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.     

Time structure of solar decametre type III radio bursts

The time structure of solar radio decametre Type III bursts occurring during the periods of enhanced emission is investigated. It is found that the time profiles can take a variety of forms of which three distinct types are the following: (1) profiles where the intensity rises to a small but steady value before the onset of the main burst, (2) the intensity of the main burst reduces to a finite level and remains steady before it decays to the base level, (3) the steady state is present during the rise as well as the decay phase of the main burst. It is shown that these profiles are not due to random superposition of bursts with varying amplitudes. They are also probably not manifestations of fundamental-harmonic pairs. Some of the observed time profiles can be due to superposition ot bursts caused by ordered electron beams ejected with a constant time delay at the base of the corona.     

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.     

Time profile of type III bursts

On the hypothesis that the time profile of a type III burst corresponds directly to the flux of electron beam, the similarity of time profile is shown to be maintained even if the electron velocity decreases with distance provided that the time is normalized to unity at the time of maximum flux. The observed time profiles of type III bursts with simple shape seem to follow the similarity law in almost all frequency range. This evidence may indicate that the time profile, both the rising and decaying phases, of a type III burst should be attributed to a common origin, e.g., the time variation of exciter determined by the initial velocity distribution in the electron beam, instead of attributing the rising time to the beam length and the decay time to the damping of plasma waves after the passage of the electron beam.     

Modeling of type III bursts dynamic spectra

The one-dimensional process of spatially limited electron stream propagation in the solar corona is simulated. It is shown that the beam instability development results either in strong relaxation in velocity space and inhibition of spatial diffusion (high-stream density) or in velocity space relaxation decrease and simultaneous growth of spatial stream length (low-stream density). Assuming a profile of background plasma density to be exponential, dynamic spectra of type III bursts are modeled, which shows that the emission source velocity is constant, and a duration of the burst emission at a given frequency reduces for high-stream densities.     

The starting frequencies of type III bursts

During the period 1960 to 1966 the monthly averages of the starting frequencies of type III bursts declined with the level of solar activity and reached a minimum near the minimum of the solar cycle in 1964. The electron densities corresponding to the observed starting frequencies are close to those expected at the base of the K corona. It is shown that sufficient free-free absorption may occur in the corona above the appropriate plasma levels to account for the observed behavior of the starting frequencies of the bursts. The daily variation in the starting frequencies is attributed to structural variation of the inner corona. Quiescent prominences may be responsible for establishing periods of anomalously low-starting frequencies.     

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.     

Pairs of non fundamental-harmonic type III bursts

Different forms of pairs of type III bursts have been discussed in the literature. We report here a new aspect revealed by high time resolution radioheliography. In some groups of these bursts, each element appears to be split into two components. These pairs recur with a characteristic time, and in a given group the time splitting of the two components of each pair is the same (one second or less). The nature of these pairs is discussed: the fundamental-harmonic hypothesis is excluded. Alternative interpretations are reviewed.     

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.     

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.     

A review of solar type III radio bursts

Solar type III radio bursts are an important diagnostic tool in the understanding of solar accelerated electron beams. They are a signature of propagating beams of nonthermal electrons in the solar atmosphere and the solar system. Consequently, they provide information on electron acceleration and transport, and the conditions of the background ambient plasma they travel through. We review the observational properties of type III bursts with an emphasis on recent results and how each property can help identify attributes of electron beams and the ambient background plasma. We also review some of the theoretical aspects of type III radio bursts and cover a number of numerical efforts that simulate electron beam transport through the solar corona and the heliosphere.     

The brightness temperatures of solar type III bursts

There is a characteristic maximum brightness temperature T _B 10¹⁵K for type III solar radio bursts in the solar wind. The suggestion is explored that the maximum observed values of T _Bmay be attributed to saturation of the processes involved in the plasma emission. The processes leading to fundamental and second harmonic emission saturate when T _Bis approximately equal to the effective temperature T _Lof the Langmuir waves. The expected maximum value of T _Bis estimated for this saturation model in two ways: from the growth rate for the beam instability, and from the maximum amplitude of the observed Langmuir turbulence. The agreement with the observed values is satisfactory in view of the uncertainties in the estimates (a) of the intrinsic brightness temperature from the observed brightness temperature, (b) of the actual growth rate of the beam instability, which must be driven by local, transient features (that are unobservable using available instruments) in the electron distribution, and (c) in the k-space volume filled by the Langmuir waves, and this is consistent with the observational data on two well-studied events at the orbit of the Earth and with statistical data for events over a range of radial distances from the Sun.     

