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

Abstract

[1] 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 ∼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 ∼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 ∼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°then 0.9°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.

Enhanced atmospheric oxygen outflow on Earth and Mars driven by a corotating interaction region

“…[22] We have analyzed the Cluster and MEX observations before and during a CIR passage. The O + outflow observed above Earth's polar region was consistent with a cusp origin scenario [e.g., Nilsson et al, 2004;Kistler et al, 2010a;Zhang et al, 2011]. The statistical studies revealed that the outflow rate from the cusp is sensitive to solar wind kinetic energy [Moore and Horwitz, 2007] but not related to geomagnetic activity level [Yau and Andre, 1997].…”

Circulation of Heavy Ions and Their Dynamical Effects in the Magnetosphere: Recent Observations and Models

“…Thus it seems that the high altitude cusp provides a likely source for the deep-tail beams. The high energy O + (∼10 keV) beams are also occasionally observed by Cluster in the near-Earth (<19 R E ) lobe when a shock compresses the magnetosphere (Zhang et al 2011b), bringing in the populations that are close to the magnetopause.…”

Acceleration of O+ from the cusp to the plasma sheet

“…Cladis et al [2000] showed an example in which the centrifugal acceleration from a sudden compression during a storm increased the O + parallel velocity by~500 eV. Zhang et al [2011] studied a case where a large change in the energy of O + was observed when there was a sharp increase in solar wind pressure and tested whether this was due to centrifugal acceleration. They found that the observed energy increase in the lobe O + beam is more likely due to field line displacement and the velocity filter effect and not due to centrifugal acceleration.…”