Density Depletion in a Coronal Flux Tube Associated With Solar Radio Emission

Wu, C. S.; Wang, Chuanbing; Lu, Quanming

doi:10.1007/s11207-006-2068-z

Cited by 10 publications

(12 citation statements)

References 30 publications

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“…This was first reported by Benson et al [1980], who studied ISIS‐1 data, and confirmed by many subsequent articles [e.g., Benson , 1985; Roux et al , 1993; Strangeway et al , 1998; Janhunen et al , 2002]. Recently, Wu et al [2006] have addressed the depletion of plasma density in a flux tube in the corona. They suggested that density depletion may also take place under conditions peculiar to active regions where solar flares occur annergetic particles are generated.…”

Section: Numerical Simulationsmentioning

confidence: 54%

“…A commonly accepted argument is that in a low‐ β plasma β (1 ≫ β ≪ m e /m i ), such as solar corona and auroral regions, a small perturbation of the ambient magnetic field in a flux tube can result in considerable density change because of pressure balance. Density depletion may also take place under conditions peculiar to active regions where solar flares occur and energetic particles are generated [ Wu et al , 2006]. Two possible theories are that density is depleted because of the presence of beams of energetic particles, and the other is attributed to magnetic compression.…”

Section: Introductionmentioning

confidence: 99%

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Density cavities associated with inertial Alfvén waves in the auroral plasma

Sharma

Singh

2009

J. Geophys. Res.

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[1] We have investigated the nonlinear interaction between inertial Alfvén waves and ion-acoustic waves by numerical simulation and showed that the system becomes more chaotic and turbulent as it evolves. The nonlinear dynamical equation satisfies modified Zakharov type of equations by taking the nonadiabatic response of the background density. The evolution of density cavities corresponding to magnetic field coherent structures are found at different times. The deepest density depletion, which is accompanied by most intense magnetic fields, has the density fluctuations of $0.4n 0 and $0.6n 0 (n 0 is electron number density) for auroral and coronal plasmas, respectively. As a result of the ponderomotive force, the magnetic field is trapped in the regions of low density. The power spectrum of magnetic field fluctuations indicates that the nonlinear interactions may be distributing energy among higher wave numbers.Citation: Sharma, R. P., and H. D. Singh (2009), Density cavities associated with inertial Alfvén waves in the auroral plasma,

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Section: Numerical Simulationsmentioning

confidence: 54%

Section: Introductionmentioning

confidence: 99%

Density cavities associated with inertial Alfvén waves in the auroral plasma

Sharma

Singh

2009

J. Geophys. Res.

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“…On the basis of ECME, a model was explored for coronal type III bursts near the active region Yoon et al 2002). According to this model, the radio emissions are produced inside a magnetic flux tube with density depletion (Duncan 1979;Wu et al 2006) and propagate in the magnetic flux tube until they arrive at a point where the frequencies of the excited waves are equal to the local exterior cutoff frequency. With this model, some longstanding problems from observations of type III bursts can be naturally resolved.…”

Section: Introductionmentioning

confidence: 99%

Solar Type Iii Radio Bursts Modulated by Homochromous Alfvén Waves

Zhao

Chen

2013

ApJ

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Solar type III radio bursts and their production mechanisms have been intensively studied in both theory and observation and are believed to be the most important signatures of electron acceleration in active regions. Recently, Wu et al. proposed that the electron-cyclotron maser emission (ECME) driven by an energetic electron beam could be responsible for producing type III bursts and pointed out that turbulent Alfvén waves can greatly influence the basic process of ECME via the oscillation of these electrons in the wave fields. This paper investigates effects of homochromous Alfvén waves (HAWs) on ECME driven by electron beams. Our results show that the growth rate of the O-mode wave will be significantly modulated by HAWs. We also discuss possible application to the formation of fine structures in type III bursts, such as so-called solar type IIIb radio bursts.

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“…The basic idea developed from observations that F/H waves with identical frequencies have overlapping source regions, as discussed previously in Section 2. In this more recent theory, we extend Duncan's notion by considering that, in the corona, there are density-depleted flux tubes in which the plasma density is much lower than the density of the exterior plasma (A model theory was proposed by Wu et al [27] to explain the origin of such density depletion). Several essential points were considered by Wu et al [23]: (i) waves emitted inside the duct have frequencies close to the local electron gyro frequency and/or second harmonic; (ii) the frequencies of the emitted waves are lower than the cutoff frequencies of the outside plasma so that the radiation cannot exit the duct; (iii) however, when an emitted wave propagates to a sufficiently high altitude where the cutoff frequency of the exterior plasma drops to a value lower than the wave frequency, it exits the duct and becomes visible; and (iv) it is therefore expected that waves having the same frequencies, regardless of where they are originated, exit at the same altitude.…”

Section: A Scenario Without Recourse To the Plasma Hypothesismentioning

confidence: 76%

On theories of solar type III radio bursts

2012

Chin. Sci. Bull.

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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.

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Density Depletion in a Coronal Flux Tube Associated With Solar Radio Emission

Cited by 10 publications

References 30 publications

Density cavities associated with inertial Alfvén waves in the auroral plasma

Density cavities associated with inertial Alfvén waves in the auroral plasma

Solar Type Iii Radio Bursts Modulated by Homochromous Alfvén Waves

On theories of solar type III radio bursts

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