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.     

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.     

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.     

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.     

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.     

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．     

Harmonic relation of type IIIb-III solar radio bursts in 6.25, 12.5, and 25.0 MHz octaves

This study of type IIIb-III evenmts strongly supports their interpretation as fundamental-harmonic burst pairs. Type IIIb chains and the related type III bursts drift from 12.5 to 6.25 MHz and from 25.0 to 12.5 MHz, respectively, during similar time intervals of 11.1 and 11.0 s, on the average. This harmonic similarity is emphasized by the fact that the drift times of type IIIb chains across the upper octave are significantly less than those of type III bursts in the lower octave, the values being around 6 and 19 s.     

Energy of solar noise-storm radio bursts

Solar noise storms (NS) are analyzed by an algorithm which separates a random signal into pulses. The burst duration distribution is shown to be inversely proportional to the squared duration of bursts. The distribution ordinates are proportional to the average pulse repetition frequency, and the distribution maximum corresponds to the limiting pulse duration equal to 0.4–0.6 s. The aggregate lifetime of all short-lasting bursts is approximately equal to the aggregate lifetime of bursts of any other duration. The energy of short-lasting bursts with a duration of 0.2–0.4 s is five times smaller than the energy of longer bursts, and it constitutes only 2–5 percent of the energy of the NS burst component. The power of bursts increases as their duration changes from 0.2 to 1.2 s until it reaches some limit at a duration of 1.2–1.4 s. The power of longer bursts remains almost unchanged up to the end of the investigated duration interval (up to durations of 300 s). Solar burst chains can be some superposition of short-lasting bursts on one longer burst. Thus, the burst energy measurements do not support the widespread point of view that solar noise storms consist of short-lasting type I bursts.     

Fine structure of decametric type II radio bursts

We have performed a comparative analysis of the fine structure of two decametric type II bursts observed on July 17 and August 16, 2002, with the 1024-channel spectrograph of the UTR-2 radio telescope in the frequency range 18.5–29.5 MHz and with the IZMIRAN spectrograph in the frequency range 25–270 MHz. The August 16 burst was weak, ～2–5 s.f.u., but exhibited an unusual fine structure in the form of broadband fibers (Δf _e > 250–500 kHz) that drifted at a rate characteristic of type II bursts and consisted of regular narrow-band fibers (Δf _e > 50–90 kHz at 24 MHz) resembling a rope of fibers. The July 17 burst was three orders of magnitude more intense (up to 4500 s.f.u. at 20 MHz) and included a similar fiber structure. The narrow fibers were irregular and shorter in duration. They differed from an ordinary rope of fibers by the absence of absorption from the low-frequency edge and by slow frequency drift (slower than that of a type II burst). Both type II bursts were also observed in interplanetary space in the WIND/WAVES RAD2 spectra, but without any direct continuation. Analysis of the corresponding coronal mass ejections (CMEs) based on SOHO/LASCO C2 data has shown that the radio source of the type II burst detected on August 16 with UTR-2 was located between the narrow CME and the shock front trailing behind that was catching up with the CME. The July 17 type II fiber burst also occurred at the time when the shock front was catching up with the CME. Under such conditions, it would be natural to assume that the emission from large fibers is related to the passage of the shock front through narrow inhomogeneities in the CME tail. Resonant transition radiation may be the main radio emission mechanism. Both events are characterized by the possible generation of whistlers between the leading CME edge and the shock front. The whistlers excited at shock fronts manifest themselves only against the background of enhanced emission from large fibers (similar to the continuum modulation in type IV bursts). The reduction in whistler group velocity inside inhomogeneities to 760 km s^?1 may be responsible for the unusually low drift rate of the narrow fibers. The magnetic field inside inhomogeneities determined from fiber parameters at 24 MHz is ～0.9 G, while the density should be increased by at least a factor of 2.     

