A Statistical Analysis of the Type-III Bursts in Centimeter and Decimeter Wavebands

A statistical analysis of the type-III bursts observed by the spectrographs in the ranges of 625∼1500 MHz, 2600-3800 MHz, and 5200-7600 MHz during the 23rd solar cycle (from 2000 July to 2004 September) is carried out. The distribution of the type-III bursts, and their durations, frequency drift rates, polarization degrees and frequency bandwidths are given in this paper. The results indicate that the average values of the frequency drift rates and frequency bandwidths increase with the frequency. The average values of the durations and polarization degrees are neither constant nor uniformly varied over a broad frequency range. Most of type-III bursts are distributed in the range from 625 to 3800 MHz, and decrease with the frequency in number. This analysis shows that the places of electron acceleration and energy release are mainly in the decimetric range, and the characteristic of this frequency range is possibly related with the magnetic configuration at the decimeter wavelengths, as well as the electron acceleration in the magnetic reconnection site close to the main flare. However, there are also a considerable number of type-III bursts in the range of 5200-7600 MHz, it means that the sites of electron acceleration are widely distributed in the coronal region. The radiation mechanisms of type-III bursts at the centimeter-decimeter wavelengths include most probably the coherent plasma radiation and the emission process of electron cyclotron maser.     

Occurrence of bidirectional type III bursts in solar flares

The 13 pairs of type III bursts with the bidirectional drift structures recorded with the spectrograph in the frequency ranges of 230–300 MHz and 625–1500 MHz at the Yunnan Observatory and 2600–3800 MHz at the Beijing National Astronomical Observatories are analyzed in this present article and the outstanding characteristics of these events are obtained. These bursts respectively reveal that the separatrix frequency between the bursts with positive and negative drifts comes between 250 MHz and 3420 MHz, with a gap being between 0.6 MHz and 110 MHz; the duration is 53 ms–1880 ms and the frequency drift rate is between 45 MHz/s and 56000 MHz/s. The drift rate at metric wavelengths is relatively low, only a few decades of MHz while it is comparatively high at microwave wavelengths, reaching 56000 MHz/s. The qualitative explanation of these events is given in this paper.     

Statistics and Analysis of Microwave Type III Bursts at Low Frequencies

68 groups of type III bursts recorded with the dynamic spectrograph (from1 to 2 GHz) at the Beijing Astronomical Observatory are statistically analyzed in this present article, with the frequency drift, duration, bandwidth and polarization of each of these events being analyzed in detail. These observational features are different from those of metric, decimetric and microwave high frequency type III bursts, showing that there possibly exist transition phenomena in the microwave low frequency band and the microwave type III bursts at low frequencies are possibly produced by the harmonic plasma radiation caused by nonthermal electronic beams and by the electron gyro-maser radiation. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Generating Region of Bidirectional Electron Beams in the Corona

Metric and decimetric type III bursts and microwave spike emissions with negative and positive frequency drift rates which were observed with radio spectrometers at Yunnan and Beijing Observatories are presented. The frequencies and heights at which the bidirectional electron beams originated are estimated. Three events reveal a separatrix frequency (at 250, 1300, and 2900 MHz) between normal- and reverse-drifting radio bursts, indicating a compact acceleration source where electron beams are injected in both upward and downward directions. These cases may indicate that the changeover frequencies of bidirectional electron beams are within a large band from 250 to 2900 MHz and the frequency bands of separatrices are in very small (4 to 100 MHz) and different bands. These type III bursts appear to be a plasma emission phenomenon from a beam of electrons which seem to have widely separated acceleration regions from the high to the low corona. These cases suggest that current sheets that separate open and closed magnetic fluxes in the low corona, and oppositely directed open field lines in the high corona are possible sites for bidirectional electron acceleration. The regions of magnetic topology from closed to open magnetic field structures should be very large (from about 20000 to 107000 km above the photosphere).     

