Auroral Substorms: Why the Sky Can Explode Without a Major Geomagnetic Storm
Many observers wait for an extremely high Kp, yet some of the most dynamic displays occur during an auroral substorm. It is a sudden release of energy stored in the magnetotail and can develop on a regional scale.
The growth phase
As solar wind transfers energy into the magnetosphere, the auroral oval may shift equatorward and a green arc becomes more defined. The scene can look quiet, but the system is being loaded with energy.
The sudden expansion
At some point the arc brightens, bends and produces rays, curtains and rapid motion. Activity expands poleward and along the oval. This is the most spectacular phase and may last from minutes to more than an hour.
Why Kp can mislead
Kp describes geomagnetic disturbance on a planetary scale and over broad time intervals. A substorm can be locally intense without becoming a major global storm. Local magnetometers, webcams and direct observation are therefore extremely valuable.
The sky may look calm at the exact moment the magnetotail is ready to release stored energy.
Signs to recognise in the field
A stable arc that steadily brightens, begins to ripple and suddenly develops vertical rays is a favourable sign. Do not leave just because the aurora looks weak: the growth phase may be preparing the expansion.