Observations of a broad and dynamic region of radiation in geospace

The solar wind particles can directly access the high-altitude cusp. They could create large diamagnetic cavities with a size of 6R _E (Earth radii) in the latitudinal direction and >10R _E in the longitudinal direction at an altitude of 7-8R _E, and up to 10.5R _E in the depth; they could depress the field from more than 260nT to near 0nT. Energetic charged particles with energies from 20keV up to 15MeV have been detected there, and their intensities are observed to increase by as much as four orders of magnitude during the diamagnetic cavity crossings. Strong electromagnetic fluctuations are measured there as well. The power spectral density of the magnetic fluctuations inside the cavities shows increases by up to five orders of magnitude at O ⁺ gyro-frequency in comparison to the outer Van Allen radiation belt. The cavity electric fields have an amplitude of up to 350mV/m, and its perpendicular component varies from -300 to 350mV/m. The phase space densities of both the He ⁺⁺ and O ⁺ ion species in the diamagnetic cavities could be more than two orders of magnitude higher than that in the outer radiation belt at a given magnetic moment. These in situ observations reveal a broad and dynamic region of radiation centered at the Earth's magnetospheric cusp, which is different from the Van Allen radiation belt centered at the Earth's magnetic equator with relatively stable magnetic field. The region of radiation is observed on both open and closed geomagnetic field lines. It can extend to low latitude to reach L-shell values of 7-12.5, providing a direct particle source for the outer radiation belt/ring current.