Depolarization of Radio Bursts Due to Reflection off Sharp Boundaries in the Solar Corona

Melrose, D. B.

doi:10.1086/498499

Cited by 20 publications

(24 citation statements)

References 16 publications

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“…A simplified theory of depolarization due to reflection was proposed by Melrose (2006). According to this theory, when a wave in one magnetoionic mode is reflected at a sharp boundary, waves in both modes are generated.…”

Section: Depolarization Due To Single Reflectionmentioning

confidence: 99%

“…To calculate the depolarization coefficient, we used the model of Melrose (2006). In this model, (1) the magnetic field is ignored in the refractive indices but is taken into account in the circular polarizations; (2) the sharp boundary layer is assumed to be parallel to the magnetic field lines and to have a thickness of less than about one wavelength; and (3) the incident waves, magnetic field, and normal to the boundary are assumed to be coplanar.…”

Section: Comparison Between Model and Observationsmentioning

confidence: 99%

“…Two different methods of reflection in the source region were considered by Melrose (2006) for fundamental emission in type I, II, and III bursts: reflection off bundles of overdense fibers for type I bursts and reflection off the walls of ducts for type II and III bursts. The polarization of a type I burst is known to depend on the location of the source region (Zlobec 1975;Wentzel et al 1986).…”

Section: Picture Of the Source Regionmentioning

confidence: 99%

“…Several mechanisms for such depolarization have been proposed (Cohen 1960;Wentzel et al 1986;Melrose 1989;Melrose 2006;Zlotnik et al 2014). One mechanism is related to the mode coupling that occurs when the waves propagate through a quasi-transverse (QT) magnetic field (Cohen 1960).…”

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confidence: 99%

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Polarization Characteristics of Zebra Patterns in Type IV Solar Radio Bursts

Kaneda

Misawa

Iwai

et al. 2017

ApJ

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The polarization characteristics of zebra patterns (ZPs) in type IV solar bursts were studied. We analyzed 21 ZP events observed by the Assembly of Metric-band Aperture Telescope and Real-time Analysis System between 2010 and 2015 and identified the following characteristics: a degree of circular polarization (DCP) in the range of 0%-70%, a temporal delay of 0-70 ms between the two circularly polarized components (i.e., the right-and left-handed components), and dominant ordinary-mode emission in about 81% of the events. For most events, the relation between the dominant and delayed components could be interpreted in the framework of fundamental plasma emission and depolarization during propagation, though the values of DCP and delay were distributed across wide ranges. Furthermore, it was found that the DCP and delay were positively correlated (rank correlation coefficient R = 0.62). As a possible interpretation of this relationship, we considered a model based on depolarization due to reflections at sharp density boundaries assuming fundamental plasma emission. The model calculations of depolarization including multiple reflections and group delay during propagation in the inhomogeneous corona showed that the DCP and delay decreased as the number of reflections increased, which is consistent with the observational results. The dispersive polarization characteristics could be explained by the different numbers of reflections causing depolarization.

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Section: Depolarization Due To Single Reflectionmentioning

confidence: 99%

Section: Comparison Between Model and Observationsmentioning

confidence: 99%

Section: Picture Of the Source Regionmentioning

confidence: 99%

mentioning

confidence: 99%

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Polarization Characteristics of Zebra Patterns in Type IV Solar Radio Bursts

Kaneda

Misawa

Iwai

et al. 2017

ApJ

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“…Since lower intrinsic polarization can occur only if the initial frequency exceeds the x-mode cutoff frequency (f x cut ), the previous expectation that LMC and nonlinear processes would produce only o-mode radiation meant that depolarization theories were mainly investigated. These include mode coupling [29], scattering by low frequency waves [30,31], and reflection at density inhomogeneities [32]. However, Mjølhus [20] showed that an incident xmode wave perpendicular to B 0 can undergo LMC into an ES wave and a reflected x-mode wave at ionospheric density irregularities.…”

