Nature 507, 338–340 (2014)

Earth's inner radiation belt consists of energetic electrons and protons, having energies up to hundreds of keV and beyond 100 MeV, respectively. They are trapped by the geomagnetic field at altitudes spanning roughly 1,000 km to 6,000 km. In addition, the energy distribution of the electrons across the inner radiation belt resembles the stripes of a zebra.

Such striations have been observed previously, and attributed to solar activity. However, Aleksandr Ukhorskiy and colleagues find that data taken during a six-month period, using instruments on board the Van Allen Probes spacecraft, reveal patterns that are more regular in the absence of geomagnetic storms.

Instead, the authors' simulations show that the Earth's rotation causes the stripes. It induces an electric field that affects the magnetospheric plasma within the 'co-rotation' region. As the Earth's rotation axis is offset from the magnetic dipole axis by 11°, this leads to an oscillation of the co-rotation region with respect to the outer magnetosphere. The resulting perturbation in the magnetic fields and, in particular, the induced electric fields then resonantly interacts with electrons on a diurnal cycle, creating the observed zebra stripes.