Chinese Solar Broadband Radio Spectrometer Research Articles

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 understanding 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), Ondřejov Radio Spectrograph in the Czech Republic (ORSC/Ondřejov), 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 located 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 Ondřejov 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 exciter, which was observed in EUV images at 171 Å and 131 Å 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.

STATISTICS AND CLASSIFICATION OF THE MICROWAVE ZEBRA PATTERNS ASSOCIATED WITH SOLAR FLARES

The microwave zebra pattern (ZP) is the most interesting, intriguing, and complex spectral structure frequently observed in solar flares. A comprehensive statistical study will certainly help us to understand the formation mechanism, which is not exactly clear now. This work presents a comprehensive statistical analysis on a big sample with 202 ZP events collected from observations at the Chinese Solar Broadband Radio Spectrometer at Huairou and the Ondrejov Radiospectrograph in Czech Republic at frequencies of 1.00 - 7.60 GHz during 2000 - 2013. After investigating the parameter properties of ZPs, such as the occurrence in flare phase, frequency range, polarization degree, duration, etc., we find that the variation of zebra stripe frequency separation with respect to frequency is the best indicator for a physical classification of ZPs. Microwave ZPs can be classified into 3 types: equidistant ZP, variable-distant ZP, and growing-distant ZP, possibly corresponding to mechanisms of Bernstein wave model, whistler wave model, and double plasma resonance model, respectively. This statistical classification may help us to clarify the controversies between the existing various theoretical models, and understand the physical processes in the source regions.

QUASI-PERIODIC WIGGLES OF MICROWAVE ZEBRA STRUCTURES IN A SOLAR FLARE

Quasi-periodic wiggles of microwave zebra pattern structures with period range from about 0.5 s to 1.5 s are found in a X-class solar flare on 2006 December 13 at the 2.6-3.8 GHz with the Chinese Solar Broadband Radio Spectrometer (SBRS/Huairou). Periodogram and correlation analysis show that the wiggles have two-three significant periodicities and almost in phase between stripes at different frequency. The Alfven speed estimated from the zebra pattern structures is about 700 Km/s. We obtain the spatial size of the waveguiding plasma structure to be about 1 Mm with the detected period of about 1 s. It suggests the ZP wiggles can be associated with the fast mag- netoacoustic oscillations in the flaring active region. The lack of a significant phase shift between wiggles of different stripes suggests that the ZP wiggles are caused by a standing sausage oscillation.

SMALL-SCALE MICROWAVE BURSTS IN LONG-DURATION SOLAR FLARES

Solar small scale microwave bursts (SMBs), including microwave dot, spike, and narrow band type III bursts, are characterized with very short timescales, narrow frequency bandwidth, and very high brightness temperatures. Based on observations of the Chinese Solar Broadband Radio Spectrometer at Huairou with superhigh cadence and frequency resolution, this work presents an intensive investigation of SMBs in several flares occurred in active region NOAA 10720 during 2005 Jan 14-21. Especially long-duration flares, SMBs occurred not only in early rising and impulsive phase, but also in the flare decay phase, and even in time of after the flare ending. These SMBs are strong bursts with inferred brightness temperature at least 8.18*10^11 - 1.92*10^13 K, very short lifetime of 5-18 ms, relative frequency bandwidths of 0.7-3.5%, and superhigh frequency drifting rates. Together with their obviously different polarizations from the background emission (the quiet Sun, and the underlying flaring broadband continuum), such SMBs should be individual independent strong coherent bursts which is related to some non-thermal energy releasing and production of energetic particles in small scale source region. These facts show the existence of small scale strong non-thermal energy releasing activities after the flare maxima, which is meaningful to the prediction of space weather. Physical analysis indicates that plasma mechanism may be the most favorable candidate for the formation of SMBs. From plasma mechanism, the velocities and kinetic energy of fast electrons can be deduced, and the region of electron acceleration can also be tracke.

