Various types of fine structure in the continuum emission of type-IV radio bursts are considered as applied to different types of radiation sources, both stationary and moving. In the case of stationary sources, the origin of the fine structure is associated both with processes in individual magnetic loops (quasi-periodic acceleration and magnetohydrodynamic waves), and with large-scale processes associated with the propagation of magnetohydrodynamic disturbances, the formation of loop arcades, and processes of discrete acceleration of particles synchronous with them, causing the pulsating nature of radio emissions. For the case of a moving source, the generation mechanism largely depends on the magnetic structure of the source (an expanding magnetic arc or an isolated plasma cloud). In this case, the connection with coronal mass ejections and shock waves is also important. Secondary pulsations are explained by a magnetohydrodynamic fluctuation source in the form of a magnetic loop or cloud. The absence of other fine structures in the continuum of moving type-IV bursts may be due to the critical angle of the loss cone for the excitation of whistlers.

ABSTRACT The fine structure in the continuum radiation of type IV solar radio bursts is very rich in various structures (zebra structure, fibres, pulsations, spikes, etc.). So far, however, no attention has been paid to the isolated fibres that occasionally appear in the metre and decimetre ranges. Here we give and discuss examples of the dynamic spectra of such bursts obtained many years ago (in the metre waveband from 1969) as well in recent years (in the decimetre and microwave wavebands from 2003–2013). Isolated fibres are observed mostly in the decimetre range (although there are examples in both the metre and microwave ranges), and they reveal a number of features of classical fibre bursts. As a generation mechanism for such fibres, the process of interaction of whistlers with Langmuir plasmons was suggested. An analysis of conditions for the realization of this process in solar magnetic arch structures and its efficiency was carried out. Estimates of the intensity of low-frequency turbulence (whistlers) and magnetic field strength in the solar corona were obtained using the data of radio fibres.

A number of phenomena with radio bursts in the decimeter and centimeter wavelength ranges similar to type-II bursts in the meter range have been considered. In all phenomena, the radio bursts are characterized by the cessation of frequency drift and its reversal. Analysis of all of the available data on relevant bursts provides evidence of termination shocks with particle accelerations in their fronts. This is confirmed by the generation of new sources of hard X-ray radiation and the radio emission of fast bursts (spikes), fibers, and zebra structures. The sources of drift stripes with a drift turn are located either between the burst loop and the lower shock wave or between the lower and upper shock waves. Estimates of the critical Mach number for common parameters of the burst plasma indicated that the values (Mcr = 1.1–1.3) can easily be observed in the given phenomena and that the radiation can be associated with the Buneman instability. The conditions necessary for the generation of observed electromagnetic radiation bursts can be provided in helmet-shaped magnetic structures in the solar corona.

We analyze complex zebra patterns and fiber bursts during type-IV solar radio bursts on August 1, 2010. It was shown that all of the main details of sporadic zebra patterns can be explained within the model of zebra patterns and fiber bursts during the interaction of plasma waves with whistlers. In addition, it was shown that the major variations in the stripes of the zebra patterns are caused by the scattering mechanism of fast particles on whistlers, which leads to the transition of whistler instability from the normal Doppler effect to an anomalous one.

An alternative mechanism is proposed for the generation of harmonics of the electron plasma frequency due to the development of explosive instability in a system of interpenetrating electron and proton flows in the solar atmosphere. The efficiency of the new mechanism in comparison with the earlier discussed mechanisms involving multistage processes of nonlinear interaction of waves in plasma is determined. It is shown that the development of explosive instability can lead to the excitation of the second and third harmonics of the plasma frequency with comparable amplitudes.

The reason for the occurrence of different elements of the fine structure of solar radio bursts in the decimeter and centimeter wavelength ranges has been determined based on all available data from terrestrial and satellite observations. In some phenomena, fast pulsations, a zebra structre, fiber bursts, and spikes have been observed almost simultaneously. Two phenomena have been selected to show that the pulsations of radio emission are caused by particles accelerated in the magnetic reconnection region and that the zebra structure is excited in a source, such as a magnetic trap for fast particles. The complex combination of unusual fiber bursts, zebra structure, and spikes in the phenomenon on December 1, 2004, is associated with a single source, a magnetic island formed after a coronal mass ejection.

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.

The Interhelioprobe mission aims to investigate the inner heliosphere and the Sun from close distances (up to 0.3 AU) and from out of the ecliptic plane (up to 30°). In this paper we present the relevance of the mission and its main scientific objectives, describe the scientific payload, ballistic scenario and orbits of the spacecraft. Possibilities of scientific cooperation with other solar and heliospheric space missions are also mentioned.

A two-step (fast + slow in comparison to the exponential law) behavior of radio-emission fluxes was discovered by the differential method in the phases of growth and decay during an analysis of radio-burst observation data received at IZMIRAN by means of meter-band spectrographs and fixed-frequency radiometers. It was shown that the observed fast mode of flux growth may be connected with the fast mode 3013 MHz of the acceleration of the electrons responsible for synchrotron radiation of the flare in the microwave band.

