The Sun’s tilted axes

As hikers and other navigators know, Earth’s magnetic axis is tilted with respect to its rotation axis. Such misalignment had not been expected in the Sun—but now it’s been seen. NASA’s Solar Dynamics Observatory (SDO) has been trained on the Sun for half of the current 11-year cycle of solar activity. Using SDO’s Helioseismic and Magnetic Imager, Adur Pastor Yabar of the Institute of Astrophysics of the Canary Islands and his colleagues created daily maps of the line-of-sight strength and polarity of the Sun’s magnetic field for each of the mission’s first 1700 days. The Sun’s rotation period varies with latitude. It’s 25.5 days at the equator and 34.4 days at the poles. To look for variations not associated with differential rotation, Pastor Yabar and his colleagues averaged each map over all longitudes in 1-degree-wide latitudinal belts. When the researchers Fourier-transformed the entire sequence of binned maps, they discovered that a more-or-less monthly oscillation showed up at every latitude on every day. Random sprouting of active regions that rotate in and out of view could conceivably account for the oscillation, but when the researchers excluded active regions, the oscillation persisted. Their latitudinal belts did not sample the same parts of the Sun, as they had assumed, but wobbled up and down over the solar disk—hence the oscillation. Based on that and other lines of evidence, Pastor Yabar and his colleagues concluded that the Sun’s magnetic and rotational axes must be misaligned. Dynamo models that presume alignment will require modification. (A. Pastor Yabar, M. J. Martínez González, M. Collados, Mon. Not. R. Astron. Soc. 453, L69, 2015. )