On the characteristics of the solar active regions responsible for the generation of type III radio bursts at hectometric frequencies in August 1968

The RAE (Radio Astronomy Explorer) satellite observed enormous numbers of type III radio bursts at hectometric wavelengths from 13 to 25 August in 1968. The drift rate of these bursts reached a maximum around the middle of 20 August. This means that the source responsible for these bursts gradually moved on the solar disk in association with the rotation of the sun. During this period, there were two large active sunspot groups, MacMath Nos. 9593 and 9597, which were located in the southern hemisphere and adjacent to each other. By examining the observational data on solar flares, type I noise storm activity and energetic electron flux increases, it is shown that the active region, MacMath No. 9597 is responsible for the generation of these type III radio bursts. The relation between type III bursts producing electron beams and type I noise activity is briefly discussed and a model of this active region is qualitatively described.NAS-NRC Associate with NASA.     

The association of solar radio bursts with auroral streams

Slow drift (Type II) radio bursts from the sun are believed to be caused by a primary disturbance moving outward through the solar atmosphere with a velocity of about 1000 km/sec. Analysis of the 2 years, 1956 October 1 through 1958 September 30, over the sunspot maximum shows that 45 per cent of these bursts are associated with the subsequent occurrence of terrestrial auroræ and magnetic storms. The mean delay between the radio bursts and the terrestrial disturbances is 33 hr, which is in good accord with the velocity for the disturbing source as deduced from the radio data. Investigation of the properties of the individual slow drift bursts and their association with other solar radio and optical phenomena reveals no completely conclusive criteria to explain why only 45 per cent of the bursts are geomagnetically important. The geomagnetic effects are enhanced, however, if the bursts occur near the equinoxes and if they are accompanied by a flare o'f importance 2 or 3, or by continuum (Type IV) radiation.

In the reverse association, with radio data available for an average 14 hr daily, it is shown that at least 60 per cent of magnetic storms are preceded, within 4 days, by a slow drift burst.     

Type III solar radio burst storms observed at low frequencies

Storms of type III solar radio bursts observed from 5.4 ot 0.2 MHz consist of a quasi-continuous production of type III events observable for half a solar rotation but persisting in some cases for well over a complete rotation (Fainberg and Stone, 1970). The observed burst drift rates are a function of the heliographic longitude of the associated active region. This apparent drift rate dependence is a consequence of the radio emission propagation time from source to observer. Based on this dependence, a least squares analysis of 2500 drift rates between frequencies in the 2.8 to 0.7 MHz range yields an average exciter speed of 0.38 c for the height range from approximately 11 to 30 R . In conjunction with the available determinations of exciter speeds of 0.33 c close to the sun, i.e. less than 3 R , and with in situ measurements of 40 keV solar electrons by space probes, the present results suggest that the exciters are electron packets which propagate with little deceleration over distances of at least 1 AU.     

Source regions for type II radio bursts

The flares associated with reported type II bursts from 1964–1973 (hereafter abbreviated source-flares) are analyzed with respect to their importance, duration, and heliographic distribution. The source-flares for type II bursts generally are normal to small in area and normal to brilliant in intensity; however, they have slightly longer durations than would be expected from flares with such small areas. Flares associated with type II bursts are distributed uniformly east and west of the central meridian. This implies that type II bursts cannot be narrow-beamed, radially-propagating radio emitters. The latitude of the occurrence of source-flares shows a drift with time that is not significantly different from that of sunspots. The drift rate is a maximum during the period of maximum solar activity. The heliographic distribution of source-flares shows large-scale organization into similarly shaped regions (source-regions) separated by 180° of longitude in each polar hemisphere. The shape of the source-regions differs between the northern and the southern hemispheres. The source-regions exhibit growth and motion characteristics which suggest that their development is due partly to the effects of differential rotation. An analysis of the plage regions which are associated with source-flares shows that relatively few plages ever produce type II bursts, yet certain plages produce type II bursts repeatedly. One active region produced type II bursts on six consecutive disc passages. Since the source-regions for type II bursts are large-scale, persistent solar features that show motion and development related to the solar cycle, an evaluation of the distinctive fluid motion characteristics of these regions appears to be an important step in the determination of specific environmental configurations which can produce solar shock waves.     

