MECHANISMS AND TIME-SCALES OF THE MAGNETOSPHERIC RESPONSE TO THE INTERPLANETARY MAGNETIC-FIELD CHANGES DURING THE 8 MAY 1986 SUBSTORM

On 8 May 1986, between 1113 and 1600 UT, an isolated magnetospheric su bstorm was observed, during which the AE-index exceeded 700 nT (CDAW 9 E event). Three available sets of measurements (a) of the solar-wind p arameters (IMP-8 satellite), (b) of the magnetotail energy flux (ISEE- 1 spacecraft), and (c) of ground magnetic observatories, allowed us to make a detailed study of the overall magnetospheric response to chang es of the interplanetary magnetic field (IMF) direction, during this e vent of weak solar-wind coupling. In order to study the mechanisms and time-delays of the magnetospheric response to the abrupt increase of the solar-wind energy input, we have evaluated the total magnetospheri c energy output U(T) following two different methods: (a) Akasofu's me thod, taking the ring current decay time tau(R) constant, and (b) Vasy liunas' method where the values of U(T) are independent of the solar-w ind energy input as determined from the epsilon parameter. Both method s suggest that the driven system has been considerably developed durin g this substorm, while an unloading event has been superposed at the e xpansion onset.