Observational characteristics of microwave type III bursts

The 266 type III bursts, observed with the 2.6–3.8 GHz high temporal resolution dynamic spectrometer of NAO during the 23rd solar cycle (from April 1998 to January 2003), are statistically analyzed. The parameters of these events, including the frequency drift, duration, polarization, bandwidth, starting and ending frequencies, are analyzed in details. The statistics on the starting and ending frequencies indicate that the starting frequency varies in a very large range from less than 2.6 GHz to greater than 3.8 GHz, while the ending frequency varies in a relatively narrow range from 2.82 GHz to 3.76 GHz. These phenomena imply that the heights where the electrons are accelerated are quite scattered, while the cutoff regions of the type III bursts are relatively restricted. The numbers of the bursts with the positive and negative drift rates are nearly equal, this may suggest that the accelerated electrons propagating upward and those propagating downward are equally proportioned in the observing frequency range. And the statistical results demonstrate that the microwave type III bursts are mainly caused by the plasma radiation and electron gyro-maser radiation.     

Type III bursts and coronal temperature

Numerical solution of the interaction of evolutionary electron beam with coronal plasma shows that the velocity of packet of the generated Langmuir's waves increases with the increase of coronal temperature. This effect can be manifested by a dependence of the drift of type III bursts on the coronal temperature.     

Relative timing of microwave bursts and metric type III bursts. Implication on the energy of type III burst exciter

The propagation speed of the exciter of solar type III bursts is derived from observations with high space and time resolution at 22 GHz and 169 MHz. A survey of an active region during two successive days revealed a high degree of association between microwave and type III bursts. From a detailed investigation of their location and timing, which requires neither a coronal density model nor the assumption of radial propagation, the exciter is found to propagate at a speed above at least 0.6c, i.e., much faster than the commonly cited value of c/3. Type III bursts in the dm-m wave band, hence, may reveal the energization of electrons up to energies far above 100 keV.     

Type III radio bursts in the outer corona

Type III solar radio bursts observed from 3.0 to 0.45 MHz with the ATS-II satellite over the period April–October 1967 have been analyzed to derive two alternative models of active region streamers in the outer solar corona. Assuming that the bursts correspond to radiation near the electron plasma frequency, pressure equilibrium arguments lead to streamer Model I in which the streamer electron temperature derived from collision damping time falls off much more rapidly than in the average corona and the electron density is as much as 25 times the average coronal density at heights of 10 to 50 solar radii (R ). In Model II the streamer electron temperature is assumed to equal the average coronal temperature, giving a density enhancement which decreases from a factor of 10 close to the Sun to less than a factor of two at large distances (> 1/4 AU). When the burst frequency drift is interpreted as resulting from the outward motion of a disturbance that stimulates the radio emission, Model I gives a constant velocity of about 0.35c for the exciting disturbance as it moves to large distances, while with Model II, there is a decrease in the velocity to less than 0.2c beyond 10 R .     

Type III solar radio bursts and the fundamental-harmonic hypothesis

The observational evidence is reviewed for the occurrence of type III solar radio bursts in pairs with frequency ratio two to one. We show that the observations can be explained under the hypothesis that there is a tendency for a type III burst to be followed by a second burst within approximately one second. This explanation leads to fewer difficulties than the hypothesis that type III bursts occur in pairs, one member being emitted at the fundamental of the local coronal plasma frequency, the other at its second harmonic. We conclude that in general, type III bursts are emitted at the second harmonic of the plasma frequency and that type III theories should account for this and only under very special circumstances (which are rare) for the emission at the fundamental and the second harmonic.     

Anisotropic Beam Model for the Spectral Observations of Radio Burst Fine Structures on 1998 April 15

A fine structure consisting of three almost equidistant frequency bands was observed in the high frequency part of a solar burst on 1998 April 15 by the spectrometer of Beijing Astronomical Observatory in the range 2.6-3.8GHz. A model for this event based on beam-anisotropic instability in the solar corona is presented. Longitudinal plasma waves are excited at cyclotron resonance and then transformed into radio emission at their second harmonic.The model is in accordance with the observations if we suppose a magnetic field strength in the region of emission generation of about 200G.     

