Investigation of the source region of ionospheric oxygen outflow in the cleft/cusp using multi-spacecraft observations by CIS onboard Cluster Original Research Article

Beams of singly ionized oxygen with narrow energy distributions originating in the dayside cleft/cusp region are frequently observed in the cleft, cusp and polar cap regions by the CODIF sensor of the CIS instrument onboard Cluster. During summer and fall of 2001, the high separation distances of ∼1Re between the spacecraft provided a good opportunity to estimate the size and location of the source of outgoing O+ ions. A statistical study shows that the source region is located near the equatorward edge of the cleft/cusp region, within a latitudinal range of around 1.5°. The longitudinal extension cannot be resolved, except that it is more than the satellite separations that was 14° in average. Cluster observations inside the source region, at 4.5–6Re, show high transverse heating of the O+ population. This process is accompanied with a sudden enhancement of low-frequency electric field waves measured by the EFW and STAFF instruments. We suggest that O+ ion outflow is caused by resonant heating by BBELF waves.