Half-sky auroras - 7.9.2015 at 22.30 - 7.9.2015 at 23.30 Ilmajoki Observation number 42119

• Aurora brightness
• • Dim auroras
• Colors with unaided eye and other features
• • Green auroras

    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.

  • White auroras

    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.

• Observed aurora forms
• • STEVE-arc

    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.

  • Veil

    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.

  • Rays

    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

  • Band

    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.

  • Arc

    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.

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