The Earth’s magnetic field is often represented by a dipole at the Earth’s center. The intersections of the axis of this dipole with the Earth’s surface are the geomagnetic poles. A better representation (the dipole field) is given by a dipole about 400 km from the center. The locations where the magnetic dip is 90° are called the magnetic dip poles; they do not coincide with the geomagnetic poles. The portion of the Earth’s field not representable by a dipole is called the nondipole field. Time-varying components of the Earth’s field are shown in Figure M-2. See also normal magnetic field.