When exploring a new country, one needs a good map. The Earth's magnetosphere is such a country, and its “maps” are models of the geomagnetic field. In essence they are mathematical prescriptions implemented by computer which, given (x,y,z) of a point and other supporting information, return the point's expected magnetic components (Bx, By, Bz) [Tsyganenko, 1990].
At present, the empirical magnetic field models by Tsyganenko [1987, 1989a, c; denoted henceforth as T87 and T89] are often used in various studies of the Earth's magnetosphere. Stern  has devised modifications of T87 with current-free tail lobes and with a plasma sheet that warps in response to dipole tilt [Peredo and Stern, 1991].