Aurora

• Auroras are the result of disturbances in the Earth’s magnetosphere caused by the solar wind
• Major disturbances result from enhancements in the speed of the solar wind from coronal mass ejections.
• These particles, mainly electrons and protons, precipitate into the upper atmosphere. The resulting ionization and excitation of atmospheric constituents emit light of varying colour and complexity.

Components

• Hemispheric Power
  • A globally integrated total energy deposition measured in GigaWatts
  • $<20$GW: there may be little or no aurora observable.
  • $>20$GW: you may need to be near the aurora to see it.
  • $>50$GW: the aurora should be quite observable with lots of activity and motion across the sky.
  • $>100$GW: this is considered to be a very significant geomagnetic storm.
• Solar Wind Speed
  • The solar wind is charged particles that travel in a stream projected from the sun. These particles include mostly protons and electrons, flowing from the Sun.
  • $<400$km/s: Normal
  • $>700$km/s: High speed
  • $>900$km/s: Very high speed
  • Helps determine the strength of the aurora (speed at which particles slam into magnetosphere)
• Density of particles
  • Solar wind density is the number of particles being carried within the solar wind stream measured in p/cm3.
  • $<8$p/cm3: Low chance
  • $>20p$/cm3: Good
• Interplanetary Magnetic Field (IMF)
  • The Earth is surrounded and protected by an invisible magnetic field generated by molten iron and nickel swirling around in Earth’s core.
  • Bt (magnitude of the field in nanoTesla (nT))
    • $<50$nT: generally good
  • Bz (direction of the field in deg):
    • negative (meaning south) good for seeing aurora
    • When the north/south direction of the IMF flips south, the lines of the magnetic field connect to the Earth’s magnetosphere, which is facing north. This causes a rift to open that allows solar wind to spill into our magnetosphere.
    • The Bz is inversely proportional to the Bt since the minimum negative value a Bz can be is the current Bt value. For example, a Bt of 10 can only go as low as a Bz of -10. If the Bt is 2.5, then the lowest the Bz can go is -2.5.

Colors

Auroras result from emissions of photons in Earth’s upper atmosphere, above 80 km (50 mi) from ionized nitrogen atoms regaining an electron, and oxygen atoms and nitrogen based molecules returning from an excited state to ground state.

• Oxygen emissions: green or orange-red, depending on the amount of energy absorbed.
• Nitrogen emissions: blue, purple or red; blue and purple if the molecule regains an electron after it has been ionized, red if returning to ground state from an excited state

Oxygen is unusual in terms of its return to ground state: it can take 0.7 seconds to emit the 557.7 nm green light and up to two minutes for the red 630.0 nm emission.

Because the highest parts of the atmosphere contain a higher percentage of oxygen and lower particle densities, such collisions are rare enough to allow time for oxygen to emit red light. Collisions become more frequent progressing down into the atmosphere due to increasing density, so that red emissions do not have time to happen, and eventually, even green light emissions are prevented.