In relation to the Sun and Earth lies a plane with an X/Y/Z axis. You can read more about that by visiting the Aurorasaurus page. The very basics of it are: Bx is the relational direction from the Sun to the Earth (towards and away), By is the east and west axis through the center of the Earth (parallel to the Sun), and Bz is the north and south axis through the center of the Earth (parallel to the sun). This is why you will see Bz referenced as north (a positive number and green in the app) or south (a negative number and red in the app). But, what does Bz really indicate?
The Earth’s core is a big magnet with north and south poles. At it’s normal, quiet state, Earth’s magnetic field is oriented to the north. The Sun is also a giant magnet. In fact, its magnetic activity is what causes sunspots to form. When solar activity increases and causes solar flares, filament eruptions, and coronal mass ejections (CMEs), the sun sheds magnetic fields. These fields can be oriented north or south as they travel toward the Earth. When these fields are oriented to the north, major effects tend not to occur because, remember, the Earth’s magnetic field is naturally north. However, when these fields reach Earth in a southward orientation, chaos tends to happen. This is why Bz is important to monitoring for potential aurora activity.
The more chaotic the flux between north and south in the Bz, the more aurora activity we tend to see. When Bz drops from north to south (a “flip”), we can see active aurora begin. This is where you really see bright green, yellow, white, pinks, and purples start to show. If the Bz remains south for a long period of time, you may or may not continue to see vibrant displays. As the Bz flips back and forth from north to south and vice versa, it is much like an electric motor generating power. These flips in the chaotic interplanetary magnetic field cause breaks in the magnetic fields and, when those break reconnect on the dark side of the Earth, electrons are excited and vibrant, colorful displays of aurora occur. Once the field stabilizes and returns back to north (and even in long periods of strong south orientation) active aurora will subside and, many times, you will see dancing aurora clouds in the dark night sky. Just remember, the bigger and more frequent the flips from north to south and back again, the more active aurora you are likely to see!
Free Versus Subscription
The free version of the app gives you approximately two hours and 30 minutes of interplanetary Bz data: the previous 90 minutes plus the next hour. The subscription version lets you go back and look at interplanetary Bz data for the past 36 hours in 6-hour increments.