Multispacecraft observations of a foreshock-induced magnetopause disturbance exhibiting distinct plasma flows and an intense density compression

Large‐scale magnetopause disturbances can result from several different types of events, including those resulting from phenomena in the foreshock region. In this observational report, we present multipoint THEMIS observations of a magnetopause disturbance along the dawnside, equatorial flank that exhibits distinct flows in the magnetospheric plasma and an abnormally strong compression of the plasma density within it, which peaks at >7X the density of the near‐Earth solar wind. We find that the fastest ion and electron flows are related to two different processes, the ion flows resulting from plasma being displaced around the disturbance and field‐aligned electron flows, possibly related to magnetic reconnection. Interestingly, the magnetospheric plasma flows around the disturbance are very similar to those previously reported around flux transfer events, but we conclude that the disturbance is most likely the result of regions of compressed and rarified plasma density in the sheath resulting from a foreshock cavity. We present simple schematics of this foreshock cavity and its leading‐edge compression region that explain many of the observed features and discuss possibilities for the intense density enhancement. Using the simultaneous THEMIS observations from the magnetosphere, magnetosheath, and solar wind, we propose that the abnormal density enhancement was the result of a combination of compression effects due to the magnetosheath and the cavity's leading‐edge compression region coupled with some complex interaction near the magnetopause along the event's distinct boundary layer. Key Points Foreshock cavities can result in extreme magnetopause deformation Plasma flows around such disturbances are similar to those for FTEs Combined compression effects can result in abnormal density enhancements