On the probability of occurrence of the type IIIb burst as a precursor

It is shown that a precursor type IIIb burst is really associated with a type III burst. The broad longitude distribution of occurrence of type IIIb bursts also suggests that these bursts are emitted at a large angle to the open magnetic field in the corona.     

Observations and interpretation of solar decameter type IIIb radio bursts

Solar decameter bursts of Type IIIb are observed with a multichannel radiometer at wavelengths around 12m. The time and frequency resolutions were 10 ms and 100 kHz. Observations on the time structure of these bursts are presented. A theoretical model which accounts for various aspects of these bursts is proposed.     

Low frequency turbulence in the solar corona and fundamental radiation of type III solar radio burst

On the basis of the previous numerical simulations, a new mechanism for the emission of the fundamental radio waves of solar radio type III bursts is presented. This hypothesis is to attribute the fundamental radio emission to the coalescence of the plasma waves with the low frequency turbulence, whistler or ion acoustic waves, pre-existing on the way of the electron beam which excite the plasma waves.It is estimated that ion acoustic waves could be occasionally unstable in the solar corona due to that drifting bi-Maxwellian distribution of electrons as observed in the solar wind, which is probably caused by collision-less heat conduction.It is also suggested that the reduced damping of the ion acoustic waves in such a distorted electron distribution in the corona may decrease the threshold electric current to cause the anomalous resistivity to be the onset of the solar flares.     

Polarization of fundamental type III radio bursts

The fundamental of type III bursts is only partially polarized, yet all theory for emission near the plasma frequency predicts pure o-mode emission. I argue depolarization is inherent in the burst itself. The o-mode radiation is intensely scattered and mode-converted when it temporarily falls behind its own source and finds itself in the medium that is already disturbed by the electron beam. In particular, mode conversion is very efficient and yet causes only modest angular scattering at the height were _p + 0.5.The predicted minimum polarization nearly equals the polarization of the harmonic, as observed. Spike polarization is naturally explained by the earlier arrival of the scattered o-mode. Additional residual polarization depends on the refraction at the site of emission; larger beam velocities imply higher polarization, as observed, because a larger fraction of the radiation escapes without mode-conversion. The polarization at the frequencies where U-bursts reverse is of particular interest.Support is acknowledged from the NSF Solar-Terrestrial Research Program.     

Solar radio burst (type III)

This article describes the observations of a type III radio burst observed at 103 MHz simultaneously by the two radio telescopes situated at Rajkot (22.3°N, 70.7°E) and Thaltej (23°N, 72.4°E). This event occurred on September 30, 1993 at about 0430 UT and lasted for only half a minute. The event consisted of several sharp spikes in a group. The rise and fall time of these are comparable, however the peaks of individual spikes varied by a factor of four. The comparison of these observations with the data of solar radio spectrograph HiRAS indicates that this was a metric radio burst giving highest emission at about 103 MHz.     

Evidence for electron excitation of type III radio burst emission

Type III radio bursts observed at kilometric wavelengths ( 0.35 MHz) by the OGO-5 spacecraft are compared with > 45 keV solar electron events observed near 1 AU by the IMP-5 and Explorer 35 spacecraft for the period March 1968–November 1969.Fifty-six distinct type III bursts extending to 0.35 MHz ( 50 R equivalent height above the photosphere) were observed above the threshold of the OGO-5 detector; all but two were associated with solar flares. Twenty-six of the bursts were followed 40 min later by > 45 keV solar electron events observed at 1 AU. All of these 26 bursts were identified with flares located west of W 09 solar longitude. Of the bursts not associated with electron events only three were identified with flares west of W 09, 18 were located east of W 09 and 7 occurred during times when electron events would be obscured by high background particle fluxes.Thus almost all type III bursts from the western half of the solar disk observed by OGO-5 above a detection flux density threshold of the order of 10^–13 Wm^–2 Hz^–1 at 0.35 MHz are followed by > 45 keV electrons at 1 AU with a maximum flux of 10 cm^–2 s^–1 ster^–1. If particle propagation effects are taken into account it is possible to account for lack of electron events with the type III bursts from flares east of the central meridian. We conclude that streams of 10–100 keV electrons are the exciting agent for type III bursts and that these same electrons escape into the interplanetary medium where they are observed at 1 AU. The total number of > 45 keV electrons emitted in association with a strong kilometer wavelength type III burst is estimated to be 5 × 10³².     

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.     

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.     