微波Ⅲ型爆发的观测特征

统计分析了国家天文台2．6-3．8 GHz高时间分辨率射电动态频谱仪在23周峰年期间(1998．4—2003．1)观测到的266个III型爆发．对这些事件的频率漂移、持续时间、偏振、带宽、开始和结束频率做了详细分析．开始和结束频率的统计分析表明,开始频率在一个非常大的范围,从小于2．6 GHz到大于3．8 GHz,而结束频率的截止区相对集中,从2．82-3．76 G．Hz．这些现象说明,电子加速的高度相当分散,在观测频率范围内具有正、负漂移率的III型爆发数基本相等,这可能意味着被加速的向上和向下传播的电子束在2．6—3．8 GHz范围有相同的比例．统计结果表明,微波III型爆发的辐射机制主要是等离子体辐射和电子回旋脉泽辐射过程．     

Observational characteristics of microwave type III bursts

The 266 type III bursts, observed with the 2.6–3.8 GHz high temporal resolution dynamic spectrometer of NAO during the 23rd solar cycle (from April 1998 to January 2003), are statistically analyzed. The parameters of these events, including the frequency drift, duration, polarization, bandwidth, starting and ending frequencies, are analyzed in details. The statistics on the starting and ending frequencies indicate that the starting frequency varies in a very large range from less than 2.6 GHz to greater than 3.8 GHz, while the ending frequency varies in a relatively narrow range from 2.82 GHz to 3.76 GHz. These phenomena imply that the heights where the electrons are accelerated are quite scattered, while the cutoff regions of the type III bursts are relatively restricted. The numbers of the bursts with the positive and negative drift rates are nearly equal, this may suggest that the accelerated electrons propagating upward and those propagating downward are equally proportioned in the observing frequency range. And the statistical results demonstrate that the microwave type III bursts are mainly caused by the plasma radiation and electron gyro-maser radiation.     

太阳微波低频段射电III型爆发的一些特性分析

本文介绍了云南天文台四波段（１．４２,２．１３,２．８４和４．２６ＧＨｚ）太阳射电高时间分辩率同步观测得到的五个微波ＩＩ型爆发事件,它们具有宽频带、长和短寿命、内向和外向快速频漂等特征．观测事例表明,非热电子束引起的等离子体辐射和电子回旋脉泽辐射两种机制都可能发生．这些观测特征既不完全同于米波—分米波ＩＩ型爆发,也不完全同于微波高频段ＩＩ型爆发,说明在微波低频段可能存在二重性或过渡现象     

Statistical analysis of high-frequency decimetric type III bursts

Here we report the statistical analysis of 160 isolated decimetric type III bursts, a majority of them (74%) having central frequency above 1000 MHz, observed in 8 flares by the spectrometer Phoenix. The most important finding of the detailed analysis is: 67% of all the bursts are reverse slope, 38% normal and 5% bi-directional. Also, we obtained the following results: (a) the best fits for the average half power duration and for the average drift rate as a function of frequency are given by t_1/2=1.7×10⁴f^–0.60and /df/dt|=(0.09±0.03)f^{(1.35 ± 0.10)}, respectively; (b) the frequency range of most of the type III bursts is less than 250 MHz; (c) the number of bursts decreases with increasing starting frequency and flux; (d) peak flux decreases with increasing frequency. The relations obtained fit well for higher frequency observations. Assuming an improved density model and type III emission at 2^ndharmonic, the beam parameters of type III bursts are determined.     