RELAXATION OF MAGNETIC FIELD RELATIVE TO PLASMA DENSITY REVEALED FROM MICROWAVE ZEBRA PATTERNS ASSOCIATED WITH SOLAR FLARES

It is generally considered that the emission of microwave zebra pattern (ZP) structures requires high density and high temperatures, which is similar to the situation of the flaring region where primary energy releases. Therefore, the parameters analysis of ZPs may reveal the physical conditions of the flaring source region. This work investigates the variations of 74 microwave ZP structures observed by Chinese Solar Broadband Radio Spectrometer (SBRS/Huairou) at 2.6-3.8 GHz in 9 solar flares, and found that the ratio between the plasma density scale height LN and the magnetic field scale height LB in emission source displays a tendency of decrease during the flaring processes. The ratio LN/LB is about 3-5 before the maximum of flares. It decreases to about 2 after the maximum. The detailed analysis of three typical X-class flares implied that the variation of LN/LB during the flaring process is most likely due to the topological changes of the magnetic field in the flaring source region, and the step-wise decrease of LN/LB possibly reflects the magnetic field relaxation relative to the plasma density when the flaring energy released. This result may also constrain the solar flare modeling to some extent.

Micorwave observations of the Chinese Solar Broadband Radio Spectrometer at Huairou

AbstractThis work presents an overview of the microwave observations of the Chinese Solar Broadband Radio Spectrometer at Huairou (SBRS/Huairou) during 1997-2011. The relationships between the microwave bursts and solar flares and the calibration of spectrometers are also studied.

Relaxation of magnetic field relative to plasma density during solar flares

AbstractWe investigated the variations of 74 microwave ZP structures observed by Chinese Solar Broadband Radio Spectrometer at 2.6–3.8 GHz in 9 solar flares, found that the ratio between the plasma density scale height LN and the magnetic field scale height LB in emission source displays a tendency of decrease during the flaring process, indicates that LB increases faster than the LN during solar flares. The detailed analysis of the step-wise decrease of LN/LB in three typical X-class flares reveals the magnetic field relaxation relative to the plasma density.

MICROWAVE BURSTS WITH FINE STRUCTURES IN THE DECAY PHASE OF A SOLAR FLARE

This paper presents the microwave bursts with fine structures (FSs) at 1.10-1.34 GHz in the decay phase of a solar flare observed by the Chinese Solar Broadband Radio Spectrometer in Huairou, which show a peak-to-peak correlation with 25-50 keV hard X-ray (HXR) bursts observed by RHESSI. In the microwave spectra, we have identified stripe-like bursts such as lace bursts, fiber structures, zebra patterns (ZPs), and quasi-periodic pulsations. We also have detected short narrowband bursts such as dots, type III, and spikes. The lace bursts had rarely been reported, but in this event they are observed to occur frequently in the decay phase of the flare. The similarity between 25 and 50 keV HXR light curve and microwave time profiles at 1.10-1.34 GHz suggests that these microwave FSs are related to the properties of electron acceleration. The electron velocity inferred from the frequency drift rates in short narrowband bursts is in the range of 0.13c-0.53c and the corresponding energy is about 10-85 keV, which is close to the energy of HXR-emitting electrons. From the Alfven soliton model of fiber structures, the double plasma resonance model of ZPs, and the Bernstein model of the lace bursts, we derived a similar magnetic fieldmore » strength in the range of 60-70 G. Additionally, the physical conditions of the source regions such as height, width, and velocity are estimated.« less

Microwave burst with fine spectral structures in a solar flare on 2011 August 9

On August 9, 2011, there was an X6.9 flare event occurred near the west limb of solar disk. From the observation obtained by the spectrometer of the Chinese Solar Broadband Radio Spectrometer in Huairou (SBRS/Huairou) around the flare, we find that this powerful flare has only a short-duration microwave burst of about only 5 minutes, and during the short-duration microwave burst, there are several kinds of fine structures on the spectrogram. These fine structures include very short-period pulsations, millisecond spike bursts, and type III bursts. The most interesting is that almost all of the pulses of very short-period pulsation (VSP) are structured by clusters of millisecond timescales of spike bursts or type III bursts. And there exists three different kinds of frequency drift rates in the VSP: the frequency drift rates with absolute value of about 55 - 130 MHz s^{-1} in the pulse groups, the frequency drift rates with absolute value of about 2.91 - 16.9 GHz s^{-1} on each individual pulse, and the frequency drift rates with absolute value of about 15 - 25 GHz s$^{-1}$) at each individual spike burst or type III burst.

