Abstract
Oxygen ions, O+, efficiently energized by weak fast shock in the
mid-altitude cusp observed by Cluster on 7 November 2004 are
investigated in this paper. An interplanetary fast shock hitting Earth
magnetosphere was observed by Geotail at 1825 UT with a sharply increase
in the solar wind dynamic pressure from 3nPa to 11nPa. There are
significant fluctuations of solar wind dynamic pressure from 1930 UT to
2000 UT during this shock compressing Earth magnetosphere. During this
pressure disturbance intervals Cluster 4 was located in the mid-altitude
cusp, (0.43, -3.25, 3.07) RE_GSE. CODIF/Cluster 4 instrument detected
O+ ions with tens keV energized by the weak fast shock with Mach number
MA~1.2 in the perpendicular direction. Those energetic
O+ ions have bunch or partial ring distributions in the mid-altitude
cusp. Our observation results put forward confident evidences on the
weak fast shock efficiently energizing heavy ions, especially, O+ ions
in the cusp.