The most well-known source of asymmetric outflow is differences in sunlight coverage due to season and time of day.  The hemisphere receiving more sunlight, and therefore extreme ultraviolet (EUV) flux, will have greater photoionization at ionospheric altitudes.  This increases the seed population for upflows and ultimately outflows.  Because outflow is driven by a variety of factors (see reviews by Hultqvist et al., 1999; Welling et al., 2015), all of which may be asymmetric themselves, the interhemispheric variance of ionospheric outflow can intensify during active periods.

Barakat et al. [2015] illustrates this effect clearly.  Figure 1 shows the total H+ (top frame) and O+ (bottom frame) outflow rate integrated over the northern (red lines) and southern (blue lines) hemispheres during a geomagnetic storm occurring from Sept. 29 to Oct. 4, 2002.  Values are obtained via the Generalized Polar Wind (GPW) model.  A clear diurnal pattern arises, favoring the southern hemisphere due the fact that it is post-equinox/southern hemispheric springtime.   Geomagnetic activity begins to intensify, as signified by the Kp index.  As activity increases, the total amount of outflow increases, as does north-south asymmetries.