On theories of solar type III radio bursts

Abstract

In solar radiophysics, many theories for type III bursts have been proposed during the past 60 years. Almost all these theories are based on the plasma hypothesis, which assumes that (i) the radiation is mainly generated by Langmuir waves via nonlinear processes and (ii) the radiation has frequencies close to the local plasma frequency and/or its second harmonic in the source region. We feel strongly that it is time to advocate an alternative approach without recourse to the plasma hypothesis. This brief discussion explains why. solar radiophysics, type III bursts, plasma hypothesis, electron cyclotron maser mechanism Citation:Wu C S. On theories of solar type III radio bursts. Chin Sci Bull, 2012Bull, , 57: 1357Bull, -1361Bull, , doi: 10.1007 Several groundbreaking papers [1-3] reported surprising observational results in the field of solar radiophysics in 1950. These "seed" results and subsequent findings gave rise to a new branch of solar physics that is presently known as solar radiophysics. The rich scientific content and exciting research in solar radiophysics have been summarized in a number of monographs and review articles [4][5][6][7][8][9]. These works describe many fascinating and intriguing phenomena that portray the complexity of the subject, and most of the observations remain unexplained. There are a number of fundamental difficulties in explaining the phenomena. First, physical processes in the solar atmosphere cannot be directly measured through in-situ observations. As a result, discussions generally rely on inferences and hypotheses. Second, it often happens that theoretical explanations are not unique. Third, understanding induced radiation processes relevant to solar radiophysics is an immature topic in plasma physics. Fourth, there could be new plasma processes that are peculiar to the solar atmosphere but of which we have little knowledge. We hope that researchers will make breakthroughs that will explain the phenomena. What we need imminently are innovative ideas. Most theories in solar radiophysics are concerned with type III bursts, which are considered later in this paper. Yet, for historical and traditional reasons, most of these discussions impose the plasma hypothesis, which is defined in the next section. The purpose of the present discussion is two-fold. First, we state that the plasma hypothesis is not proven, and second, we make general comments on existing theories.The paper is organized as follows. Section 1 outlines the important observational results of type III radio bursts. Section 2 outlines the traditional approach in which enhanced Langmuir waves are thought to play deterministic roles. Motivated by an outstanding issue that was raised by observations made in the 1970s, we present a scenario of type III bursts that is based on a totally different notion in Section 3. Section 4 presents concluding remarks and a summary.

A self‐consistent mechanism for electron cyclotron maser emission and its application to type III solar radio bursts

“…Since the ECM emission mechanism was proposed almost simultaneously with the plasma emission mechanism at the end of the 1950s, it has not been adopted extensively in the solar radiophysical community because of some heavy difficulties (e.g., the escape difficulty, the inversion difficulty, and the polarization difficulty) in the explanation of SRBs [ Wild , ; C. S. Wu , ; D. J. Wu , ; Wu et al , ]. However, the discussions in the last section imply that taking account of the effects of the self‐generated AW of an FEB, the novel ECM emission mechanism can probably overcome these difficulties of the conventional ECM emission mechanism.…”

“…Since the ECM emission mechanism was proposed almost simultaneously with the plasma emission mechanism at the end of the 1950s, it has not been adopted extensively in the solar radiophysical community because of some heavy difficulties (e.g., the escape difficulty, the inversion difficulty, and the polarization difficulty) in the explanation of SRBs [Wild, 1985;C. S. Wu, 2012;.…”

“…In order to remove this dilemma, Duncan [] proposed that the radiation is ducted outward in the corona by reflecting off highly collimated overdense fibers. However, the existence and maintenance of these fibers require a strongly magnetized plasma environment, in which the ambient magnetic field may remarkably influence exciting processes of type III SRBs [ Dulk , ; C. S. Wu , ; D. J. Wu , ]. On the other hand, the plasma emission models usually assume that the effect of the ambient magnetic field on the exciting process of type III SRBs is unimportant and ignorable.…”

“…In general, this ECM emission might be more efficient than the plasma emission because the linear instability, in principle, should be a preferential developing process rather than the nonlinear instability when these two simultaneously take place in a plasma. However, the ECM emission mechanism has been not adopted extensively in the solar radiophysical community as expected because there are some heavy difficulties in the application to SRBs [ Wild , ; C. S. Wu , ; D. J. Wu , ; Wu et al , ].…”

“…In order to remove this dilemma, Duncan [1979] proposed that the radiation is ducted outward in the corona by reflecting off highly collimated overdense fibers. However, the existence and maintenance of these fibers require a strongly magnetized plasma environment, in which the ambient magnetic field may remarkably influence exciting processes of type III SRBs [Dulk, 1985;C. S. Wu, 2012;D.…”