Polarization changes with time during solar microwave impulsive bursts

The evolution with time of circular polarization (t) from solar bursts at 7 GHz presents, in the majority of cases, a polarization degree peak before the maximum flux time. The subsequent evolution of (t) is continuous and usually increasing. The changes could be caused by superimposed polarization effects, due to the fast emissive electrons (dominant in the first phase), and to the propagation effects caused by the coronal condensation where the event occurred (dominant in the second phase). In an approximate approach, (t) is connected to the movement of the source in the second phase, being qualitatively sound, but limited to the lack of knowledge on acceleration processes and on magnetic field topology in the active region where the flares take place.     

Polarization features of type IV bursts

Using both the polarization records of our institute at seven different frequencies and polarization records from other stations, the spectral diagrams of some important type IV bursts are completed by polarization diagrams. Combining both types of diagrams and adding the results of optical observations and X-ray data it is possible to come to a deeper understanding of the processes taking place during strong solar radio bursts.     

Spectral distributions of microwave bursts

The lack of open literature publication of the distributional properties of the cm-λ spectra of solar microwave bursts has lead to some erroneous concepts of the typical characteristics of these spectra. To provide more accurate information, this paper sets forth various distributions of the peak flux density spectra of large numbers of bursts, based on observations of the Sagamore Hill Radio Observatory at nine discrete frequencies between 245 and 35000 MHz over the years 1968–1971. As a foundation for the distribution studies, the basic spectral classification system is outlined. The majority of burst spectra were found to contain a cm-λ component having a single spectral maximum in the 1400 to 35000 MHz range; such spectra are designated C type. A study of the correlation of the spectral maximum frequency f _max of the cm component and the photospheric magnetic field strength of the associated region shows a tendency for greater correlation at higher f _max for stronger magnetic sssfields. A study of the correlation for C type spectra between f _max and the quasi-cutoff frequency f _qc on the low-frequency side shows that for bursts of moderate peak flux density (50–500 sfu) f _qc is well correlated with f _max; a good fit to the relation f _max=A f _qc is found with A =3.4. The possible attenuating mechanisms responsible for the spectral shaping of the cm component are discussed.     

Kilometric shock-associated events and microwave bursts

The peak times of impulsive microwave bursts are compared with those of shock-associated (SA) kilometric radio events. The first peaks in these two frequency regimes are usually well-correlated in time, but the last peaks of the SA events observed at 1 MHz occur an average of 20 min after the last impulsive microwave peaks. In some cases, the SA events overlap in time with the post-burst increases of microwave bursts; sometimes there is general correspondence in their intensity time profiles. These observations suggest that the earlier components of the SA events are usually caused by electrons accelerated in or near the microwave source region. We discuss the possibility that the later components of some SA events could be associated with nonthermal electrons responsible for microwave post-burst increases, although they have traditionally been attributed to electrons accelerated at type II burst producing shocks in the upper corona.     

Quasi-periodic structure in solar microwave bursts

High sensitivity, high time resolution recordings of microwave radio bursts show a number of periodic and quasi-periodic bursts which exhibit intervals of the order of 10–20 s. Some of the bursts are accompanied by simultaneous pulsations of the same interval detected in X-rays, type III-m, and extreme ultraviolet emissions. Mechanisms to explain solar radio pulsations are reviewed to see which can explain or be extended to explain these observations.Supported by a company-financed research program of The Aerospace Corporation.     

Spatially coherent oscillations in microwave bursts

Solar microwave burst observations (made with the WSRT) with high time and high spatial resolution show large-scale (> 8000 km) short-period (1.5 s) modulations of the source. It is argued that an interpretation in terms of Alfvén oscillations in the microwave source is ruled out by this observation. Instead it must be the source of the fast electrons, that produce the microwaves, that is oscillating. The fluctuating acceleration region is identified with a volume where a sheared field is compressed against a flux tube by an unstable current. MHD oscillations in the overlying fluxtube are caused by the pushing force. The rapidly expanding current plays a major role in the flare theory of van Tend and Kuperus (1978).     

Fine structure in solar microwave bursts

We have designed and constructed a new multi-channel radio spectrograph for the study of short-lived structures in solar microwave bursts. It measured the integrated flux over the whole solar disc in two circular polarizations at 36 frequencies between 4 and 8 GHz, with a time constant of 0.5 ms. We have analyzed all 119 recorded bursts observed in 1981 and 1983. We focused our attention on events with a lifetime of less than 1 s. Fine structure occurs in about 30% of the observed bursts, and can be as rich in detail as in bursts observed at lower frequencies. We found at least four different classes of events. In one event neither bandwidth nor time resolution of the receiver appear to be sufficient to resolve the fine structure. The bulk of the drifts is found to be towards higher frequencies. Periodic flux variations were found in two cases.     

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.     

The intensity decrease of microwave bursts

A typical microwave burst on 1968 January 11, 1700 UT is used to demonstrate that the radiation spectrum at maximum phase can be described by gyromagnetic absorption. A model for the source is derived from the observed spectrum. With the aid of this model, we try to explain the decreasing phase of the burst intensity. Satisfactory agreement with observation is obtained, when one assumes that the cooling of the burst plasma is caused by heat conduction parallel to the magnetic field lines.     

