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The local and global plasma properties in the magnetosheath play afundamental role in regulating solar wind–magnetosphere coupling processes.However, the magnetosheath is a complex region to characterise as it has beenshown theoretically, observationally and through simulations that plasmaproperties are inhomogeneous, non-isotropic and asymmetric about theSun-Earth line. To complicate matters, dawn–dusk asymmetries are sensitive tovarious changes in the upstream conditions on an array of timescales. Thepresent paper focuses exclusively on dawn–dusk asymmetries, in particularlythat of ion density. We present a statistical study using THEMIS data ofthe dawn–dusk asymmetry of ion density in the dayside magnetosheath and itslong-term variations between 2009 and 2015. Our data suggest that, ingeneral, the dawn-side densities are higher, and the asymmetry grows fromnoon towards the terminator. This trend was only observed close to themagnetopause and not in the central magnetosheath. In addition, between 2009and 2015, the largest asymmetry occurred around 2009 decreasing thereafter.We also concluded that no single parameter such as the Alfvén Mach number,plasma velocity, or the interplanetary magnetic field strength couldexclusively account for the observed asymmetry. Interestingly, the dependenceon Alfvén Mach number differed between data sets from different timeperiods. The asymmetry obtained in the THEMIS data set is consistent withprevious studies, but the solar cycle dependence was opposite to an analysisbased on IMP-8 data. We discuss the physical mechanisms for this asymmetryand its temporal variation. We also put the current results into context withthe existing literature in order to relate THEMIS era measurements to thosemade during earlier solar cycles

The dawn–dusk asymmetry of ion density in the dayside magnetosheath and its annual variability measured by THEMIS