The coronal disturbance of 1972, August 12

Using the observed data for metric and hectometric type III radio bursts, the dependence of burst characteristics on the solar longitude has been examined over a wide frequency range. It is found that there exists an east-west asymmetry for the extension of metric type III bursts into hectometric wavelength range. In particular, hectometric bursts are rarely observed for solar flares associated with metric bursts eastward solar longitude 60°E. Furthermore, for eastern longitudes, the low frequency radio observations show a large dispersion in drift time interval.     

Solar S-bursts at Frequencies of 10 – 30 MHz

Solar S-bursts observed by the radio telescope UTR-2 in the period 2001 – 2002 are studied. The bursts chosen for a detailed analysis occurred in the periods 23 – 26 May 2001, 13 – 16 and 27 – 39 July 2002 during three solar radio storms. More than 800 S-bursts were registered in these days. Properties of S-bursts are studied in the frequency band 10 – 30 MHz. All bursts were always observed against a background of other solar radio activity such as type III and IIIb bursts, type III-like bursts, drift pairs and spikes. Moreover, S-bursts were observed during days when the active region was situated near the central meridian. Characteristic durations of S-bursts were about 0.35 and 0.4 – 0.6 s for the May and July storms, respectively. For the first time, we found that the instantaneous frequency width of S-bursts increased with frequency linearly. The dependence of drift rates on frequency followed the McConnell dependence derived for higher frequencies. We propose a model of S-bursts based on the assumption that these bursts are generated due to the confluence of Langmuir waves with fast magnetosonic waves, whose phase and group velocities are equal.     

Variations of type III burst parameters during a decametric solar storm

An analysis has been made of type III bursts recorded during a decametric solar storm observed from July 29 to August 16, 1975 with the UTR-2 antenna (Kharkov, IRE Acad. Sci. Ukr. SSR). The bursts were recorded with a dynamic spectrograph and radiometers at 25.0, 20.0, 16.7, and 12.5 MHz. Daily observations have yielded histograms of the type III burst distribution with respect to the frequency drift rate in three subbands between 25.0 and 12.5 MHz. During the middle stage of the storm the drift rate was about twice as high as at the onset and the final stage of the storm. Abrupt changes in the mean frequency drift rate were registered some two to three days after the active region McMath 13790 had come onto the limb and also before it disappeared behind the solar disk. Sudden changes in the drift rates of the type III bursts were accompanied by sudden changes of their mean duration. The rather long burst durations observed at 25.0 MHz at the beginning and the end of the radio storm coincided with such at the twice lower frequency, i.e. 12.5 MHz, during the period when an increased drift rate was observed.Similar variations of type III burst parameters can be interpreted in the framework of the plasma mechanism of burst generation in the corona, assuming that at the middle stage of the storm the bursts observed in the 25.0–12.5 MHz range were emitted at the fundamental whereas when the emitting region was near the limb the bursts received corresponded to the second harmonic of the Langmuir oscillations in the range of 12.5 to 6.25 MHz excited at greater heights.     

Statistical Analysis of Decimetric Type III Bursts

Detailed statistics and analysis of 264 type III bursts observed with the 625–1500 MHz spectrograph during the 23rd solar cycle (from July 2000 to April 2003) are carried out in the present article. The main statistical results are similar to those of microwave type III bursts presented in the literature cited, such as the correlation between type III bursts and flares, polarization, duration, frequency drift rate (normal and reverse slopes), distribution of type III bursts and frequency bandwidth. At the same time, the statistical results also point out that the average values of the frequency drift rates and degrees of polarization increase with the increase in frequency and the average value of duration decreases with the increase in frequency. Other statistical results show that the starting frequencies of the type III bursts are mainly within the range from 650 to 800 MHz, and most type III bursts have an average bandwidth of 289 MHz. The distributions imply that the electron acceleration and the place of energy release are within a limited decimetric range. The characteristics of the narrow bandwidth possibly involve the magnetic configuration at decimetric wavelengths, the location of electron acceleration in the magnetic field nearto the main flare, the relevant runaway or trapped electrons, or the coherent radio emission produced by some secondary shock waves. In addition, the number of type III bursts with positive frequency drift rates is almost equal to that with negative frequency drift rates. This is probably explained by the hypothesis that an equal number of electron beams are accelerated upwards and downwards within the range of 625 to 1500 MHz. The radiation mechanism of type III bursts at decimetric wavelengths probably includes these microwave and metric mechanisms and the most likely cause of the coherent plasma radiation are the emission processes of the electron cyclotron maser.     

