Analyzing Global Coronal Magnetic Fields using the Coronal Multi-channel Polarimeter
The determination of the magnetic field across different layers of the solar atmosphere, particularly in the corona, provides vital insights into key physical processes governing solar activity. In the paper "Global maps of the magnetic field in the solar corona" by Yang et al. (2020), the authors extend the capability of imaging spectroscopy for coronal magnetic field diagnostics. Leveraging the observations made with the Coronal Multi-channel Polarimeter (CoMP), they offer global maps of the plane-of-sky component of the coronal magnetic field, with derived field strengths ranging from 1 to 4 Gauss between 1.05 and 1.35 solar radii.
Methodology
The authors employ observations of the Fe XIII emission lines at 1074.7 nm and 1079.8 nm. These lines afford insights into plasma density through their intensity ratios, which are sensitive to electron density. They further harness the Doppler velocity from these spectral lines to identify transverse magnetohydrodynamic (MHD) waves and calculate their phase speeds. Through combining these measures, Yang et al. derive the plane-of-sky component of the magnetic field utilizing the kink speed formula within the coronal plasma environment, assuming the predominant influence of magnetic pressure and therefore adjusting magnetic permeability constants appropriately.
Key Results
The paper establishes that magnetic field strengths in the studied coronal areas predominantly range from 1 to 4 Gauss. Notably, this is consistent with fields in localized coronal regions deduced through other methodologies such as radio and spectropolarimetric observations. Despite demonstrating coherency on a large scale with the extrapolated potential field source surface (PFSS) model, the measurements depict discrepancies such as variance seen at scales below ~200 arcsec. The inherent limitations of using the PFSS model, such as assuming a current-free coronal field and reliance on synoptic magnetograms, could account for these mismatches.
Implications
This work underscores the potential for imaging-spectroscopy-based methods to contribute to routine coronal magnetic field measurements, a practice currently impeded by limitations in conventional methods such as the Zeeman effect. The approach could facilitate better predictions of solar phenomena influenced by coronal magnetism, such as solar flares and CMEs, which have substantive effects on space weather. The current analysis, requiring up to two hours of data collection under stable conditions, needs further development for applicability in dynamic conditions, such as during solar eruptions.
Future Prospects
Improvements in instrumentation could enhance resolution capabilities, facilitating exploration of localized structures within the coronal field. The integration of advanced modeling techniques and real-time data processing could yield more comprehensive views of coronal dynamics. Additionally, further refinement of the assumptions involved (e.g., line-of-sight effects, assumptions of isotropic density distributions) could decrease uncertainty margins, amplifying the method’s accuracy. As solar observatories deploy more advanced observational frameworks, the assimilation of data from instruments like CoMP is poised to enhance our understanding of both global and localized coronal dynamics.