The viscous-like magnetospheric convection and the length of the earth's magnetotail

An estimate of an upper bound on the length of the Earth's magnetotail has been made under the assumptions that (1) the flux of open magnetic field lines in the polar caps never vanishes and (2) the annihilation of the open field line flux is extremely low during magnetically quiet periods. These two basic assumptions are discussed and justified. It is found that the tail may be as long as 6000 Re (0.25 a.u.) after a prolonged quiet time period. On the other hand, the length of the shortest tail, which would result from a sequence of strong magnetospheric substorms, is estimated to be ～ 600 Re. These events are thus capable of shortening the tail by an order of magnitude.