Solar Drift Pair Bursts in the Decameter Range

The results of observations of solar decametric drift pair bursts are presented. These observations were carried out during a Type III burst storm on July 11–21, 2002, with the decameter radio telescope UTR-2, equipped with new back-end facilities. High time and frequency resolution of the back-end allowed us to obtain new information about the structure and properties of these bursts. The statistical analysis of more than 700 bursts observed on 13–15 July was performed separately for “forward” and “reverse” drift pair bursts. Such an extensive amount of these kind of bursts has never been processed before. It should be pointed out that “forward” and “reverse” drift pair bursts have a set of similar parameters, such as time delay between the burst elements, duration of an element, and instant bandwidth of an element. Nevertheless some of their parameters are different. So, the absolute average value of frequency drift rate for “forward” bursts is 0.8 MHz s⁻¹, while for “reverse” ones it is 2 MHz s⁻¹. The obtained functional dependencies “drift rate vs. frequency” and “flux density vs. frequency” were found to be different from the current knowledge. We also report about the observation of unusual variants of drift pairs, in particular, of “hook” bursts and bursts with fine time and frequency structure. A possible mechanism of drift pairs generation is proposed, according to which this emission may originate from the interaction of Langmuir waves with the magnetosonic waves having equal phase and group velocities.     

Type IIIb radio bursts: 80 MHz source position and theoretical model

We present Culgoora spectrograph and radioheliograph observations as well as a model of type IIIb bursts; the latter are defined as chains of striae of slow or no frequency drift, the chain as a whole drifting like a normal type III burst.The 80 MHz source positions are studied for a group of IIIb bursts, a IIIb precursor and harmonic pairs of 1:2 frequency ratio. It is found that the IIIb position may vary in a IIIb group. No significant difference was found between the source positions of a IIIb precursor and the following III burst. For one event we found that the fundamental IIIb burst showed a high degree of circular polarization (46%), while its second harmonic, a normal type III burst, was unpolarized.We suggest that the main cause for the striae in type IIIb bursts is the existence of filamentary, density irregularities along the path of the electron stream. The denser filaments initially reduce the value of the density gradient along the electrons' path and thereby enhance their emissions over a small range of plasma frequencies. If the radio emission from the filaments dominates the emission from the ambient rarified plasma, striae appear in the spectrum and a type IIIb burst results. This condition is more easily satisfied at the fundamental frequency and for electron streams of relatively high density.Radiophysics Publication RPP 1758, October, 1974, (2nd version).On leave from the Dept. of Astronomy, University of Tokyo, Tokyo, Japan.On leave from the Dept. of Astronomy, Cairo University, Cairo, Egypt.     

Type III solar radio burst storms observed at low frequencies

Storms of type III solar radio bursts observed from 5.4 to 0.2 MHz, indicate the quasicontinuous production of type III events observable for a half solar rotation but persisting in some cases for well over a complete rotation. The characteristics of these storms, including the dependance of occurrence and apparent drift rates on the disc position of the associated active region are discussed. The drift rate dependance is shown to be a consequence of the propagation time of emission from the source to the observer. The occurrence rate of a burst every 10 sec observed near CMP implies that if this level of activity persists, then about a quarter of a million exciter packets are released into the interplanetary plasma during a complete rotation. Storm bursts are less intense than most isolated type III's and occur over a more limited frequency range. There appears to be a very close relation between these storms and decametric continuum.     

Type II Radio Bursts with High and Low Starting Frequencies

We report on the detailed analysis of i) differences between the properties of type IIs with various starting frequencies (high: ≥100 MHz; low: ≤50 MHz; mid: 50 MHz ≤f≤ 100 MHz) and ii) the properties of CMEs and flares associated with them. For this study, we considered a sample of type II radio bursts observed by Culgoora radio spectrograph from January 1998 to December 2000. The X-ray flares and CMEs associated with these events are identified using GOES and SOHO/LASCO data. The secondary aim is to study the frequency dependence on other properties of type IIs, flares, and CMEs. We found that the type IIs with high starting frequencies have larger drift rate, relative drift rate, and shock speed than the type IIs with low starting frequencies. The flares associated with high frequency type IIs are of impulsive in nature with shorter rise time, duration and delay between the flare start and type II start times than the low frequency type IIs. There is a distinct power – law relationship between the flare parameters and the starting frequencies of type II bursts, whereas the trend in the CME parameters shows low correlation. While the mean speed of CMEs is larger for the mid-frequency group, it is nearly the same for the high and low frequency groups. On the other hand, the percentage of CME association (90%) is larger for low frequency type IIs than for the high frequency type IIs (75%).     

