On the solar wind flow control of the polar cusp

The position of the polar cusp in the dayside magnetosphere is known to be controlled by the solar wind dynamics and the interplanetary magnetic field. Using plasma data from the Viking and IMP 8 satellites, we have done a statistical analysis of the influence of the solar wind azimuth flow component on the cusp position in magnetic local time (MLT). We show that the position of the cusp in MLT is strongly coupled to the azimuthal (east‐west) flow component of the solar wind. For instance, the cusp in the northern hemisphere moves toward prenoon when the unaberrated solar wind appears to come from the west of the Sun (as compared to the radial flow), and vice versa. This suggests, first of all, a strong dynamical coupling of the cusp to the solar wind flow. Second, the dawn‐dusk cusp displacement with respect to the azimuthal flow component indicates a transfer of angular momentum from the Sun, potentially by a J × B force in the solar wind as suggested in the “hetegonic nebulae” theory by Alfven and Arrhenius [1976]. This lateral deflection force plays, on the average, a more important role for the cusp location than the aberration effect of the Earth's prograde motion across the expanding solar wind. In previous studies similar displacements of the cusp have been mainly related to the IMF By component; for example, a positive By leads to a cusp displacement to the postnoon side and vice versa. We compare these results and discuss them in context with the dynamical solar wind flow control of the cusp.