Cusp for high and low merging rates

Abstract

The traditional distinction between the cusp for northward and southward interplanetary magnetic field (IMF) conditions is deficient in that many, if not most, “northward” IMF cusp encounters closely resemble southward IMF encounters. Partly for that reason, “southward” IMF cusps have been far more often examined than “northward. ” Recently, Newell et al. (2007) showed that the magnetopause merging rate, dΦMP/dt, much better predicts cusp latitude than does Bz (or Bs). Here, we investigate the extent to which high and low merging rate conditions better separates cusp encounters into mutually distinct classes. Indeed, high merging rate cusps (those with dΦMP/dt > 2* 〈dΦMP/dt〉) differ from low merging rate cusps (those with dΦMP/dt < 0.5〈dΦMP/dt〉) in several quantitative and even qualitative ways. High‐energy (tens of keV) ions, apparently of magnetospheric origin, do not extend into the high merging rate cusp but frequently do for low merging rates. However, the dispersion curve for ions, apparently of magnetosheath origin, extends to several keV for high merging rates but only to 1–2 keV for low merging rates. The local time extent of the cusp is 2.33 hours MLT for low merging rate conditions and 3.45 hours MLT for high merging rate conditions. All high merging rate cusps show clear forward dispersion (declining energy with increasing latitude) with low‐energy ion cutoffs within that dispersion. Low merging rates cusps rarely show forward dispersion, but about half, or slightly less, show reverse dispersion (declining energy with decreasing latitude) at the poleward edge of the cusp. Low‐energy ion cutoffs in the low‐merging rate cusp primarily occur only within that reverse dispersion. “Double” cusps, some with very clear latitudinal separation, occur in some high merging rate cases, but no low merging rate cases. The high merging rates cases also typically have a “shadow” region of electron‐only precipitation at roughly polar rain energies and intensities, lying immediately equatorward of the main cusp. For low‐merging rate conditions, the region immediately equatorward of the cusp (main magnetosheath ion population) contains ions as well as electrons, and forms a boundary layer.