First measurement of ∼10 keV neutral atoms in the low‐latitude ionosphere

Abstract

Energetic neutral atoms (ENAs) with energies of 4 to 35 keV were measured at altitudes of 170 to 570 km by a new ENA instrument on board a sounding rocket. The instrument measured particles precipitating into the ionosphere from the equatorial region of the magnetosphere at a magnetic local time of ∼1830. The geomagnetic activity was quiet for a prolonged period before the launch. The measured ENA flux was ∼10² (cm² s str keV)−1 at energies of ∼10 keV. The energy spectrum is in a good agreement with an expected spectrum of hydrogen atoms originating from the ring current region as reported by Milillo et al. [1996]. The altitude profile is also discussed in terms of collisional interaction of ENAs with upper‐atmospheric constituents.