Meter-wavelength observations of the solar radio burst storm of August 17–22, 1968

Meter-wavelength observations are presented for the solar radio storm of August 17–22, 1968. The data comprise dynamic spectra and high-resolution brightness distributions from the 80 MHz radioheliograph.It is found that the storm consisted essentially of type III bursts at the lower frequencies and type I at the higher frequencies; the transition, usually near 60 MHz, was fairly sharp. The type I source was located over an active region associated with a large sunspot group. The type III position was displaced about 0.5 R transversely from the type I, in a region of low magnetic field.The evident close association between the two types of emission can best be explained by disturbances originating in the type I region, propagating outwards through a region of weak magnetic field, and triggering an electron acceleration process, probably at the cusp of a helmet structure. The observed frequency and spatial relationship between the type I and type III components in events of this kind follow as a natural consequence of this model.A comparison of these results with the hectometer-wavelength satellite observations of the 1968 August event makes possible a qualitative estimate of the outward path of the type III exciters through the corona, and it is apparent that below the solar wind region of the corona this path departs considerably from the radial direction.     

Evidence for a common origin of the electrons responsible for the impulsive X-ray and type III radio bursts

Observations of impulsive solar flare X-rays 10 keV made with the OGO-5 satellite are compared with ground based measurements of type III solar radio bursts in 10–580 MHz range. It is shown that the times of maxima of these two emissions, when detectable, agree within 18 s. This maximum time difference is comparable to that between the maxima of the impulsive X-ray and impulsive microwave bursts. In view of the various observational uncertainties, it is argued that the observations are consistent with the impulsive X-ray, impulsive microwave, and type III radio bursts being essentially simultaneous. The observations are also consistent with 10–100 keV electron streams being responsible for the type III emission. It is estimated that the total number of electrons 22 keV required to produce a type III burst is 10³⁴. The observations indicate that the non-thermal electron groups responsible for the impulsive X-ray, impulsive microwave, and type III radio bursts are accelerated simultaneously in essentially the same region of the solar atmosphere.     

A metric type III burst asymmetry relative to simple bipolar active regions

Metric type III solar radio burst positions are compared spatially and temporally to underlying active region geometry. The positions of these radio bursts have an asymmetric location distribution relative to simple bipolar regions. The type III bursts show a tendency to occur nearer the leading active region - an association shown before from type III burst and magnetic field polarity measurements. The type III bursts also generally occur to the left of the outward to inward directed magnetic field. The asymmetry relative to the outward directed magnetic field has a sense that is consistent with a mechanism of type III burst production that involves a pre-existing coronal current system situated between expanding closed and open magnetic field lines.     

CHANGES IN SUNSPOT AND FLOCCULAR SOURCES OF RADIO EMISSION PRECEDING AN IMPORTANCE 2N FLARE ON 23 AUGUST 1988

Based on observations from the Siberian solar radio telescope, and invoking data from other observatories, we investigate preflare changes in the sunspot and floccular sources of radio emission and the development of an importance 2N flare in the chromosphere and corona in the active region on August 23, 1988.It has been ascertained that preflare changes became observable six hours prior to the flare onset and manifested themselves in intense flux fluctuations above the sunspot and in an enhancement of the source emission flux above the flocculus.It is shown that the flare onset is associated with a newly emerged magnetic flux in the form of a pore near the filament and with the appearance of radio sources above the filament. The flare was accompanied by type III radio bursts and a noise storm at meter wavelengths. Coronal mass ejection parameters are estimated from type III burst observations.     

Electron plasma oscillations associated with type III radio emissions and solar electrons

An extensive study of the IMP-6 and IMP-8 plasma and radio wave data has been performed to try to find electron plasma oscillations associated with type III radio noise bursts and low-energy solar electrons. This study shows that electron plasma oscillations are seldom observed in association with solar electron events and type III radio bursts at 1.0 AU. In nearly four years of observations only one event was found in which electron plasma oscillations are clearly associated with solar electrons. For this event the plasma oscillations appeared coincident with the development of a secondary maximum in the electron velocity distribution functions due to solar electrons streaming outwards from the Sun. Numerous cases were found in which no electron plasma oscillations with field strengths greater than 1 μV m^?1 could be detected even though electrons from the solar flare were clearly detected at the spacecraft. For the one case in which electron plasma oscillations are definitely produced by the electrons ejected by the solar flare the electric field strength is relatively small, only about 100 μV m^?1. This field strength is about a factor of ten smaller than the amplitude of electron plasma oscillations generated by electrons streaming into the solar wind from the bow shock. Electromagnetic radiation, believed to be similar to the type III radio emission, is also observed coming from the region of the more intense electron plasma oscillations upstream of the bow shock. Quantitative calculations of the rate of conversion of the plasma oscillation energy to electromagnetic radiation are presented for plasma oscillations excited by both solar electrons and electrons from the bow shock. These calculations show that neither the type III radio emissions nor the radiation from upstream of the bow shock can be adequately explained by a current theory for the coupling of electron plasma oscillations to electromagnetic radiation. Possible ways of resolving these difficulties are discussed.     

