TIL Bz is one component of a 3-vector, but Bx and By orientations just don't interact with Earth's magnetic field

I was digging into topics for the Extra exam questions.

The interplanetary magnetic field is a vector quantity with a three axis component, two of which (Bx and By) are orientated parallel to the ecliptic. The Bx and By components are not important for auroral activity and are therefor not featured on our website. The third component, the Bz value is perpendicular to the ecliptic and is created by waves and other disturbances in the solar wind.

When the north-south direction (Bz) of the the interplanetary magnetic field is orientated southward, it will connect with Earth’s magnetosphere which points northward. Think of the ordinary bar magnets that you have at home. Two opposite poles attract each other! A (strong) southward Bz can create havoc with Earth’s magnetic field, disrupting the magnetosphere and allowing particles to rain down into our atmosphere along Earth’s magnetic field lines. When these particles collide with the oxygen and nitrogen atoms that make up our atmosphere, it causes them to glow and emit light which we see as aurora.

For a geomagnetic storm to develop it is vital that the direction of the interplanetary magnetic field (Bz) turns southward.

I was looking at E3C05 on HamStudy, which I’m enjoying as a more productive alternative to doomscrolling.

What orientation of Bz (B sub z) increases the likelihood that charged particles from the Sun will cause disturbed conditions?