Comment on “MeV magnetosheath ions energized at the bow shock” by S.‐W. Chang et al.

Abstract

[1] From a study of the 4 May 1998 storm event, Chang et al. [2001] (hereinafter referred to as CETAL01) suggested that ‘‘ions are accelerated at the quasi-parallel bow shock to energies as high as 1 MeVand subsequently transported into the magnetosheath during this event’’ and mentioned that ‘‘This is confirmed by a comparison of energetic ion fluxes simultaneously measured in the magnetosheath and at the quasi-parallel bow shock when both regions are likely connected by the magnetic field lines’’ (see their Abstract). After an inspection of the measured energetic ion data, however, one finds that CETAL01 have misplotted the observed ion energy spectrum in the ‘‘magnetosheath’’ (near the cusp) to lower energy which brings it in closer agreement to the flux measured near the quasi-parallel bow shock, making their analysis suspect. In fact, simultaneous measurements at this time indicate that (1) the energetic ion flux near the cusp was about one order of magnitude higher than that near the quasi-parallel bow shock, (2) the energetic ion time signatures were seen first near the cusp then near the bow shock, and (3) the energetic ion flux observed near the bow shock was independent of bow shock geometry. Each of these three facts is sufficient to demonstrate that the quasi-parallel bow shock was not the main source of the energetic ions near the cusp during this event. [2] CETAL01 stated that ‘‘A comparison of Interball and Polar ion spectra can potentially falsify our bow shock source hypothesis and is now the focus of our analysis.’’ In Figure 11 of CETAL01, accordingly, they compared the energetic ion flux measured by Interball near the quasiparallel bow shock with that measured by Polar near the cusp during the interval 1101–1142 UT on 4 May 1998, where their Polar/CEPPAD energetic ion data (open circles in their Figure 11) were taken only from the ion sensor that was looking 90 from the Polar spin axis. Our Figure 1 replots the Interball data (stars) and the Polar/CEPPAD data (open squares) for the same time interval. Comparing Figure 11 of CETAL01 to our Figure 1, we find that they have misplotted the CEPPAD ion energy spectrum near the cusp to the lower energies which reduces the difference between Interball and Polar ion fluxes. A closer inspection of their Figure 11 suggests that for each energy interval (channel) near the cusp they used the lower energy threshold to represent it without taking into account the effective energy passband [McKinnon and Fritz, 1976] of the steep energy spectrum, and the resulting energy spectrum near the cusp shown in their Figure 11 is thus lower than the actually observed spectrum as shown in our Figure 1. [3] This is not the only case where CETAL01 misplotted the observed ion energy spectrum around the cusp to lower than the actually observed spectrum, for in an earlier paper, Chang et al. [1998] (hereinafter referred to as CETAL98) misplotted the MICS (Magnetospheric Ion Composition Sensor) lower energy limit from 1 keV/e to 0.6 keV/e which brought the cusp fluxes into better alignment with ‘‘bow shock ion spectra,’’ and in addition, misplotted the HIT (Heavy Ion Telescope) helium data point in the cusp below the actual observed value of 0.22 He ions (cm-srs-keV/e) 1 at 0837:40-0845:00 UT on 27 August 1996 [CETAL98, Figure 3]. Based upon such misplotted data, CETAL01 stated in their introduction that CETAL98 ‘‘showed that cusp energetic ion spectra (<300 keV e ) matched very well with a large body of bow shock ion spectra.’’ [4] Yet even after misplotting the observed CEPPAD ion spectrum near the cusp to below the actual observed one, the Interball ion flux near the bow shock was still lower than the Polar ion flux near the cusp as shown in their Figure 11, so CETAL01 made another spectral adjustment by increasing the measured Interball ion flux near the bow shock by about 300% to match the repositioned Polar ion flux to obtain their Figure 13. They called it a ‘‘distance correction’’ and made it central to their argument stating that ‘‘an important piece of evidence for the bow shock source is demonstrated in Figure 13.’’ However, it has been reported [Lee, 1982] that the proton flux at 7 RE from the bow shock is almost the same as that at the bow shock at energies larger than 60 keV; that is, no correction is needed JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. A8, 1311, doi:10.1029/2002JA009634, 2003