How are aroras created? Which molecules contribute to it? What are the colors and wavelengths? What does G4 mean in this context?

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Auroras, also known as the Northern and Southern Lights, are natural light displays predominantly seen in high-latitude regions around the Arctic and Antarctic. They are created through a complex interaction between the solar wind and Earth's magnetic field and atmosphere.

Creation of Auroras

  1. Solar Wind Interaction: The process begins with the solar wind, a continuous stream of charged particles (mainly electrons and protons) emitted by the sun. When these particles reach Earth, they interact with its magnetic field, which channels them towards the polar regions.
  2. Magnetic Field and Atmosphere: As these charged particles enter the Earth's atmosphere, they collide with gas molecules, primarily in the ionosphere, which is located about 100-600 km above the Earth's surface. These collisions transfer energy to the gas molecules, exciting their electrons to higher energy states.
  3. Photon Emission: The excited electrons eventually return to their original, lower energy states, releasing the absorbed energy in the form of photons (light). The specific wavelengths (and thus colors) of the emitted light depend on the type of gas molecule and the energy levels involved in the transition.

Contributing Molecules and Colors

Different gases in the Earth's atmosphere contribute to the various colors observed in auroras:
  1. Oxygen:
    • Green: The most common auroral color, green light, is emitted by oxygen atoms at altitudes of around 100 km. This light has a wavelength of approximately 557.7 nm.
    • Red: At higher altitudes (above 200 km), oxygen can also emit red light with wavelengths of 630 nm and 658 nm. These emissions occur due to longer-lived excited states of oxygen atoms.
  2. Nitrogen:
    • Purple/Violet: Nitrogen molecules, particularly ionized nitrogen (N2+), emit purple or violet light at wavelengths around 427.8 nm. This occurs at higher altitudes (above 400 km) where oxygen is less prevalent.
    • Blue: Excited nitrogen molecules can also emit blue light when they return to a more stable energy state. This typically happens at altitudes between 100-300 km.
  3. Other Colors:
    • Yellow-Green: A mix of emissions from oxygen and nitrogen can produce a yellow-green color.
    • Pink/Orange: When red light from nitrogen overlaps with blue light, it can create pink or orange hues.

G4 Context

In the context of auroras, "G4" refers to a geomagnetic storm classification. The National Oceanic and Atmospheric Administration (NOAA) uses a scale from G1 (minor) to G5 (extreme) to categorize the intensity of geomagnetic storms. A G4 storm is classified as "severe" and can significantly enhance auroral activity, making the lights visible at much lower latitudes than usual. During such storms, the increased influx of charged particles can lead to more intense and widespread auroral displays. In summary, auroras are created by the interaction of solar wind particles with Earth's magnetic field and atmosphere, leading to the excitation and subsequent photon emission from oxygen and nitrogen molecules. The colors and wavelengths of the auroras depend on the type of gas and the altitude at which the interactions occur. The term "G4" indicates a severe geomagnetic storm, which can amplify auroral activity.
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