Radio signature of magnetic reconnection and bi-directional shock waves in a flare-CME event on April 15, 1998

A flare-CME event on April 15, 1998 is studied with data of Nobeyama Radio Polarimeters (NoRP) and Heliograph (NoRH), the radio spectrometers of Chinese National Astronomical Observatories (1.0–2.0 GHz and 2.6–2.8 GHz), and the Astrophysical Institute of Postdam (200–800 MHz), as well as the data of YOHKOH, SOHO, BATSE, and GOES. There were strong fluctuations superposed on the initial phase of the BATSE hard X-ray burst, and the radio burst at 1.0–2.0 GHz with a group of type III-like positive and negative frequency drift pairs, which may be interpreted as the process of magnetic reconnection or particle acceleration in corona. A type II-like burst with a series of pulsations at 200–800 MHz followed the maximum phase of the radio and hard X-ray burst, and slowly drifted to lower frequencies with typical zebra feature. After 10 min of that, a similar dynamic spectrum was recorded at 2.6–3.8 GHz, where the type II-like signal drifted to higher frequencies with a series of pulsations and zebra structures. The polarization sense was strongly RCP at 2.6–3.8 GHz, and weakly LCP at 1.0–2.0 GHz, which was confirmed by the observations of NoRP. The radiation mechanism of these pulsations may be caused by the electron cyclotron maser instability. The local magnetic field strength and source height are estimated based on the gyro-synchrotron second harmonic emission. The ambient plasma density is calculated from the YOHKOH/SXT data. The ratio between the electron plasma frequency and gyro-frequency is around 1.3, which corresponds to the reversal value from extraordinary mode (LCP) to ordinary mode (RCP). Moreover, both the time scale and the modularity of an individual pulse increase statistically with the increase in the burst flux, which may be explained by the acceleration process of non-thermal electrons in the shock wave-fronts propagated upward and downward. Therefore, the radio observations may provide an important signature that flare and CME are triggered simultaneously by magnetic reconnection and are associated with the formation of bi-directional shock waves.     

Discussion on the coronal structure related to type III bursts

Some statements recently published on the coronal structure related to type III bursts are discussed. The indirect approach based upon computed coronal magnetic field, as proposed by Kuiper (1973), is examined; it is concluded that some doubt exists about the suitability of this method. The problem of the relationship between the existence of filaments inside an active center and type III bursts production is analysed: considering the respective locations of flaring sites and filaments as well as their place inside the magnetic network of the active center, one cannot deduce any evidence of type III inhibition by filaments. There is no direct conclusive evidence of the association of type III's with known coronal structures. Some observations suggest that at least in some cases, the coronal streamers are involved.     

Effects of turbulence on the electron cyclotron-maser mechanism for solar microwave spike bursts

It is shown that small magnetic perturbations can significantly alter the rates of cyclotron growth, absorption, mode conversion, and refraction because of the sensitive dependence of these processes on the field strength in narrow layers. In particular, growth lengths are increased, absorption depths decreased, mode conversion becomes more effective, and turbulent refraction leads to isotropization of the emission. The criteria for significant effects to occur are derived and it is shown that they can be met by the few-percent field perturbations observed in coronal loops. Relative to the theory of cyclotron-maser emission in smoothly varying plasmas, perturbations enable fundamental o-mode (o1) and second-harmonic x-mode (x2) radiation to saturate more effectively, increase the chance of x1, o1, and x2 radiation escaping to infinity through absorption and mode-coupling windows, and partially isotropize radiation emitted near the x-mode cutoff. It is concluded that o1 and x1 emission are both likely to be present, and that x2 emission is possible under some circumstances. However, x1 radiation can escape only at near-parallel propagation ( 0) or via mode conversion to the o-mode at 90°, whereas o1 and x2 emission can escape for a wide range of angles around = 0 and, under many circumstances, near = 90°.     

