The Aurora Borealias (northern lights) is putting on a show tonight, that will be better than the one in May this year. Geo-magnetic index Kp is around 9 right now, equiv to a G4 (strong) event. Aurora sightings have been reported as far south as Tennessee.
Social media is buzzing with images taken by amateurs and professionals alike.
Auroras and CMEs
Coronal Mass Ejections (CMEs) are large expulsions of plasma and magnetic field from the Sun’s corona. They can eject billions of tons of coronal material and carry an embedded magnetic field. CMEs travel outward from the Sun at speeds ranging from slower than 250 km/s to as fast as near 3000 km/s. www.swpc.noaa.gov/...
This space storm is a result of an X1.8 solar flare and a fast (1200 to 1300 km/s) coronal mass ejection (CME) that erupted from the Sun on the evening of 8 October, 2024.
The X1.8 flare was observed on Oct 9 at 0156 UTC from Region 3848. Active Region 3848 is currently just above the Sun's equator and slightly to the east as shown in the Solar Synoptic Map below.
The SOHO LASCO Coronagraph image below shows the solar flare and CME on Oct 9. The white streak to the right is Comet C/2023 A3 (Tsuchinshan–ATLAS)!
Earlier this month, the Sun emitted X7.1 and X9.0 solar flares and CMEs on Oct 1 and 3 resp. The X9.0 flare was the strongest of this 11-year solar cycle. However, they did not create such strong geo-mag storms and auroras.
The colors of the Aurora
Auroras are caused by emissions from O atoms and N2 molecules. High energy electrons in the CME cause the energy states of O and N2 to rise (aka excitation) which result in photon emissions at specific wavelengths (colors) as the O and N2 energy falls back to its ground state.
- O atoms below 200 km emit in the familiar green color.
- O atoms above 200 km emit in red (Why? See below).
- N2 near 200 km emits in blue.
- N2 below 100 km emits in reds and blues that mix to produce pink.
Intense solar storms cause lower altitude N2 emissions.
How about the red colors often seen high in the sky?
Atomic O is unusual in terms of its return to ground state after excitation: it takes ~0.7 seconds to emit green light and up to 2 minutes for red.
- At low altitudes, collisions with other atoms or molecules absorb the excitation energy and prevent emissions.
- At 100-200 km, O density is sufficiently low to allow green emission.
- Above 200 km, O density is very low which allows time to emit red light.
For more info about aurora colors, see en.wikipedia.org/... and www.nps.gov/....
Epilogue
So, let’s step out and take some pictures. Even if the aurora is not visible to the naked eye, phone cameras are quite good at capturing them and their colors. Try it.
