Density Turbulence and the Angular Broadening of Outer Heliospheric Radio Sources at High Latitudes and in the Ecliptic Plane

Abstract

Density irregularities are responsible for the scattering of radio waves in the solar wind and astrophysical plasmas. These irregularities significantly affect the inferred physical properties of radio sources, such as size, direction, and intensity. We present here a theory of angular broadening due to the scattering of radio waves by density irregularities that improves the existing formalism used to investigate radio wave scattering in the outer heliosphere and the very local interstellar medium. The model includes an inner scale and both latitudinal and radial dependencies for the density fluctuation spectra and propagation paths for the radiation both near and out of the ecliptic plane. Based on the pickup-ion-mediated solar wind model (PUI model) of Zank et al., we estimate the turbulence and solar wind quantities for the high-latitude fast solar wind. The predictions include the density variance, inner/dissipation scale, velocity correlation length, mean magnetic field, and proton temperature. The density turbulence amplitude is estimated in two ways. First, a simple scaling technique is used to extend the theoretical predictions of the PUI model for the high-latitude wind beyond the heliospheric termination shock. Second, the solar wind and turbulence quantities are calculated near the ecliptic plane using plasma and magnetometer data from the Voyager 2 spacecraft over the period 1977–2018. Based on the turbulence models and observations, we calculate the scattering angle of the radio sources in the high-latitude and near-ecliptic wind. Finally, we compare the numerical results with the analytic predictions from Cairns and Armstrong et al. and the observed source sizes.

Linear Mode Decomposition in Magnetohydrodynamics Revisited

“…It is possible, however, as discussed by Bandyopadhyay & McComas (2021), Zhao et al (2022b), that the slab component may become more dominant as distance from the Sun decreases, and the manner in which turbulence anisotropy evolves with heliocentric distance remains an important and open question. Finally, density fluctuations (Telloni et al 2009;Bruno et al 2014;Zank et al 2021a) are of course present in the solar wind, and it has been argued that some of them are advected entropy-related density fluctuations (Hunana & Zank 2010;Adhikari et al 2017Adhikari et al , 2020bAdhikari et al , 2020aZank et al 2017;Asgari-Targhi et al 2021;Tasnim et al 2022).…”

An Anisotropic Density Turbulence Model from the Sun to 1 au Derived from Radio Observations

“…´-cm −6 . The exponent in this expression is close to four and therefore is not inconsistent with the n r i 2 4 d á ñ µexpectation (Zank et al 2017;Tasnim et al 2022) associated with nearly incompressible magnetohydrodynamic turbulence. Comparison of the model with observations does, however, suggest that a photosphere-rooted scaling (with a dependence on the quantity (r/R e − 1)) could be a better rough symmetry than a simple heliocentric symmetry (i.e., ∝r −4 ), probably due to the turbulence evolution along flux tubes rooted into the photosphere.…”

“…where q (cm −1 ) is the wavevector associated with a density fluctuation, 〈L〉 denotes an ensemble average, and n(r) is the background plasma density. A typical wavenumber spectrum (e.g., Alexandrova et al 2013;Tasnim et al 2022) has three main domains…”

“…We note that the density fluctuation power spectrum shows evidence of flattening between the inertial and dissipation scales (e.g., Celnikier et al 1987;Coles & Harmon 1989;Šafránková et al 2015), which we do not consider here for simplicity. Qualitatively, since the wavenumber volume element d 3 q = 4π q 2 dq, the wavenumber spectrum (for an illustrative isotropic density fluctuation spectrum) has three major parts (left panel of Figure 1; see also Figure 2 of Tasnim et al 2022): q 2 S(q) q − γ for large wavelengths (q < q o ), with values of γ between 0 and 1 (e.g., Marsch & Tu 1990;Bird et al 2002;DeForest et al 2018), the (Kolmogorov) inertial range q 2 S(q) ∼ q −5/3 between the outer scale q o and the inner scale q i (e.g., Marsch & Tu 1990), and the dissipation range q 2 S(q) ∼ q − δ at small wavelengths (q > q i ; e.g., Celnikier et al 1983).…”

Verification of the Standard Theory of Plasma Emission with Particle-in-cell Simulations

“…The standard theory has been widely used to explain radio bursts in space and astrophysical plasmas, such as solar radio bursts in terms of type I-V (see, e.g., Chen et al 2014;Vasanth et al 2016;Lv et al 2017;Li et al 2017;Vasanth et al 2019, for the latest observational studies), emissions from planetary electron foreshocks (e.g., Moses et al 1984;Etcheto & Faucheux 1984;Cairns 1987;Kuncic & Cairns 2005;Píša et al 2017) and the outer heliosphere (Kurth et al 1984;Zank et al 1994;Gurnett & Kurth 1995;Cairns 1995Cairns , 1998Gurnett et al 1998;Cairns & Zank 2002;Tasnim et al 2022). The theory involves the evolution of turbulence and complicated multistep nonlinear processes.…”