Why Bz is the north-south (z-axis) component of the interplanetary magnetic field carried outward by the solar wind. When Bz turns southward (negative), it is opposite to Earth's own field at the magnetopause and the two fields reconnect, letting solar wind energy pour into the magnetosphere. That produces geomagnetic storms and auroras, which usually degrade HF paths, especially polar ones. A northward (positive) Bz tends to shield the magnetosphere and keeps conditions quieter.
Watch out Geomagnetic field stability is what the K and A indices report, not Bz; the critical frequency for straight-up transmissions is foF2, measured by ionosondes.
Bz is the z (north-south) axis of the IMF: Bz south = storm, Bz north = calm.