A New Catalogue of Fine Structures Superimposed on Solar Microwave Bursts

The 2.6-3.8 GHz, 4.5-7.5 GHz, 5.2-7.6 GHz and 0.7-1.5 GHz component spectrometers of Solar Broadband Radio Spectrometer (SBRS) started routine observations, respectively, in late August 1996, August 1999, August 1999, and June 2000. They just managed to catch the coming 23rd solar active maximum. Consequently, a large amount of microwave burst data with high temporal and high spectral resolution and high sensitivity were obtained. A variety of fine structures (FS) superimposed on microwave bursts have been found. Some of them are known, such as microwave type Ⅲ bursts, microwave spike emission, but these were observed with more detail; some are new. Reported for the first time here are microwave type U bursts with similar spectral morphology to those in decimetric and metric wavelengths, and with outstanding characteristics such as very short durations (tens to hundreds ms), narrow bandwidths, higher frequency drift rates and higher degrees of polarization. Type N and type M bursts were also observed. Detailed zebra pattern and fiber bursts at the high frequency were found. Drifting pulsation structure (DPS) phenomena closely associated with CME are considered to manifest the initial phase of the CME, and quasi-periodic pulsation with periods of tens ms have been recorded. Microwave “patches”, unlike those reported previously, were observed with very short durations (about 300ms), very high flux densities (up to 1000 sfu), very high polarization (about 100% RCP), extremely narrow bandwidths (about 5%), and very high spectral indexes. These cannot be interpreted with the gyrosynchrotron process. A superfine structure in the form of microwave FS (ZPS,type U), consisting of microwave millisecond spike emission (MMS), was also found.     

The Source Regions of Impulsive Solar Electron Events

Low-energy (2–19 keV) impulsive electron events observed in interplanetary space have been traced back to the Sun, using their interplanetary type III radiation and metric/decimetric radio-spectrograms. For the first time we are able to study the highest frequencies and thus the radio signatures closest to the source region. All the selected impulsive solar electron events have been found to be associated with an interplanetary type III burst. This allows to time the particle events at the 2 MHz plasma level and identify the associated coronal radio emissions. Except for 5 out of 27 cases, the electron events were found to be associated with a coronal type III burst in the metric wavelength range. The start frequency yields a lower limit to the density in the acceleration region. We also search for narrow-band spikes at the start of the type III bursts. In about half of the observed cases we find metric spikes or enhancements of type I bursts associated with the start of the electron event. If interpreted as the plasma emission of the acceleration process, the observed average frequency of spikes suggests a source density of the order of 3×10⁸ cm^–3 consistent with the energy cut-off observed.     

Different time constants of solar decimetric bursts in the range 100–1000 MHz

Between 1980, January 1 and 1981, December 31 a total of 664 decimetric pulsation events, abbreviated DCIM, were observed with the Zürich spectrometers in the frequency range 100 to 1000 MHz. All of these events were recorded on film, allowing an effective resolution in time of 0.5 s, and 5 MHz in frequency. Some of these events were also recorded digitally with higher time and frequency resolution.The class of DCIM bursts can be divided into two groups depending on their duration and thus reflecting different physical mechanisms. Each of the two groups can be further divided into small and large bandwidth subgroups, reflecting differences in the source parameters. Short decimetric events ( 1s) are most abundant in this frequency range. They may be caused by fast transients in the solar atmosphere. The half-power bandwidth of the shortest DCIM bursts, the millisecond spikes, were found to be 6 to 12 MHz. A single event may consist of more than 1000 individual spikes. The long lasting DCIM bursts (5 s to 300 s) exhibit a gradual and smooth time profile. Such long lasting events indicate the presence of trapped particles in magnetic fields. In some events decimetric gyrosynchrotron radiation was observed below 1000 MHz as a continuation of corresponding microwave events.Some of the decimetric events exhibit very large drift rates (2000 MHz s^-1). Such large values request either a drastic reduction of the effective scale height of the active region in the beam model or a different explanation than the conventional beam model.     

