On the Formation of the E Region Layers at Low Altitudes—Theory

Abstract

An analysis of E region ion layer formation is presented, including the effects of a non‐dipole geomagnetic field and gravity on vertical ion drifts, with a particular focus on E region layers at lower altitudes (ELLAs). For nonzero magnetic declination, meridional winds can have an effect similar to those of zonal winds. At ELLA altitudes (∼90–110 km) this non zero magnetic declination effect can dominate the usual meridional wind effect and rival the usual zonal wind effect. In regions with negative magnetic declination, the meridional wind contribution to the vertical ion drift can reverse direction at ELLA altitudes. Analysis of gravity‐driven ion drifts shows that an ion freely falling at the terminal velocity ( m/s) moves more slowly than observed descending motions of ion layers in the upper E region. The slower descent rate of ELLAs is, we find, closer to the terminal velocity. Because the terminal velocity in the highly collisional E region decreases with decreasing altitude, a gravity‐driven accumulation of ions could occur. This not‐previously‐considered mechanism is consistent with previously published SAMI3 (Sami3 is another model of the ionosphere) simulations of E region metallic ion layers.