The relationship of electron plasma oscillations to type III radio emissions and low-energy solar electrons

An extensive study of the IMP-6 and IMP-8 plasma and radio wave data has been performed to try to find electron plasma oscillations associated with type III radio noise bursts and low energy solar electrons. This study shows that electron plasma oscillations are seldom observed in association with solar electron events and type III radio bursts at 1.0 AU. In nearly four years of observations only one event was found in which electron plasma oscillations are clearly associated with solar electrons. Numerous cases were found in which no electron plasma oscillations with field strengths greater than 1 V/m could be detected even though electrons from the solar flare were clearly detected at the spacecraft.For the one case in which electron plasma oscillations are definitely produced by the electrons ejected by the solar flare, the electric field strength is very small, only about 100 V/m. This field strength is about a factor of ten smaller than the amplitude of electron plasma oscillations generated by electrons streaming into the solar wind from the bow shock. Electromagnetic radiation, believed to be similar to the type III radio emission, is also observed coming from the region of more intense electron plasma oscillations upstream of the bow shock. Quantitative calculations of the rate of conversion of the plasma oscillation energy to electromagnetic radiation are presented for plasma oscillations excited by both solar electrons and electrons from the bow shock. These calculations show that neither the type III radio emissions nor the radiation from upstream of the bow shock can be adequately explained by a current theory for the coupling of electron plasma oscillations to electromagnetic radiation. Possible ways of resolving these difficulties are discussed.     

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.     

Energetic electrons associated with solar flares and their relation to type I noise activity

The generation of energetic electrons is always associated with the solar flares which occur within the sunspot groups that are highly active in emitting type I noise storms. The number of the solar flares which are associated with the distinct electron events observed at the earth tends to increase in association with the westward movement of these active groups. This tendency is not contradictory to the close association between electron producing solar flares and type I active regions if we take into account the limited directivity of type I noise storms associated with these sunspot groups.The acceleration of the energetic electrons associated with solar flares seems to be closely related to the type I active regions where the enormous numbers of suprathermal electrons exist and play a role in generating these radio noise storms.NAS-NRC Associate with NASA.     

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.     

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.     

Combined Radio and Space-Based Solar Observations: From Techniques to New Results – Preface

The phenomena observed at the Sun have a variety of unique radio signatures that can be used to diagnose the processes in the solar atmosphere. The insights provided by radio observations are further enhanced when they are combined with observations from space-based telescopes. This Topical collection demonstrates the power of combination methodology at work and provides new results on i) type I solar radio bursts and thermal emission to study active regions; ii) type II and IV bursts to better understand the structure of coronal mass ejections; and iii) non-thermal gyro-synchrotron and/or type III bursts to improve the characterisation of particle acceleration in solar flares. The ongoing improvements in time, frequency, and spatial resolutions of ground-based telescopes reveal new levels in the complexity of solar phenomena and pose new questions.     

Electron Acceleration in the Corona

The series of nine impulsive, highly collimated beams of near-relativistic electrons seen by ACE/EPAM on 26 and 27 June 2004 occurred at a quiet time with respect to solar flare and CME production. However, they were accompanied by decametric type III radio bursts observed by WIND/WAVES, which had progressively higher starting frequencies, suggestive of coronal acceleration. There were no CMEs seen by SOHO/LASCO in association with any of the type III bursts except possibly the first. The energy spectrum of the electrons was soft, typically E^−4.5 but extended up to at least ∼200 keV. We suggest that the source region for these events is in the high corona. We discuss this result in the context of solar electron acceleration at other times.     

Interaction of Weak Shock Waves with Current Sheets in an Active Region to Produce Nanoflares and Chains of Type I Radio Bursts

Interaction of weak shock waves with a current sheet is investigated by a two-dimensional numerical magnetohydrodynamic model. In accordance with solar coronal conditions, a ratio of thermal to magnetic pressures of 0.1 and a shock Alfvén Mach number slightly above 1 are considered. It is found that even weak shock waves trigger magnetic field reconnection in current sheets. Based on this result, it is suggested that drifting chains of type I radio bursts are radio manifestations of the interactions of weakly super-Alfvénic shock waves with pre-existing current sheets distributed in an active region. This model of type I noise storms is then discussed in connection with the concept of nanoflares (localized reconnections) and the heating of the solar corona.     

