The distant cusp and the surrounding magnetopause: A view in snapshots from polar

In 1996 and 1997 during months when the orbit apogee was at XGSM⩾0 (May–October 1996 and April–August 1997), NASA's Polar spacecraft was skimming the high-latitude dayside magnetopause whenever the solar wind dynamic pressure (pdyn) was larger than ∼ 4 nPa. Magnetopause crossings occurred mostly under a northward and/or strongly dominating duskward-dawnward interplanetary magnetic field and took place at the poleward or dawn/dusk edge of the distant cusp. Polar data reveal that the indentation of cusp magnetopause may be deep, ∼2.5 RE below the axisymmetric surface of empirical magnetopause models. A significant IMF BY-dependence for probability of magnetopause encounter by Polar in a given local time sector suggests a formation of localized valleys extending away from the cusp proper along the merging lines on the magnetopause. Multipoint data in the distant cusp with the spacecraft separation on a ∼1–2 RE spatial scale could help to confirm this finding.