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.