Magnetotail Ion Structuring by Kinetic Ballooning‐Interchange Instability

By combining three‐probe THEMIS observations and 3‐D Particle‐in‐Cell simulations, we identify key structures on the ion gyroradius scale that occur in connection with ballooning‐interchange instability heads in the Earth's magnetotail. The mesoscale structures occur at sites of strong ion velocity shear and vorticity where the thermal ion Larmor radius is about half of the width of the head. Finer structures occur at the smaller scales characterizing the wavelength of the electromagnetic ion cyclotron waves generated at the heads. These two processes act to erode and thin the current sheet, thereby forming a local magnetotail configuration that is favorable for reconnection. Plain Language Summary Ballooning‐interchange heads are fundamental dynamic objects in the nightside Earth's magnetosphere, which represent magnetic structures that are similar in shape to the atmospheric mushroom clouds resulting from a sudden formation of a volume of lower‐density gases. The heads appear in series and drive geomagnetic east‐west oriented aurora, which glows like a pearl necklace across the night sky. Magnetospheric ions appear to sense the heads and modify their circular paths perpendicular to the magnetospheric magnetic field, because their gyration radius is on the order of the head's size. Such a modification of the ion motion appears to lead to ion redistribution on the two sides of the head according to the ion energy. Together with a finer ion structuring, which occurs along the magnetic field lines, these processes cause changes in the nightside magnetic field, which may lead to severe magnetospheric disturbances and prolonged bright aurora. Key Points Key ion structures were identified in connection with ballooning‐interchange instability heads in the Earth's magnetotail Mesoscale structures were found at strong ion velocity shear/vorticity sites, where the thermal ion gyroradius is half of the head's width Finer structures were generated at scales characterizing the wavelength of the electromagnetic ion cyclotron waves