Cluster observations of unusually high concentration of energetic O + carried by flux ropes in the nightside high‐latitude magnetosheath during a storm initial phase

We present measurements from Cluster spacecraft to investigate the energetic singly charged oxygen ions, O + , within the flux ropes in the nightside high‐latitude magnetosheath during the initial phase of an intense storm on 24 October 2011. Three magnetic flux ropes were identified by Cluster 4 in the intervals from 20:10 UT to 20:20 UT. Unusually, large number density of energetic O + ions at energy of tens of keV was detected within these flux ropes. The number density of O + ions was above 0.1 cm −3 and the maximum value was about 0.25 cm −3 , 1 order of magnitude larger than the ambient value (~0.01 cm −3 ) in the magnetosheath. The O + /H + ratio is as large as ~0.08 within the flux ropes. Enhanced convection electric fields E y (~10 mV/m) are associated with the flux rope and the high concentrations of energetic O + . The flux ropes, which are presumably produced by magnetic reconnection at the dayside magnetopause or cusp, are convected at a larger velocity than the tailward velocity of ambient flows in the magnetosheath. These observations together show that abundant energetic O + ions are carried by the flux ropes toward tail in the nightside magnetosheath. Our observations present new evidence for a chain linking the dayside to the nightside in the global O + transport process.