Shock-driven variation in ionospheric outflow during the 11 October 2001 moderate storm

The Composition and Distribution Function (CODIF) Analyzer on board the Cluster spacecraft frequently observes narrow-energy, field-aligned O+ ions over the polar caps and in the lobes, particularly during storm times. This population is due to ion outflow from the cusp. During the 11 October 2001 moderate storm, CODIF, located in the southern lobe at similar to 12 Earth radii (R-E) down the tail and moving inbound, observed a change in the outflow. A sudden increase in solar wind dynamic pressure at 1702 UT led to a sudden increase in the energy of peak O+ flux from similar to 0.2 to 5 keV or an abrupt increase in parallel velocity from 54.0 +/- 3.8 to 212.5 km/s. This high-energy population gradually decreased in energy over the next similar to 2 h. The energy increase resulted from the effects of the solar wind shock on preexisting outflow. To model the effects of the shock, we first determined how the magnetic field configuration changed, by comparing the local magnetic field before and after the shock with the predictions of the T04s empirical model. The local field, reasonably well reproduced by T04s, experienced a 0.8-R-E perpendicular displacement and a first 8.3 degrees then 0.9 degrees rotation in 3.8 min when the solar wind shock hit. We used local measurements of the H+ drift velocity and magnetic field to calculate if the observed velocity increase was due to centrifugal acceleration. We found that at the beginning centrifugal acceleration contributed only 11.4%, i.e., 18.0 km/s, to the sudden O+ parallel velocity increase; the net effect of the centrifugal acceleration is against the velocity increase. We conclude that the increase was actually due to the field displacement, which brought a higher velocity population to the Cluster location. A higher velocity is expected at higher latitudes due to the velocity filter effect that results from higher-energy ions moving further along the field line than lower-energy ions, as the field line convects over the polar cap and into the tail.