What orientation of Bz (B sub z) increases the likelihood that charged particles from the Sun will cause disturbed conditions?
Why Bz is the north-south component of the interplanetary magnetic field carried by the solar wind. Earth's own field points northward at the dayside magnetopause, so when the arriving Bz points southward the two fields are antiparallel and magnetic reconnection occurs, opening a door for solar wind particles and energy to pour into the magnetosphere. That coupling drives geomagnetic storms, auroras, and absorption or disruption of HF paths, especially over polar routes.
Watch out A northward Bz is the opposite case: the fields are aligned, reconnection is inefficient, the magnetosphere stays largely shielded, and conditions tend to be quiet. East-west orientations are not the component reported for this coupling effect.
South opens the door: Bz south means storms. Think 'Bz south, bands go south.'