Coronal Electron Density Fluctuations Inferred from Akatsuki Spacecraft Radio Observations
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Coronal Electron Density Fluctuations Inferred from Akatsuki Spacecraft Radio Observations D. Wexler1,2 · T. Imamura3 · A. Efimov4 · P. Song5 · L. Lukanina4 · H. Ando6 · E. Jensen7 · J. Vierinen8 · A. Coster2
Received: 31 March 2020 / Accepted: 28 July 2020 © Springer Nature B.V. 2020
Abstract Trans-coronal radio observations were taken during the 2011 observing campaign of the Akatsuki spacecraft through superior conjunction. The observed X-band (8.4 GHz) signals exhibit frequency fluctuations (FF) that are produced by temporal variations in electron density along the radio ray path. A two-component model for interpretation of the FF is proposed: FF scales largely with acoustic wave amplitude through the inner coronal regions where the sound speed dwarfs the solar wind outflow speed, while FF in the region of solar wind acceleration is dominated by the increased density oscillation frequency on the sensing path that results from bulk advection of the plasma inhomogeneities. An estimate of fractional electron density fluctuation is obtained from the mid-corona. A radial profile of slow solar wind speed is determined in the extended corona using mass-flux continuity principles. The coronal sonic point for slow solar wind is estimated to range from 4 to 5 solar radii from the heliocenter. Keywords Coronal acoustic waves · Solar wind · Electron density oscillations, radio frequency fluctuations
B D. Wexler
[email protected]
1
University of Southern Queensland, Toowoomba QLD 4350, Australia
2
MIT Haystack Observatory, Westford, MA 01886, USA
3
University of Tokyo, Kashiwa, Chiba, 277-8561, Japan
4
Kotelnikov Institute of Radioengineering and Electronics, Fryazino Branch, Russian Academy of Sciences, Fryazino, Russia
5
University of Massachusetts, Lowell, MA 01854, USA
6
Kyoto Sangyo University, Kyoto, 603-8555, Japan
7
Planetary Science Institute, Tucson, AZ 85719, USA
8
University of Tromsø, 9019 Tromsø, Norway
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1. Introduction Radio occultation studies of the solar corona reveal ubiquitous frequency fluctuations that imply electron density disturbances at all heliocentric distances and heliolatitudes. The intensity and spectral form of these inferred density fluctuations are of great interest in the study of coronal turbulence and mechanisms of solar wind acceleration. Alfvén waves generated by transverse motions of magnetic structures in the photosphere propagate up into the corona, but the dissipation process to release energy for coronal heating and solar wind acceleration remains unclear (Cranmer and Winebarger, 2019; Arregui, 2015; De Moortel and Browning, 2015). One theory holds that locally generated compressive (acoustic or magnetosonic) waves play a role in the release of Alfvén wave energy into the corona. Outgoing Alfvén waves subjected to magnetohydrodynamic (MHD) speed gradients may partially reflect and give rise to density oscillations through non-linear interactions, see e.g. (Cranmer et al., 2015; Matthaeus and Velli, 2011; Matthaeus et al., 199
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