers in Huairou [1].

Frequency (GHz)	Diameter of Antenna (m)	Δf (MHz)	Δt (ms)	Polarization Accuracy
1.1–2.06 (1.10–1.34)	7.3	4	5 (1.25)	<10%
2.60–3.80	3.2	10	8	<10%
5.20–7.60	3.2	20	5	<10%
2.84	2.0	$\pm 40$ MHz	200	<10%

. The antenna with a diameter of 7.3 m recorded the radio signal at 1.10–2.06 GHz with 5 ms temporal resolution and 4 MHz frequency resolution. It has been uprated to a higher level of 1.25 ms temporal resolution at the 1.1–1.34 GHz band. The antenna with a diameter of 3.2 m observed the signal at 2.60–3.80 GHz and at 5.20–7.60 GHz with 5/8 ms temporal resolution and 10/20 MHz frequency resolution. All antennas had very high sensitivity (<2%) and recorded with dual polarization (right and left polarized signal). The accuracy of the polarization degree is less than 10%. There is also a single frequency radiometer at 2.84 GHz with an antenna diameter of 2.0 m, which provides daily F10.7 flux data for space weather forecasting.

2. Studies Based on the SBRS Observations in Decimeter–Centimeter Wavelengths

As we all know, in the radio spectrum, many fine structures (FSs) can be shown and labeled, like type I, type II, type III, type IV, continuum, spike, fiber, zebra pattern, quasiperiodic pulsations, patches, and so on, as shown in the work of [35,36,37,38,39]. These structures consist of different spectral parameters, like bandwidth, duration, frequency drift rate, polarization degree, and so on. These parameters help us to understand the dynamic process of the radiation and diagnose the physical condition of the emission source. In the observations of SBRS/Huairou, almost all kinds of fine structures have been recorded. Moreover, in the very-high-temporal and -frequency resolution data, many new and superfine structures have been identified and studied. These new spectral features promote the research of radio radiation mechanisms and plasma dynamic processes in solar atmosphere. Based on the observations of SBRS/Huairou since 1999, more than 200 peer-reviewed articles have been published in international journals which reported many important discoveries related to the solar radio emission mechanism or provided new clues to understand the evolution of solar activity. In the following, we present a brief introduction of some of the outstanding results obtained from the SBRS/Huairou observations.

2.1. Statistical Study of Radio Bursts and Radio Fine Structures During Flare/CME Events

Since 1999, SBRS has been in the frame of regular observation and it has recorded a great number of flare/CME events during the daytime in Beijing. Yan et al. [40] reported 91 radio burst events at 1–7.6 GHz, which were accompanied by flares and CMEs in 1997–2003 in the 23rd solar activity cycle. They found that 38 (41.7%) of the 91 events contained FSs (or FS groups); 26 FSs occurred during the rising phase, 16 during the maximum, and 8 in the decaying phase. Huang et al. [41] investigated 27 solar microwave burst events at 2.6–3.8 GHz, accompanied by M/X class flares and fast CMEs. In these events, 234 fine structures were distinguished and classified. They found that more than 70% of the fine structures occur before the microwave peak at 2.84 GHz. They conducted a further survey at three bands (1.1–2.06, 2.6–3.8, 5.2–7.6 GHz) [42]. It was found at the lower frequency band, the amount and species of radio FS are larger than the higher band. The pulsations and type III bursts display the most frequent occurrence and more radio FSs occur before the soft X-ray (SXR) maximum in almost all the events. These results show us the scene that more radio FSs take place at the higher altitude above the active region, which must be around the reconnection region or particle acceleration site. The physical condition there could be very complex to produce fast and various plasma dynamic processes. In addition, more FSs appear before or during the flare peak, which suggests that radio bursts are closely related to energy release, particle acceleration, and propagation.

2.2. New Features of Radio Fine Structures and Their Diagnostic Parameters

With high time and frequency resolution, many new features and superfine structures of FSs have been discovered and studied. From their spectral parameters, the conditions of the emission source, like the magnetic field strength and orientation, plasma temperature, and density, the moving speed of the source or electrons could be deduced and investigated. In the following, we show several typical samples of events and report the methods to deduce the physical condition of the sources.