An extreme example of a radio relic in Abell 4038

A radio source with a spectral index of −2.2 between 0.08 and 1.425 GHz has been observed at the Very Large Array (VLA) at 1.425 GHz with resolution ∼ 3 arcsec. The projected linear length of the source is 56 kpc with an average projected distance of 42 kpc from the centroid of the rich southern cluster Abell 4038, assuming the source is in the cluster. The physical parameters of the source include a high minimum-energy field ( B _me) of 38 μG, which is unusual for a source of low surface brightness and relaxed appearance, but is explained by its unusually steep spectrum. Although its radio morphology has some characteristics of a narrow-angle-tail source (NAT), the absence of an identified host galaxy ( m _R ≥ 23.0) makes it unlikely that the source is a working radio galaxy. The relic is probably the remains of an FR II radio galaxy that was once energized by a particular bright cluster elliptical now 18 kpc to its east. The density of the intracluster gas has been sufficient to confine the source and preserve its morphology, permitting the source to age and its spectrum to steepen through synchrotron and inverse Compton energy losses.     

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.     

An example of radio and optical homologous flares

Fokker (1967) has raised the question of whether the optical and radio homologies of flares are correlated. Two 2b flares occurring nearly 54 h apart (July 9 and 12, 1968) were observed at 29 wavelengths from H+4.00 Å to H–4.26 Å and at 10-cm radio. Adjacent pictures were spaced 0.295 Å and 2 sec apart. The time resolution of the radio traces was about 10 sec. Detailed comparison of the pictures showed near-perfect similarities in the two events. These similarities included flare shape, filament agitation, rising arch formation and surges with line-of-sight velocities nearly 200 km/ sec observed. Comparion of the microwave radio flux traces showed detailed similarities in the shapes and simultaneity of at least eight features.Research supported in part by the Atmospheric Sciences Section, National Science Foundation, NSF Grant GA-4184, while at Harvey Mudd College, Claremont, Calif.     

Evidence for halo-like radio sources from kilometric type III burst observations

The radio azimuths for many kilometric type III bursts that originate near or behind the limb of the Sun are observed to drift far to the east or far to the west of the spacecraft-Sun line. It is shown that the behavior of the observed burst parameters for these events corresponds to the response of a spinning dipole antenna to halo-like sources of radiation around the Sun. Our results provide evidence for a previous suggestion that behind-the-limb type III events should appear as halo-like sources of radiation to an observer on the opposite side of the Sun, due to scattering of the radiation from the primary source back around the Sun.     

A comparison of type III solar radio burst theories using satellite radio observations and particle measurements

The required electron density to excite a type III solar burst can be predicted from different theories, using the low frequency radio observations of the RAE-1 satellite. Electron flux measurements by satellite in the vicinity of 1 AU then give an independent means of comparing these predicted exciter electron densities to the measured density. On this basis, one theory predicts the electron density in closest agreement with the measured values.NAS/NRC Postdoctoral Resident Research Associate.     

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.     

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.     

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.     

Coronal densities and magnetic fields from K-coronameter and type IV radio burst data

From K-coronameter data we have obtained an electron density profile above the active region responsible for the Type IV burst observed on 14 September 1966. If the observed frequency cutoff in the burst's spectrum is caused by the Razin effect, then the coronal electron density may be derived from the intensity variation in the burst as it propagates outwards from the Sun. We show that the electron density profiles obtained from K-coronameter data (appropriate to 1.125 <r/R < 2.0) and from the radio data (2.2< r/R < 2.5) form a continuous distribution. We conclude that the cutoff is due to the Razin effect, and that radiation in the burst is due to relativistic electrons having a steep inverse power-law energy distribution. From the electron density profile derived from the radio data, we find that the coronal magnetic field was 0.26 G at r/R = 2.2.     

A type II solar radio burst observed in the corona and in interplanetary space

Solar Physics - Possible mechanisms of the third harmonic generation in solar radio bursts are investigated. It is shown that the most essential is the coupling of plasma waves in a source with...     

Harmonic structure of type IIIb and III bursts

A decameter solar radio storm of type IIIb and III bursts has been analysed, using single frequency records at frequencies 12.5 and 25.0 MHz.Several kinds of burst associations are classified. As a result it is shown that in double oblique burst-traces of type IIIb + III on the frequency-time plane the type III burst is shifted by an octave above the type IIIb burst at any moment of the IIIb + III pair's lifetime. In particular, the harmonic structure of the spectrum is peculiar to the event of type IIIb + III in the initial and the final stages. This property of the pair is clear if the type IIIb and III radiations occur at the fundamental coronal plasma frequency and its harmonic respectively. On the other hand, if it is assumed that a type IIIb burst is the precursor of a type III one, there is no reason why the two bursts should be harmonically related.     

Quantitative comparisons of type III radio burst intensity and fast electron flux at 1 AU

We compare the flux of fast solar electrons and the intensity of the type III radio emission generated by these particles at 1 AU. We find that there are two regimes in the generation of type III radiation: one where the radio intensity is linearly proportional to the electron flux, and the second regime, which occurs above a threshold electron flux, where the radio intensity is proportional to the 2.4 power of the electron flux. This threshold appears to reflect a transition to a different emission mechanism.