The magnetic boundary between the Earth's field and the shocked solar wind, that is essentially a boundary between the Magnetosphere and the surrounding plasma flowing from the Sun, is often called the Magnetopause. It has a bullet-shaped front, gradually changing into a cylinder, with a cross-section is that is approximately circular. The location of the magnetopause is determined by the balance between the pressure of the dynamic planetary magnetic field and the dynamic pressure of the solar wind. As the solar wind pressure increases and decreases, the magnetopause moves inward and outward in response.
Average distances in the magnetosphere are often measured in Earth radii (RE), with one Earth radius amounting to 6371 km, and in these units, the distance from the Earth's center to the "nose" of the magnetosphere is about 11 RE and to the flanks abreast of the Earth about 15 RE, while the radius of the distant tail is 25-30 RE. By way of comparison, the moon's average distance is about 60 RE. The solar wind, that is supersonic, passes through a bow shock where the direction of flow is changed so that most of the solar wind plasma is deflected to either side of the magnetopause.. The zone of shocked solar wind plasma is the magnetosheat.
The process of magnetic reconnection of solar wind and magnetospheric field lines, allows solar wind plasma to enter and primarily populate Earth's magnetosphere, becoming trapped within, but also allows plasma of terrestrial origin (especially ionospheric ions from the auroral regions and polar caps) to escape from the magnetosphere and enter the solar wind. At Earth, the solar wind plasma which enters the magnetosphere forms the plasma sheet. The amount of solar wind plasma and energy that enters the magnetosphere is regulated by the orientation of the interplanetary magnetic field, which is embedded in the solar wind. There can be different mathematical equestions used in order to reproduce the actual measurements, and the ones used with our Backtracing Code are described here: Magnetopause Models