米波窄带快频漂射电爆发──“Blips”的观测和证认

用云南天文台高时间分辨率（１０ｍｓ）高频率分辨率（０．５ＭＨｚ）的射电频谱仪观测分析证认了米波窄带短持续时间快频漂爆发的存在．这种爆发既不同于经典的ＩＩＩ型爆发，也不同于ｓｐｉｋｅ和Ｉ型爆发，是一种新的米波爆发型别．它的特性与分米波的“ｂｌｉｐｓ”相近．     

Electron beams and associated rapid fluctuations in solar flares

Recent observations show that the rapid fluctuations in radio, hard X-ray, and H emissions are closely associated with type III and microwave (or decimetric) bursts during the impulsive and/or preimpulsive phases of solar flares.In order to clarify the physical processes of these observed phenomena, this paper proposes a tentative model of two acceleration regions A (source of type III bursts) and B (source of microwave or decimetric bursts) formed in the neutral sheet and at the top of a flaring loop, respectively; and also suggests that the electron beams streaming from region A and/or region B downward to the chromosphere are responsible for the rapid fluctuations in the different emissions mentioned above during the impulsive and/or pre-impulsive phases of solar flares.     

Electron beams in the low corona

Selected high-resolution spectrograms of solar fast-drift bursts in the 6.2–8.4 GHz range are presented. The bursts have similar characteristics as metric and decimetric type III bursts: rise and decay in a few thermal collision times, total bandwidth 3% of the center frequency, low polarization, drift rate of the order of the center frequency per second, and flare association. They appear in several groups per flare, each group consisting of some tens of single bursts. Fragmentation is also apparent in frequency; there are many narrowband bursts randomly scattered in the spectrum. The maximum frequency of the bursts is highly variable.The radiation is interpreted in terms of plasma emission of electron beams at plasma densities of more than 10¹¹ cm^–-3. At this extremely high frequency, emission from the plasma level even at the harmonic is only possible in a very anisotropic plasma. The scale lengths perpendicular and parallel to the magnetic field can be estimated. A model of the source region and its environment is presented.Paper presented at the 4th CESRA Workshop in Ouranopolis (Greece) 1991.     

由对偶的Ⅲ型射电爆发观测探讨日冕双向电子束的起源问题

介绍云南和北京天文台射电频谱仪观测到的3个对偶的米波－－微波Ⅲ型爆发，估计了双向电子束起源的频率和高度，3个事件分别揭示了在正向和反向漂移爆发之间的分界频率（约为250，1300和2900MHz),它们指出了一个致密的电子加速源，在这个源中产生了向上和向下两个方向注入的电子束，从这些事例可以表明不同事件的双向电子束的分界频率有一个相当大的范围（250－2900MHz)，而它们的起源范围却是在一个很小的（大约4－100MHz)和不同的频段范围内。最后讨论了日冕磁结构的拓扑范围、电子加速源构造的空间尺度、电子束运动速度和对偶Ⅲ型爆发的产生机制。     

太阳米波和分米波Ⅱ型、Ⅲ型射电暴及其精细结构观测研究进展

太阳米波和分米波的射电观测是对太阳爆发过程中耀斑和日冕物质抛射现象研究的重要观测手段。米波和分米波的太阳射电暴以相干等离子体辐射为主导,表现出在时域和频域的多样性和复杂性。其中Ⅱ型射电暴是激波在日冕中运动引起电磁波辐射的结果。在Ⅱ型射电暴方面,首先对米波Ⅱ型射电暴的激波起源问题和米波Ⅱ型射电暴与行星际Ⅱ型射电暴的关系问题进行了讨论;其次,结合Lin-Forbes太阳爆发理论模型对Ⅱ型射电暴的开始时间和起始频率进行讨论:最后,对Ⅱ型射电暴信号中包含的两种射电精细结构,Herringbone结构(即鱼骨结构)和与激波相关的Ⅲ型射电暴也分别进行了讨论。Ⅲ型射电暴是高能电子束在日冕中运动产生电磁波辐射的结果。在Ⅲ型射电暴方面,首先介绍了利用Ⅲ型射电暴对日冕磁场位形和等离子体密度进行研究的具体方法;其次,对利用Ⅲ型射电暴测量日冕温度的最新理论进行介绍;最后,对Ⅲ型射电暴和Ⅱ型射电暴的时间关系、Ⅲ型射电暴和粒子加速以及Ⅲ型射电暴信号中包含的射电精细结构(例如斑马纹、纤维爆发及尖峰辐射)等问题进行讨论并介绍有关的最新研究进展。     

