Partial penetration of IMF By into the magnetosphere

Recent observations of partial penetration of the IMF B_y into the magnetosphere (Fairfield, 1979; Cowley and Hughes, 1983) are shown to agree with the idea of a magnetopause current K_y, induced by IMF B_y (Primdahl and Spangslev, 1983). The slow decay of K_y, caused by Joule heat losses in the cusp ionospheres is responsible for the appearance (on the average) of a fraction of IMF B_y inside the magnetosphere, and this also explains the large statistical scatter of the data. A decay time constant of 4–5 days is derived from the average fraction of IMF B_y observed inside the magnetosphere in good agreement with the 7-day time constant previously proposed.