Abstract. Recent studies reported on a new magnetospheric phenomenon called cusp energetic particle (CEP) events. It has been suggested that these energetic particles with significant fluxes up to several hundred keV/e are accelerated locally in the cusp. An alternative explanation for the energetic particle events is that they are accelerated at the quasi-parallel bow shock, then transported downstream and enter the cusp along newly reconnected field lines or some other solar wind entry mechanism. It is well known that shock-accelerated ions have characteristic abundance ratios, composition ratios relative to the solar wind composition, temperatures, and spectral dependency on solar wind conditions. These parameters are used to distinguish between a local (cusp) and remote (bow shock) acceleration.Our results show that average values of these parameters in the cusp are comparable to those at quasi-parallel shocks. Furthermore, changes in some of these parameters indicate a solar wind entry process that depends on energy and mass. No local acceleration is required to explain the observed CEP events up to 150 keV/e. For ions above 150 keV/e the magnetosphere itself may be responsible for the observed cusp fluxes.

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Data from two near‐conjugate passes of DE 1 and DE 2 through the cusp/cleft region of the Earth's magnetosphere are presented and compared with model calculations of particle transport from the solar wind to spacecraft locations in the magnetosphere. Comparison of the observed and calculated particle spectra shows that the model can successfully match the spectra at both spacecraft using the same model parameters. This demonstrates that the modeling technique is applicable at both high and low altitudes. We are also able to conclude that the particles originate from a fairly narrow spatial region on the magnetopause even though magnetosheath plasma has access to the magnetosphere over the entire magnetopause in the model. The success of the model in reproducing key features of the observed spectra and the fact that the two satellites in near magnetic conjunction but at different altitudes observed similar, distinctive features at times separated by 10–20 min demonstrates that there are quasi‐stationary, spatial features in the cusp/cleft region of the Earth's magnetosphere.

Recent interpretations of cusp energetic ions observed by the POLAR spacecraft have suggested a new energization process in the cusp [Chen et al., 1997; 1998]. Simultaneous enhancement of H+, He+2, and O>+2 fluxes indicates that they are of solar wind origin. In the present study, we examine H+ and He+2 energy spectra from 20 eV to several 100 keV measured by the Hydra, Toroidal Imaging Mass‐Angle Spectrograph (TIMAS), and Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE) on POLAR. The combined spectrum for each species is shown to be continuous with a thermal distribution below 10 keV/e and an energetic component above 20 keV/e. Energetic ions with comparable fluxes and a similar spectral shape are commonly observed downstream from the Earth's quasi‐parallel (Q∥) bow shock. In addition to the similarity in the ion spectra, electric and magnetic field noise and turbulence detected in the cusp by the Plasma Wave Instrument (PWI) and Magnetic Field Experiment (MFE) onboard POLAR are similar to the previously reported observations at the bow shock. The waves appear to be coincidental to the cusp energetic ions rather than causal. We suggest that these ions are not accelerated locally in the cusp. Rather, they are accelerated at the Q∥ bow shock and enter the cusp along open magnetic field lines connecting both regions.

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