Plain Language Summary Mercury's magnetosphere was suggested to be particularly sensitive to solar wind conditions. The applicability of the component reconnection model on IMF B-y indicates that at Mercury reconnection occurs at lower shear angles than at Earth. These dependences are explained in terms of the IMF B-x-controlled dayside magnetospheric topology, the component reconnection model applied to IMF B-y and B-z, and the variability of solar wind ram pressure associated with heliocentric distance r(sun). Azimuthal shifts are associated with both IMF B-y and r(sun): the cusp moves toward dawn when IMF B-y or r(sun) decrease. The comparison illustrates that the disturbance peak over the cusp is strongest and widest extending in local time for negative IMF B-x and negative IMF B-z, and when Mercury is around the perihelion. The assessment entails four steps: (1) propose and validate an approach to estimate the solar wind magnetic field (interplanetary magnetic field (IMF)) for MESSENGER's cusp transit (2) define an index s measuring the intensity of the magnetic disturbance which significantly peaks within the cusp and serves as an indicator of the cusp activity level (3) construct an empirical model of sigma as a function of IMF and Mercury's heliocentric distance r(sun), through linear regression and (4) use the model to estimate and compare the polar distribution of the disturbance s under different conditions for a systematic comparison. This study assesses the response of the cusp to solar wind changes comprehensively, using 2848 orbits of MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) observation.
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