Polarization measurements of solar type III radio bursts at 25.3 MHz

We report the results of 1966, 1968, and 1969 polarization measurements of solar type III radio noise bursts made by recording the output of two orthogonally polarized receiving channels and subsequent digital processing of selected data. The processed data yield total intensity, degree of polarization, ellipticity, and polarization ellipse orientation at 1 second intervals. The measurements are made in a 100 Hz bandwith to minimize the influence of the propagating medium on the measurements. The mean degree of polarization was found to be about 65% in contrast to previous studies which indicated that type III events were more weakly polarized. By assuming that type III bursts are flare related we study the polarization characteristics of type III bursts as a function of the solar longitude of the related flares. The relation between type III event polarization characteristics and flare importance is also investigated. The significance of polarization measurements in studies of solar radio events is pointed out and suggestions for further theoretical research are given.     

The initial stage of development of type IV radio bursts and the relation to expanding magnetic bottles

Using the observed data for wide-band type IV solar radio bursts, the onset time differences between the microwave and metric frequencies and the peak flux intensities of the metric component are analyzed as a function of the longitudinal position of the associated flares on the solar disk. It is shown that this time difference is dependent on the position of the associated flare and that the peak flux intensity reaches maximum when a flare occurs in the region 10 to 40 ° west of the central meridian of the solar disk. These results are explained by taking into account the eastward expansion of magnetic bottles which trap mildly relativistic electrons responsible for type IV bursts. Discussion is given on the relation between these magnetic bottles and shock waves which excite type II radio bursts.NASA Associate with University of Maryland, Astronomy Program.     

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.     

On the Nature of Modulation of Radio Emission During Solar Flares

The distribution of pauses between subsequent elements of a periodic process is symmetric, while a random process produces an asymmetric exponential distribution. The third moment of the pause distribution, which is sensitive to the asymmetry, can therefore be used to discriminate between perodic and random processes. With such a method we analyze the observations of 19 series of solar type III radio bursts and find with a confidence of 0.99 that, on average, the bursts are randomly distributed in time. Only one series can be considered to be periodic with a confidence 0.5. The bandwidth of the repetition frequency of most bursts corresponds to the quality of oscillations of Q¯ = 1.0±0.6 that does not indicates a resonance. Therefore, the modulation of particle beams and intensity of type III radio emission should be considered mainly as the result of random processes. Thus, these properties observed in the majority of radio type III bursts do not support the existence of any periodic or resonant oscillations in the solar corona during flares, although some periodic processes in active regions cannot entirely be ruled out.     

Homologous type II radio bursts and coronal transients

The homology of seven successive type II solar radio bursts, which occurred at the times of flares from an active region near the solar west limb on 1980, July 27–29, is described, together with evidence for coronal mass outflows accompanying these bursts. It is argued that homologous type II bursts imply that the corona is restructured in a similar manner by successive coronal transients.     

Very-Large-Array Observations of Evolving Type I Noise Storms and Nonthermal Bursts In Association With Flares And Coronal Mass Ejections

Very-Large-Array (VLA) observations of the Sun at 20, 91 and 400 cm have been combined with data from the SOHO, TRACE and Wind solar missions to study the properties of long-lasting Type I noise storms and impulsive metric and decimetric bursts during solar flares and associated coronal mass ejections. These radio observations provide information about the acceleration and propagation of energetic electrons in the low and middle corona as well as their interactions with large-scale magnetic structures where energy release and transport takes place. For one flare and its associated CME, the VLA detected impulsive 20 and 91 cm bursts that were followed about ten minutes later by 400 cm burst emission that appeared to move outward into the corona. This event was also detected by the Waves experiment on Wind which showed intense, fast-drifting interplanetary Type III bursts following the metric and decimetric bursts detected by the VLA. For another event, impulsive 91 cm emission was detected about a few minutes prior to impulsive bursts at 20.7 cm, suggesting an inwardly propagating beam of electrons that excited burst emission at lower levels and shorter wavelengths. We also find evidence for significant changes in the intensity of Type I noise storms in the same or nearby active region during impulsive decimetric bursts and CMEs. These changes might be attributed to flare-initiated heating of the Type I radio source plasma by outwardly-propagating flare ejecta or to the disruption of ambient magnetic fields by the passage of a CME.     

Radio and soft X-ray investigation of the solar flares of February 4, 1986

In this paper, the 3B flare of February 4, 1986 is studied comprehensively. The escape electrons accelerated to 10–100 keV at the top of coronal loop are confirmed by III type bursts. The energetic electron beams moved downward trigger the eruptions in the low layer of solar atmosphere. The radio and soft X-ray bursts are interpreted, respectively, by the maser mechanism and evaporation effect. Finally, the important role of energetic electron beams in solar flares is pointed out.