How the Tilt of the Dipole Could Affect the Rate of Reconnection at the Earth's Magnetopause

The semiannual variation of geomagnetic activity was attributed by Russell and McPherron (1973) to reconnection modulated by the varying angle of the magnetic field at the nose of the magnetosphere to the statistically Parker-spiral-oriented IMF. The division of the semiannual variation of geomagnetic activity into two annual variations according to the polarity of the IMF clearly demonstrates that reconnection is the cause of the semiannual variation. The so-called Russell-McPherron mechanism does not explain the diurnal variation of geomagnetic indices, albeit it does explain the annual variation of this diurnal variation. O'Brien and McPherron (2002) have recently demonstrated that if the reconnection rate depends on the tilt of the dipole an improved prediction of both the semiannual and diurnal variation results, but they do not provide a credible explanation for tilt dependence. Nevertheless, a very simple explanation does exist based on simple geometric arguments following those of Crooker (1979) and Luhmann et al (1984) who predicted the sites of reconnection for a dipole perpendicular to the solar wind flow. If reconnection is initiated where the IMF is antiparallel to the magnetospheric field and if the rate of reconnection depends on the solar wind pressure normal to the magnetopause, then the rate maximizes for the 0 deg. tilt (0 deg. magnetic latitude of the subsolar point) and lessens as either pole of the dipole tilts toward the Sun. In short, the simple merging law used by Russell and McPherron may need tuning, but the basic mechanism is valid as proposed.