According to Boller and Stolov (1974), the probability of instabilities (and the magnetic activity) should be proportional to A0+A1 sin2 &psgr;M, where A0 and A1 are constant and &psgr;M is the angle between the solar wind and the dipole axis of the earth. Such an expression permits one to fit very well, in phase and in amplitude, both the semiannual and the daily variation of the magnetic activity as defined by aa or am indices.