Mid-coronal magnetic field intensity: Transcoronal Spacecraft Radio Observations

Abstract:

Knowledge of magnetic field structure and intensities is essential for coronal MHD wave energy calculations. Faraday rotation (FR) of polarized radio signals passing through the corona provides the line-of-sight (LOS) integrated product of the aligned magnetic field components and electron density. Frequency shifts of the same radio signals occur in accordance with the time rate of change in column electron density. Concurrent analysis of FR and radio frequency shifts, with simplifying assumptions, potentially allows the magnetic field contribution to the FR to be isolated from the density component. We analyzed transcoronal MESSENGER spacecraft X-band radio observations from 2009 and 2013 and compared calculated field strengths to estimates from the Community Coordinated Modeling Center (CCMC) 3-D MAS/CORHEL coronal model. The observed FR was evaluated assuming the field strength and electron density are greatest at the point of closest solar approach (at heliocentric distance r in units of solar radius, Rs) and representative over an effective LOS scaling length. A segmental analysis was performed. In data segments with little radio frequency shift, the evolution of FR was attributed primarily to the change in B strength. The 2013 closed-field region sampled at r =1.65 Rs had calculated magnetic field strength of 101,00 nT, which compared favorably to the 96,000 nT value from the proximate point on the corresponding CCMC magnetic field map. In contrast, the 2009 data yielded a field strength of only 4800 nT at 1.61 Rs, versus 6000 nT on the CCMC map. These results suggest that low field strengths may present in the interspace between widely-separated adjacent closed field regions. Our study illustrates the complementarity of Carrington-rotation specific CCMC models with transcoronal radio FR observations, and prompts us to reassess the limits of global-average power law models for magnetic field strength at mid-coronal heights.