Acceleration of O⁺ from the cusp to the plasma sheet

Abstract

Heavy ions from the ionosphere that are accelerated in the cusp/cleft have been identified as a direct source for the hot plasma in the plasma sheet. However, the details of the acceleration and transport that transforms the originally cold ions into the hot plasma sheet population are not fully understood. The polar orbit of the Cluster satellites covers the main transport path of the O + from the cusp to the plasma sheet, so Cluster is ideal for tracking its velocity changes. However, because the cusp outflow is dispersed according to its velocity as it is transported to the tail, due to the velocity filter effect, the observed changes in beam velocity over the Cluster orbit may simply be the result of the spacecraft accessing different spatial regions and not necessarily evidence of acceleration. Using the Cluster Ion Spectrometry/Composition Distribution Function instrument onboard Cluster, we compare the distribution function of streaming O + in the tail lobes with the initial distribution function observed over the cusp and reveal that the observations of energetic streaming O + in the lobes around À20 R E are predominantly due to the velocity filter effect during nonstorm times. During storm times, the cusp distribution is further accelerated. In the plasma sheet boundary layer, however, the average O + distribution function is above the upper range of the outflow distributions at the same velocity during both storm and nonstorm times, indicating that acceleration has taken place. Some of the velocity increase is in the direction perpendicular to the magnetic field, indicating that the E × B velocity is enhanced. However, there is also an increase in the parallel direction, which could be due to nonadiabatic acceleration at the boundary or wave heating.

Atmospheric loss from the dayside open polar region and its dependence on geomagnetic activity: implications for atmospheric escape on evolutionary timescales

“…In statistical studies of the polar cap, data with the constraint that β is less than 0.01 are used (e.g. Liao et al, 2010Liao et al, , 2015. Therefore, a constraint of β > 0.1 in the dayside magnetosphere will exclude typical polar cap data.…”

“…1) depending on how effectively it is accelerated: (1) low-energised ion populations will convect anti-sunward across the polar cap and further downtail and towards the plasma sheet, where they end up on closed field lines Liao et al, 2015); (2) sufficiently energised ions will reach the plasma mantle with typical velocities high enough to pass the tail X-line and consequently escape in the distant tail ; (3) highly energised ions may escape into the dayside magnetosheath directly from the cusps (Slapak et al, , 2013. Heavy (e.g.…”

Relative outflow enhancements during major geomagnetic storms – Cluster observations

“…This result was corroborated by Liao et al (2010), who found that the occurrence frequency of O + observations in the lobes increases during the storm main phase. Liao et al (2015) discussed the influence of the geomagnetic activity on the velocity increase in O + as it is transported from the cusp to the tail lobe. The authors found that from the cusp to the polar cap and to the tail lobes, the acceleration of oxygen ions is not significant.…”

The Earth: Plasma Sources, Losses, and Transport Processes

“…11. Liao et al (2015), compared the phase space density of the individual O + lobe beams with the phase space density of the outflowing cusp density and confirmed that the observed beam flux and the increase in energy of the beams down the tail are consistent with the velocity filter effect during quiet times, with no significant acceleration of the O + along this path. A small increase due to centripetal acceleration, however, as suggested by , is not excluded.…”