Observations of energetic O+ ions with strong velocity shear in the low latitude boundary layer during an intense storm main phase

Using particle and electromagnetic field data from Magnetospheric Multiscale Spacecraft (MMS), we investigate energetic O⁺ ion characteristics in the strong velocity shear regions in the dusk-side low-latitude boundary layer (LLBL) during the main phase of an intense storm on 13 October 2016. In the large velocity reversal regions, O⁺ ion number density is very high, N_o+ ~ 0.3 cm^-3. The pitch angle distributions of these energetic O⁺ ions vary distinctly across different energy ranges. The pitch angles of the lower energetic (3 keV to 10 keV) O⁺ ions are mostly less than 45 degrees and show a quasi-parallel distribution. Conversely, the pitch angles of the higher energetic (20 keV to 40 keV) O⁺ ions are dominantly in the range from 45 to 135 degrees, suggesting a quasi-perpendicular distribution. The quasi-parallel distribution of lower energetic O⁺ ions implies that these O⁺ ions are outflow along the magnetic field line from the dayside high-latitude ionosphere. Intense electric fields in the strong shear flow region can accelerate O⁺ ions to higher energy, altering their motion from along the magnetic field to the transverse direction in the dusk-side LLBL. Our studies present evidence for strong shear flow in the dusk-side LLBL driving energetic O⁺ ions to traverse the magnetic field motion. The quasi-perpendicular distribution of higher energetic O⁺ ions, in the inner edge of the dusk-side LLBL, may provide a new source of ring current energetic particles during the main phase of the intense storm.