Reading the numbers
Kp and the G scale, Bz, solar wind speed and density, and hemispheric power, and what values matter for aurora.
Kp and the G scale
Kp measures how disturbed Earth's magnetic field has been over a 3-hour period, from 0 (quiet) to 9 (extreme), worked out from magnetometers around the world. The higher it is, the further from the pole the aurora reaches. NOAA's storm scale starts at Kp 5:
| Kp | NOAA scale | Name | About how often |
|---|---|---|---|
| 5 | G1 | Minor | 900 days per cycle |
| 6 | G2 | Moderate | 360 days per cycle |
| 7 | G3 | Strong | 130 days per cycle |
| 8 | G4 | Severe | 60 days per cycle |
| 9 | G5 | Extreme | 4 days per cycle |
A cycle is the Sun's roughly 11-year activity cycle. Kp is a global average, so what it means for you depends on where you are: see Magnetic latitude. Polar Lights works out the Kp your place Needs from your magnetic latitude.
Bz
Bz is the north–south part of the magnetic field carried by the solar wind, in nanotesla (nT). It's the main switch for the aurora:
- South (negative): the solar wind's field links up with Earth's and lets energy in. Around −5 nT for a while helps; −10 nT or lower for 15 minutes or more often brings strong aurora.
- North (positive): little energy gets in, even if the wind is fast.
Bz can swing back and forth within minutes, which is why aurora can flare up and fade quickly.
Solar wind speed
Typical wind is 300 to 400 km/s. Above about 500 km/s it carries more energy; 600 to 700 km/s or more is fast wind from a coronal hole or a CME. A sudden step up together with a jump in density is a shock, often the leading edge of a CME.
Density
How many protons there are in each cubic centimetre of solar wind (p/cm³), typically 2 to 10. Denser wind pushes harder on Earth's field, but density on its own does little for the aurora. It matters most when Bz is south.
Hemispheric power
NOAA's estimate of how much energy the aurora is putting into the upper atmosphere over a whole hemisphere, in gigawatts (GW):
- below about 20 GW, quiet aurora far to the north;
- around 30 to 50 GW, usually visible at high latitudes;
- above about 50 GW, bright aurora spreading towards lower latitudes.
A rise of tens of gigawatts within an hour often marks a substorm.