Decameter storm radiation,II

The physical properties of six decametric storms, observed at Clark Lake Radio Observatory are studied. The height of the storm continuum sources was determined from the rotation rate. Assuming that the radiation originates at the plasma frequency we computed the gradient of electron density for the regions where the storms originate. The mean angular size of the decametric continuum sources is large; it increases with decreasing frequency. The storm continuum is found to be strongly directive toward the disk center. The east-west asymmetry, well observed at meter wavelengths is also observed at decameter wavelengths. The occurrence of two distinct classes of type III bursts in storms is discussed: ‘off-fringe’ and ‘onfringe’ type Ill's. The ‘off-fringe’ Ill's are found to be displaced in position from the continuum source ; on the other hand, the ‘on-fringe’ ones coincide in position with the continuum. These two kinds of bursts differ in other properties as well. A model of the storm region is proposed. The continuum radiation and the ‘on-fringe’ type III's are believed to originate above closed magnetic loops, in regions of diverging field lines; the ‘offfringe’ type Ill's are thought to be excited by energetic electron streams, having access to open magnetic field lines at the base of the loops.     

Chains of type I stormbursts

From radio spectra between 160 and 320 MHz of chains of type I bursts it appears that their duration distributions allow an exponential fit, and that those of samples containing long and short chains respectively, taken from the same storm, have virtually the same characteristic time (logarithmic slope). On the average this figure decreases - as a function of the frequency - at about 1 s per 10 MHz. The high frequency cut-off of chain activity (noise storms) is mainly a consequence of the frequency dependence of the probability for the first burst of a chain to appear. Given the density of type I bursts in a chain, it is concluded that the probability of a type I burst to be followed by another one is at least 90% below 250 MHz and 70–80% at 300 MHz, which makes it essential for type I theories to include a mechanism to this effect. The drift rate distribution for chains is symmetrical with a peak at-10 MHz/s. The statistics is indicative of a correlation between drift rate and duration. No evidence has been found for the occurrence of chain pairs or frequency splitting in chains, nor for an association between chains and type III bursts.     

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.     

Observations of Solar Type II bursts at frequencies 10–30 MHz

We present the results of radio telescope UTR-2 observations of solar Type II radio bursts in the 10–30 MHz frequency range. These events possess a fine structure consisting of fast drift sub-bursts similar to Type III bursts. The frequency drift rate of the Type II bursts at decameter wavelengths is smaller than 0.1 MHz s^–1. One of these bursts with herringbone structure has a wave-like backbone that almost does not drift. The features of the observed bursts are discussed.     

Harmonic components of decametric solar radio bursts

Type-IIIb, IIId, and III solar decametric radio bursts, being distinguished by the typical negative drift rate of their dynamic spectra, are compared. Observational data were obtained with a UTR-2 antenna during the period 1973–1982. During the analysis of the bursts of all these spectral varieties, the frequency drift time (drift delay) was measured in the ranges 25 to 12.5 MHz, 25 to 20 MHz, and 12.5 to 10 MHz. Durations of type-III bursts were determined at the harmonically-related frequencies of 25 and 12.5 MHz; radio source locations were also used.It is shown that these decametric bursts are distinctly divided into two groups: (1)type-IIIb chains of simple stria bursts and also normal type-III storm bursts observed at central regions constitute a group of events with a fast drifting spectrum; (2) type-III bursts from type-IIIb-III pairs and the limb variant of normal III bursts, as well as peculiar type-IIId chains of diffuse striae and related chains with an echo component, constitute a second group of events with comparatively slow drift rates.The first group of the phenomena is associated with the fundamental F frequency and the second one, with the harmonic H of the coronal plasma frequency. The results of the present investigation agree well with earlier conclusions on the harmonic origin of decametric chains and type-III bursts. Measurements of drift delays in narrow frequency ranges, an octave apart, as well as type-III burst durations at harmonically-related frequencies confirm the existence of both F and H components in the solar radiation. The essential result of 10 years of decametric observations is that the frequency drift rates and durations are rather stable parameters for the various type-III bursts and stria-burst chains. The stability characterizes some unspecified conditions of burst generation in the middle corona.     