Evolvement of microwave spike bursts in a solar flare on 2006 December 13

Solar radio spikes are one of the most intriguing spectral types of radio bursts. Their very short lifetimes, small source size and super-high brightness temperature indicate that they should be involved in some strong energy release, particle acceleration and coherent emission processes closely related to solar flares. In particular, for the microwave spike bursts, their source regions are much close to the related flaring source region which may provide the fundamental information of the flaring process. In this work,we identify more than 600 millisecond microwave spikes which recorded by the Solar Broadband Radio Spectrometer in Huairou(SBRS/Huairou) during an X3.4 solar flare on 2006 December 13 and present a statistical analysis about their parametric evolution characteristic. We find that the spikes have nearly the same probability of positive and negative frequency drifting rates not only in the flare rising phase, but also in the peak and decay phases. So we suppose that the microwave spike bursts should be generated by shockaccelerated energetic electrons, just like the terminational shock(TS) wave produced by the reconnection outflows near the loop top. The spike bursts occurred around the peak phase have the highest central frequency and obviously weak emission intensity, which imply that their source region should have the lowest position with higher plasma density due to the weakened magnetic reconnection and the relaxation of TS during the peak phase. The right-handed polarization of the most spike bursts may be due to the TS lying on the top region of some very asymmetrical flare loops.     

Numerical simulation of type III solar radio bursts on meter- and hectometer-waves

Numerical simulation of type III bursts is made by the use of fully numerical scheme showing a general rule for obtaining a numerically stable difference scheme. Although the electron distribution function is one-dimensional in velocity space, the plasma waves is cylindrically symmetric two-dimensional in K-space.It is confirmed that the previous simulation made by the use of semi-analytical method assuming the plateau distribution of electron distribution is qualitatively correct, but the number density of electron beam to have a typical type III burst was overestimated by a factor of about 3.It is demonstrated that a tentative neglection of a term for the induced scattering of plasma waves into nonresonant K-range gives no remarkable effect on the energy loss of the electron beam, though the scattering is strong. The reason is that the scattering reduces the saturation level of plasma waves resulting in a reduction of the energy loss, while a part of the energy of electron beam is indirectly lost by the scattering.     

Some studies on the solar microwave bursts in relation to the slowly varying component

The occurrences of 5772 microwave bursts recorded by the Sagamore Hill and Manilla Solar Radio Observatories over the period January 1968 to July 1970, covering the maximum phase of the current solar cycle at frequencies 2695, 4995 and 8800 MHz and their energy excesses have been examined in relation to the S-component of solar radio emission. The average slowly varying component has been determined by the superposed epoch method commonly known as the Chree analysis. Similar treatment of the bursts, data, mentioned above has been made to examine any probable 27-day variation and the results obtained have been compared with that of the S-component. Further, spectra of the microwave bursts under the so-called spectral type - inverted U, particularly those having a peak at 4995 MHz, have also been examined and compared with the average spectrum of the S-component. Some of the important results obtained from the present analysis are: (1) the nature of variation of both the average number of occurrences and energy excesses of the microwave bursts follow in general the average 27-day variation of the S-component, (2) the number of occurrences and energy excesses of the microwave bursts are comparatively greater in the ascending phase of the 27-day cycle than those in the descending phase, (3) bursts at progressively higher frequencies originate at lower levels in the solar atmosphere than those of the associated S-component, and (4) the average spectrum of the microwave bursts of inverted U spectral type having a peak at 4995 MHz is quite identical in nature to that of the S-component.     

A rapidly evolving active region NOAA 8032 observed on April 15th, 1997

The active region NOAA 8032 of April 15, 1997 was observed to evolve rapidly. The GOES X-ray data showed a number of sub-flares and two C-class flares during the 8–9 hours of its evolution. The magnetic evolution of this region is studied to ascertain its role in flare production. Large changes were observed in magnetic field configuration due to the emergence of new magnetic flux regions (EFR). Most of the new emergence occured very close to the existing magnetic regions, which resulted in strong magnetic field gradients in this region. EFR driven reconnection of the field lines and subsequent flux cancellation might be the reason for the continuous occurrence of sub-flares and other related activities.     

Preliminary results on the apparent size of the sources of type III bursts observed at low frequencies

We present preliminary results on the apparent angular size of the sources of four type III bursts observed between 3500 and 50 kHz from the IMP-6 spacecraft. The observations were made with a dipole rotating in the plane of the ecliptic where the sources are assumed to be. The apparent angular sizes obtained are unexpectedly large. We discuss different explanations for the results. It seems that the scattering of radio waves by electron density inhomogeneities is the most likely cause.We report a temporal increase of the apparent angular size of the source during the burst lifetime for some bursts. From its characteristics it apears to be a real effect.