Auroras appear in many different forms and no two nights are the same. The British Astronomical Association (BAA) provides some standard guidance to classifying aurorae. In practice, most of these types are not discrete entities. Rather, they transition into each other (sometimes rapidly) and there are many intermediates. It is often the case that more than one form is present at a time. Some of the variation in perceived appearance is also simply the result of the aurora changing in intensity and position relative to the observers' own location, rather than an actual change in structure. Scientific descriptions of aurorae take into account movement, structure, form, brightness and colour, and therefore there are numerous variations to the main forms that I describe below:
Below I am providing photographs of some of the main types of aurora that I have observed in Iceland. I have based these on the BAA classifications, but with some amendments for what I specifically see in Iceland. Please note that the naked eye colours described below relate to my own personal experiences - your eyes may differ in sensitivity to mine and you may perceive aurora colours differently.