Penetration of the By component of the IMF into the magnetotail

ISEE-1 measurements in the magnetotail show that, on the average, about 60 percent of the By component penetrates into the plasma sheet. An analysis is made of the characteristics of field-aligned currents in the nocturnal sector of the magnetospheric-ionospheric system connected with the effect of the By component. Possible consequences of the efficient penetration of the By component into the plasma sheet are examined.