Проанализированы записи радиоизлучения Солнца, полученные на спектрографе ИЗМИРАН (25–270 МГц) для события солнечной вспышки 12 февраля 2010 г. Отмечено, что в трех больших группах всплесков III типа наблюдалась разнообразная тонкая структура на фоне невысокого континуума. Согласно данным радиогелиографа Нансэ, источники всех трех групп всплесков располагались в одной активной области 11046 и их излучение сопровождалось всплесками в мягком рентгеновском диапазоне (космический аппарат GOES): в 07:21 UT С7.9, в 09:40 UT В9.6 и в 11:25 UT М8.3. После первой группы всплесков наблюдались классические волокна (fiber bursts) в сочетании с обратно дрейфующими волокнами с необычным дугообразным дрейфом. После третьей (самой мощной) группы наблюдались стабильные секундные пульсации и медленно дрейфующие волокна. Мгновенная полоса частот у них на порядок превышала полосу частот классических волокон, а частотный дрейф был в несколько раз меньше. Более сложные волокна наблюдались в самой слабой группе в интервале 09:40:30–09:42:00 UT. Они представляли собой узкополосные ( 0.5 МГц) волокна, периодически повторяющиеся в небольшой полосе частот (5–6 МГц) в течение нескольких секунд. Особенностью данного явления является также присутствие множества хаотически дрейфующих ансамблей волокон, пересекающихся и накладывающихся друг на друга. Предполагается, что появление таких структур может быть связано с существованием множества мелких ударных фронтов за передним краем коронального выброса массы.

Solar radio emission records received at the IZMIRAN spectrograph (25–270 MHz) during the solar flare event of February 12, 2010 are analyzed. Different fine structures were observed in three large groups of type III bursts against a low continuum. According to data from the Nancay radioheliograph, sources of all three groups of bursts were located in one active region, 11046, and their emissions were accompanied by soft X-ray bursts (GOES satellite): C7.9 at 0721 UT, B9.6 at 0940 UT, and M8.3 at 1125 UT. After the first group of bursts, classical fiber bursts were observed in combination with reverse-drift fiber bursts with unusual arc drift. After the third (the most powerful) group, stable second-length pulsations and slow-drift fiber bursts were observed, the instantaneous frequency bands of which were an order of magnitude larger than the frequency band of classical fiber bursts, and the frequency drift was several times lower. More complex fiber bursts were observed in the weakest group in the time range 0940:39–0942:00 UT. They were narrow-band (∼0.5 MHz) fiber bursts, periodically recurring in a narrow frequency band (5–6 MHz) during several seconds. The presence of many chaotically drifting ensembles of fibers, crossing and superimposing on one another, is a feature of this event. It is assumed that occurrence of these structures can be connected with the existence of many small shock fronts behind the leading edge of a coronal mass ejection.

An alternative mechanism for the excitation of electron Langmuir frequency harmonics as a result of the development of explosive instability in a weakly relativistic beam-plasma system in the solar atmosphere is proposed. The efficiency of the new mechanism as compared to the previously discussed ones is analyzed.

Type III radio bursts are produced near the local electron plasma frequency fp and near its harmonic 2fp by fast electrons ejected from the solar active regions and moving through the corona and solar wind. The coronal bursts have dynamic spectra with frequency rapidly falling with time, the typical duration being about 1–3 s. In the present paper, 37 well-defined coronal type III radio bursts (25–450 MHz) are analyzed. The results obtained substantiate an earlier statement that the dependence of the central frequency of the emission on time can be fitted to a power-law model, f(t) ∝ (t − t0)−α, where α can be as low as 1. In the case of negligible plasma acceleration and conical flow, it means that the electron number density within about 1 solar radius above the photosphere will decrease as r−2, like in the solar wind. For the data set chosen, the index α varies in the range from 0.2 to 7 or bigger, with mean and median values of 1.2 and 0.5, respectively. A surprisingly large fraction of events, 84%, have α ⩽ 1.2. These results provide strong evidence that in the type III source regions the electron number density scales as n(r) ∝ (r − r0)−β, with minimum, mean, and median β = 2α of 0.4, 2.4, and 1.0, respectively. Hence, the typical density profiles are more gently sloping than those given by existing empirical coronal models. Several events are found with a wind-like dependence of burst frequency on time. Smaller power-law indices could result from the effects of non-conical geometry of the plasma flow tubes, deceleration of coronal plasma, and/or the curvature of the magnetic field lines. The last effect is shown to be too weak to explain such low power-law indices. A strong tendency is found for bursts from the same group to have similar power-law indices, thereby favoring the hypothesis that they are usually produced by the same source region.

The nature of the zebra pattern in continual type-IV solar radio bursts is discussed. It is shown that, when a weakly relativistic monoenergetic proton beam propagates in a highly nonisothermal plasma, the energy of the slow beam mode can be negative and explosive instability can develop due to the interaction of the slow and fast beam modes with ion sound. Due to weak spatial dispersion, ion sound generation is accompanied by cascade merging, which leads to stabilization of explosive instability. The zebra pattern forms due to the scattering of fast protons by ion sound harmonics. The efficiency of the new mechanism is compared with that of previously discussed mechanisms.