The earth behaves as an enormous dipole magnet. This large dipole magnet is not directly aligned with the earth's spin axis, but tilted by about 11 degrees.
The planetary magnetic field of the earth is an obstacle to the solar wind. When the supersonic solar plasma strikes the magnetic field of the earth a bow-shock is formed where the plasma is slowed to less than supersonic speeds. This effect is similar to the bow of a ship cutting through water or the flow of air over a physical object at supersonic speed.
The bullet-shaped magnetosphere is a direct result of the solar wind impacting the earths magnetic field.
The solar wind compresses the sunward side of the magnetosphere to 6-10 earth radii and draws out the farside into a long comet-like tail (magnetotail). The magnetotail extends to about 1000 earth radii away from the sun.
The magnetic field of the earth has been studied for several decades and enough has been learned to construct models of its shape and size.
The most used models are those developed by Dr. Tsyganenko from 1987 and 1989 (referred to as T87 and T89). Through these models a field line can be traced from a given point in space.
The models shown in figure 6 demonstrate how the seasonal change due to the tilt of the earths rotational axis effect the shape of the magnetosphere.