How to Read the Bz Magnetic Field
Of all the numbers on a real-time space weather dashboard, Bz is the one most closely tied to whether the aurora actually lights up. It measures the north-south orientation of the interplanetary magnetic field (IMF) as it arrives at Earth.
Southward Bz Opens the Gate
Earth's magnetic field points northward at the equatorial magnetopause. When the incoming IMF points southward (Bz negative), the two fields reconnect, letting solar wind energy pour into the magnetosphere. Northward Bz keeps that gate mostly shut, and even a fast solar wind may produce very little aurora.
The Threshold Numbers
Bz values between 0 and -5 nT produce mild activity. Between -5 and -10 nT is meaningful and usually visible from high latitudes. Below -10 nT is strong; below -20 nT indicates severe geomagnetic storming, often with widespread mid-latitude aurora.
Bz Is Volatile
Bz can flip from strongly southward to strongly northward within minutes. This is why aurora chasers watch the DSCOVR real-time plot rather than relying only on the 3-hour Kp.
Frequently Asked Questions
Why does 'southward' mean 'negative'?
By convention, the z-axis of the GSM coordinate system points from Earth toward the geomagnetic north. A field pointing the opposite way โ southward โ is recorded as a negative number.
How long does Bz need to be southward?
Sustained southward Bz for 30 minutes or more is generally required to drive a substorm and visible aurora.