Fine structure of large flare radioemission

Several recent phenomena with zebra patterns (ZPs) and fiber bursts on the dynamic spectra of solar type IV radio bursts have been complexly analyzed using all available ground-based and satellite data (SOHO, TRACE, RHESSI). ZPs and fiber bursts were observed at frequencies of 50–3800 MHz. The main relative spectral parameters and the degree of circular polarization of ZPs and fiber bursts are almost identical. The fine structure was observed in powerful and weak phenomena (and was more impressive in weak phenomena) during impulsive and decline phases at instants of recurring continuum bursts. The shape of the fine structure depends on that of the magnetic loops in a radio source, the type of fast particle acceleration (impulsive or prolonged), and the presence of shock waves and coronal mass ejections. Several new effects of the interaction between zebra stripes and fiber bursts have been detected. Specifically, up to 40 fiber bursts with different frequency ranges were simultaneously observed in the frequency range 1–2 GHz against a background of sudden absorptions. It has been indicated that different effects in the ZP stripe behavior can be explained within the scope of the model with whistlers, if the quasi-linear diffusion of fast particles on whistlers (which deforms the particle velocity distribution function) is taken into account.     

Spiky Fine Structure of Type III-like Radio Bursts in Absorption

An uncommon fine structure in the radio spectrum consisting of bursts in absorption was observed with the Chinese Solar Broadband Radiospectrometer (SBRS) in the frequency range of 2.6?–?3.8 GHz during an X3.4/4B flare on 13 December 2006 in active region NOAA 10930 (S05W33). Usual fine structures in emission such as spikes, zebra stripes, and drifting fibers were observed at the peak of every new flare brightening. Within an hour at the decay phase of the event we observed bursts consisting of spikes in absorption, which pulsated periodically in frequency. Their instantaneous frequency bandwidths were found to be in the 75 MHz range. Moreover, in the strongest Type III-like bursts in absorption, the spikes showed stripes of the zebra-pattern (ZP) that drifted to higher frequencies. All spikes had the duration as short as down to the limit of the instrument resolution of ≈8 ms. The TRACE 195 Å images indicate that the magnetic reconnection at this moment occurred in the western edge of the flare loop arcade. Taking into account the presence of the reverse-drifting bursts in emission, in the course of the restoration of the magnetic structures in the corona, the acceleration of the beams of fast particles must have occurred both upward and downward at different heights. The upward beams will be captured by the magnetic trap, where the loss-cone distribution of fast particles (responsible for the emission of continuum and ZP) were formed. An additional injection of fast particles will fill the loss-cone later, breaking the loss-cone distribution. Therefore, the generation of continuum will be quenched at these moments, which was evidenced by the formation of bursts in absorption.     

Loss-Cone Instability and Formation of Zebra Patterns in Type IV Solar Radio Bursts

The loss-cone instability of energetic electrons at double plasma resonance is considered. Conditions required for the formation of a zebra pattern in type IV solar radio bursts are determined. It is shown that electrons with a power-law energetic spectrum can effectively excite upper-hybrid waves at double plasma resonance. Stripes of a zebra pattern become more pronounced with an increase of the loss-cone opening angle and the power-law spectral index. The growth rate at the resonance frequencies decreases with an increase of the cyclotron harmonic number. Interpretation of observations and diagnostics of plasma for the April 21, 2002, event are performed. Conclusions about the impulsive mode of injection of energetic electrons into a coronal arc are made.     

Unusual Zebra Patterns in the Decimeter Wave Band

An analysis of new observations showing fine structures consisting of narrowband fiber bursts as substructures of large-scale zebra-pattern stripes is carried out. We study four events using spectral observations taken with a newly built spectrometer located at the Huairou station, China, in the frequency range of 1.1 – 2.0 GHz with extremely high frequency and time resolutions (5 MHz and 1.25 ms). All the radio events were analyzed by using the available satellite data (SOHO LASCO, EIT, and MDI, TRACE, and RHESSI). Small-scale fibers always drift to lower frequencies. They may belong to a family of ropelike fibers and can also be regarded as fine structures of type III bursts and broadband pulsations. The radio emission was moderately or strongly polarized in the ordinary wave mode. In three main events fiber structure appeared as a forerunner of the entire event. All four events were small decimeter bursts. We assume that for small-scale fiber bursts the usual mechanism of coalescence of whistler waves with plasma waves can be applied, and the large-scale zebra pattern can be explained in the conventional double plasma resonance (DPR) model. The appearance of an uncommon fine structure is connected with the following special features of the plasma wave excitation in the radio source: Both whistler and plasma wave instabilities are too weak at the very beginning of the events (i.e., the continuum was absent), and the fine structure is almost invisible. Then, whistlers generated directly at DPR levels “highlight” the radio emission only from these levels owing to their interaction with plasma waves.     

