ARC The arcs are wider than the bands and do not fold as strongly. The arcs are normally neither very bright nor active.

The arc is probably the most common form of aurora. When aurora show is a calm arc in the low northern sky it often doesn’t evolve to anything more during night. In more active shows the arc is often the first form to appear and the last to disappear.

The lower edge of the arc is usually sharp but the upper edge can gradually blend into the background sky. As activity increases rays and folds normally develop, and the arcs turn gradually into bands.

An aurora arc runs across the picture. Vertical shapes are rays. Photo by Atacan Ergin.

Aurora Arc. Photo by Mauri Korpi.

Aurora Arc. Photo by Anna-Liisa Sarajärvi.

Aurora Arc. Photo by Matti Asumalahti.

CoronaA corona is a hand fan shaped structure, it usually forms south of the observer's zenith, most commonly consisting of rays or bands. The corona is usually the most beautiful part of the aurora show. It is bright and active, but on the other hand also short-lived.

Aurora corona. Photo by Anna-Liisa Sarajärvi.

Aurora corona. Photo by Merja Ruotsalainen.

Corona formed from bands. Photo by Markku Ruonala.

Aurora corona. Photo by Tapio Koski.

Bands are usually narrower, more twisty at the bottom, brighter, and more active than arches. Bands usually develop from arches.

Bands can form J and U shapes, sometimes even full spirals. The corona can also arise from bands. Bands are a fairly common form of aurora.

Aurora band. Photo by Merja Ruotsalainen.

Aurora band. Photo by Matias Takala.

Aurora band. Photo by Lea Rahtu-Korpela.

Aurora bands. Photo by Lauri Koivuluoma.

Aurora band. Photo by Matias Takala.

The raysare parallel to the lines of force of the magnetic field, i.e. quite vertical, usually less than one degree thick light streaks. The rays can occur alone or in connection with other shapes, mainly with arcs and bands. Short rays are usually brightest at the bottom but dim quickly. The longest rays, even extending almost from the horizon to the zenith, are usually uniformly bright and quite calm, and unlike the shorter rays, most often occur in groups of a few rays or alone. Rays, like bands, are a very typical form of aurora.

Artificial light pillars, which are a halo phenomenon visible in ice mist, can sometimes be very similar to the rays of aurora. Confusion is possible especially when the lamps that cause the artificial light pillars are far away and not visible behind buildings or the forest. The nature of the phenomenon is clear at least from the photographs.

Rays. Picture of Tom Eklund.

Rays. Photo by Mika Puurula.

Two beams rise from the aurora veil. Photo by Anssi Mäntylä.

Two radial bands. Show Jani Lauanne.

Radial band and veil. Photo by Jussi Alanenpää.

Two rays. Photo by Aki Taavitsainen.

It may be possible to confuse such rays with artificial light columns. Compare the image below. Picture of Tom Eklund.

There is no aurora in this image, but all the light poles - including the wide and diffuse bar seen at the top left - are artificial light pillars born of ice mist. Photo by Sami Jumppanen.

Aurora and artificial light pillars. All the radial shapes in the picture above are probably artificial light pillars that coincide appropriately with the aurora band. In the image below, the aurora band has shifted and does not overlap with the pillars produced by the orange bulbs. There is no orange in auroras. Photo by Katariina Roiha

Veil
Veil is the most bland and very common form of aurora. It usually covers its homogeneous dim glow over a wide area of the sky at once. Most often, the veil is seen in the calmer and quiet phase of the night after the aurora maximum as a background for other forms. The veil can also occur alone and in that case it will be quite difficult to reliably identify as an aurora, especially at a observation site which has a lot of light pollution.

A similar glow of light can also be caused by airborne moisture, smoke, or a very thin layer of clouds that reflects the light that hits them. However, clouds can also be used to identify veil, especially if the middle or upper cloud appears dark against a lighter background, then it is very likely to be aurora veil if the brightness of the background sky is not due to the rising or falling Moon or Sun. When photographing, very long exposure times usually reveal the green colour of the veil auroras.

Veil and rays. Photo by Esa Palmi.
 

Red aurora veil. Photo by Marko Mikkilä.

Veil. Photo by Milla Myllymaa.

Aurora veil that changes color from green at the lower edge through purple to blue at the top. Photo by Jaakko Hatanpää.

Dim green veil. Photo by Jarmo Leskinen.

Radial aurora band surrounded by veil. Photo by Jussi Alanenpää.

The Stable Auroral Red arcs (SAR arcs)  are usually clearly distanced to the south from the aurora oval and is a very opaque and normally red ribbon. In most cases SAR arcs are only visible in the photo or on the liveview screen of the SLR camera. Using a camera with very high sensitivity is the best method for capturing these faint arcs. The arch usually settles between east and west.

A stable red arc of aurora is a rare phenomenon. In some rare occasions, several SAR arcs may be simultaneously visible.

The first SAR arcs of the Skywarden were observed on nights between November 3-4. and 4-5. days in 2015 in the latitudes of central Finland.   

