Interactive Polarity Explorer
The Bz is the north-south direction of the solar wind's magnetic field. A southward Bz (negative value) is the secret ingredient for a strong aurora. Select a polarity to see its effect.
Impact on Earth's Magnetic Field:
Magnetic Field Polarity
Why Does Southward Bz Matter?
Think of magnets. Opposite poles attract (North and South), while like poles repel (North and North). Earth's own magnetic field points north at its outermost boundary where it meets the solar wind.
When the solar wind's Bz is also pointing north (positive), the two magnetic fields repel each other. Earth's shield remains strong, and very little solar wind energy gets through.
But when the Bz turns south (negative), the two fields are opposites. They attract, connect, and effectively open a gateway. This allows a flood of energy and particles to pour into our atmosphere, creating a vibrant and active aurora.
How to Interpret Bz Data
South is the Key
The first thing to look for in any forecast is a negative Bz value. The more negative (e.g., -10 nT is better than -5 nT), the better.
Sustained is Best
A Bz that stays southward for a long period allows more energy to accumulate in the system, often leading to a more impressive and long-lasting light show.
Quick Bz Reference Guide
đ Typical Bz Values
- -20 to -30 nT: Extremely favorable - Major storm possible
- -10 to -20 nT: Very favorable - Strong aurora likely
- -5 to -10 nT: Favorable - Good aurora displays
- -5 to +5 nT: Neutral - Limited aurora activity
- +5 to +10 nT: Unfavorable - Aurora unlikely
đ IMF (Interplanetary Magnetic Field)
Bz is one component of the IMF - the magnetic field carried by the solar wind through space. The IMF has three components:
- Bx: Sun-Earth direction
- By: East-West direction
- Bz: North-South direction (most important for auroras)
⥠Magnetic Reconnection
When the Bz turns southward, it enables a process called magnetic reconnection. This is where Earth's magnetic field lines and the solar wind's field lines connect and merge, allowing energy transfer. This reconnection is the key mechanism that powers geomagnetic storms and brilliant auroral displays.
đ¯ Forecasting Tips
For the best aurora viewing, look for:
- Duration: Bz southward for 2-3 hours or more
- Magnitude: Values below -10 nT are ideal
- Stability: Consistent southward orientation
- Combined factors: Fast solar wind speed (>500 km/s) plus southward Bz creates optimal conditions