Energetic neutral atom images of a narrow flow channel from the plasma sheet: Astrid-1 observations

[1] We present two energetic neutral atom (ENA) images in the energy range 26- 37 keV from the ENA imager on board the Swedish microsatellite Astrid- 1. The images are taken 24 hours apart from below the ring current at 0300 magnetic local time (MLT) around the equator at 1000 km altitude and show a narrow region of ENA emissions extending along the field lines in the postmidnight region. A parameterized model of the ring current is used to extract the ion distribution. The extracted ion distribution is 25degrees wide in longitude within L = 4 6 with an almost isotropic pitch angle distribution (PAD). One ENA image from a polar vantage point shows that the loss cones of the PAD are partially filled. The z component of the interplanetary magnetic field (IMF) was +2 and 3 nT at the times of the two observations. For IMF B-z = 3 nT the ENA flux is stronger than that for the observation when IMF B-z = +2 nT. We use a kinetic ring current model to show that the morphology of the extracted ion distribution can be explained by a narrow flow channel from the plasma sheet at L = 10 and 0200- 0300 MLT to L = 5 and 0100- 0200 MLT for 26 keV protons. In order to match the absolute flux of the extracted ion distribution with the results from the kinetic model, the plasma sheet density at L = 10 had to be set to 10 cm 3. This value is unreasonably high, and we discuss some scenarios that may justify a more reasonable value. During the previous 5 days the D-st had been between 50 and 15 nT, and the interplanetary magnetic field B-z had been fluctuating between 10 and +10 nT.