Corona

Corona diagram

Corona diagram

All images, except where noted, credit Wiebke Salzmann CC-BY-SA. Click images for larger versions.

Cloud of the Day – Corona

Here is a meteorological phenomenon that is often misnamed “halo.” A corona is similar to a halo in that they both form rings around the Sun and Moon. The Sun’s corona (the one formed in Earth’s atmosphere, not the one around the actual Sun) is hard to see because the Sun is so bright. A corona is a more subtle effect and needs the more muted light of the Moon to really show itself.

Corona-bright-Wiebke-Salzmann-cc-by-saWhile haloes result from the light being refracted by ice crystals high in the atmosphere, coronae are caused by the diffraction of light scattered by particles – water droplets, ice crystals, dust motes, etc – in the lower atmosphere. A corona can also form on a foggy window pane. Haloes have fixed dimensions, calculable from the known refractive index of ice. Coronae come in various sizes due to the variability in the size of the light-scattering particles. Smaller droplets make larger coronae. In addition to the light scattered from the surface of the particle, small contributions to the corona are made by light that reflects directly off the droplet, or passes through it.

Corona around street lamps through an aspirated window pane.

Corona around street lamp through an aspirated window pane.

Artificial corona around LED lamps of different colors, created with lycopodium spores. As can be seen the diffraction rings of red light have a greater radius than those of blue light.

Artificial corona around LED lamps of different colors, created with lycopodium spores. As can be seen the diffraction rings of red light have a greater radius than those of blue light.

Image credit - Florian Marquardt - CC-BY-SA

Interference patterns – Florian Marquardt – CC-BY-SA

A classic corona consists of a bright aureole in the center, with one or more colorful rings around it. For the sharpest coronae, the droplets must be all close to the same size, so the interference pattern in the light can be well defined. It is constructive and destructive interference among the scattered light waves, where they add to make bright regions and subtract to make dark regions, that make the alternating rings of bright and dark.

rjb

  

About arjaybe

Jim has fought forest fires and controlled traffic in the air and on the sea. Now he writes stories.
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11 Responses to Corona

  1. emmylgant says:

    The things I learned from hanging out here! :-)
    Thank you professor.

  2. arjaybe says:

    So, if I’m the Professor, are you Maryanne?

  3. emmylgant says:

    Hmmm. Lemme see… kinda, with undertones of Gilligan?
    and may be even a sprinkle of ginger now and again.

  4. arjaybe says:

    Now that’s an image I’ll have trouble getting out of my mind: Gilligan crossed with Ginger.-)

  5. emmylgant says:

    As long as it doesn’t give you nightmares….

  6. emmylgant says:

    And no I don’t look like either Gilligan or Ginger but I sometime act/think a little bit like that. That should be easier to forget..

  7. arjaybe says:

    That’s a relief.-)

  8. emmylgant says:

    :-) Pigtails is laughing. Something about Gilligan with red hair and lipstick has her in stitches.
    I should have been more explicit.

  9. arjaybe says:

    Yeah. If Gilligan is going to have red hair, then it has to be Pippi Longstocking pigtails, right?

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