The origin of solar energetic particles and type II meter radio bursts

The relationship between the proton intensity in the interplanetary space and radio bursts of type II for 78 proton events for the period of 1989–2005 is studied based on the data of the Radio Solar Telescope Network. Two families of events have been revealed in plots describing the dependence of the intensity of protons with different energies and the rate of the frequency drift of meter-decameter radio bursts. This suggests the generation of shock waves both in the region of flare energy release and at the fronts of coronal mass ejection.     

Numerical simulation of type III solar radio bursts caused by high-density electron beam

Numerical simulation for the type III solar radio bursts in meter wavelengths was made with the electron beam of a high number density enough to emit fundamental radio waves comparable in intensity with the second harmonic.This requirement is fulfilled if the optical thickness ₁ for the negative absorption (amplification) becomes -23 to -25. Since ₁ is roughly proportional to the time-integral of the electron flux of the beam, the intensity of the fundamental waves depends strongly on the parameters which determine the electron flux. Therefore, it is most unlikely that the harmonic pairs of type III bursts of the first and the second harmonics occur frequently with comparable intensities in a wide frequency range, say 200 MHz to 20 MHz, if we take the working hypothesis that the fundamental waves are caused by the scattering of electron plasma waves by thermal ions and amplified during the propagation along the beam.However, we cannot rule out the possibility that single type III bursts with short durations or group of such bursts are the fundamental waves emitted by the above mechanism, but only if the observed large size of the radio source can be attributed to the radio scattering alone.     

Trajectories followed by U-like solar radio bursts

Radiospectrographic observations of some U-like bursts have been employed in combination with a model coronal condensation due to Waldmeier to derive trajectories along which the disturbing agency, which excites the radio emission, may have travelled. Such trajectories as connect regions of opposite magnetic polarity within one centre of activity should have a parachute-like shape in order to account for the observations. Travelling velocities are of the order of 35000 to 55000 km/sec. Moreover, the distribution of U-like bursts in heliographic longitude is investigated and an attempt is made to explain the fact that the second branches of U-like bursts are less developed than the first branches.     

Electron-cyclotron maser emission in solar microwave spike bursts

A new model is developed for electron-cyclotron maser emission from flaring loops, which incorporates competition between driving of the instability and maser-induced relaxation, together with interactions between small neighboring regions of unstable plasma. This results in a picture in which radiation is emitted in bursts from regions whose length scale is determined self-consistently by previous bursts, while the unstable plasma fluctuates about the point, close to marginal stability, at which driving of the instability is balanced by relaxation due to maser-induced electron diffusion. Under the conditions applicable to flaring loops, time scales of fundamental x-mode (x1) driving and saturation are approximately equal at 1 ms, resolving a (10⁴–10⁶)-fold discrepancy in previous models and agreeing with the observed time scales of microwave spike bursts. Saturation effects are found to be especially effective in suppressing amplification of the most strongly growing modes. This suppression enables fundamental o-mode (o1) and second-harmonic x-mode (x2) emission to compete more effectively against x1 emission for the available free energy than has previously been estimated. Consideration of mode competition, burst time scales, suppression of growth due to overlap between amplification and absorption bands, and escape of radiation through absorption layers to the observer, implies that the observed radiation probably escapes from the corona principally in the o-mode, either emitted directly as o1 radiation or mode converted from x1 emission.     

Interplanetary phenomena and solar radio bursts

The relationship between solar radio emissions and transient interplanetary phenomena is reviewed. It is believed that the most significant advance in recent years has come from coordinated studies of coronal mass ejections and moving type IV bursts, where the evidence appears to favour the Langmuir wave hypothesis as the emission mechanism. Type II bursts are not generally a signature of the main energetic particle acceleration in flares. They do, however, occasionally propagate to 1 AU, and beyond, where they are normally accompanied by protons in the 20 MeV region. Apart from the impulsive microwave burst, there is no reliable radio signature associated with energetic particle acceleration in flares, although many phenomena have high correlations with radio emissions. The exceptions suggest that such correlations may be incidental. Therefore, it is concluded that attention should also be given to events with a positive absence of radio emission in order to make progress in understanding solar processes.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985.     

