Driven by the Sun's proton emissions and its interaction with the Earth's magnetic field the upper atmosphere, (between 90km and 120km), becomes ionized. Then when the ions return to their ground states the recombination spectra they form produces the colours which we know as the aurora.
Auroral activity is rated as either not visible, low, moderate, high, very high or extremely high. These classifications are defined according to the brightness achieved by auroral activity, visual activity (ie. changes of form or structure), whether the aurora is pulsating, and according to the intensity and fluctuations of colour in the aurora. The various levels of activity are defined below:
- Not visible: Self-explanatory.
- Low: Low intensity aurorae consisting mostly of diffuse, dim, and lifeless activity. Generally no rapid changes in form or structure are discerned with auroral activity that is classified as "low."
- Moderate: Moderate intensity auroral activity which consists of diffuse aurorae intermixed with curtain aurorae or other forms of relatively-low activity aurorae. Moderate activity may include beams or rays of aurorae which travel either east or west with time. No colour fluctuations or significant brightenings of aurorae are associated with moderate intensities.
- High: High intensity auroral activity is activity associated with very bright, active displays that may exhibit changes of colour and rapid pulsations. High activity is generally confined to curtain aurorae and moderate-intensity pulsating aurorae.
- Very High: Very high intensity auroral activity is usually only experienced over the high latitude regions where zenith aurorae and significant auroral displays occur. Activity classified as very high consists of most auroral forms of activity, but the activity is always very bright and extremely active. Curtain aurorae may change form and colour rapidly. Zenith aurorae may become exceedingly bright and colourful.
- Extremely High: Extremely high auroral activity is only rarely encountered. Activity at this level of intensity is most often experienced within the middle and/or low latitude zones during significant periods of geomagnetic activity. The expansion of the auroral zone equatorward and poleward produces the intense activity over regions equatorward of the normal position of the auroral oval. This activity usually consists of exceedingly bright, rapidly fluctuating, strongly pulsating, colour-varying auroral activity. Levels of auroral activity this high are usually only associated with "rogue flares", which may occur only once or twice during a solar cycle.
| NORTHERN HEMISPHERE: High latitudes >= 55 deg. N. Middle latitudes >= 40 < 55 deg. N. Low latitudes < 40 deg. N. |
SOUTHERN HEMISPHERE: High latitudes >= 55 deg. S. Middle latitudes >= 30 < 55 deg. S. Low latitudes < 30 deg. S. |