The importance of source positions during radio fine structure observations

The measurement of positions and sizes of radio sources in the observations of the fine structure of solar radio bursts is a determining factor for the selection of the radio emission mechanism. The identical parameters describing the radio sources for zebra structures（ZSs） and fiber bursts confirm there is a common mechanism for both structures. It is very important to measure the size of the source in the corona to determine if it is distributed along the height or if it is point-like. In both models of ZSs（the double plasma resonance（DPR） and the whistler model） the source must be distributed along the height, but by contrast to the stationary source in the DPR model, in the whistler model the source should be moving. Moreover, the direction of the space drift of the radio source must correlate with the frequency drift of stripes in the dynamic spectrum. Some models of ZSs require a local source, for example,the models based on the Bernstein modes, or on explosive instability. The selection of the radio emission mechanism for fast broadband pulsations with millisecond duration also depends on the parameters of their radio sources.     

Interference origin of the fine structure in the dynamic spectra of solar radio bursts

The multibeam propagation of radio waves in the solar plasma is analyzed, because the emission from a solar flare passes through an inhomogeneous solar atmosphere on its way to the observer. A formula (a mathematical model) for calculating the structure of the dynamic spectrum for flare radio bursts has been obtained. Comparison of the calculated spectra with the observed ones shows that the results of interference explain the formation of a zebra structure and the separation of its stripes into individual spikes, describe the time profile of the spikes, and explain the properties of fibers, ropes of fibers, and chains of “point” bursts. The similarity of the dynamic spectra testifies that the fine structure of the spectra is formed not in the emission source but as a result of the propagation of waves through the solar corona and interplanetary space.     

Effects of the distribution function of non-thermal electrons and of cyclotron resonant absorption on millisecond radio spikes

A theoretical analysis of electron-cyclotron maser instabilities indicates that the distribution function of non-thermal electrons influences millisecond radio spikes in solar flares, and that a hollow beam distribution is more likely than a loss-cone distribution. The restrictions of classical theories of cyclotron resonant absorption are discussed and a formula is derived for the absorption coefficient near the resonant frequency. Finally, the computations show that for typical coronal parameters, the growth rates of the fundamental of fast extraordinary modes are much faster than those of their second harmonics; and because the directional angle of the fundamental is smaller, its resonant absorption may be neglected. Moreover, the band-width of the fundamental is consistent with observation of radio spikes; therefore, we claim that the millisecond radio spikes in the decimetric range are composed mainly of fundamentals of the fast extraordinary modes. The second harmonics of fast extraordinary modes may be generated for directions near to the vertical to the magnetic field, but it is impossible to observe both fundamental and second harmonics in the same direction.     

Interpretation of a special fine structure in type-IV solar radio bursts

A special fine structure (slowly drifting chains of narrowband fiber bursts), firstly observed during the solar type-IV radio burst on April 24, 1985, is interpreted as the radio signature of whistler waves periodically excited by a switch-on/switch-off process of a loss-cone instability in a localized wave packet of the fast magnetoacoustic mode.     

Solar Decameter Spikes

We analyze and discuss the properties of decameter spikes observed in July?–?August 2002 by the UTR-2 radio telescope. These bursts have a short duration (about one second) and occur in a narrow frequency bandwidth (50?–?70 kHz). They are chaotically located in the dynamic spectrum. Decameter spikes are weak bursts: their fluxes do not exceed 200?–?300 s.f.u. An interesting feature of these spikes is the observed linear increase of the frequency bandwidth with frequency. This dependence can be explained in the framework of the plasma mechanism that causes the radio emission, taking into account that Langmuir waves are generated by fast electrons within a narrow angle θ≈13^°?–?18^° along the direction of the electron propagation. In the present article we consider the problem of the short lifetime of decameter spikes and discuss why electrons generate plasma waves in limited regions.     

