First measurement of similar to 10keV neutral atoms in the low-latitude ionosphere

Energetic neutral atoms (ENAs) with energies of 4 to 35 keV were measured a t altitudes of 170 to 570 km by a new ENA instrument on board a sounding ro cket. The instrument measured particles precipitating into the ionosphere f rom the equatorial region of the magnetosphere at a magnetic local time of similar to 1830. The geomagnetic activity was quiet for a prolonged period before the launch. The measured ENA flux was similar to 10(2) (cm(2) s str keV)(-1) at energies of similar to 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 pro file is also discussed in terms of collisional interaction of ENAs with upp er-atmospheric constituents.