# Has a magnetic field flip of a distant star ever been measured?

The magnetic field of the Sun flips during each solar cycle, with the flip occurring when sunspot cycle is near its maximum. Levels of solar radiation and ejection of solar material, the number and size of sunspots, solar flares, and coronal loops all exhibit a synchronized fluctuation, from active to quiet to active again, with a period of 11 years.

Has this phenomenon ever been measured, directly or indirectly, on stars outside the solar system?

The Sun's magnetic activity cycle of $$\sim 22$$ years involves a large-scale reversal of the polarity of the magnetic field every $$\sim 11$$ years.
To directly measure the reversing cycles in magnetic polarity requires spatially resolved maps of the vector magnetic field. Such spatially resolved maps are possible for fast-rotating, and hence highly magnetically active stars through Zeeman Doppler Imaging. In general, highly magnetically active stars appear not to show magnetic activity variations as strongly as the Sun. Nevertheless, recent instrumental developments have led to (difficult) observations of some solar-type stars with intermediate rotation rates. There is now plenty of evidence for magnetic polarity reversals in many of these (e.g. in Chi$$^1$$ Ori, Rosen et al. 2016; in LQ Hya, Lehtinen 2019; in V1358 Ori, Willamo et al. 2021).