Comment on ”Correlation of cusp MeV helium with turbulent ULF power spectra and its implications”

Abstract

In a recent paper, Chen and Fritz [1998] (CF) reported on a new magnetospheric phenomenon called cusp energetic particle (CEP) events.These energetic particles have significant fluxes up to several hundred keV/e.CF compared fluxes of energetic ions in the cusp with those simultaneously observed by Geotail upstream and downstream from the bow shock.The authors found the fluxes in the cusp to be substantially higher and concluded that CEP's are accelerated locally in the cusp.This is in disagreement with Chang et al. [1998] who showed that spectra upstream/downstream from a quasi-parallel shock are similar to CEP spectra.In this comment we address the flux comparison in CF.Using the same data sets, we show that the average spectra they used are not accurate representations of bow shock associated energetic ions.Our analysis demonstrates that no additional acceleration mechanism in the cusp is required to reconcile the Polar and Geotail data presented by CF.A number of studies have established the existence of two different classes of energetic particles in the upstream region: the so-called magnetospheric bursts and the bow shock-associated particles.Magnetospheric bursts have been studied by, e.g.$arris et al. [1976] who have presented evidence that energetic (100's of keV/e) magnetospheric ions can traverse the magnetosheath and escape into the upstream region.However, considerable evidence has been accumulated by the ISEE mission that the bow shock itself is the dominant source of energetic ions below 200 keV/e.Gosling et al. [1978] pointed out that the population of bow shock accelerated ions again consists of a reflected and a diffuse component.The reflected component is associated with the quasi-perpendicular bow shock (0B >45 , where 0B is the angle between the magnetic field line and the shock normal) and shows the characteristics of a several keV/e beamlike distribution.The diffuse component is associated with the quasi-parallel bow shock (On <45 ) and is generally more isotropic with energies up to several hundred keV/e [e.g., Mb'bius et al., 1987; $choler et al., 1989; Fuselief et al., 1995].Many observed features of the diffuse ion distribution, such as their general flow patterns, spectra, spatial distribution in front of and behind the quasi-parallel bow shock, and composition, have been successfully interpreted using models and simulations of a first order Fermi mechanism [e.g., Lee, 1982; $choler et al., 1992].Bonifazi and Moreno [1981] reported that energetic particle events occurred over periods when 0<45 , with 75% of the events for periods with 0 <25 .A comparison of CEP events with upstream/downstream