Shapes and types of aurora
Shapes and types of aurora: When we think of the aurora, the first image that comes to mind is one with green swirling bands dancing all night across the arctic skies. In reality, the aurora can adopt different shapes and we discern several fixed patterns.
The aurora will look significantly different depending on where you see it from. However there are three main patternsthat seem to always show up. They can appear alone or in combos:
arcs are the least structured patterns. They resemble a uniform ‘rainbow’ spanning from one horizon to the other. Arcs can be diffuse or structured with rays but they mostly appear either when the aurora conditions are quiet or growing in intensity. At low latitudes aurora shows usually begin with an arc on the horizon.
Typical arc structure at high latitudes (here northern Norway). The arc stretches from north-east to north-west and splits into two on the right-hand side. There is very little pillar structures so it’s mainly a diffuse arc. It happened during low auroral activity. The shutter of the camera had to be left open longer (15 seconds) to get the colors and the milky way at once!
Typical arc structure at lower geomagnetic latitude during the growing phase of a geomagnetic storm (Kp 6). The stable and diffuse arc hung on the northern horizon of Denmark for about an hour before getting brighter and more structured (pillars).
The two previous pictures showed you distant arcs. Being under an arc looks a bit different! This hind-view shows an actually rather thick arc at the base. We can discern more structures but this arc is still quite diffuse. It’s typical of medium auroral activity at high latitudes or of the growth phase of a substorm.
Bands have more primary structure. They are ribbon-like patterns that exhibit swirls, folds and snake-like structures. They can be diffuse or more defined with pillars as well. Bands are what people picture when they think about the aurora. They the testament to more active auroral conditions. Bands can move slowly or with moderate speed. When conditions intensify they can start ‘dancing’. When the auroral activity increases the bands seem to shrink in diffuse thickness and get concentrated into structured pillars. It’s like the light and color ‘huddle up’ into those giant rays that shine brighter and more vibrant than the rest, as shown in the picture below:
Pillars are also commonly dubbed beams or rays. For many chasers they’re the most spectacular feature of the aurora. They’re huge colored columns of light that soar towards space. They are the sign of increased auroral activity. Therefore they’re often accompanied by increased brightness and movement.Looking at pillars from the side (here about 400km from the display) allows you to gaze at the whole ‘height’ of the aurora and makes you realize how immense it is. Up to 600km can separate the green base and the purple canopy! At low geomagnetic latitudes pillars will usually show during periods of increased activity, typically during a geomagnetic storm or sporadic substorm. They usually ‘dance’ on the horizon. While your longer distance to them will make them look fainter and duller, a camera can capture their beauty quite well (see picture below).
When you stand right under a display of pillars, your unique perspective makes them look completely different. They start appearing as striped drapes moving rather quickly across the sky. When a drape is right over you (at zenith), you get this mind-boggling vortex effect coveted by so many aurora chasers. As the rayed drapes develop quickly right above you, they usually create what we call an auroralcorona. The base of the aurora (here underlined in white) is rather close to you (80-100km above) while the top end of the pillars (black arrow heads) are several hundreds of kilometers further up. This makes the pillars appear to be converging towards the zenith and create a crownshape!
Coronas are probably the most astonishing, yet rare auroral phenomenon, and for good reasons. They usually only happen at high latitudes during substorms but sporadically and very elusively. At lower latitudes you need to wait for a major solar event to push the aurora above you. Coronas are so psychedelic that they make the aurora play tricks on your imagination. Some people see birds, dragons, angels, butterflies and all other sorts of shapes.
Coronas are the ultimate goal for aurora watchers and chasers. They’re the brightest, fastest and most colorful auroras. They typically allow you to see colors with the naked eye! Unfortunately lots of people never get to see them because of their elusive nature. Higher geomagnetic activity usually allows a mixture of aurora shapes. This configuration is often observed at higher latitudes around unsettled to active auroral conditions. In this panorama, a diffuse and less active arc is visible (more equatorward), some active and more structured bands develop (poleward). Read more about aurora coronas and understand them better
Mix of shapes and types of aurora
Higher geomagnetic activity usually allows a mixture of aurora shapes. This configuration is often observed at higher latitudes around unsettled to active auroral conditions. In this panorama, a diffuse and less active arc is visible (more equatorward), some active and more structured bands develop (poleward). Sometimes the auroral activity is so high that the whole sky ‘explodes’ into different shapes and colors. The aurora dances so fast that only pictures can freeze them. Then you can easily have all three major shapes and all their combinations in one frame!
Have a look at Adriens page and work here