This paper presents a simultaneous observation of the bow shock and magnetopause by THEMIS probes that allows determination of the actual magnetosheath thickness at the subsolar point. Moreover, Geotail located at the dusk dayside magnetosheath registered a brief excursion to the magnetosphere in this time. The spacecraft configuration reveals a significant deformation of the magnetopause surface that locally decreases its curvature radius. The highly curved magnetopause results in the decrease of the magnetosheath thickness to about half of its standard value in a particular observation point. The observed phenomenon is attributed to a rotation of the interplanetary magnetic field (IMF). Although it is generally expected that the bow shock and magnetopause move in accord, being driven mainly by the solar wind dynamic pressure, we suggest that the local and transient thinning of the magnetosheath can result from different responses of its boundaries to a sudden change of the pressure and/or IMF orientation.
