Latitude distribution of vertically precipitating energetic neutral atoms observed at low altitudes

Earlier modeling work of the latitude distribution of earthward traveling energetic neutral atoms (ENA) originating in the ring current have suggested that an isotropic ring current with an empty loss cone will deposit most ENA away from the magnetic equator, while a ring current with an equatorial pitch angle distribution peaked at 90° will deposit most ENA at the magnetic equator. We present the first direct low altitude measurements of energetic particles at low and equatorial latitudes supporting this model. The evolution from an isotropic ring current to a more anisotropic one is manifested in the latitude distribution of the low altitude energetic particles during a geomagnetic storm. In the growth and main phase of the storm the latitude distribution of the energetic particles is wide and often show secondary peaks away from the equator, while in the recovery phase the latitude distribution narrows and peaks at the magnetic equator.