Quiet aurora band - 23.3.2023 at 21.26 Växjö, Ruotsi Observation number 113786

• Aurora brightness
• • Bright auroras
• Observed aurora forms
• • 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.

  • 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.

  • Stable Auroral Red (SAR) arc

    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-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.

• 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.

  • Red coloration of the shapes top

    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.

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