Dual spacecraft determinations of magnetopause motion

We examine the motion of Earth's magnetopause for 16 dawnside traversals of this boundary by the sister spacecraft AMPTE/UKS and IRM in December, 1984, when their separation was 400-900 km. We compare magnetopause normal vectors, n, and speeds of motion, u n , obtained separately from each spacecraft by use of three different methods, and also compare those u n to corresponding speeds, u * n , obtained from observed time lags between the two spacecraft. Agreement between u n values and n vectors determined from the three methods ranges from poor to excellent. Comparing u * n and u n values, we find a clear tendency for |u * n | to be larger than |u n |: While slightly less than half of the results show reasonable agreement (0.5 < u n /u * n < 2), there are about as many results in the range 0 < u n /u * n < 0.5, and a few cases give the wrong sign of u n .