Fundamental and harmonic radiation in solar type III bursts

There are three kinds of observations that provide indirect evidence for the contentions that (a) some type III radiation is fundamental radiation; and (b) type III's are at times emitted simultaneously as fundamental and second-harmonic plasma radiation.Presented by D. Melrose     

Remote flare brightenings and type III reverse slope bursts

We present two large flares which were exceptional in that each produced an extensive chain of H emission patches in remote quiet regions more than 10⁵ km away from the main flare site. They were also unusual in that a large group of the rare type III reverse slope bursts accompanied each flare.The observations suggest that this is no coincidence, but that the two phenomena are directly connected. The onset of about half of the remote H emission patches were found to be nearly simultaneous with RS bursts. One of the flares (August 26, 1979) was also observed in hard X-rays; the RS bursts occurred during hard X-ray spikes. For the other flare (June 16, 1973), soft X-ray filtergrams show coronal loops connecting from the main flare site to the remote H brightenings. There were no other flares in progress during either flare; this, along with the X-ray observations, indicates that the RS burst electrons were generated in these flares and not elsewhere on the Sun. The remote H brightenings were apparently not produced by a blast wave from the main flare; no Moreton waves were observed, and the spatially disordered development of the remote H chains is further evidence against a blast wave. From geometry, time and energy considerations we propose: (1) That the remote H brightenings were initiated by direct heating of the chromosphere by RS burst electrons traveling in closed magnetic loops connecting the flare site to the remote patches; and (2) that after onset, the brightenings were heated by thermal conduction by slower thermal electrons (kT1 keV) which immediately follow the RS burst electrons along the same loops.     

Solar radio type III bursts and coronal density structures

The comparison of solar radio type III bursts measured at 169 MHz with K corona observations leads to the conclusion that about 75% of the active regions over which type III bursts occur are associated with low density coronal structures. The comparison with X-ray maps of the solar disk shows that all these regions are located in low intensity regions.It is concluded that the idea generally accepted that the type III bursts are associated with dense coronal structures and travel in these structures is not at all proven for a large number of cases.     

Interplanetary baseline observations of type III solar radio bursts

Simultaneous observations of type III radio bursts from spacecraft separated by 0.43 AU have been made using the solar orbiters HELIOS-A and HELIOS-B. The burst beginning at 19:22 UT on March 28, 1976 has been located from the intersection of the source directions measured at each spacecraft, and from burst arrival time differences. The source positions range from 0.03 AU from the Sun at 3000 kHz to 0.08 AU at 585 kHz. The electron density along the burst trajectory, and the exciter velocity (=0.13c) were determined directly, without the need to assume a density model as has been done with single-spacecraft observations. The separation of HELIOS-A and -B has also provided the first measurements of burst directivity at low frequencies. For the March 28 burst the intensity observed from near the source longitude (HELIOS-B) was 3–10 dB greater than that from 60° west of the source (HELIOS-A).     

The third harmonic of type III solar radio bursts

The harmonic ratios of a large sample of inverted-U bursts are found to be smaller at the turning frequency than at the starting frequency. Ratios <2.0 are explained by postulating that the lowest fundamental frequencies emitted are prevented from escaping from the corona by an evanescent region between the source and the observer. This concept is used to construct a source model for inverted-U bursts where the density is lower inside a magnetic flux tube than it is outside.     

Low frequency spectra of type III solar radio bursts

Flux density spectra have been determined for ninety-one simple type III solar bursts observed by the Goddard Space Flight Center radio astronomy experiment on the IMP-6 spacecraft during 1971 and 1972. Spectral peaks were found to occur at frequencies ranging from 44 kHz up to 2500 kHz. Half of the bursts peaked between 250 kHz and 900 kHz, corresponding to emission at solar distances of about 0.3 to 0.1 AU. Maximum burst flux density sometimes exceeds 10^–14 W m^–2 Hz^–1. The primary factor controlling the spectral peak frequency of these bursts appears to be variation in intrinsic power radiated by the source as the exciter moves outward from the Sun, rather than radio propagation effects between the source and IMP-6. Thus, a burst spectrum strongly reflects the evolution of the properties of the exciting electron beam, and according to current theory, beam deceleration could help account for the observations.