The generation of solar type II radio bursts in coronal magnetic loops

It is shown that the existing theory of type II bursts, based on a model of the emission from the shock wave front, has difficulties when compared with observational data. We suggests a new model for type II bursts. According to this model, in an expanding magnetic loop a cluster of energetic electrons acts to excite the cyclotron instability of plasma waves. The waves are excited on surfaces where the cyclotron resonance condition is satisfied, and are then transformed into electromagnetic emission by merging. Our proposed model may be useful to explain some observational facts, such as the narrow-band character of the emission and the space-time relationship between the harmonics. Some tests to check the validity are proposed.     

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.     

Flux-rms relation in solar radio bursts

One recent discovery that provides a strong constraint on the mechanisms of astrophysical activities is the correlation between the flux and the root-mean-squared (rms) variability of X-ray emission. In this work we study the flux-rms relation of solar radio bursts. Four flares observed by the Solar Radio Broadband Spectrometer (SRBS) of China are analyzed. In these flares, fine structures (FSs) emerge at least in one frequency band of SRBS. We find that the flux-rms relation consists of two components. One relates to the non-FS emission and the other to the FS emission. The flux-rms relationship for the non-FS part of the radio bursts is clearly different from that for the FS part. The former shows a curve-like behavior, while the latter shows a dramatic variation. We propose a model to describe the flux-rms relation of the non-FS part. Our results imply that the non-FS part emission could be triggered by some multiplicative processes. On the contrary, multiplicative mechanisms should be excluded from the explanations of FSs in the radio bursts.     

Pulsating type IV solar radio bursts

Several models for pulsating type IV radio bursts are presented based on the assumption that the pulsations are the result of fluctuations in the synchrotron emission due to small variations in the magnetic field of the source. It is shown that a source that is optically thick at low frequencies due to synchrotron self-absorption exhibits pulsations that occur in two bands situated on either side of the spectral peak. The pulsations in the two bands are 180° out of phase and the band of pulsations at the higher frequencies is the more intense. In contrast, a synchrotron source that is optically thin at all frequencies and whose low frequency emission is suppressed due to the Razin effect develops only a single band of pulsations around the frequency of maximum emission. However, the flux density associated with the later model would be too small to explain the more intense pulsations that have been observed unless the source area is considerably larger than presently seems reasonable.     

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.     

The high-frequency characteristics of solar radio bursts

We compare the millimeter, microwave, and soft X-ray emission from a number of solar flares in order to determine the properties of the high-frequency radio emission of flares. The millimeter observations use a sensitive interferometer at 86 GHz which offers much better sensitivity and spatial resolution than most previous high-frequency observations. We find a number of important results for these flares: (i) the 86 GHz emission onset appears often to be delayed with respect to the microwave onset; (ii) even in large flares the millimeter-wavelength emission can arise in sources of only a few arc sec dimension; (iii) the millimeter emission in the impulsive phase does not correlate with the soft X-ray emission, and thus is unlikely to contain any significant thermal bremsstrahlung component; and (iv) the electron energy distributions implied by the millimeter observations are much flatter (spectral indices of 2.5 to 3.6) than is usual for microwave or hard X-ray observations.     

Wide-band average spectra of solar radio bursts

Peak flux spectra of solar radio bursts in a wide frequency band have been statistically determined for different morphological types of bursts, for various ranges of magnetic field of the burst-associated sunspots and also for the bursts occurring in the central and limb region of the solar disk. Important results obtained are: (i) The generalised spectra have two peaks, one near to meter-wave and the other in the centimeter-wave region, the former peak being more pronounced than the latter; (ii) identical spectral shape is observed for the great and impulsive types and also for GRF and PBI types of bursts; (iii) the radio emission intensity is relatively higher in the central part than that in the limb part of the solar disk for frequencies 1–10 GHz, while the reverse is true for frequencies 0.245–1 GHz and 10–35 GHz; (iv) the optical depth of the absorbing layer above the source of a burst is found to be the same for meter to centimeter-wavelength bursts, implying that the radio sources in this wide band have uniform characteristics with respect to optical thickness; (v) in case of simultaneous emission in the dekameter to X-ray band, most of the decimetric bursts are seen to be very prompt and coincident with the associated flare's starting time. The interpretations of the obtained spectra give an insight into the possible generation mechanisms, pointing to the location of the source region in the solar atmosphere.     