SAR arc photographed by Lasse Nurminen 2018. Observation of the Skywarden 79113.

STEVE (Strong Thermal Emission Velocity Enhancement) is an aurora-like phenomenon that can be observed in middle latitudes. STEVE does not belong to traditional auroras as a phenomenon, but may appear at the same time with them.

STEVE looks like a narrow, white or mauve arc that is clearly separate from the rest of the aurora oval in the pole-ward side of the sky. In Northern Europe, STEVE can sometimes be seen quite far from the oval and be visible across the southern half of the sky.

A mauve STEVE with a clear gap to the oval. Image: Keijo Lehtimäki

It forms a long and narrow east-west aligned dim line that goes across the night sky. The length of the arc can be over 2 000 km and it is located at altitudes of 110-300 kilometers.

The best time for observing STEVE is when the active aurora of a substrom have subsided.

The arc is white, grey or light purple in color, but the appearance of the arc may vary slightly. In more colorful versions, the bottom edge of the arc shows while/grey color, whereas the upper part has more purple shade. These colorful versions are called Double-Layer STEVE.

Double-Layer STEVE. Image: Eero Karvinen

STEVE's appearance can resemble single, detached rays or there can be rays within the arc itself.

A ray-like STEVE. Photo by: Riku Talvio

Quite often purple rays within the aurora oval itself get mistakenly identified as STEVE. While within the aurora oval the purple rays tend to disappear relatively fast, STEVE-events can last from ten minutes to hours.

STEVE very rarely shows green color, whereas lower edges oval's traveling rays tend to be clearly green. However, occasionally there may also be a green, "toothed" band called ”picket fence” below and aligned with the STEVE arc.

STEVE with picket fence. Image: Sirpa Pursiainen

SAR arcs are a different phenomenon happening in the same region that sometimes gets mixed up with STEVE. SAR-arcs, however, are more diffuse, dimmer or fainter, purely red, and lasts longer than STEVE, even days.

This rare image shows a red SAR-arc with a white STEVE arc. There is an area without aurora light around STEVE. Image: Atacan Ergin

When photographing a suspected STEVE, it is important to try capturing both the arc form and the arc's location in relation to the rest of the oval.

In some cases, fine westward-moving structures can be observed within the arc. These can be captured by taking videos of the event or capturing short-exposure animations.

A picket fence is a green form of aurora (northern lights) with short vertical beams.

Picket fence often occurs under the purple-gray STEVE arc, but can also be an independent green radial arc.

In the picture, the board fence is shown in the upper right corner under the STEVE arch. Photo by Sirpa Pursiainen

The picket fens gets sometimes mixed up with normal green belt with rays. In Picket Fence, the individual rays are clearly separated from each other and the arc of the rays is clearly separated from the rest of the northern lights. The fences of the fence also do not form a single curtain extending up to the pole of the sky.

Picket fence can also occur independently without a STEVE arc. The picture shows very well how the Picket Fence is separated from the rest of the northern lights and does not expand upwards. Photo by John Andersen

Dunes

The dunes are a dim and very rare shape that has so far been associated with the aurora visible in early evening.

Aurora dunes can be most easily confused to ribbons on lower clouds. In order to fill in the description of the phenomenon, a striped pattern formed by parallel lines must also appear in the aurora. The stripes are most easily recognizable right at the front edge of the aurora but they may also occur among the rest of the aurora.

The dune auroras are visible as a parallel striped float. Photo by Tapio Terenius

Raidallisen dyynilautan reunassa voi toisinaan olla voimakastakin aaltoilua.

There can sometimes be strong ripples at the edge of a striped dune float. The rippling of the edge of the dune float can vary from minor to large (pictured). Photo by Pirjo Koski

Pulsating patches or -aurora typically have more or less regular shape and size. They typically appear in the later part of an aurora display. They are almost always pulsating in variable periods.

Red Arc with Green Diffuse Aurora (RAGDA), is a two-component form of northern lights that occurs south of the oval.

Both parts of the aurora are formed when positive particles from the magnetosphere hit the Earth's upper atmosphere. The phenomenon occurs before magnetic midnight during large aurora substorms and is best distinguished when it deteches southward from the bright rays of the substorm aurora.

The phenomenon consists of a faint red arc, which looks a lot like a Stable Auroral Red (SAR)-arc. The common factor for these two red arcs is the reaction of the ring current with the substorm.

A Red Arc with Green Diffuse Aurora
Photo: Antero Ohranen

Below the red arc are green diffuse patches or sausage-like shapes. RAGDA's green aurora is essentially featureless and without any rays. It may appear slightly more bluish in camera images compared to the green aurora of the oval.

RAGDA's green has slightly more bluish shade than the rest of the oval.
Photo: Lasse Nurminen

Sometimes the red arc and the green patches are clearly separated from the aurora oval and sometimes almost in contact with the southward edge of the oval.

During an active substorm, the green northern lights can sometimes be seen with red tops. These usually have rays that RAGDA's green aurora lacks.