Observations of a complex solar radio burst with fine structure on 3 May 1973

The simultaneous high resolution recordings of dynamic spectra in the range 93–220 MHz and polarization at 204 MHz of a complex type II–IV event which started at 08:33 UT on 3 May 1973 shown a sporadic zebra pattern. In contrast with the unpolarized type II burst, the stripes in the emission and absorption of the zebra pattern were fully polarized and most likely corresponded to the ordinary wave. As to spectral and polarization characteristics, the fiber bursts with intermediate frequency drift did not differ from the stripes of the zebra pattern. The microstructure of the type II burst was characterised by a lot of spikes with variable frequency drift, duration 0.1 s and instantaneous bandwidth ≈1 MHz.     

Correlation of solar decimetric radio bursts with X-ray flares

Several hundred radio bursts in the decimetric wavelength range (300–1000 MHz) have been compared with simultaneous soft and hard X-ray emission. Long lasting (type IV) radio events have been excluded. The association of decimetric emission with hard X-rays has been found to be surprisingly high (48%). The association rate increases with bandwidth, duration, number of structural elements, and maximum frequency. Type III-like bursts are observed up to the upper limit of the observed band. This demonstrates that the corona is transparent up to densities of about 10¹⁰ cm^–3, contrary to previous assumptions. This can only be explained in an inhomogeneous corona with the radio source being located in a dense structure. The short decimetric bursts generally occur during the impulsive phase, i.e. simultaneously with hard X-rays. The times of maximum flux are well correlated (within 2 s). The HXR emission lasts 4 times longer then the radio emission in the average. This work finds a close relationship between decimetric and HXR emission with sufficient statistics offering additional information on the flare process.     

Electron acceleration in flares inferred from radio and hard X-ray emissions

Properties of electron acceleration in flares, especially the density structure in the acceleration region, are deduced from a correlation study between decimetric type III, spike, and hard X-ray (HXR) bursts. The high association rate found (71%) strongly suggests that spikes also originate from energetic electrons. Spikes and type III bursts have been found to be easily identified by their different polarizations. The two types of emission generally do not overlap in frequency. A reliable lower limit to the density is derived from the starting frequency of type III and U bursts. The spike emission very likely yields an upper limit. The density inhomogeneity in the acceleration region spans more than one order of magnitude and is more than one order of magnitude larger in the associated type U sources. A peak-to-peak correlation does not always exist between type III, spike and HXR bursts. This discrepancy can be interpreted in terms of the different source conditions and propagation properties. Whereas spikes need special conditions to become visible, type III and peaks of HXR may be the product of many elementary accelerations.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985.     

Temporal Correlation of Hard X-Rays and Meter/Decimeter Radio Structures in Solar Flares

We investigate the relative timing between hard X-ray (HXR) peaks and structures in metric and decimetric radio emissions of solar flares using data from the RHESSI and Phoenix-2 instruments. The radio events under consideration are predominantly classified as type III bursts, decimetric pulsations and patches. The RHESSI data are demodulated using special techniques appropriate for a Phoenix-2 temporal resolution of 0.1 s. The absolute timing accuracy of the two instruments is found to be about 170 ms, and much better on the average. It is found that type III radio groups often coincide with enhanced HXR emission, but only a relatively small fraction (∼20%) of the groups show close correlation on time scales < 1 s. If structures correlate, the HXRs precede the type III emissions in a majority of cases, and by 0.69 ± 0.19 s on the average. Reversed drift type III bursts are also delayed, but high-frequency and harmonic emission is retarded less. The decimetric pulsations and patches (DCIM) have a larger scatter of delays, but do not have a statistically significant sign or an average different from zero. The time delay does not show a center-to-limb variation excluding simple propagation effects. The delay by scattering near the source region is suggested to be the most efficient process on the average for delaying type III radio emission.