, can drive storm activity, but several outstanding questions remain concerning dropouts and the precise channels to which outer belt electrons are lost during these events. By analysing data collected at multiple altitudes by the THEMIS, GOES, and NOAA-POES spacecraft, we show that the sudden electron depletion observed during a recent storm's main phase is primarily a result of outward transport rather than loss to the atmosphere.Trapped radiation belt electrons undergo three characteristic types of motion: gyro-motion around magnetic field lines due to any velocity component perpendicular to the field, bounce-motion along field lines between magnetic mirror points due to any velocity component parallel to the field, and drift-motion around the Earth resulting from magnetic gradient and curvature drifts. Associated with each of these oscillatory motions are adiabatic invariants, which are conserved so long as electric and/or magnetic fields do not change on scales similar to those of the associated motions. The first invariant conserves the magnetic moment of the particle and is proportional to the perpendicular momentum squared divided by the local magnetic field strength; the second and third invariants conserve the integral of parallel momentum over one full bounce period and the magnetic flux through a particle's drift orbit, respectively. Magnetospheric changes on timescales much longer than electron drift periods are considered fully adiabatic, that is, they are fully reversible. Initially, it was thought that the observed flux dropouts were fully adiabatic changes in the system. Essentially, electrons moved radially outward (inward) during a storm's main (recovery) phase to conserve their third invariant as Earth's magnetic field was altered by the field produced by an enhanced (weakening) ring current 7,8 , which is a magnetospheric current system resulting from charge-dependent particle drift. As electrons moved radially outward (inward) in the field, their fluxes decreased (increased) for fixed energy as the first adiabatic invariant was also conserved. It was later shown that although this 'Dst effect' (after the disturbance storm time (Dst) geomagnetic index (Kp) used to indicate storm activity) does play a role in the flux dynamics, many flux dropouts do not return to the pre-storm flux level

[1] We report on internal, magnetospheric processes related to markedly different storm-time responses of phase space density (PSD) in invariant coordinates corresponding to equatorially mirroring, relativistic electrons in Earth's outer radiation belt. Two storms are studied in detail, selected from a database of 53 events (Dst min < À40 nT) during the THEMIS era thus far (December 2007-August 2012. These storms are well covered by a number of in situ THEMIS spacecraft and complemented by additional ground-based and in situ observatories, and they epitomize the divergent behaviors that the outer radiation belt electrons can exhibit during active periods, even during otherwise similar Dst and auroral electrojet (AE) profiles. From our statistical results with the full database, the changes in the radial profile peak in PSD reveal notably consistent behavior with prior studies: 58% of geomagnetic storms resulted in PSD peak enhancements, 17% resulted in PSD peak depletions, and 25% resulted in no significant change in the PSD peak after the storm. For the two case studies, we examined the PSD at multiple equatorial locations (using THEMIS), trapped and precipitating fluxes from low-Earth orbit (using POES), and chorus, hiss, EMIC, and ULF waves (using THEMIS spacecraft, ground observatories, and the GOES spacecraft). We show that (1) peaks in PSD were collocated with observed chorus waves outside of the plasmapause during the most active periods of the PSD-enhancing storm but not during the PSD-depleting storm, providing evidence for the importance of local acceleration by wave-particle interactions with chorus; (2) outer belt dropouts occurred following solar wind pressure enhancements during both storms and were consistent with losses from magnetopause shadowing and subsequent outward radial transport; during the PSD-enhancing storm, this revealed how the outer belt can replenish itself seemingly independently of the remnant of the pre-existing belt leftover after a dropout, which in this case resulted in a double-peaked outer belt distribution; (3) slow decay in PSD was associated with corresponding locations in L* and enhanced wave amplitudes of plasmaspheric hiss; (4) precipitation loss associated with wave-particle interactions with hiss and EMIC waves appeared to be significantly more important during the PSD-depleting storm than the PSD-enhancing storm; and (5) PSD transport during the recovery phase of both storms and throughout the PSD-enhancing storm was consistent with ULF-wave-driven radial diffusion away from maxima in PSD; this indicates the importance of ULF waves in redistributing outer belt PSD after local acceleration occurs. We conclude that these source, transport, and loss processes, individually well characterized by previous studies, do indeed appear to act in concert, leading to predominance of local acceleration in one case and loss in another. These processes can therefore conspire toward optimal source or loss of outer belt electrons under suitable external drivers, and the conditions ...

[1] The ion foreshock is a source of energy for magnetospheric ULF waves, but it is usually only considered effective at driving ULF waves with frequencies above the Pc5 (2-7 mHz) range. We present observations for an 8 h high speed solar wind interval on 14 July 2008 during which three distinct types of transient ion foreshock phenomena (TIFP) were observed just upstream of the dayside bow shock. We demonstrate that TIFP generate global magnetospheric Pc5 ULF waves with amplitudes as large as 10 mV/m in the electric field and 10 nT in the magnetic field. We characterize the magnetospheric ULF response to several different TIFP that occur during this interval, including the first report of the ULF response to a foreshock bubble. Using a novel spacecraft configuration, we find that the local time with the highest Pc5 wave amplitude is closely related to the location of the ion foreshock. Statistical studies of Pc5 ULF wave activity, other case studies of ULF waves driven by processes in the ion foreshock, and recent theoretical and simulation work on TIFP place these results in context: TIFP are an important energy source for Pc5 ULF waves in the magnetosphere.

The abrupt boundary between a magnetosphere and the surrounding plasma, the magnetopause, has long been known to support surface waves. It was proposed that impulses acting on the boundary might lead to a trapping of these waves on the dayside by the ionosphere, resulting in a standing wave or eigenmode of the magnetopause surface. No direct observational evidence of this has been found to date and searches for indirect evidence have proved inconclusive, leading to speculation that this mechanism might not occur. By using fortuitous multipoint spacecraft observations during a rare isolated fast plasma jet impinging on the boundary, here we show that the resulting magnetopause motion and magnetospheric ultra-low frequency waves at well-defined frequencies are in agreement with and can only be explained by the magnetopause surface eigenmode. We therefore show through direct observations that this mechanism, which should impact upon the magnetospheric system globally, does in fact occur.