Global Effects of a Polar Solar Eclipse on the Coupled Magnetosphere‐Ionosphere System

It is well‐known that solar eclipses can significantly impact the ionosphere and thermosphere, but how an eclipse influences the magnetosphere‐ionosphere system is still unknown. Using a coupled magnetosphere‐ionosphere‐thermosphere model, we examined the impact on geospace of the northern polar‐region eclipse that occurred on June 10, 2021. The simulations reveal that the eclipse‐induced reduction in polar ionospheric conductivity causes large changes in field‐aligned current, cross‐polar cap potential and auroral activity. While such effects are expected in the northern hemisphere where solar obscuration occurred, they also occurred in the southern hemisphere through electrodynamic coupling. Eclipse‐induced changes in monoenergetic auroral precipitation differ significantly between the northern hemisphere and southern hemisphere while diffuse auroral precipitation is interhemispherically symmetric. This study demonstrates that the geospace response to a polar‐region solar eclipse is not limited just to the eclipse region but has global implications. Plain Language Summary A solar eclipse occurs when the moon obscures the sunlight reaching Earth. This obscuration inhibits the photoionization of the upper atmosphere, thereby reducing the ionospheric electron density and electrical conductivity of the ionized portion of the upper atmosphere. Since the polar ionosphere is effectively a resistive load on the current system flowing between the magnetosphere and ionosphere, we may expect the state of this system to be affected by the eclipse. The June 10, 2021 solar eclipse provided an unprecedented opportunity to investigate such impacts because it shadowed much of the polar region in the northern hemisphere. We used a whole geospace simulation model to examine the possible effects and found that the eclipse causes oscillations—a form of ringing—in polar ionospheric electrical currents and associated electron precipitation responsible for aurora. Interestingly, the solar eclipse also changed the electrical currents flowing into the non‐obscured (southern) hemisphere and auroral activity there. This study provides new insights into the effects of solar eclipses on auroral activity and the whole geospace. Key Points Eclipse‐induced depletion of ionospheric conductance causes oscillations in auroral activity, currents, and the coupled M‐I system The northern polar solar eclipse impacts the southern ionosphere through electrodynamic coupling Monoenergetic precipit