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.     

Observations of a post-flare radio burst in X-rays

More than six hours after the two-ribbon flare of 21 May 1980, the hard X-ray spectrometer aboard the SMM imaged an extensive arch above the flare region which proved to be the lowest part of a stationary post-flare noise storm recorded at the same time at Culgoora. The X-ray arch extended over 3 or more arc minutes to a projected distance of 95 000 km, and its real altitude was most probably between 110 000 and 180 000 km. The mean electron density in the cloud was close to 10⁹ cm^–3 and its temperature stayed for many hours at a fairly constant value of about 6.5 × 10⁶ K. The bent crystal spectrometer aboard the SMM confirms that the arch emission was basically thermal. Variations in brightness and energy spectrum at one of the supposed footpoints of the arch seem to correlate in time with radio brightness suggesting that suprathermal particles from the radio noise regions dumped in variable quantities into the low corona and transition layer; these particles may have contributed to the population of the arch, after being trapped and thermalized. The arch extended along the H = 0 line thus apparently hindering any upward movement of the upper loops reconnected in the flare process. There is evidence from Culgoora that this obstacle may have been present above the flare since 15–30 min after its onset.     

Source parameters of a solar radio microwave burst

Choosing a representative solar radio microwave burst, a typical burst which contains a precursor and an impulsive burst followed by a slowly decaying component, as an example, we have researched its radio emission mechanism and calculated the two important theoretical characteristic parameters, intensity and dimension, of its various sources.     

Direct observations of low-energy solar electrons associated with a type III solar radio burst

A highly anisotropic packet of solar electron intensities was observed on 6 April 1971 with a sensitive electrostatic analyzer array on the Earth-orbiting satellite IMP-6. The anisotropies of intensities at electron energies of several keV were factors 10 favoring the expected direction of the interplanetary magnetic lines of force from the Sun. The directional, differential intensities of solar electrons were determined over the energy range 1–40 keV and peak intensities were 10² cm^–2 s^–1 sr^–1 eV^–1 at 2–6 keV. This anisotropic packet of solar electrons was detected at the sattelite for a period of 4200 s and was soon followed by isotropic intensities for a relatively prolonged period. This impulsive emission was associated with the onsets of an optical flare, soft X-ray emission and a radio noise storm at centimeter wavelengths on the western limb of the Sun. Simultaneous measurements of a type III radio noise burst at kilometric wavelengths with a plasma wave instrument on the same satellite showed that the onsets for detectable noise levels ranged from 500 s at 178 kHz to 2700 s at 31.1 kHz. The corresponding drift rate requires a speed of 0.15c for the exciting particles if the emission is at the electron plasma frequency. The corresponding electron energy of 6 keV is in excellent agreement with the above direct observations of the anisotropic electron packet. Further supporting evidence that several-keV solar electrons in the anisotropic packet are associated with the emission of type III radio noise beyond 50R is provided by their time-of-arrival at Earth and the relative durations of the radio noise and the solar electron packet. Electron intensities at E 45 keV and the isotropic intensities of lower-energy solar electrons are relatively uncorrelated with the measurements of type III radio noise at these low frequencies. The implications of these observations relative to those at higher frequencies, and heliocentric radial distances 50R , include apparent deceleration of the exciting electron beam with increasing heliocentric radial distance.Research supported in part by the National Aeronautics and Space Administration under contracts NAS5-11039 and NAS5-11074 and grant NGL16-001-002 and by the Office of Naval Research under contract N000-14-68-A-0196-0003.     

Millisecond spikes in a short decimetric solar radio burst

The characteristics of the millisecond spikes with short duration and weak flux density which were observed with high time resolution (1 ms) at 1420, 2000 and 2840 MHz during the great type IV solar radio burst of 30 July 1990 are introduced in detail in this article. The time profiles of the spikes are statistically analyzed and the parameters of the spike source are also estimated.     

An analysis of fine structures in a solar radio burst at 10 cm and a coexisting hard X-ray flare

The solar burst event of 1992-06-07 is analyzed in this paper using HXR material of the Yohkoh satellite and radio data at 2840 MHz observed at Beijing Astronomical Observatory. The results show that during the impulsive phase, the pulsational component had two time scales, a longer one of about 30 s, and a shorter one of 1–4 s. The pulsations on the longer scale are found to be correlated with a series of variations in the HXR images of the source region. A physical picture comprising loop-loop interaction and MHD oscillation modulation is presented.     

Observations of fine time structure in solar flare hard X-ray bursts

Hard X-ray (?100 keV) time histories of solar flares which occurred on 1978 December 4 and 1979 February 18 are presented. The first flare was observed by 3 identical instruments from near-earth orbit (Prognoz 7) and interplanetary space (Venera 11 and 12). Fine time structure is present down to the 55 ms level for the e-folding rise and fall times. These data may be used to localize the emission region by the method of arrival time analysis.     

