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 vect ors, n, and speeds of motion, u(n), obtained separately from each spacecraf t by use of three different methods, and also compare those u(n) to corresp onding speeds, u(n)*, obtained from observed time lags between the two spac ecraft. Agreement between u, 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 slight ly 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).