Generation of Intermediate Drift Bursts by Magnetohydrodynamic Waves in the Solar Corona

The plasma mechanism of radio emission generation in an inhomogeneous medium is investigated. In the model under study, the electron beam with loss-cone distribution generates upper-hybrid waves that, in turn, are transformed into radio emission. It is shown that the influence of the plasma density inhomogeneity limits the plasma waves’ intensity considerably due to variation in their wave vector. The results are used to interpret the intermediate drift (IMD) bursts. A model is proposed in which these bursts are reflections of propagating small-scale (with amplitudes of about 1% and sizes of hundreds of kilometers) magnetohydrodynamic (MHD) disturbances of magnetic tubes. It is shown that this model allows us to explain the spectral parameters of the bursts in question. At present, the lack of precise and independent data about the magnetic field does not allow us to decide definitively between the existing models (whistler or MHD waves) of the IMD bursts; nevertheless, if the proposed model is correct, it can be used to determine the characteristics of the coronal MHD waves.     

A mechanism for pulsation in solar microwave bursts

I suggest that the pulsation in solar microwave bursts is a modulation of gyro-synchrotron radiation. Whistler waves at the foot of a coronal loop (radio source) interact with nonthermal electrons with loss-cone distribution at the top. As a consequence, electrons outside the loss-cone diffuse into the loss-cone, pass through the loop foot, sink in the atmosphere, and emit gyro-synchrotron radiation as additional pulses. Electrons remaining outside the loss-cone give the background radiation of the burst.Assuming the configuration of a magnetic dipole lying below the photosphere, I calculated the period of pulsation to be 1 s- 1 min. The ratio of the pulse peak to background intensity is calculated to be 0 – 100%; the calculated pulse width is about 0.3 – 50 s. These values are consistent with the observed values. A brief discussion of the probable interpretation of fast, millisecond structures is also given.     

Plasma emission and the decimetre portion of type-IV solar bursts

An attempt is made to account for the decimetre portion of the Type-IV solar radio bursts by plasma emission. Non-thermal electrons (E ～ 500 keV) trapped in a magnetic mirror (IV_dm, burst source) having loss-cone gap distribution excite plasma waves which are transformed into transverse waves through non-linear scattering by ions. A good agreement was reached between the calculated spectrum and the observed fluxes for the event of 1972 August 2. A distribution of the number of non-thermal electrons with height, and a total number of 10³², were obtained. Also it was found that the Langmuir waves can accelerate some background thermal electrons to the MeV range.     

Radiation from Instabilities in Space Plasmas

Some successful features of the theory of radiation from plasma instabilities in space plasmas are reviewed, with emphasis on plasma emission in type III solar radio bursts due to the bump-in-tail instability, and planetary radio emissions due to loss-cone driven electron cyclotron maser emission. The emission occurs in sporadic, localized bursts, and the theory for the instability needs to be combined with some statistical ideas to model the observed emissions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of Radio Wave Propagation on the Properties of Solar Microwave Bursts

Scintillation of radio signals passing through the solar corona is considered. An expression describing the dynamic spectrum of these scintillations on the basis of multibeam propagation of radio waves is derived. Properties of the analytically calculated spectrum are shown to coincide with zebra-structure properties of solar radio bursts. It is determined that the time profile of the scintillations caused by multibeam propagation may appear as impulses of emission or absorption or may have a sawtooth form. It is concluded that assuming specific emission source features is not the only way to explain the zebra structure, since the effect of multibeam propagation of radio waves through the solar corona and interplanetary space yields a simple explanation of the phenomenon discussed.     

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.     

