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

• Published in: Space Weather Vol. 15; no. 2; pp. 310 - 324
• Main Authors: Wexler, D B, Jensen, E A, Hollweg, J V, Heiles, C, Efimov, A I, Vierinen, J, Coster, A J
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.02.2017
• Subjects: Alfven waves; Corona; Egress; Energy flux; Faraday effect; Faraday rotation; Field strength; Fluctuations; Flux density; Geophysics; Kinetic energy; Magnetic fields; Magnetohydrodynamics; MESSENGER Spacecraft; Parameter uncertainty; Plasma density; Power law; Radio signals; Radio transmissions; Solar corona; Solar cycle; Solar energy; Solar minimum; Solar physics; Solar rotation; Solar wind; Solar wind properties; Spacecraft; Wave energy; Wave power
• ISSN: 1539-4964; 1542-7390
• DOI: 10.1002/2016SW001558

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 8.4GHz 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 (FRFs) 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 3.3mHz and 6.1mHz. Our results confirm the presence of coronal FRF similar to those described previously at greater solar offset. Interpreted as Alfvén waves crossing the line of sight 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. Key Points Wave-like Faraday rotation fluctuations were identified in the lower corona The wave energies scale as a power law in the 1-20mHz frequency regime Faraday rotation fluctuation analysis enables tracking of modeled Alfven wave energy flux