The Polarization of Metre-Wave Solar Emission

Suzuki, S.; Sheridan, K. V.

doi:10.1017/s1323358000018786

Cited by 4 publications

(7 citation statements)

References 15 publications

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“…So far, we believe that VLA observations of noise storms at 327 MHz indicate that at the higher-frequency two-component noise storms are also bipolar, even at the limb. Thus the prediction by that higher-frequency observations of noise storms might show evidence for weak coupling, which the model of Suzuki and Sheridan (1980) is consistent with, are not confirmed: we believe that there continues to be no evidence for weak coupling in QT layers in observations of noise storms at metric wavelengths.…”

Section: Mode Coupling and Noise Stormsmentioning

confidence: 67%

“…However, we need to take into account the fact that f~ for lines of sight at 1.5 GHz probably differs from f~ for lines of sight at 5 GHz. (In effect the model of Suzuki and Sheridan (1980) incorporates this result, and they indeed find that the region of strong coupling is larger at low frequencies than at high frequencies). The value of ft is most sensitive to the magnetic field strength, and the magnetic field in the QT layer along the line of sight at a given frequency is probably related to the strength of the field in the source.…”

Section: Mode Coupling and The Microwave S Compouentmentioning

confidence: 76%

“…Based on typical parameters (4) requires B < 0.2 G for the 327 MHz QT layer (as discussed in Section 2 this value is only weakly sensitive to the values of n e and L), and this implies even lower values of B in the QT layers at lower frequencies (cf. limits of B < 1.5 G for the 200 MHz QT layer discussed by Suzuki and Sheridan, 1980). For sources near the limb the height of the QT layer will be similar to the height of the source, and then the magnetic fields in the two locations may be comparable.…”

Section: Mode Coupling and Noise Stormsmentioning

confidence: 92%

“…However, as is usual in type III bursts, the degree of polarization was small in all cases. Observations of U bursts at lower frequencies do not show weak coupling, i.e., the two legs show opposite circular polarizations at 80 MHz (Sheridan, McLean, and Smerd, 1973;Suzuki, 1978;Suzuki and Sheridan, 1978). Suzuki and Sheridan (1980) argue that in fact this trend is consistent with models for the corona, but the result implies that magnetic fields in QT layers for U bursts are generally stronger than they are for noise storms at the same frequency, whereas the low degree of polarization suggests that the magnetic field is larger in the U burst source than in noise storms.…”

Section: Mode Coupling and Noise Stormsmentioning

confidence: 99%

“…The theory of mode coupling in cold plasmas has been studied in considerable detail specifically with reference to solar observations (Piddington and Minnett, 1951;Cohen, 1960;Melrose, , 1974aMelrose, , b, 1980aZheleznyakov, 1977;Zheleznyakov andZlotnik, 1978, 1988;Suzuki and Sheridan, 1980;Bandiera, 1982). There are two electromagnetic modes which can propagate in a cold plasma: the ordinary (o) and extraordinary (x).…”

Section: Introductionmentioning

confidence: 99%

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Observations of mode coupling in the solar corona and bipolar noise storms

1992

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We review high-spatial-resolution observations of the Sun which reflect on the role of mode coupling in the solar corona, and present a number of new observations. We show that typically polarization inversion is seen at 5 GHz in active region sources near the solar limb, but not at 1.5 GHz. Although this is apparently in contradiction to the simplest form of mode coupling theory, in fact it remains consistent with current models for the active region emission. Microwave bursts show no strong evidence for polarization inversion. We discuss bipolar noise storm continuum emission in some detail, utilizing recent VLA observations at 327 MHz. We show that bipolar sources are common at 327 MHz. Further, the trailing component of the bipole is frequently stronger than the leading component, in apparent conflict with the 'leading-spot' hypothesis. The observations indicate that at 327 MHz mode coupling is apparently strong at all mode-coupling layers in the solar corona. The 327 MHz observations require a much weaker magnetic field strength in the solar corona to explain this result than did earlier lower-frequency observations: maximum fields are 0.2 G. This is a much weaker field than is consistent with current coronal models.

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Section: Mode Coupling and Noise Stormsmentioning

confidence: 67%

Section: Mode Coupling and The Microwave S Compouentmentioning

confidence: 76%

Section: Mode Coupling and Noise Stormsmentioning

confidence: 92%

Section: Mode Coupling and Noise Stormsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Observations of mode coupling in the solar corona and bipolar noise storms

1992

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Polarization of fundamental type III radio bursts

Wentzel

1984

Sol Phys

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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 ~o _~ % + 0.5f2.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.

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