A New Route to Coronal Vector Magnetometry
| Nugget | |
|---|---|
| Number: | 534 |
| 1st Author: | Alin PARASCHIV |
| 2nd Author: | |
| Published: | September 1, 2026 |
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Introduction
The magnetic field of the solar corona remains observationally elusive. At the same time, it is absolutely the most fundamental property defining all coronal activity. One of the best approaches to characterizing the field is exploitation of the magnetic sensitivity of the forbidden Fe XIII coronal emission lines in the near infrared (wavelengths around 1 μ). For a long time, the rule in coronal magnetism seemed simple: if you wanted the full three-dimensional magnetic field in the corona you needed the complete polarization signal, in two different spectral lines. Coronal light generally carries a polarization fingerprint that encodes information about the magnetic field, described with four so-called Stokes parameters, I, Q, U, and V. The V parameter is the circular polarization, which can be very strong at radio wavelengths but which can be weak in the visible/IR range.
Getting at the Stokes parameters
The I parameter is just the total intensity; the linear polarization (Q, U) come from from the saturated Hanle effect. This is measurable with current coronal instruments but is sensitive to the field orientation only. The circular polarization from the weak-field Zeeman effect (Stokes V) is significantly harder to measure, while being critically needed for measuring the field strength. Measurements demand large apertures and excellent SNR. Only the DKIST Cryo-NIRSP instrument has thoroughly delivered on such measurements so far (Ref. [1]); meanwhile, wide-FOV, small- aperture instruments like CoMP/UCoMP can't realistically chase Stokes V, even though they are the instruments capable of producing large- scale, high-cadence maps of the plane-of-the-sky projection of B, via Alfvénic wave tracking (Ref. [2]). This, in other words, is an imaging method for determining BPOS, the plane-of-the sky field intensity. Figure 1 illustrates the CoMP view of the global corona, showing its full image capability.
A solution
Our study (Ref. [3]) asks whether this Doppler seismology-derived BPOS can be included in an inverse reconstruction, rather than being treated as a separate, independent diagnostic. The result is a new "IQUD" inversion mode where Fe XIII 1074.7/1079.8 nm IUQ observations are further constrained by a wave-derived B_POS (the "D" for Doppler-oscillation diagnostic). The propagating-kink- wave dispersion relation is used in a POS-projected form, that doesn't force the field to lie exactly in the plane of sky. Ref. [3] investigates test cases, showing that IQUD matches ground truth (IQUV) statistically indistinguishable, aside from degeneracy multiplicity (IQUD alone returns a 4-fold-degenerate B). Under perturbation, matches stay above 96% up to reasonable and observationally achievable relative-uncertainty levels. Inversions still exceeded 77% even at a fairly adverse uncertainties. In other words, performance is limited by the usual suspects: LOS integration and the single-point-approximation that are the bane of remote-sensing the optically thin and highly structured solar corona. Figure 2 illustrates the four-fold degeneracy issue for the IQUD inversions, bearing in mind the two-fold degeneracy of IQUV.
Conclusion
Our alternative IQUD approach holds great promise for improvements in global coronal magnetometry, and we are grateful for the Hawaiian observatories on Mauna Kea (DKIST) and Mauna Loa (UCoMP) that make this possible.
References
[1] "Mapping the Sun's coronal magnetic field using the Zeeman effect"