Revisiting the Frequency Drift Rates of Decameter Type III Solar Bursts Observed in July – August 2002

Estimating for the frequency drift rates of type III solar bursts is crucial for characterizing their source development in the solar corona. According to Melnik et al. (Solar Phys.269, 335, 2011), the analysis of powerful decameter type III solar bursts, observed in July?–?August 2002, found a linear approximation for the drift rate versus frequency. The conclusion contradicts reliable results of many other well-known solar observations. In this paper we report on the reanalysis of the solar data with a more advanced method. Our study shows that the decameter type III solar bursts of July?–?August 2002, as standard type III bursts, follow a power law in frequency drift rates. We explain the possible reasons for this discrepancy.     

The spectrum and position of solar noise storms at decameter wavelengths

Decametric storm radiation during the period July–August 1970 has been observed simultaneously with a high sensitivity spectrograph at Arecibo Observatory and with the log-periodic, swept-frequency array of the Clark Lake Radio Observatory. The observations complement each other; different types of fine structure emissions can be easily identified on the spectrograph records and their position can be determined from the swept-frequency recordings. We study the relative positions of the different emissions which have been observed during the storms. Four distinct sources appeared to be present. The continuum emission, the type I bursts and the flare-related type III's were all emitted at different locations. The storm type III bursts, type IIIb's and drift pairs overlapped in position, but appeared at different locations than the previously mentioned sources.On leave of absence from Instituto Argentino de Radioastronomia, Argentina.     

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．     

Bursts with temporal fine structure at 5730 MHz

An analysis of solar radio bursts with temporal fine structure (TFS) at 5730 MHz in relation to the magnetic configuration of the corresponding active regions (AR) is presented. We found that the occurrence of TFS bursts increases with increasing complexity of the AR's magnetic configuration. The degree of polarization of TFS bursts varies over a wide range. Most of these fast bursts with a high degree of polarization were observed in active regions with a simple magnetic configuration β. Most of the unpolarized fast bursts were observed in active regions with the most complicated configuration βγδ. Because bursts that are polarized in different modes have different displacements of position with respect to that of associated microwave bursts, we conclude that there are at least two types of TFS bursts at 5730 MHz. We think that fast bursts that are polarized in the ordinary mode are due to microwave type III bursts.     

Solar radio type III bursts and coronal density structures

The comparison of solar radio type III bursts measured at 169 MHz with K corona observations leads to the conclusion that about 75% of the active regions over which type III bursts occur are associated with low density coronal structures. The comparison with X-ray maps of the solar disk shows that all these regions are located in low intensity regions.It is concluded that the idea generally accepted that the type III bursts are associated with dense coronal structures and travel in these structures is not at all proven for a large number of cases.     

Directivity of low frequency solar type III radio bursts

The occurrence rate of type III solar bursts in the frequency range 4.9 MHz to 30 kHz is analyzed as a function of burst intensity and burst arrival direction. We find that (a) the occurrence rate of bursts falls off with increasing flux, S, according to the power law S ^–1.5, and (b) the distribution of burst arrival directions at each frequency shows a significantly larger number of bursts observed west of the Earth-Sun line than east of it. This western excess in occurrence rate appears to be correlated with the direction of the average interplanetary magnetic field, and is interpreted as beaming of the observed burst radiation along the magnetic field direction.Presently at the University of Maryland, College Park, Maryland.     

The role of energetic electrons in the correlation of meter and decimeter type III bursts with 4 keV X-ray emission

The correlation of type III burst-groups with 4 keV solar X-ray emission is examined. A total of 151 burst-groups reported by the Fort Davis Observatory were compared with X-ray emission observed by the Naval Research Laboratory experiment on the OGO-5 satellite. A higher X-ray correlation is found for type III burst-groups when: (1) the bursts are observed on the decimeter band and (2) the bursts are more intense. The bremsstrahlung flux resulting from the proposed coronal loss of the E< 10 keV type III electrons is shown to be below the detection threshold of the OGO-5 experiment. No fine structure is found in the correlated impulsive X-ray bursts with a time scale on the order of one second. It is proposed that electrons are accelerated over a time of 10–100 s or more and that the type III bursts are the result of the occasional escape of a small fraction of the energetic electrons from the acceleration region.