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Magnetic field in the chromosphere
NLFFF extrapolation assumes that the plasma beta is low. This assumption is not valid in the photosphere, which is the very layer that provides the boundary condition. Measuring the magnetic field directly at greater heights — in the chromosphere — is therefore essential, both to test how far photospheric extrapolation can be trusted and to observe magnetic reconnection where it actually happens.
How much does photospheric extrapolation miss?
We measured the chromospheric magnetic field in two active regions through spectropolarimetric observations at He I 10830 Å, and compared the results with potential fields and NLFFFs extrapolated from the photosphere. This comparison allows the uncertainty of photospheric extrapolation to be estimated quantitatively.
The observed chromospheric magnetic field can be more non-potential than the photospheric field, and this large non-potentiality at chromospheric heights is not reproduced by NLFFF extrapolation from the photosphere. At some locations the underestimation reaches 30°–40° in shear signed angle. We attribute this deviation to the non-force-freeness of the photosphere. The result argues that measured chromospheric magnetic fields should be incorporated into NLFFF modeling in order to improve coronal extrapolation.
→ Y. Kawabata, A. Asensio Ramos, S. Inoue, and T. Shimizu, The Astrophysical Journal, 898, 32, 2020
Diagnosing the height of magnetic reconnection
Magnetic reconnection in the lower atmosphere drives chromospheric phenomena such as Ellerman bombs and UV bursts. How the atmosphere is heated depends strongly on the ionization degree and the plasma beta, both of which vary with height, so the height at which reconnection occurs must be diagnosed.
To test whether multiwavelength spectropolarimetry can do this, we synthesized Stokes profiles from a realistic magnetohydrodynamic simulation containing two reconnection regions at different heights. When reconnection occurs at low altitude, both red- and blueshifted components produced by the bidirectional reconnection flow appear in the photospheric lines Fe I 8468 Å and K I 7664/7698 Å. When it occurs at high altitude, the chromospheric Ca II 8498/8542 Å lines instead show emission caused by heating in the upper part of their formation layer. Near-infrared multiline spectropolarimetry can therefore distinguish the height at which reconnection takes place.
→ Y. Kawabata, C. Quintero Noda, Y. Katsukawa, M. Kubo, T. Matsumoto, and T. Oba, The Astrophysical Journal, 960, 26, 2024
Resolving the 3D structure of Ellerman bombs
The prediction above was tested with real observations. Using the Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) on the 1 m balloon-borne telescope SUNRISE III, we observed an emerging flux region. Being above most of the atmosphere, the telescope delivers seeing-free, high-spatial-resolution data, and SCIP provides seamless multiline spectropolarimetry from the photosphere to the lower chromosphere.
We analyzed the multiline Stokes profiles of the photospheric Fe I and K I lines and the chromospheric Ca II lines, and applied the weak field approximation to the K I and Ca II lines to reconstruct the three-dimensional magnetic field. Blue- and red-wing brightenings of the Ca II 8542 Å line appear at spatially offset locations, indicating bidirectional reconnection flows.
The reconstructed 3D field shows that the opposite-polarity structure reaches different heights in the two events analyzed. In one event it is confined to the lower layers and is absent at the formation height of the Ca II 8542 Å core, which shows no intensity enhancement. In the other it extends up to the Ca II 8542 Å core formation height, where enhanced line-core intensity is also observed. We interpret this as the reconnection current sheet reaching different altitudes.
→ Y. Kawabata et al., The Astrophysical Journal Letters, 1007, L39, 2026