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

Wexler, D. B. and Jensen, E. A. and Hollweg, J. V. and Heiles, C. and Efimov, A. I. and 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. ISSN 1539-4956


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.

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Item Type: Article (Commonwealth Reporting Category C)
Refereed: Yes
Item Status: Live Archive
Additional Information: Permanent restricted access to Published version, in accordance with the copyright policy of the publisher.
Faculty / Department / School: Current - Faculty of Health, Engineering and Sciences - School of Agricultural, Computational and Environmental Sciences
Date Deposited: 02 May 2017 05:49
Last Modified: 16 Apr 2018 23:37
Uncontrolled Keywords: coronal radio sounding; Faraday rotation; solar physics; Alfven waves
Fields of Research : 02 Physical Sciences > 0201 Astronomical and Space Sciences > 020109 Space and Solar Physics
Socio-Economic Objective: E Expanding Knowledge > 97 Expanding Knowledge > 970102 Expanding Knowledge in the Physical Sciences
Identification Number or DOI: 10.1002/2016SW001558
URI: http://eprints.usq.edu.au/id/eprint/30499

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