Interferometer observation of pulsating sources associated with a type IV solar radio burst

An extremely complex outburst, part of which showed unsually rapid intensity fluctuations of a few second interval, was observed on 1970 November 5 with the 160 MHz interferometer of the Nobeyama Solar Radio Station. The pulsating source, which was stable in position and strongly circularly polarized ( 60 %), had an extension as large as 17 (7.5 × 10⁵ km) in the east-west direction. The structure of the source remained unchanged while the source darkened and brightened repeatedly. The change of the source brightness occurred in a time shorter than a second.Two alternative mechanisms responsible for the pulsating phenomenon are suggested; (1) gyroresonance absorption of continuum radiation by a fast particle beam injected in a quasi-periodic manner into a large region of weak magnetic field, or (2) magnetohydrodynamic oscillation of the continuum source itself, which is intrinsically much smaller than observed. It is observed as a large source as a consequence of scattering of the emitted radiation in a region situated above the source.     

Type III radio burst productivity of solar flares

We study the statistical relationship between type III radio bursts and optical flares, using the comprehensive flare data base at the NOAA National Geophysical Data Center (Boulder, Colorado), and the radio observations obtained with the ARTEMIS multichannel spectrograph in Nancay (France), operating at 500–100 MHz.At variance with previous results, we find that type III probability of occurrence depends only weakly upon the spatial extension of the flare observed in H, but strongly upon its brightness. We also confirm that type III probability increases with proximity to sunspots and with mass motions (surges and prominence activity); in addition, our statistical data are consistent with both relations holding at fixed flare brightness. Thus, some of the conditions favorable to type III occurrence are characteristic of compact flares, while others are characteristic of large and long-duration flares, which are often related to mass ejections. This apparent paradox suggests that particle acceleration and magnetic expansion are at work simultaneously in the ejection of electron streams out of flaring sites.     

Type III radio burst productivity of solar flares

We study the occurrence probability of type III radio bursts during flares as a function of the flare position on the Sun. We find that this probability peaks around 30° east of the central meridian, which points to a reciprocal tilt of the average radiation pattern of type IIIs. We argue that anisotropic scattering of the radiation by overdense coronal fibers parallel to the magnetic field is the dominant factor determining the orientation of radiation patterns. It follows that the average magnetic field appears to be tilted 30° west from the vertical. We also find that within a given active region, the average type III production rate of flares peaks 1° west of the center of gravity of all the flares of this active region.We infer that the coronal magnetic field above active regions presents a strong east-west asymmetry, resulting from the well known asymmetry at the photospheric level. As the west side of an active region covers a smaller area with stronger magnetic field than the east side, western flares are generally closer to open field lines than eastern flares. As a consequence, accelerated particles on the trailing (east) side of active regions usually stay trapped in magnetic loops, while on the leading (west) side they are more likely to escape along open lines into interplanetary space. As a result of the initial westward tilt of these open lines, we estimate that the corresponding Archimedean spiral is on average (apparently) rooted 15° west of the flare.     

Cyclotron wave instability in the corona and origin of solar radio emission with fine structure

The longitudinal waves (Bernstein modes and plasma waves near the hybrid frequency) in a mixture of equilibrium coronal plasma and a small group of energetic electrons are investigated. The energetic electrons have a nonequilibrium momentum distribution inherent in trapped particles. The frequency dependence of the cyclotron instability increments is studied. Attention is paid to a significant role of the relativistic effects for the cyclotron instability of longitudinal waves. For sufficiently large velocity of nonequilibrium electrons the increments are shown to increase when the hybrid frequency coincides with one of the gyrofrequency harmonics (double plasma resonance). The results obtained are used in Parts II and III to explain tadpoles and zebra-pattern in solar radio bursts.     

Characteristics of a type II solar radio burst on 22 March 1998

A solar radio type II burst (which was seen as two patches of emission, one during 07:00–07:13 UT and other one during 07:20–07:35 UT) was observed on 22 March 1998 using the Madurai radio spectrograph. A broad range of data (from Culgoora and Hiraiso spectrographs, white-light data from SOHO/LASCO and X-ray data from Yohkoh and GOES satellites) was also studied for this event, which was analyzed in comparison with these supplementary data. In addition, the conditions associated with this shock were analyzed quantitatively. From the above investigations, the following conclusions have been made. The temporal relationship between H-alpha flare and burst has shown that the active region AR 8185 is the source of this type II burst. A bright front feature observed with LASCO is also associated with this type II burst and active region AR 8185. The time profile of the shock derived from the first patch of this type II burst coincides with the flare starting time. Also, within error limits, the start time of the CME is same as the flare. Hence, it is not possible to decide whether the type II originated in the flare or was driven by CME. In addition, the investigations of the second patch alone has provided the following results. The inferred shock speed for the second patch of emission is lower than the first and closer to the CME speed. The emission occurred below 50 MHz. These conditions imply that this patch may be a separate burst which might have been produced by the CME alone.     

