The CDAW 6 data base is used to compare solar wind and magnetospheric pressures. The flaring angle of the tail magnetopause is determined by assuming that the component of solar wind pressure normal to the tail boundary is equal to the total pressure within the tail. Results indicate an increase in the tail flaring angle from 18¿ to 32¿ prior to the 1055 substorm onset and a decrease to 25¿ after the onset. This behavior supports the concept of tail energy storage before the substorm and subsequent release after the onset.