Angular Broadening: Effects of Nonzero, Spatially Varying Plasma Frequency between the Source and Observer

Cairns, Iver H.

doi:10.1086/306215

Cited by 20 publications

(35 citation statements)

References 19 publications

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“…Lee and Jokipii, 1975;Rickett, 1977;Coles and Harmon, 1989;Bastian, 1994;Cairns, 1998;Spangler, 2002] …”

Section: Density Turbulencementioning

confidence: 99%

“…Equations (16) and (22) of Cairns [1998] include the effects on refractive scattering that arise from f p (R) being nonzero and varying with position between the source and observer. By analogy with these equations we write…”

Section: Structure Functionmentioning

confidence: 99%

“…The process is similar to the twinkling of stars and modified "seeing" caused by density turbulence in Earth's atmosphere and ionosphere. This scattering process has been investigated for many years using geometrical optics [e.g., Steinberg et al, 1971] and the parabolic wave equation [e.g., Lee and Jokipii, 1975;Coles and Harmon, 1989;Bastian, 1994;Cairns, 1998]. …”

Section: Introductionmentioning

confidence: 99%

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Constraints on coronal turbulence models from source sizes of noise storms at 327 MHz

Subramanian

Cairns

2011

J. Geophys. Res.

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[1] We seek to reconcile observations of small source sizes in the solar corona at 327 MHz with predictions of scattering models that incorporate refractive index effects, inner scale effects, and a spherically diverging wavefront. We use an empirical prescription for the turbulence amplitude C N 2 (R) based on very long baseline interferometry observations by Spangler et al. of compact radio sources against the solar wind for heliocentric distances R ≈ 10-50 R . We use the Coles and Harmon model for the inner scale l i (R), which is presumed to arise from cyclotron damping. In view of the prevalent uncertainty in the power law index that characterizes solar wind turbulence at various heliocentric distances, we retain this index as a free parameter. We find that the inclusion of spherical divergence effects suppresses the predicted source size substantially. We also find that inner scale effects significantly reduce the predicted source size. An important general finding for solar sources is that the calculations substantially underpredict the observed source size. Three possible, nonexclusive, interpretations of this general result are proposed. First and simplest, future observations with better angular resolution will detect much smaller sources. Consistent with this, previous observations of small sources in the corona at metric wavelengths are limited by the instrument resolution. Second, the spatially varying level of turbulence C N 2 (R) is much larger in the inner corona than predicted by straightforward extrapolation sunward of the empirical prescription, which was based on observations between 10 and 50 R . Either the functional form or the constant of proportionality could be different. Third, perhaps the inner scale is smaller than the model, leading to increased scattering. These results and interpretations are discussed and compared with earlier work.

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“…Lee and Jokipii, 1975;Rickett, 1977;Coles and Harmon, 1989;Bastian, 1994;Cairns, 1998;Spangler, 2002] …”

Section: Density Turbulencementioning

confidence: 99%

Section: Structure Functionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Constraints on coronal turbulence models from source sizes of noise storms at 327 MHz

Subramanian

Cairns

2011

J. Geophys. Res.

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“…For the power spectrum offluctuations of type P n (q) ¼ C 2 N q À , Thejappa et al (2007) have derived the following equation based on equations (24) and (27) of Cairns (1998):…”

Section: Scatteringmentioning

confidence: 99%

Effects of Scattering on Radio Emission from the Quiet Sun at Low Frequencies

Thejappa¹,

MacDowall²

2008

ApJ

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The observations of the quiet Sun at meter and decameter wavelengths show that its brightness temperatures can be $1 order of magnitude lower than the expected values of 10 6 K and the apparent diameters can be very large. We examine whether this unusual behavior is due to refraction in the smoothly varying coronal plasma and scattering by random density fluctuations using an improved Monte Carlo simulation technique. We use a three-dimensional model for the electron density, N e , and a power-law spectrum for the density fluctuations with an exponential index of . We consider two cases: (1) ¼ 11/3 (Kolmogorov spectrum), relative level of density fluctuations ¼ ÁN e /N e ¼ 0:1, and scale heights ranging from the inner scale l i ¼ 684 (N e cm À3 ) À1/2 to outer scale l o ¼ 10 6 l i , and (2) ¼ 3 (flat spectrum), ¼ 0:02, and scale heights ranging from 50 to 75 km. We consider the fluctuations to be anisotropic with axial ratios of $10. The Kolmogorov type of fluctuations yield slightly lower brightness temperatures in comparison with those of a flat spectrum. The brightness temperature distributions, east-west diameters, and central brightness temperatures at various frequencies indicate that the refraction and scattering are probably the underlying reasons for the unusual behavior of the quiet-Sun radio emission. This study clearly demonstrates that Monte Carlo techniques can be very effective in extracting the coronal electron temperatures accurately from the radio data, provided the information about the density distributions and density fluctuations is known by some independent methods.

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“…In the following work we have calculated the scattering on the assumption that the wavelength in the plasma is the same as the free space wavelength. When the observing frequency approaches the local plasma frequency, this will underestimate the scattering [Cairns, 1998]. This effect is negligible for cases in which the small-angle forward scattering assumption is valid.…”

Section: Scattering Modelmentioning

confidence: 99%

Radio wave scattering in the outer heliosphere

Armstrong¹,

Coles²,

Rickett³

2000

J. Geophys. Res.

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Abstract. The Voyager 1 and 2 plasma wave instruments have observed lowfrequency (1.5-4 kHz) radio waves apparently generated near the heliopause. The waves are found in two distinct bands. Power at the lower frequencies centered on 1.78 kHz shows no modulation when the spacecraft is rolled about the Earthspacecraft line, indicating that the radiation is isotropic. Power in the higher band centered on 3.11 kHz shows roll modulation as high as 60%, indicating that the source of the radiation is quite compact, subtending an angle <1 rad at the spacecraft. Simple estimates of the scattering of 3.11 kHz radiation from electron density fluctuations indicate that, if the radiation had originated from the distance of the heliopause, the scattering would be so large that no roll modulation should be observed. Here we show that these earlier scattering estimates were too high because they ignored the latitude variation of scattering and the inner scale of the electron density fluctuation spectrum. With these two effects properly included, the expected scattering is consistent with the observations and the postulate that the radiation originates from near the nose of the heliosphere.

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Angular Broadening: Effects of Nonzero, Spatially Varying Plasma Frequency between the Source and Observer

Cited by 20 publications

References 19 publications

Constraints on coronal turbulence models from source sizes of noise storms at 327 MHz

Constraints on coronal turbulence models from source sizes of noise storms at 327 MHz

Effects of Scattering on Radio Emission from the Quiet Sun at Low Frequencies

Radio wave scattering in the outer heliosphere

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