Section: Introductionmentioning

confidence: 99%

Mode Conversion of Langmuir to Electromagnetic Waves with Parallel Inhomogeneity in the Solar Wind and the Corona

Kim¹,

Cairns²,

Robinson³

2008

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Linear mode conversion of Langmuir waves to radiation near the plasma frequency at density gradients is potentially relevant to multiple solar radio emissions, ionospheric radar experiments, laboratory plasma devices, and pulsars. Here we study mode conversion in warm magnetized plasmas using a numerical electron fluid simulation code with the density gradient parallel to the ambient magnetic field B0 for a range of incident Langmuir wavevectors. Our results include: (1) Both oand x-mode waves are produced for Ω ∝ (ωL) 1/3 (ωc/ω) 1, contrary to previous ideas. Only o mode is produced for Ω 1.5. Here ωc is the (angular) electron cyclotron frequency, ω the angular wave frequency, and L the length scale of the (linear) density gradient. (2) In the unmagnetized limit, equal amounts of o-and x-mode radiation are produced. (3) The mode conversion window narrows as Ω increases. (4) As Ω increases the total electromagnetic field changes from linear to circular polarization, with the o-and x-mode signals remaining circularly polarized. (5) The conversion efficiency to the x mode decreases monotonically as Ω increases while the o-mode conversion efficiency oscillates due to an interference phenomenon between incoming and reflected Langmuir/z modes. (6) The total conversion efficiency for wave energy from the Langmuir/z mode to radiation is typically less than 10%, but the corresponding power efficiencies differ by the ratio of the group speeds for each mode and are of order 50 − 70%. (7) The interference effect and the disappearance of the x mode at Ω 1 can be accounted for semiquantitatively using a WKB-like analysis. (8) Constraints on density turbulence are developed for the x mode to be generated and be able to propagate from the source. (9) Standard parameters for the corona and the solar wind near 1 AU suggest that linear mode conversion should produce both o-and x-mode radiation for solar and interplanetary radio bursts. It is therefore possible that linear mode conversion under these conditions might explain the weak total circular polarizations of type II and III solar radio bursts.

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The Morphology of Decimetric Emission from Solar Flares: GMRT Observations

et al. 2006

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Abstract.Observations of a solar flare at 617 MHz with the Giant Meter-wave Radio Telescope are used to study the morphology of flare radio emssion at decimetric wavelengths. There has been very little imaging in the 500 -1000 MHz frequency range, but it is of great interest since it corresponds to densities at which energy is believed to be released in solar flares. This event has a very distinctive morphology at 617 MHz: the radio emission is clearly resolved by the 30 beam into arc-shaped sources seeming to lie at the tops of long loops, anchored at one end in the active region in which the flare occurs, with the other end lying some 200000 km away in a region of quiet solar atmosphere. Microwave images show fairly conventional behaviour for the flare in the active region: it consists of two compact sources overlying regions of opposite magnetic polarity in the photosphere. The decimetric emission is confined to the period leading up to the impulsive phase of the flare, and does not extend over a wide frequency range. This fact suggests a flare mechanism in which the magnetic field at considerable height in the corona is destabilized a few minutes prior to the main energy release lower in the corona. The radio morphology also suggests that the radiating electrons are trapped near the tops of magnetic loops, and therefore may have pitch angles near 90 • .

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Depolarization of Radio Bursts Due to Reflection off Sharp Boundaries in the Solar Corona

Cited by 20 publications

References 16 publications

Polarization Characteristics of Zebra Patterns in Type IV Solar Radio Bursts

Polarization Characteristics of Zebra Patterns in Type IV Solar Radio Bursts

Mode Conversion of Langmuir to Electromagnetic Waves with Parallel Inhomogeneity in the Solar Wind and the Corona

The Morphology of Decimetric Emission from Solar Flares: GMRT Observations

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