伴随CME的微波爆发的特征

分析了与日冕物质抛射(CME)有关的太阳微波爆发(SMB)的特征,包括持 续时间、峰值流量、爆发类型、谱指数等．选取了从1999年11月至2003年9月的136 个事件,包括60个部分晕状CME(120°<宽度<360°)／晕状CME(宽度=360°)和 76个正常CME(20°<宽度<120°)／窄CME(0°<宽度<20°)． 研究发现: (1)与正常CME／窄CME有关的微波爆发持续时间较短,与部分晕状 ／晕状CME有关的微波爆发持续时间有长有短; (2)与慢CME有关的微波爆发持续时 间较短,与快CME有关的微波爆发持续时间可长可短;(3)与正常／窄CME有关的微 波爆发峰值辐射流量比较小,与部分晕状／晕状CME有关的微波爆发峰值辐射流量有大 有小;(4)与慢CME有关的微波爆发峰值辐射流量较小,与快CME有关的微波爆发峰 值辐射流量可长可短; (5)与正常／窄CME有关的微波爆发绝大多数为简单(simple) 型,与晕状CME有关的微波爆发绝大多数为复杂(C)／大爆发(GB)型; (6)与CME 有关的事件在频率,f相似文献   

High-resolution microwave spectra of solar bursts

Microwave observations with exceptionally high spectral resolution are described for a set of 49 solar flares observed between May and October 1981. Total power data were obtained at 40 frequencies between 1 and 18 GHz by the Owens Valley frequency-agile interferometer with 10 s time resolution. Statistical analysis of this sample of microwave bursts established the following significant characteristics of their microwave spectra: (i) Most ( 80%) of the microwave events displayed complex spectra consisting of more than one component during some or all of their lifetime. Single spectral component bursts are rare. It is shown that the presence of more than one component can lead to significant errors when data with low spectral resolution are used to determine the low-side spectral index. (ii) The high-resolution data show that many bursts have a low-side spectral index that is larger than the maximum value of about 3 that might be expected from theory. Possible explanations include the effect of the underlying active region on the perceived burst spectrum and/or the necessity for more accurate calculations for bursts with low effective temperatures, (iii) the peak frequencies of the bursts are remarkably constant during their lifetimes. This is contrary to expectations based on simple models in which the source size and ambient field remain constant during the evolution of a burst.Swiss National Science Foundation Fellow from the University of Bern.     

Homologous microwave bursts and associated solar flares

Homologous characteristics of radio bursts at 3000 MHz and associated optical flares are studied. It is found that flares associated with homologous radio bursts are also homologous optically.Published with the permission of the Director-General of Observatories, New Delhi.     

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.     

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.     

A mechanism for pulsation in solar microwave bursts

I suggest that the pulsation in solar microwave bursts is a modulation of gyro-synchrotron radiation. Whistler waves at the foot of a coronal loop (radio source) interact with nonthermal electrons with loss-cone distribution at the top. As a consequence, electrons outside the loss-cone diffuse into the loss-cone, pass through the loop foot, sink in the atmosphere, and emit gyro-synchrotron radiation as additional pulses. Electrons remaining outside the loss-cone give the background radiation of the burst.Assuming the configuration of a magnetic dipole lying below the photosphere, I calculated the period of pulsation to be 1 s- 1 min. The ratio of the pulse peak to background intensity is calculated to be 0 – 100%; the calculated pulse width is about 0.3 – 50 s. These values are consistent with the observed values. A brief discussion of the probable interpretation of fast, millisecond structures is also given.     

Interpretation of time characteristics of solar X-ray bursts referring to associated microwave bursts

It has been controversial whether the flare-associated hard X-ray bursts are thermal emission or non-thermal emission. Another controversial point is whether or not the associated microwave impulsive burst originates from the common electrons emitting the hard X-ray burst.It is shown in this paper that both the thermal and non-thermal bremsstrahlung should be taken into account in the quantitative explanation of the time characteristics of the hard X-ray bursts observed so far in the photon energy range of 10–150 keV. It is emphasized that the non-thermal electrons emitting the hard X-rays and those emitting the microwave impulsive burst are not common. The model is as follows, which is also consistent with the radio observations.At the explosive phase of the flare a hot coronal condensation is made, its temperature is generally 10⁷ to 10⁸K, the number density is about 10¹⁰ cm^–3 and the total volume is of the order of 10²⁹ cm³. A small fraction, 10^–3–10^–4, of the thermal electrons is accelerated to have power law distribution. Both the non-thermal and thermal electrons in the sporadic condensation contribute to the X-ray bursts above 10 keV as the bremsstrahlung. Fast decay of the harder X-rays (say, above 20 keV) for a few minutes is attributed to the decay of non-thermal electrons due to collisions with thermal electrons in the hot condensation. Slower decay of the softer X-rays including around 10 keV is attributed to the contribution of thermal component.The summary of this paper was presented at the Symposium on Solar Flares and Space Research, COSPAR, Tokyo, May, 1968.     

The secondary spectral component of solar microwave bursts

Microwave observations in the range 1 to 18 GHz with high spectral resolution (40 frequencies) have shown that many events display a complex microwave spectrum. From a set of 14 events with two or more spectral components, we find that two different classes of complex events can be distinguished. The first group (4 events) is characterized by a different temporal evolution of the spectral components, resulting in a change of the spectral shape. These events probably can be explained by gyrosynchrotron emission from two or more individual sources. The second class (10 events) has a constant spectral shape, so that the two spectral components vary together in intensity. For all ten events in this second class, the ratio of primary to secondary peak frequencies is remarkably similar, exhibiting an average value of 3.4, and both components show a common circular polarization. These properties suggest either a common source for the different spectral components or several sources which are closely coupled. An additional example of this class of burst was observed interferometrically to provide spatial resolution. This event suggests that the primary and secondary components have a similar location, but that the surface area of the secondary component is larger.Swiss National Science Foundation Fellow from the University of Bern.