MeV ion event observed at 0950 UT on 4 May 1998 at a quasi-perpendicular bow shock region: New observations and an alternative interpretation on its origin

Abstract

[1] The region near the Earth's bow shock provides a chance to test and compare various charged particle acceleration models (magnetospheric versus bow shock acceleration processes). This kind of research attracted the interest of a large part of the space community in the last four decades. During the last decade, some ion events with spectrum extending to high energies (greater than ∼1.3 MeV) observed by Polar on 4 May 1998 provided the opportunity for a new debate. These ion events were analyzed in a large number of papers and were discussed in terms of acceleration processes in the cusp of the magnetosphere or at a quasi-parallel region of the bow shock. In this paper, we reexamine in detail one of the ion events observed during the period of interest, around ∼0950 UT, and we provide new crucial information and interpretation by focusing on data obtained very close to the shock (not examined so far). At ∼0950 UT, Polar crossed a quasi-perpendicular bow shock under special solar wind speed/bow shock conditions, which favor the efficient acceleration of the solar ambient energetic population via the shock drift acceleration (SDA) mechanism. We analyze in detail observations upstream and downstream from the bow shock crossing of 0945 UT, for instance, flux-time profiles, energy spectra, ion composition, and pitch angle distributions, and we find them, as expected, in excellent agreement with the predictions of the SDA theory.