Polar, Cluster and SuperDARN Evidence for High-Latitude Merging during Southward IMF: Temporal/Spatial Evolution

Magnetic merging on the dayside magnetopause often occurs at high latitudes. Polar measured fluxes of ac- celerated ions and wave Poynting vectors while skimming the subsolar magnetopause. The measurements indicate that their source was located to the north of the spacecraft, well removed from expected component merging sites. This rep- resents the first use of wave Poynting flux as a merging dis- criminator at the magnetopause. We argue that wave Poynt- ing vectors, like accelerated particle fluxes and the Wal ´ en tests, are necessary, but not sufficient, conditions for iden- tifying merging events. The Polar data are complemented with nearly simultaneous measurements from Cluster in the northern cusp, with correlated observations from the Super- DARN radar, to show that the locations and rates of merging vary. Magnetohydrodynamic (MHD) simulations are used to place the measurements into a global context. The MHD simulations confirm the existence of a high-latitude merging site and suggest that Polar and SuperDARN observed effects are attributable to both exhaust regions of a temporally vary- ing X-line. A survey of 13 merging events places the location at high latitudes whenever the interplanetary magnetic field (IMF) clock angle is less than 150 . While inferred high- latitude merging sites favor the antiparallel merging hypoth- esis, our data alone cannot exclude the possible existence of a guide field. Merging can even move away from equatorial latitudes when the IMF has a strong southward component. MHD simulations suggest that this happens when the dipole tilt angle increases or when IMF BX increases the effective dipole tilt.