EXTERNAL AND INTERNAL INFLUENCES ON THE SIZE OF THE DAYSIDE TERRESTRIAL MAGNETOSPHERE

ISEE-1 and -2 observations of the position of the magnetopause during solar cycle 21 are used to determine quantitatively the role of both internal and external influences. We find that the magnetopause is a function of the direction of the IMF and the magnitude of the solar wind momentum flux but depends only weakly on the ring current intensity. The magnetopause stand-off position is found to lie at a distance similar to that expected from theory for northward IMF, but is found to lie 0.5 Re closer to the Earth for southward IMF. When the position of the magnetopause is normalized for variations in the solar wind dynamic pressure, the magnetopause is found to move earthward 1 Earth radius for every 7.4 nT southward IMF, but remains at nearly a constant distance for northward IMF.