Direct assessment of far-side helioseismology using so/phi magnetograms

(Solar Orbiter nugget #8 Dan Yang¹, Laurent Gizon^1,2,3, Hélène Barucq^4,5 and the SO/PHI team)

Helioseismic imaging is a powerful method to detect active regions on the Sun's far side by using observations of the solar acoustic oscillations. Acoustic waves propagate through the solar interior and connect the far side to the visible side of the Sun (see Fig. 1a for an illustration). Since waves travel faster in magnetised regions, they have the potential to detect active regions along their paths of propagation. Lindsey & Braun (2000) proved this concept to work by cross-correlating the forward and backward propagated waves. This method is known as helioseismic holography and is used routinely to monitor the Sun’s far side. Recent developments in helioseismic holography (Gizon et al.2018; Yang et al. 2023) led to significant improvements in the ability to image active regions on the far side (Fig. 1b), which may also be seen in images of the chromosphere and low corona by NASA’s STEREO spacecrafts (Fig. 1c). 

Figure 1: Panel a shows a wave packet travelling from the visible to the far side of the Sun under the ray approximation (Lindsey & Braun, 2000). Panel b shows an example map of the far side on 4 March 2014 (blue) obtained using helioseismology (Yang, Gizon & Barucq, 2023), together with a magnetogram of the front side from SDO/HMI (grey). Panel c shows a contemporaneous STEREO/EUVI image (orange) of the chromosphere and low corona.

The STEREO images of the solar chromosphere are however fuzzy and do not inform us directly about the exact locations of active regions at the Sun’s surface. Photospheric magnetograms are the optimal data to be used for the validation and the calibration of far-side helioseismology. These have only recently become available for the far side during the cruise phase of the Solar Orbiter mission of ESA and NASA. Figure 2 shows the SO/PHI (Solanki et al. 2020) magnetogram on 18 November 2020 (panel a) and the contemporaneous helioseismic image deduced from the SDO/HMI Dopplergrams (panel b). The three largest active regions are clearly seen at the same locations on both the SO/PHI magnetogram and the helioseismic image. This provides the strongest evidence so far that far-side helioseismology works.

Solar Orbiter has entered its nominal mission phase, and the SO/PHI synoptic program will provide magnetograms of the Sun from various vantage points with a cadence of multiple images per day. This means that many more active regions will be used in the future to calibrate the helioseismic far-side maps, and study the emergence and evolution of active regions over the entire solar surface. In particular, it will be possible to convert the signal from far-side helioseismology into active-region magnetic field strength.

Figure 2: Magnetic activity on the entire solar surface on 18 November 2020 during Carrington Rotation CR 2237. The SO/PHI and SDO/HMI magnetograms are shown in panel a. The contemporaneous seismic image is shown in panel b.

This work has been accepted for publication in Astronomy and Astrophysics (forthcoming article) and is available online at https://doi.org/10.1051/0004-6361/202346030.

Affiliations:

(1) Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

(2) Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany

(3) Center for Space Science, NYUAD Institute, New York University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE

(4) Makutu project team, Inria, TotalEnergies, 64000 Pau, France

(5) LMAP, UMR 5142, Université de Pau et des Pays de l'Adour, 64000 Pau, France

References:

[1] L. Gizon, D. Fournier, D. Yang, et al., Astron. Astrophys. 620, A136 (2018)

[2] C. Lindsey & D. C. Braun, Science 287, 1799 (2000)

[3] S.K. Solanki, J.C. del Toro Iniesta, J. Woch, et al, Astron. Astrophys. 642, A11 (2020)

[4] D. Yang, L. Gizon, & H. Barucq, Astron. Astrophys. 669, A89 (2023)

[5] D. Yang, L. Gizon, H. Barucq, et al., Forthcoming Article in Astron. Astrophys. (2023)

Further reading:

L. Gizon, A. C. Birch, & H. C. Spruit, Annu. Rev. Astron. Astrophys. (2010)