A Special Radio Spectral Fine Structure Used for Plasma Diagnostics in Coronal Magnetic Traps

A specific combination of spectral fine structures in meter –  decimeter dynamic spectra of solar radio burst emission is reported in observations carried out at the Astrophysical Institute Potsdam. We describe and interpret the occurrence of zebra patterns in fast drifting (type III burst-like) envelopes of absorbed continuum emission. A possible mechanism of the origin of such an involved spectral pattern is put forward, leading to a necessarily multinonequlibrium component coronal plasma. The suggested mechanism is based on the fact that during the passage of a fast electron beam through the corona the loss cone instability (which is caused by electrons captured in a magnetic trap generating the continuum) is quenched. As result, a fast drift burst appears in absorption, and the zebra pattern becomes visible on the low background emission. This zebra pattern is generated by a group of electrons with a nonequilibrium distribution over transverse velocities. In the absence of the beam the pattern is invisible against the background of the stronger continuum. It is shown that the mechanism is sensitive to the distribution parameters of the different electron ensembles. Therefore the effect in dynamic radio spectra is comparatively rare but its proper existence underlines that the simultaneous presence of different ensembles of electrons in the flaring corona can be quite a frequent situation. This can explain some problems in deconvolving X-ray photon spectra to electron energy spectra.     

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。     

A Study of Solar Radio Bursts with Fine Structures during Flare/CME Events

We have selected 27 solar microwave burst events recorded by the Solar Broadband Radio Spectrometer (SBRS) of China, which were accompanied by M/X class flares and fast CMEs. A total of 70.4% of radio burst events peak at 2.84 GHz before the peaks of the related flares’ soft X-ray flux with an average time difference of about 6.7 minutes. Almost all of the CMEs start before or around the radio burst peaks. At 2.6?–?3.8 GHz bandwidth, 234 radio fine structures (FSs) were classified. More often, some FSs appear in groups, which can contain several individual bursts. It is found that many more radio FSs occur before the soft X-ray maxima and even before the peaks of radio bursts at 2.84 GHz. The events with high peak flux at 2.84 GHz have many more radio FSs and the durations of the radio bursts are independent of the number of radio FSs. Parameters are given for zebra patterns, type III bursts, and fiber structures, and the other types of FSs are described briefly. These radio FSs include some special types of FSs such as double type U bursts and W-type bursts.     

Formation of zebra patterns of solar microwave bursts as a result of propagation of radio waves through the inhomogeneous corona

The geoefficiency of solar bursts is diagnosed using the dynamic radio emission spectrum. At certain time intervals, the spectrum exhibits nearly parallel narrow-band emission strips termed the zebra pattern. Although there are many hypotheses of its origin, all of them do not take into account changes in the signal parameters upon signal propagation through the solar corona. Our analysis shows that the propagation effects form a dynamic spectrum that contains a zebra pattern. The properties of the modeled spectrum are shown to coincide with the basic properties of the observed spectrum. It is clarified that the spike structure of strips is a natural consequence of the interference of radio waves, and the occurrence of this structure is considered to be evidence in favor of the interference nature of the zebra pattern formation. Consequently, the zebra pattern can be formed not in the radiation source itself, but rather can arise as a result of propagation of radio waves through an inhomogeneous refracting medium of the solar corona.     

Generation of intermediate drift bursts in solar type IV radio continua through coupling of whistlers and Langmuir waves

The possible generation of intermediate drift bursts in type IV dm continua through coupling between whistler waves, traveling along the magnetic field, and Langmuir waves, excited by a loss-cone instability in the source region, is elaborated. We investigate the generation, propagation and coupling of whistlers. It is shown that the superposition of an isotropic background plasma of 10⁶K and a loss-cone distribution of fast electrons is unstable for whistler waves if the loss-cone aperture 2α is sufficiently large (sec α?4); a typical value of the excited frequencies is 0.1 ω _ce (ω _ce is the angular electron cyclotron frequency). The whistlers can travel upwards through the source region of the continuum along the magnetic field direction with velocities of 21.5–28 v _A (v _A is the Alfvén velocity). Coupling of the whistlers with Langmuir waves into escaping electromagnetic waves can lead to the observed intermediate drift bursts, if the Langmuir waves have phase velocities around the velocity of light. In our model the instantaneous bandwith of the fibers corresponds to a frequency of 0.1–0.5 ω _ce and leads to estimates of the magnetic field strength in the source region. These estimates are in good agreement with those derived from the observed drift rate, corresponding to 21.5–28 v _A, if we use a simple hydrostatic density model.