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.     

Expanding arch structure of a solar radio outburst

A flare-associated complex outburst was observed on 1968, October 23–24 with the 80 MHz Culgoora radioheliograph. Two harmonic type II bursts were followed by two successive extended sources with arch structure which appeared further beyond the optical limb than the preceding sources. The second arch showed a remarkable expansion with a projected velocity of 1200 km/sec. At its maximum the arch extended to a height of 2R . The height-time plots derived from both the radioheliograph and spectrum observations suggest that two shock waves of different propagation velocities were initiated at the flash phase of the flare: the faster one was responsible for the first type II burst and the first radio-emitting arch; the slower one for the second type II burst and the second arch whose expansion advanced with the shock front.     

Spectral features of burst radio emission over the solar cycle

We have investigated common burst spectral features for the 20th cycle of solar activity. The maximum daily radio fluxes in 8 frequency ranges are analysed. For every year the classification of these daily spectra is obtained by cluster analysis methods. There are two spectral minima for average spectra of clusters (in frequency ranges 4–3 and 0.5–0.25 GHz). As a rule their positions do not change during the solar cycle.Every annual spectrum of weak bursts has three minima (in frequency ranges 4–3, 2–1, and 0.5–0.25 GHz). The positions of these minima remain invariable during the solar cycle. But anuual spectra of strong bursts depend essentially on the phase of solar activity.The basic features of most burst spectra can be explained by gyrosynchrotron radiation of thermal and nonthermal electrons and plasma radiation at the plasma frequency and its second harmonic.     

A complex solar radio burst and the characteristics of its related microwave sources and EUV coronal loops

On 2001 Oct. 19, a very complex solar radio burst with a host of interesting features was observed with a broadband (0.7–7.6 GHz) solar radio spectrometer. Combining with the data of NoRH (Nobeyama Radio Heliograph) and TRACE (Transition Region and Corona Explorer), the spectral features of the radio burst, the evolution of the microwave radio sources, and relations with the complex EUV coronal loops are analyzed. The burst is the radio manifestation of a large double-ribbon flare; it consisted of two stages. The earlier stage was dominated bya broadband burst in the centimeter-meter waveband from gyro-synchrotron emission of sources at the footpoints of the loop. The later stage was dominated by a narrow-band decimeter wave burst in the decimetermeter waveband, from a combination of plasma emission and gyro-resonance emission from sources in the top of the loop.     

Observations of radio emission from the cosmic gamma-ray burst GRB 030329

We present the radio observations of the afterglow from the intense cosmic gamma-ray burst GRB 030329 performed with the radio telescopes of the Institute of Applied Astronomy, Russian Academy of Sciences, at the Svetloe (λ=3.5 cm) and Zelenchuk (λ=6 cm) Observatories. The difference between the fluxes measured in two different polarization modes suggests the existence of a circular polarization in the radio afterglow from GRB 030329. However, since the measurement errors of the fluxes with different circular polarizations are large, we cannot draw a firm conclusion about its detection; we can only set an upper limit on its value. An analysis of the possible generation mechanisms for the circular polarization of the relativistic jet suggests that there is a helical magnetic field in the jet. The existence of significant flux densities at various wavelengths during a long (≥10 days) period leads us to conclude that the hydrodynamic evolution of the relativistic bow shock takes place in the stellar wind, not in the interstellar medium. We have estimated the total GRB energy (E=10⁵¹ erg) (under the assumption of isotropic radiation) and the plasma density of the stellar wind from the presupernova (n=3 cm^?3). The magnetic-field strength in the relativistic jet can be estimated as B≈100 G.     

The harmonic structure of a type II burst on 12 May, 1983

We report on the observation, in the 12 May 1983 type II radio burst, of the fundamental, the second, third, and, possibly, fourth harmonics. The emission on the first three harmonics starts almost simultaneously but ceases at different moments of time. The emission intensity of the third harmonic is much smaller than is that of the fundamental and second harmonics.It is suggested that the emission is observed on the first harmonics of the electron-cyclotron frequency and originates in regions satisfying the conditions for double plasma resonance. The magnetic field estimated in these regions exceeds the generally accepted value by one order of magnitude.