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Type III bursts

Type III bursts are typical spectral structures in the dynamic spectrum, which can appear in both low and high frequency ranges. The structure of type III bursts is shown as a bright stripe drifting from high to low frequency or vice versa [10]. In one group of type III bursts, the stripes always drift in the same direction, but pairs of type III bursts with inverse frequency drifting can also be recorded in the microwave spectrum. As seen in Figure 2, the type III bursts at 2.6–3.1 GHz shift from high to low frequency, while those at 3.5–3.8 GHz shift from low to high frequency. It is proposed that they are emitted by electron beams traveling along the magnetic field through plasma emission. On 16 February 1999, SBRS recorded several type III bursts with different frequency drifting rates. Ning et al. [43] proposed that they were emitted by the accelerated electrons with different speeds.

In the observations of SBRS at 5.2–7.6 GHz, Altyntsev et al. [44] found a sub-second type III burst, which displayed a very fast frequency drift rate (6 GHz s⁻¹). With one-dimensional scans of SSRT 5.7 GHz, they measured the relative positions of burst sources and their velocities along a flare loop from soft X-ray and extreme-ultraviolet images. They put forward a new explanation for the very fast drifting type III burst, i.e., the drifting structure is caused by the dynamic density increase in the region where emission occurred. The quantitative estimations are also consistent with the theoretical models of magnetic reconnection with reasonable boundary conditions. Chen and Yan [45] presented new features of short-lived absorptive type III-like microwave bursts, which consist of a number of absorptive short-duration “spikes”. For each stripe, the duration at a single frequency band is less than the instrument resolution of 8 ms. They suggested that fragmented electron injections with durations of as short as several milliseconds into the loss cone produce short-lived spikes along fast-drifting lines.

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Zebra patterns

Microwave zebra patterns are the most interesting and intriguing phenomenon observed during solar flares (Figure 3). Many people have contributed to studies of the formation and application of microwave zebra patterns [46,47,48,49,50,51,52,53,54,55,56,57]. Chernov et al. [46,47,48,49] made a series of work to analyze the spectral feature and model of zebra patterns. In the work of [46], the authors found that the stripes of zebra patterns at 2.6–3.8 GHz display a superfine structure, consisting of separate spike-like pulses of millisecond duration. They considered the superfine structure of the zebra pattern bursts as the coupling of plasma electrostatic waves with whistlers. The work of [48] presents another feature of superfine structure in zebra patterns. They found fine structures consisting of narrow-band fiber bursts as substructures of large-scale zebra-pattern stripes. They assumed two different radiation models produce this complex structure of zebra pattern. The coalescence of whistler waves with a plasma wave produced the superfine fiber bursts and the conventional double plasma resonance model emitted the large-scale zebra pattern structures. Altyntsev et al. [50] reported a group of infrequent zebra patterns at 5.2–7.6 GHz of SBRS data. They proposed the generation mechanism of zebra structures in terms of the conversion of plasma waves to electromagnetic emission on the double plasma resonance surfaces distributed across a flare loop.

Quasiperiodic structures usually display in the spectral structure of zebra patterns. On the event of 21 April 2002, Chen and Yan [51] found pulsating superfine structures on the typical zebra pattern. The observed that quasiperiodic pulsations with about 30 ms periods could probably have resulted from relaxation oscillations, which modulate the process of wave–particle interactions to produce oscillatory electron cyclotron maser emissions to form the zebra stripes. During an X class flare on 13 December 2006, Yu et al. [52] analyzed the quasiperiodic wiggles of zebra patterns at 2.6–3.8 GHz. They proposed that these wiggles can be associated with the fast magnetoacoustics oscillations in the flaring active region. These modulate the local plasma condition and produce quasiperiodic emissions in zebra patterns. In the work of [54], the authors performed numerical simulations of standing and propagating sausage oscillations in a coronal loop modeled as a straight, field-aligned plasma slab to interpret the zebra pattern wiggles.

