Northern lights – color and shape
The different Colors of Northern Lights
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The color the Northern Lights display can vary, although green is by far the most common color. The color they display depends on the type of gas that is involved in the process as well as the energy level the sun particles embody. While green, yellow and red colored aurora is associated with excited oxygen, blue and purplish colors are caused by interaction with nitrogen.
Whether a Northern Light is green or red depends on the altitudes at which the oxygen emits. For different altitudes, the energy level the particles have can vary. The higher the altitude is the higher is the energy level. If the collision occurs at a relatively high altitude, the color of the Northern Light is red. For this to happen, a higher intensity of the activity of the solar wind is needed. On the other hand, green Northern Lights are created at lower altitudes and hence at lower energetic levels. Therefore, a lower degree of solar activity is needed to be present in order to create green northern lights.
The blue and the purple colored Northern Lights are rarely seen. That is because Northern Lights need to go above the shadow of the earth and be directly exposed to the light of the sun to appear in these colors. In order for this to happen, the sun needs to be located in a certain position. The sun needs to be positioned slightly below the horizon in a so-called deep twilight. A coloring of this style is referred to as “resonance scattering” and can also happen when the light of the sun is reflected by the moon.
Yellow Northern Lights are caused by a combination of the different colors mentioned above.
The different shapes of Northern Lights
The aurora can appear in many forms. The basic forms are: Arcs, Corona, Diffuse, and Drapery. Drapery is the shape that is seen most often. Drapery aurora is commonly described as flickering curtains. Arcs have shapes that remind us of rainbows, while Corona is seen in the shape of a crown. Diffuse aurora is only seen on rare occasion and does not take on a shape with specific features. Moreover, they cannot be seen easily, without the help of certain devices.
Scientists have not yet found an exact answer to the question of what causes these different shapes. It is for example believed that the shapes depend on the electrons’ source and the way these electrons incident into our atmosphere. This process varies due to the fact that the magnetic field of the earth is responding in an ever-changing way to the particles originating from the sun. Moreover, the relative position between the Northern Lights and the person observing plays a role in determining the shape of the Northern Light that can be seen.
Aurora, Lights of the northern sky., Sigurður H. Stefnisson og Jóhann Ísberg. Reykjavík. JPV publisher, 2012.