Eclipse Camera Catches Hidden Magnetic Fingerprints in the Sun’s Green Corona

For more than a century, the glowing green ring of light that appears around the Sun during a total solar eclipse has tantalized astronomers. The emission comes from highly ionized iron atoms, Fe XIV, sitting in a corona heated to millions of degrees, yet the magnetic fields that sculpt this searing atmosphere have remained stubbornly difficult to measure directly. Now a team led by Hervé Haudemand of the Astrophysical Observatory of Turin, part of Italy’s National Institute for Astrophysics, has reported the first results from a purpose-built coronal magnetograph that observed the April 8, 2024 total solar eclipse from Durango, Mexico. Writing in the journal Solar Physics, the researchers show that the direction of linearly polarized light in the famous Fe XIV 530.3 nanometer line carries a measurable imprint of the coronal magnetic field, a signature produced by a quantum optical phenomenon known as the saturated Hanle effect.

The physics behind the measurement is subtle but elegant. When iron ions in the corona are excited by anisotropic photospheric radiation and then re-emit light, the scattered radiation is linearly polarized. A magnetic field perturbs the motion of the bound electrons, rotating and depolarizing the emitted light, which is the essence of the Hanle effect, first described in the 1920s and historically important in the development of quantum physics. For forbidden lines like the coronal green line, which arises from a magnetic-dipole transition within the ground term of Fe XIV, the critical magnetic field required for the effect is only a few times ten to the minus five gauss. Because typical coronal fields range from a thousandth of a gauss up to ten gauss, the effect is saturated: the polarization no longer depends on the field’s strength, only on its direction. That saturation is a gift for observers, because the angle of linear polarization becomes a direct tracer of magnetic geometry rather than a tangled combination of field strength and orientation.

The theory makes a specific prediction. The polarization direction is governed by the angle between the magnetic field and the local solar vertical. When the field is close to radial, the polarization is parallel to the field’s projection on the plane of the sky, with a maximum degree of polarization. As the field tilts toward the Van Vleck angle of 54.7 degrees, the degree of polarization falls to zero and the emission becomes completely unpolarized. Beyond that critical angle, for fields nearly tangent to the solar limb, the polarization flips to a direction perpendicular to the field projection. In all cases the polarization of the forbidden line should be predominantly radial. Deviations from that radial expectation, the team reasoned, encode information about the local magnetic topology, including loop-like structures that arch above the limb.

Capturing that signal from the ground is extraordinarily difficult. The solar corona shines at roughly one millionth to one ten-billionth of the disk’s brightness, while daytime sky brightness at most observatories is many orders of magnitude higher. Total solar eclipses are the great equalizer: the Moon acts as a near-perfect natural occulter with negligible diffraction, and the lunar shadow suppresses atmospheric scattering by several orders of magnitude. The Coronal Magnetograph, or CorMag, an internally occulted coronagraph developed at the Turin Observatory under the European HEMERA stratospheric balloon program, was pointed at the eclipsed Sun from Ciudad Juárez in Durango, where totality lasted four minutes and nineteen seconds. The eclipse occurred near the maximum of solar cycle 25, offering unusually complex magnetic configurations to test the technique against.

The observing strategy was unorthodox. CorMag’s internal occulter, designed for non-eclipse operation, blocks the inner corona out to 1.4 solar radii, so rather than disassembling the telescope and risking damage to its field lens, the team off-pointed the instrument westward and let the image of the eclipsed Sun drift across the detector. They tracked the drifting lunar disk using a circle-detection algorithm, anchored the solar position with ephemeris data, and co-registered sequences of images to build composite views of the corona from 1.05 to 2 solar radii. A camera restart cost part of the precious window, and thin clouds dimmed the first half of the data, so the analysis focused on the cleaner western limb. Even so, the instrument achieved a signal-to-noise ratio sufficient to map the polarization angle across extended coronal structures.

One further hurdle remained: the green-line filter also admits continuum light from the K-corona, the Thomson-scattered glow of photospheric light off coronal electrons, which is polarized tangentially to the limb and would contaminate any Hanle signature. Since CorMag could not safely tilt its filter to take off-band images during the short eclipse, the team turned to the K-Coronagraph at the Mauna Loa Solar Observatory in Hawaii, which was specially reactivated on April 9, 2024 to support eclipse campaigns. By matching the spatial resolution of the two instruments and scaling the K-corona brightness from its 735 nanometer bandpass to 530.3 nanometers using a model of the solar spectrum convolved with thermally broadened line profiles, the researchers subtracted the continuum pixel by pixel and isolated the polarized Fe XIV signal.

The comparison with theory produced a striking result. The measured angle of linear polarization was, on average, shifted toward the tangential direction rather than the radial orientation predicted for saturated Hanle polarization. The team attributes this systematic offset to a residual K-corona continuum signal that survives the subtraction and, near the Van Vleck angle where the line emission is strongly depolarized, can dominate the observed polarization entirely. Intriguingly, the same tangential tendency was reported in eclipse observations of the green line from 1991, hinting at a possible limitation in currently accepted theories or in the treatment of the continuum. Nevertheless, within the region above the southwest limb, where loop-like structures were visible, the measured polarization angles traced the geometry of the magnetic field, with the largest deviations occurring where the field was nearly radial, exactly as the saturated Hanle formalism requires.

To validate the interpretation, the team compared their polarization maps with a Potential-Field Source-Surface model constructed from Helioseismic and Magnetic Imager magnetograms aboard the Solar Dynamics Observatory. In this approximation the coronal field is current-free and anchored to measured photospheric flux, with a radial boundary imposed at 2.5 solar radii. The modeled field lines and the observed polarization patterns showed clear consistency in the regions under study, and a statistical analysis confirmed that the Fe XIV polarization angle correlates with the direction of the local magnetic field while the K-corona continuum, as expected, shows no such dependence. A complementary comparison with the Upgraded Coronal Multi-channel Polarimeter, which observed the Fe XIII 1074.7 nanometer line the following day, revealed matching latitude-dependent trends in the two lines’ polarization angles, further supporting the magnetic interpretation despite the one-day separation and a coronal mass ejection that forced the analysis away from the northwest quadrant.

The study validates CorMag as an effective instrument for constraining coronal magnetic field models and demonstrates, in practice, that the saturated Hanle effect in forbidden lines can be exploited as a diagnostic of coronal magnetic topology. The authors emphasize that future campaigns, with the internal occulter removed, proper solar tracking, and off-band filter tilting to fully remove the continuum, will deliver the degree of linear polarization directly and enable rigorous forward modeling against more sophisticated force-free and magnetohydrodynamic simulations. Because coronal magnetic fields govern solar wind acceleration, coronal heating, and the eruptions that drive space weather at Earth, a reliable optical magnetometer for the corona would fill one of the most consequential gaps in heliophysics, turning each total solar eclipse into a magnetic survey of our star’s outer atmosphere.

Subject of Research: Measurement of solar coronal magnetic field topology via the saturated Hanle effect in the polarized Fe XIV 530.3 nm forbidden emission line observed during the April 8, 2024 total solar eclipse

Article Title: Signature of the Saturated Hanle Effect in the Linearly Polarized Coronal Fe XIV Emission Line

Article References: Signature of the Saturated Hanle Effect in the Linearly Polarized Coronal Fe XIV Emission Line. (n.d.). https://doi.org/10.1007/s11207-026-02753-8

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02753-8

Keywords: solar corona, magnetic fields, Hanle effect, Fe XIV 530.3 nm, total solar eclipse, coronal magnetograph, polarimetry, forbidden lines, PFSS model, K-corona, space weather, Solar Physics