MICROWAVE ZEBRA PATTERN STRUCTURES IN THE X2.2 SOLAR FLARE ON 2011 FEBRUARY 15

Zebra pattern structure (ZP) is the most intriguing fine structure on the dynamic spectrograph of solar microwave burst. On 15 February 2011, there erupts an X2.2 flare event on the solar disk, it is the first X-class flare since the solar Schwabe cycle 24. It is interesting that there are several microwave ZPs observed by the Chinese Solar Broadband Radiospectrometer (SBRS/Huairou) at frequency of 6.40 ~ 7.00 GHz (ZP1), 2.60 ~ 2.75 GHz (ZP2), and the Yunnan Solar Broadband Radio Spectrometer (SBRS/Yunnan) at frequency of 1.04 ~ 1.13 GHz (ZP3). The most important phenomena is the unusual high-frequency ZP structure (ZP1, up to 7.00 GHz) occurred in the early rising phase of the flare, and there are two ZP structure (ZP2, ZP3) with relative low frequencies occurred in the decay phase of the flare. By scrutinizing the current prevalent theoretical models of ZP structure generations, and comparing their estimated magnetic field strengths in the corresponding source regions, we suggest that the double plasma resonance model should be the most possible one for explaining the formation of microwave ZPs, which may derive the magnetic field strengths as about 230 - 345 G, 126 - 147 G, and 23 - 26 G in the source regions of ZP1, ZP2, and ZP3, respectively.

Spiky Fine Structure of Type III-like Radio Bursts in Absorption

An uncommon fine structure in the radio spectrum consisting of bursts in absorption was observed with the Chinese Solar Broadband Radiospectrometer (SBRS) in the frequency range of 2.6 – 3.8 GHz during an X3.4/4B flare on 13 December 2006 in active region NOAA 10930 (S05W33). Usual fine structures in emission such as spikes, zebra stripes, and drifting fibers were observed at the peak of every new flare brightening. Within an hour at the decay phase of the event we observed bursts consisting of spikes in absorption, which pulsated periodically in frequency. Their instantaneous frequency bandwidths were found to be in the 75 MHz range. Moreover, in the strongest Type III-like bursts in absorption, the spikes showed stripes of the zebra-pattern (ZP) that drifted to higher frequencies. All spikes had the duration as short as down to the limit of the instrument resolution of ≈8 ms. The TRACE 195 A images indicate that the magnetic reconnection at this moment occurred in the western edge of the flare loop arcade. Taking into account the presence of the reverse-drifting bursts in emission, in the course of the restoration of the magnetic structures in the corona, the acceleration of the beams of fast particles must have occurred both upward and downward at different heights. The upward beams will be captured by the magnetic trap, where the loss-cone distribution of fast particles (responsible for the emission of continuum and ZP) were formed. An additional injection of fast particles will fill the loss-cone later, breaking the loss-cone distribution. Therefore, the generation of continuum will be quenched at these moments, which was evidenced by the formation of bursts in absorption.

Broadband radio spectral observations of the solar eclipse on 2008-08-01 and its implications on the quiet Sun atmospheric model

Based on the joint-observations of the radio broadband spectral emissions of the solar eclipse on August 1, 2008 at Jiuquan (total eclipse) and Huairou (partial eclipse) at the frequencies of 2.00–5.60 GHz (Jiuquan), 2.60–3.80 GHZ (Chinese solar broadband radiospectrometer, SBRS/Huairou), and 5.20–7.60 GHz (SBRS/Huairou), the authors assemble a successive series of broadband spectra with a frequency of 2.60–7.60 GHz to observe the solar eclipse synchronously. This is the first attempt to analyze the solar eclipse radio emission under the two telescopes located at different places with broadband frequencies in the periods of total and partial eclipses. With these analyses, the authorsmade a semiempirical model of the coronal plasma density of the quiet Sun, which can be expressed as ne ≃ 1.42×109(r−2+1.93r−5) (cm−3), in the space range of r=1.039–1.212 R⊙, and made a comparison with the classic model.