Based on the abundant spectral feature of zebra patterns and many proposed models, Tan et al. [55] classified the microwave zebra patterns into three kinds according to frequency separation and its variation. The Bernstein wave model produces the zebra patterns with equidistant frequency separation, the whistler wave model emits zebra patterns of variable-distant separation, and the double plasma resonance model emits growing-distant zebra patterns, respectively. Each kind of zebra pattern shows characteristic features and their formation mechanism can interpret those spectral features appropriately. At the same time, as the frequency separation of zebra stripes is proportional to the magnetic field, we may also apply it to directly diagnose the magnetic field in the source region [56]. In addition, we can also use the distribution and parameter variation to understand the dynamics of energetic electrons during solar flares. In the impulsive phase of the M class flare on 1 December 2004, Huang et al. [57] found two groups of zebra patterns of different frequency separation between two stripes. The appearance of zebra patterns also corresponds to impulsive peaks of HXR emissions. They proposed that the decreasing of the separation of zebra patterns is caused by the decrease in the magnetic field strength of the flaring loop. The occurrence of the zebra patterns indicates the propagation of energetic electrons inside the flaring loop and coupling between the energetic electrons and the local plasma.

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Quasiperiodic pulsations

In the spectrum, quasiperiodic pulsations (QPPs) are displayed as a group of arranged, bright stripe structures, and each stripe may show a frequency drifting structure. They are usually produced by plasma emissions of energetic electron beams. QPPs are an important oscillated signal of the source region for their physical parameters and dynamic processes. Due to the very wide frequency range and very high temporal–spectral resolutions of radio broadband spectrum observations, we may extract many parameters of QPPs (including period, duration, lifetime, bandwidth, frequency drifting rate, polarization degree, brightness temperature, etc.) and diagnose the detailed information of the source region and its dynamic processes [58,59,60,61,62,63,64]. Sometimes QPPs display as a spectral drift of the global structure, which is named DPS [60,61]. Karlicky et al. [60] conducted a study of drifting structures in the spectra using SBRS data and the observations of the Ondrejov radiospectrographs from the Czech Republic. The work of [61] presents that the QPPs at 200–700 MHz band show a frequency drift of the global structure towards the lower frequency range. It is proposed that DPS is produced by the electron beams inside the plasmoid, which involves the upward movement of the source and, thus, the global frequency shift in QPPs. In addition, QPPs usually appear in conjunction with other fine structures, like zebra pattern, spikes, type III bursts, patches, and so on [41,62]. Figure 4 show a sample of QPPs with spikes filling in each bright pulse. It indicates that there should be more than one plasma processes to produce the radio bursts.

QPPs may occur at any stage of the solar flares, i.e., before the onset of the flare [63], in the impulsive and peak phase [60,61], and also in the decay phase [59]. From the statistical study, we found that they prefer to occur in the impulsive and maximum phases [41,42]. Based on the observations of SBRS/Huairou in an X3.4 flare/CME event on 2006 December 13, Tan et al. [64] found that the flaring process behavior produced multi-timescale microwave QPPs. The timescale ranged from hectoseconds to milliseconds and formed a broad hierarchy, which implied that the source region should have complex plasma loop systems and be undergoing a highly dynamic processes with different time and space scales and energy releases.

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Microwave spike bursts

Microwave spike bursts should be the shortest-duration and narrowest-band bursts related to solar eruptions (Figure 5). The observations of SBRS/Huairou at decimeter–centimeter wavelengths indicated that the microwave spike bursts have a lifetime of milliseconds, a bandwidth of about 1% of the central frequency, strong polarization, and a super high brightness temperature (up to ${10}^{16}$ K). With these characteristics, we may deduce that the source region should be much smaller than 1 arc-second. Spikes could be recorded at broad frequency ranges, which are proposed to be produced by energetic electrons in the frame of cyclotron maser radiation or plasma emission.

Spikes always appear in groups with random distribution in the spectrum, and the whole group of spikes shows a systematic frequency shift towards low or high bands. It is interesting that, in the observations of SBRS/Huairou, the spikes can also be structured by other spectral fine bursts. The whole group of FSs displays as microwave zebra patterns, QPPs, or type III bursts for a relatively long duration. When we zoom in to see the detailed structure, it is found that the bright structure is composed of a number of short and narrow-band spikes [45,62,65]. Therefore, many people have proposed that the microwave spike bursts should be the elementary bursts (EBs), which represent the smallest energy release process. A large number of EB clusters may form an intense flaring microwave burst and may even form a flare [62,66,67].

