THEMIS multi-spacecraft observations of magnetosheath plasma penetration deep into the dayside low-latitude magnetosphere for northward and strong By IMF

On 2007‐06‐03 the five THEMIS spacecraft consecutively traversed the dayside (13.5 MLT) magnetopause during northward IMF with strong By. While one spacecraft monitored the magnetosheath, the other four encountered an extended region of nearly‐stagnant magnetosheath plasma attached to the magnetopause on closed field lines. This region was much denser than, but otherwise similar to, the nightside cold‐dense plasma sheet. At two points in time this region was bordered by two spacecraft, revealing that its thickness grew from 0.65 RE to 0.9 RE in ∼25 minutes. There was no evidence for Kelvin‐Helmholtz waves nor diffusion at the local magnetopause. Our observations suggest that even when the IMF clock angle was as large as 60°, substantial solar wind entry across the dayside magnetopause occurred due to reconnection either poleward of both cusps or poleward of one cusp in one hemisphere and equatorward‐of‐the‐cusp in the other.