Propagation and absorption of cyclotron maser radiation in solar microwave spike bursts

It has been argued that the loss-cone-driven electron cyclotron maser instability can account for the properties of millisecond microwave spike bursts observed during some solar flares. However, as it propagates outward from the corona, maser radiation undergoes gyroresonance absorption when its frequency is a harmonic of the local electron-cyclotron frequency. Existing analytical models using slab geometries predict that this absorption should be sufficiently strong to prevent the radiation from being seen at the observed levels, except under highly restrictive conditions or for unrealistic plasma parameters. A more comprehensive analysis is presented here to determine if and when maser radiation can escape to produce microwave spike bursts. This analysis employs numerical raytracing and incorporates propagation and absorption of fundamental maser emission in a realistic model of a coronal flux loop. It is found that ranges of physical conditions do exist under which maser radiation can escape to an observer and that these conditions are much more limiting for fundamental emission in the extraordinary ()-mode than in the ordinary (o)-mode. Detailed investigation implies that escaping radiation in the -mode is highly directional and chiefly observable toward the center of the solar disk, while escapingo-mode radiation is found to emerge from the corona over a much wider range of directions, with some cases corresponding to radiation observable near the solar limb.     

The solar elongation distribution of low-frequency radio bursts

Over 500 days of low-frequency (<1 MHz) radio observations from the IMP-6 spacecraft have been accumulated to produce a two-dimensional map (frequency vs elongation) of solar type III burst occurrences. This map indicates that most solar bursts in this frequency range are observed at the second harmonic of the plasma frequency rather than the fundamental. The map also shows that the solar wind electron density varies as R ^?γ , where γ can be somewhat less than 2 to perhaps 3 or higher.     

On the spatial directivity of solar radio bursts

In this paper, a new method of estimating the spatial directivity (in the form of center-to-limb variation) of microwave burst emission is proposed and derived. Estimations of radioemission directivity values vs observation frequency are obtained. Results are compared to the radio source model using an inhomogeneous magnetic field, source size and particle density, and show a high degree of agreement. Values of model parameters from earlier estimations are confirmed.     

Frequency and time splitting of decameter solar radio bursts

The paper deals with the observations of the fine structure of type III bursts in the 12.5–25 MHz band using the UTR-2 (IRE AN UkSSR, Kharkov) radio telescope. A fine structure arises in the form of chains of short-lived narrow-band bursts. The chains have a frequency drift analogous to type III bursts. Observations allow two different-type chains to be singled out. Ordinary stria-bursts, split-pairs and triplets belong to the first type chains. They may also involve the echo-type phenomena The second type chains (IIId) involve diffusive stria-bursts, diffusive split-pairs and triplets. The analysis of a harmonic structure of chains incidates that the first type chains are generated at the frequencies close to the local plasma electron frequency _pe . The second type chains and, consequently, diffusive stria-bursts correspond to the second harmonic of the plasma frequency 2 _pe . Experimental data evidence that the type III bursts with a fine structure are excited by the faster particle streams than the ordinary type III bursts with a diffusive character both of the fundamental and the second harmonic.     

Solar radio bursts and acceleration of solar energy particles

The correlation between the proton flux intensity I _p with the energies E _p > 1?100 MeV and radio burst parameters for 107 solar energetic events is considered using the observation data for 1989?C2005 obtained with GOES and Wind satellites, as well as the Radio Solar Telescope Network (RSTN). It has been revealed that 73 and 77% of the events were accompanied by type-II radio bursts in the meter (m II, 25?C299 MHz) and the decameter-hectometer (DH II, 20 kHz?C14 MHz) wavelength ranges, respectively. The correlation coefficient between I _p and the frequency drift velocity of the type-II bursts V _II did not exceed 0.40. As V _II increased, the intensity of I _p increased for the m-II bursts and decreased for the DH-II bursts. Coronal shock waves accelerate protons more efficiently than interplanetary waves, and their contribution to acceleration increases with an increase in the particle energy E _p . The acceleration of solar energetic particles in the region of the flare energy release is predominant.     

A study of type V solar radio bursts

A model for the solar Type V event is developed. This model assumes that the basic difference between Type III and Type V bursts is the evolution of the electron beam. For a Type V this beam rapidly elongates, so that it takes progressively longer times to pass higher plasma levels. Physical process influencing the beam development, including Coulomb collisions, non-linear interactions with Langmuir waves and wave-particle scattering from various hydromagnetic wave modes is discussed. The model is compared with previously derived models and with observations.Operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.     

A study of Type V solar radio bursts

Results of an observational study of Type V bursts are presented. Observations were made using the C.S.I.R.O. radioheliograph at Culgoora. Source parameters studied included flux evolution, polarization, size, shape, position, motions and brightness temperature at 160, 80 and 43 MHz. Comparisons of source characteristics observed at different frequencies are made.Operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.