In the paper we examine the numerical results for the case of aligned interstellar MHD flow with fixed boundary conditions for the solar wind and the interstellar plasma whose magnetic field strength is varied between 1.0 to 3.0 ¿G. The results for aligned flow show how variations of the upstream Alfv¿n Mach number can cause substantial changes in the location of the bow shock and heliopause near the stagnation line. The heliopause experiences a squeezing effect, which increases with decreasing Alfv¿n Mach number. At a critical magnetic field strength the structure of the heliospheric boundary changes dramatically, moving outward from the Sun by virtue of a sucking effect. The purpose of the paper is to explain the mechanisms which can give rise to a sucking effect of the heliopause.