Bz is a measure of the direction of the magnetic field at ace. The field is three dimensional, so there are three components in the magnetic force, Bx, By, and Bz. Bz is the strength of the field from the sun out through the Earth, By is the strength along the parallel cutting from East to West across the equator, and Bz is the parallel from the South to North Pole. Of the three, Bz has the most impact on aurora. When Bz is north (positive on the chart) the magnetic arriving from space line up with Earth and there is little interaction. When Bz is south (negative on the chart) it is opposite Earth’s poles and there can be significant interaction – producing aurora! Thus, a negative Bz is almost required for aurora. All the other factors can be favorable, but if Bz is positive, pack up your camera and go home. The longer the Bz has been negative, and the strong it is negative the stronger the aurora will be. However, if the strength of the magnetic field is low, and the Bz is only registering -1 or -2 nT, aurora will be unlikely.