Short‐Lived Absorptive Type III–like Microwave Bursts as a Signature of Fragmented Electron Injections

In this paper, we devote ourselves to interpreting the short-lived absorptive type III-like microwave bursts in the 2006 December 13 flare event observed with high temporal and spectral resolutions (8 ms and 10 MHz) by the Chinese Solar Broadband Radio Spectrometer (SBRS/Huairou) at 2.6-3.8 GHz. In the decimeter-centimeter wavelength range, we first present the observations of short-lived bursts represented as a number of absorptive "spikes" superposed on the type IV continuum that can be connected by fast-drifting lines. The mean drift rate, the instantaneous bandwidth, and the absorption depth of these absorptive spikes are about –12 GHz s−1, 70 MHz, and 40%, respectively. The duration at a single frequency band can be less than the instrument resolution of 8 ms. On the basis of numerical investigations of the loss-cone instability, we suggest that fragmented electron injections with durations of as short as several milliseconds into the loss cone could be the most appropriate mechanism with which to explain the bursts. The length of an electron beam is estimated to be about 400 km, on the basis of the observational results. These injections may be related to the fragmented energy release processes during the flare. We also observe some absorptive type III-like bursts accompanying ordinary type III bursts with reverse drifts. They start at the same frequency, and the starting frequency slowly drifts to the low-frequency region. This could be a signature of propagating bidirectional electron beams originating near the reconnection region.

Observable Parameters of Solar Microwave Pulsating Structure and Their Implications for Solar Flare

From the observations with the Chinese Solar Broadband Radiospectrometer (SBRS/Huairou) in the frequency range of 1.10 – 2.06 GHz and 2.60 – 3.80 GHz during 2004 – 2006, we select 14 flare events which were associated with numerous fast microwave subsecond pulsating structures (period: P<0.5 s). In order to describe these subsecond pulsating structures comprehensively, we defined a set of observable parameters including emission frequency (f 0), bandwidth (b w), polarization degree (r), period (P), duration (D), modulation depth (M), quality factor (Q), single pulse frequency drifting rate (R spfd), global frequency drifting rate (R gfd), and symmetrical factor of the pulse profile (S). Then based on a detailed analysis of the spectrograms of the fast pulsations which occurred in one of these flares (an X3.4 flare/CME event occurred on 13 Dec. 2006), we discuss the possible relations among these observable parameters and their physical implications for the dynamical processes of solar eruptive events, and applied them to interpret the nature of the pulsations in the flare/CME event. Such study of microwave periodic pulsations provides us with a useful tool to probe the details of the flare kernels, and understand the physical mechanism of solar eruptive processes.

Correlation between the Sharp Variation of the Transport Rate of Magnetic Helicity and Solar Eruptive Events

In this Letter we report a close relationship between the variations of the transport rate of magnetic helicity (dH/dt) and a microwave burst. The latter may be regarded as a prompt signal of nonthermal energetic particles originating from the magnetic reconnection during solar flaring events. We analyze the observations of magnetograms of MDI/SOHO and SOT/Hinode and the high-cadence microwave observation at 2.84 GHz obtained by the Chinese Solar Broadband Radiospectrometer (SBRS/Huairou) of a flare/CME event that occurred in NOAA Active Region 10930 on 2006 December 13. We find that there is a sharp jump of dH/dt around the onset and quench of a microwave burst at a frequency of 2.84 GHz: the rate of dH/dt changes from negative to positive around the start of the eruption and recovers to negative when the eruption stopped. Furthermore, the temporal profile of dH/dt is consistent with that of a microwave burst. These results indicate that sharp variations of dH/dt are closely related to the solar eruption.

The Microwave Pulsations and the Tearing Modes in the Current‐Carrying Flare Loops

Solar microwave observations of the X3.4 Flare/CME event observed in Chinese solar broadband radiospectrometer (SBRS/Huairou) on 2006 December 13 show a series of very short period pulsations (VSP) with the period of <1.0 s in the frequency range of 2.60-3.80 GHz. Many pulsating events have the period of only several tens of milliseconds. These pulsations are quasi-periodic, broad bandwidth, and ubiquitous during all the phases of the flare/CME event. Based on theoretical analysis of the temporal behavior of the resistive tearing mode in the electric current-carrying flare loops, we propose that microwave pulsations are a result of the modulation of the tearing-mode oscillations in the current-carrying flare loops. Our calculation of the period of the tearing-mode oscillations are in good agreement with the observations.