The plasma density gradients in the outer drift shells of the plasmasphere are generally stable with respect to convective instability during quiet geomagnetic-geoelectric conditions. When the IMF turns southward before the onset of a magnetic substorm, this situation gradually changes: the magnetospheric convection electric field is then enhanced and penetrates deeper in the nightside plasmasphere. The sunward and eastward magnetospheric convection velocity is enhanced in the post-midnight local time sector at L>4, and occasionally down to L=2.5. As a result of this enhancement of the azimuthal component of the convection velocity, centrifugal effects are augmented in the distant region of the magnetosphere; the field-aligned potential energy of ions and electrons is reduced along the equatorial portion of magnetic field lines; the field-aligned plasma density distribution, initially in mechanical equilibrium, is accelerated and becomes convectively unstable. As a consequence, a field-aligned polar-wind-like flow of H+-ions (and He+-ions) is driven upward and the mid-latitude ionosphere gets depleted of its light ions. The plasma density at high altitudes diminishes in all flux tubes whose angular speed has been enhanced. It does not change significantly, however, on lower L-shells where the convection velocity has not changed. As a result of the shear in the upward ionization flow, a "knee" develops in the cross-L plasma density distribution along the drift shell which is tangent to a surface which has been called the Roche limit surface or zero-parallel-force surface. As a result of Coulomb collisions and wave-particle interaction, the upflowing particles that are able to overcome the reduced potential barrier may become trapped, and will tend to accumulate in the equatorial potential well beyond this zero-parallel-force surface. But, quasi-interchange - a type of ballooning instability driven by gravitational force or centrifugal effects - prevents this accumulation from taking place; it detaches from the plasmasphere any equatorial plasma cloud of trapped ions and electrons with negative gradients of the density and kinetic pressure beyond the Roche limit surface. The detached plasma is driven away from the unperturbed plasmasphere by quasi-interchange motion and by ballooning instability.