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Other spectral fine structures

Additionally, SBRS/Huairou observed many unique and interesting spectral fine structures associated with solar flares, such as the microwave fish-group structure [68], fiber bursts [69,70], lace bursts (Figure 6) [59], M-shaped structure [71], U-shaped structure [72,73], and many other peculiar structures. So far, we are not clear on the formation mechanism of these spectral fine structures. They possibly reflect the coupling process between the nonthermal particles and the plasma waves in the source regions.

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Diagnostics of magnetic fields in corona

Radio emissions in the atmosphere display various features, which are produced by different mechanisms. The emission of the quiet Sun is emitted by thermal plasma through bremsstrahlung. The gradual intense emission of the active region without solar activity is produced by cyclotron resonance radiation. In solar activity, radio emissions include gyrosynchrotron emissions, plasma emissions, and cyclotron maser radiation. The radio emission of the Sun spreads a wide space from the chromosphere, transition region, and corona. In solar physics, it is a big challenge to measure the coronal magnetic fields. Until now, it has been difficult to measure the coronal magnetic field by direct or indirect measurement. The continuous observation in frequency and high accuracy in intensity and polarization allow us to infer the strength and orientation of the magnetic field of the emitting sources in corona. Although the emission processes of radio bursts are complex and the emission mechanisms involve a lot of parameters, diagnostics of magnetic fields in corona from radio emission could be regarded as a feasible and referential approach.

Using the spectral observations of SBRS on 13 December 2006, Yan et al. [74] found various fine structures (like spikes, reverse slope-type III bursts, type-U burst, V-shaped burst, pulsations, spiky zebra patterns) in the X 3.4 class flare. They studied the spectral features of these fine structures and estimated the coronal magnetic field and the source density in the rising phase and around the flare maximum, respectively. Based on the spectral parameters of various fine structures and their proposed models, Tan et al. [75] proposed a set of explicit diagnostic functions for corona magnetic fields from the solar radio observations, which included emission mechanisms of bremsstrahlung, cyclotron, gyrosynchrotron, synchrotron, and the coherent plasma emissions, and covered the solar quiet region, active region, and flare source regions.

3. Future: Open a New Field—Spectral Observations in Millimeter Wavelengths

So far, the spectral observations in centimeter, decimeter, meter, and even decameter wavelengths are mature and have accumulated a great amount of data. However, the spectral observations in millimeter wavelengths are still very rare and almost nonexistent in the world. At the same time, millimeter wave emissions mainly come from the solar transition region, which is a key layer for understanding the mystery of coronal heating, the origin of solar flares, and solar winds. The super-wide broadband spectral dynamic observation in the millimeter wavelength will add new information for the study of the solar transition region. Therefore, we propose a new plan to observe the nonthermal or thermal emission signals of the solar transition region at millimeter wavelengths.

First, we will construct a tested broadband millimeter spectrometer (test-SUBMS) in Saisteng Mountain in Qinghai province, China. There is a ready-made astronomical observation station, and the altitude is 4030 m, far from big cities and modern industrial centers. The air there is very clean and very dry with an annual humidity <10%. Figure 7 displays the antenna and the location for test-SUBMS. The parameters of test-SUBMS can be found in the work of [76].

The second step is to set up a space-based solar ultra-wide broadband millimeter wave spectrometer (SUBMS) with an orbital altitude of 400–450 km and an inclination angle of 42–43°. Such an orbit can completely avoid the absorption of water and oxygen molecules and transparency fluctuations in the Earth’s atmosphere, thus achieving complete and stable observation of the Sun in full frequencies. When the SUBMS is built and put into observation, we will be the first in the world to obtain ultra-wide frequency range and high temporal–spectral resolution solar dynamic spectrum observations in the millimeter wavelength. The observations will fill the gap for the radio signal detection in the solar transition region and open a new window for the study of the origin of solar eruptions and the mystery of coronal heating.