On the possibility of determining the coronal magnetic field from solar type III radio burst observations

A simple method of estimating the coronal magnetic field is suggested. It is based on the observational fact that the duration of the highly polarized part in type III bursts can be different, varying from a small fraction of the burst length to its total duration. We suggest that this difference is determined by the relation between the size of the region where only the ordinary wave can propagate and the size of the region where the burst is generated at a fixed frequency. The magnetic field is estimated at several tens of gauss in regions emitting highly polarized type III bursts at frequencies over 200 MHz. Density and magnetic field scales are estimated.     

Millisecond structures in solar radio emission close to 264 MHz

High-time resolution spectral measurements of solar radio emission close to 264 MHz are reported. Instrumental resolutions of the order of a few kHz in frequency and tenths of milliseconds in time were used to resolve the burst fine structure in the time-frequency plane. Fine structures, having narrow bandwidths and durations of some 5 to 30 ms, have been observed mostly in association with type I and type III bursts. These very short duration bursts have negative frequency drifts of about - 50 to- 60 MHz s^-1. They can occur individually or in small groups where they sometimes display a quasi-periodicity of a few milliseconds.     

Observations of Powerful Type III Bursts in the Frequency Range 10 – 30 MHz

The properties of powerful (flux >10⁻¹⁹ W m⁻² Hz⁻¹) type III bursts observed in July – August 2002 by the radio telescope UTR-2 at frequencies 10 – 30 MHz are analyzed. Most bursts have been registered when the active regions associated to these bursts were located near the central meridian or at 40° – 60° to the East or West from it. All powerful type III bursts drift from high to low frequencies with frequency drift rates 1 – 2.5 MHz s⁻¹. It is important to emphasize that according to our observations the drift rate is linearly increasing with frequency. The duration of the bursts changes mainly from 6 s at frequency 30 MHz up to 12 s at 10 MHz. The instantaneous frequency bandwidth does not depend on the day of observations, i.e. on the disk location of the source active region, and is increasing with frequency.     

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.     

On a sequence of remarkable fine structures in the type IV burst of 24 April, 1985

During the type IV burst on 24 April, 1985 we observed at 234 MHz an untypical, strong, nearly six hours lasting continuum emission incorporating several groups of broadband pulsations, zebra patterns, fiber bursts, and a new fine structure phenomenon. The power spectra of the groups of broadband pulsations reveal no simple structure. There is only one common periodic component between 0.3 s and 0.4 s. Slowly drifting chains of narrowband fiber bursts are described as a new fine structure by spectrograms and simultaneously recorded single frequency intensity profiles. A qualitative model of this new fine structure is suggested.     

Unusual Solar Radio Burst Observed at Decameter Wavelengths

An unusual solar burst was observed simultaneously by two decameter radio telescopes UTR-2 (Kharkov, Ukraine) and URAN-2 (Poltava, Ukraine) on 3 June 2011 in the frequency range of 16?–?28 MHz. The observed radio burst had some unusual properties, which are not typical for the other types of solar radio bursts. Its frequency drift rate was positive (about 500 kHz?s^?1) at frequencies higher than 22 MHz and negative (100 kHz?s^?1) at lower frequencies. The full duration of this event varied from 50 s up to 80 s, depending on the frequency. The maximum radio flux of the unusual burst reached ≈10³ s.f.u. and its polarization did not exceed 10 %. This burst had a fine frequency-time structure of unusual appearance. It consisted of stripes with the frequency bandwidth 300?–?400 kHz. We consider that several accompanied radio and optical events observed by SOHO and STEREO spacecraft were possibly associated with the reported radio burst. A model that may interpret the observed unusual solar radio burst is proposed.     

The relation between the surges and solar radio emission

The 120 limb surges which have been observed by means of Wrocaw Observatory coronagraph from September 1966 to November 1977 are investigated. The evolution of surges was compared with the radio data during the surges. A correlation between radio bursts and the surges was found, particularly with chains of type I radio bursts, which is the first reliable correlation found of these bursts with non-radio events. The type I correlation only applied for surges without accompanying flare, of which 43% are correlated with this type of radio emission. In 23 of 30 associated events the start of a surge coincided within 5 minutes with the start or an enhancement of the type I storm. If flares were present, the association was not significant.We also compared the maximum height reached by a surge with the frequencies of the radio bursts emitted at the same time and the maximum velocity of the rising surge with the frequency drift of type I chains. No such a correlation was however found.We discuss the possibility that surges are the result of a sudden energy input into the chromosphere related to the type I source in the corona.