The Northern Lights visit the Methow Valley
Early this week Methow residents were treated to a view of the Northern Lights. From the Valley floor, the northern horizon glowed green as if there was a large city along the southern boundary of the Pasayten Wilderness. The green glow also formed an arc higher in the sky. From the Pine Forest neighborhood, curtains of glowing light were clearly visible!
The Physics behind the Green Curtain
The aurora is associated with the arrival of highly energetic plasma and magnetic field energy released by the sunlast Saturday- not unlike a literal fireball. Thiscoronal mass ejectionitself was sourced by a relatively large flare up of energy near the Sun’s surface.
The plasma ball slammed into the Earth’s magnetic field and enegized a lot of the particles in the upper atmosphere.
The green color is associated to thequantum transitionof electrons in oxygen atoms. High in the atmosphere, oxygen atoms arecommonly found alone, rather than in molecular pairs. While virtually all of the particles up there are energized by those impeding solar particles, theexcited oxygen atoms decay very slowly to light with a wavelength of 557.7nm. The essential physics behind that decay isphosphorescencefamiliar from glow-in-the-dark toys.
For the experts:
This is a so-called forbidden transition, where the emitted photon is required to carry more than one integral unit (hbar) of angular momentum. This can happen when the photon is emitted with additional orbital angular momentum with respect to the recoiling oxygen atom. For the 557.7 nm line, it’s a D_2 -> S_0 transition, induced by the electric quadruple moment of the oxygen atom.
If the quantum nature of light and its interaction with. Matter interested you, consider signing up for our course: Quantum Mechanics for the Working Professional.
