Faraday rotation fluctuations of MESSENGER radio signals through the equatorial lower corona near solar minimum

Article


Wexler, D. B., Jensen, E. A., Hollweg, J. V., Heiles, C., Efimov, A. I., Vierinen, J. and Coster, A. J.. 2017. "Faraday rotation fluctuations of MESSENGER radio signals through the equatorial lower corona near solar minimum." Space Weather. 15 (2), pp. 310-324. https://doi.org/10.1002/2016SW001558
Article Title

Faraday rotation fluctuations of MESSENGER radio signals through the equatorial lower corona near solar minimum

ERA Journal ID34283
Article CategoryArticle
AuthorsWexler, D. B. (Author), Jensen, E. A. (Author), Hollweg, J. V. (Author), Heiles, C. (Author), Efimov, A. I. (Author), Vierinen, J. (Author) and Coster, A. J. (Author)
Journal TitleSpace Weather
Journal Citation15 (2), pp. 310-324
Number of Pages15
Year2017
Place of PublicationUnited States
ISSN1539-4956
1542-7390
Digital Object Identifier (DOI)https://doi.org/10.1002/2016SW001558
Web Address (URL)http://onlinelibrary.wiley.com/doi/10.1002/2016SW001558/abstract;jsessionid=46DDF9355DE076140BABAE7A1CABA47C.f04t02
Abstract

Faraday rotation (FR) of transcoronal radio transmissions from spacecraft near superior conjunction enables study of the temporal variations in coronal plasma density, velocity and magnetic field. The MESSENGER spacecraft 8.4 GHz radio, transmitting through the corona with closest line-of-sight approach 1.63-1.89 solar radii and near-equatorial heliolatitudes, was recorded soon after the deep solar minimum of solar cycle 23. During egress from superior conjunction, FR gradually decreased, and an overlay of wave-like FR fluctuations (FRF) with periods of hundreds to thousands of seconds was found. The FRF power spectrum was characterized by a power-law relation, with the baseline spectral index being -2.64. A transient power increase showed relative flattening of the spectrum and bands of enhanced spectral power at 3.3 mHz and 6.1 mHz. Our results confirm the presence of coronal FRF similar to those described previously at greater solar offset. Interpreted as Alfvén waves crossing the LOS radially near the proximate point, low-frequency FRF convey an energy flux density higher than that of the background solar wind kinetic energy, but only a fraction of that required to accelerate the solar wind. Even so, this fraction is quite variable and potentially escalates to energetically significant values with relatively modest changes in estimated magnetic field strength and electron concentration. Given the uncertainties in these key parameters, as well as in solar wind properties close to the Sun at low heliolatitudes, we cannot yet confidently assign the quantitative role for Alfvén wave energy from this region in driving the slow solar wind.

Keywordscoronal radio sounding; Faraday rotation; solar physics; Alfven waves
ANZSRC Field of Research 2020519999. Other physical sciences not elsewhere classified
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Byline AffiliationsComputational Engineering and Science Research Centre
Planetary Science Institute, United States
University of New Hampshire, United States
University of California, United States
Russian Academy of Sciences, Russia
Massachusetts Institute of Technology, United States
Institution of OriginUniversity of Southern Queensland
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