An emissionless gap without any aurora light can be observed between the red arc and green diffuse aurora. The two features don't seem to be connected. Of the two aligned structures, the red one is located ~ 100 kilometers higher than the green aurora.

There seems to be an empty area without any aurora light between the red and green aurora.
Photo: Markku Ruonala

The event is dynamic. It sometimes starts with green blobs, then a red arc appears. These two can also appear in the sky at the same time. The shapes move often from east to west. Then the Red Arc with Green Diffuse Aurora fades away and only the red arc remains visible in the sky. The red arc is recognized as the SAR arc.

Observing this phenomenon is easier when the night sky is clear and dark before magnetic midnight. When looking for the red arc with green diffuse aurora, the best results can be achieved by pointing the camera towards south or southwest of the brightest part of the oval.

A wide-angle lens is recommended for photographing the shape of the red arc going over the sky, but a regular lens can also be used. The red arc is quite dim, so the exposure times needed are typically longer than the ones for northern lights.

Two RAGDA-arcs
Photo: Pirjo Koski

If the red arc cannot be distinguished in the images, the phenomenon identification 'Diffuse green auroras' should be used for isolated hazy green blobs/arc.

Dunes in RAGDA's green aurora
Photo: Matias Takala

Form not identifiable
Sometimes auroras have to be observed in such poor conditions that it is not possible to reliably identify the shape even if for example the structure and conditions could be recognized. Such a situation could be the outcome of for example alight background sky, cloud cover or a covered horizon.

Paljain silmin valkoinen väri näkyy useimmiten himmeissä näytelmissä, kun silmä ei kykene erottamaan mitään varsinaista väriä. Harvoin kirkkaissa näytelmissä valkoinen väri voi myös syntyä sopivista vihreän, punaisen ja sinisen yhdistelmistä.

Green, seen with the naked eye, is one the most common colors of the aurora. The green color is derived from atomic oxygen.

Green auroras. Lea Rahtu-Korpela.

Green auroras. Photo by Juha Ojanperä.

Red lower edge visible with the naked eye. The bands which are starting to level up their activity and are green colored have quite often a narrow red lower edge. This is the most common form of red color which is derived from molecular nitrogen.

Aurora band with purple lower edge. Photo by Ilmo Kemppainen.

The low hanging brightest aurora band is colored red at the lower edge. Photo by Tero Ohranen.

Narrow purple reddish tones at the lower part of this aurora band. Photo by Merja Ruotsalainen.

Purple band at the bottom. Photo by Panu Lahtinen.

Auroras which have red top part that can be seen with naked eye are most often observed in the bands and long rays. In this case the lower parts are usually green. If the upper parts are in sunlight, red may be stronger than green. This shade of red is due to the discharge of the excitation state of the atomic oxygen.

Aurora that shift to reddish towards the top. Photo by Karri Pasanen. 

Red top in a aurora band. Photo by Simo Aikioniemi.

Red at the top of the aurora. Picture of Tom Eklund.

Completely dark red aurora is a very rare and strikingly handsome revelation. This phenomenon is also due to the discharge of an excitation state of an atomic oxygen.

Throughout red aurora. Photo by Tobias Billings.

Yellow aurora color that can be seen with naked eye is a rarity that can arise from suitable combinations of green, red, and blue in bright shows.

Clearly blue auroras can be seen only during the best aurora displays close to the maximum phase or soon after it. Sometimes blue auroras can be seen shortly after the sunset at the top part of the auroral shapes, specially rays. It is created by the mission of the ionized nitrogen molecules created by the suns radiation.

Strongly colored blue auroras. Photo by Jorma Mäntylä.

Blue top parts of the aurora. Image by Tom Eklund. 

Blue top parts of the aurora. Image by Jaakko Hatanpää.

Partly blue corona. Photo by Tapio Koski.

Faintly blue top parts of an aurora veil. Photo by Jaakko Hatanpää.

Pulsating aurora. The brightness of the pulsating aurora usually varies rhythmically over a period that can be only a fraction of a second at its fastest, but can also be several minutes. Pulsing usually only occurs in(strong auroral conditions) higher quality shows , especially towards the end of them. However, the pulsation may be followed by yet another eruption. Sometimes the variation in brightness is at the same stage in the whole form, whereby the whole form "turns on and off" at the same time. Pulsation is also found in arches and bands, but above all in spots..

Flaming. This rare subclass of aurora does not mean so much a single shape, but a large area in the sky. In the flaming aurora, bright waves that are sweeping upward towards the magnetic zenith emerge in the sky. Very rarely waves can wipe downwards. Bands are usually reported during flaming, less often spots.

Flickering. This rare subclass refers to a situation where irregular variations in brightness occur in aurora, such as in fluttering flames.

Streaming. In streaming aurora fast irregular variations in brightness occur along the horizontal dimension of homogeneous shapes.

Usually in Lapland or even in the south you can see purple auroras in stronger aurora shows. The most common color in auroras along with green and red.