How One Scientist Decoded the Mysterious Sounds of the Northern Lights

For more than 15 years, a lone scientist in southern Finland has spent countless winter nights among the snowy fields and frozen lakes around his village, in pursuit of one of the most ephemeral mysteries of the heavens: the faint, almost phantasmagorical sounds heard during intense displays of the aurora borealis, or northern lights.

The epic study by acoustician Unto K. Laine includes the first audio recordings of the muffled crackling or popping sometimes heard overhead during spectacular aurora displays.

Laine has shown the sounds are real, and he thinks he has found what causes them: sparks of electricity discharging beneath the aurora in an inversion layer of the atmosphere that can form in clear and calm weather conditions.

Here's an explanation as to what causes the various colors of an Aurora.

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Auroras result from emissions of photons in the 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.[33] They are ionized or excited by the collision of particles precipitated into the atmosphere. Both incoming electrons and protons may be involved. Excitation energy is lost within the atmosphere by the emission of a photon, or by collision with another atom or molecule:

oxygen emissions
green or orange-red, depending on the amount of energy absorbed.
nitrogen emissions
blue or red; blue if the atom 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 three quarters of a second to emit green light and up to two minutes to emit red. Collisions with other atoms or molecules absorb the excitation energy and prevent emission. Because the highest atmosphere has a higher percentage of oxygen and is sparsely distributed such collisions are rare enough to allow time for oxygen to emit red.

Collisions become more frequent progressing down into the atmosphere, so that red emissions do not have time to happen, and eventually even green light emissions are prevented.

This is why there is a color differential with altitude; at high altitudes oxygen red dominates, then oxygen green and nitrogen blue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything. Green is the most common color.

Then comes pink, a mixture of light green and red, followed by pure red, then yellow (a mixture of red and green), and finally, pure blue.

Auroras and the ionosphere

Bright auroras are generally associated with Birkeland currents (Schield et al., 1969;[34] Zmuda and Armstrong, 1973[35]), which flow down into the ionosphere on one side of the pole and out on the other. In between, some of the current connects directly through the ionospheric E layer (125 km); the rest (”region 2”) detours, leaving again through field lines closer to the equator and closing through the “partial ring current” carried by magnetically trapped plasma. The ionosphere is an ohmic conductor, so some consider that such currents require a driving voltage, which an, as yet unspecified, dynamo mechanism can supply. Electric field probes in orbit above the polar cap suggest voltages of the order of 40,000 volts, rising up to more than 200,000 volts during intense magnetic storms. In another interpretation the currents are the direct result of electron acceleration into the atmosphere by wave/particle interactions.

Ionospheric resistance has a complex nature, and leads to a secondary Hall current flow. By a strange twist of physics, the magnetic disturbance on the ground due to the main current almost cancels out, so most of the observed effect of auroras is due to a secondary current, the auroral electrojet. An auroral electrojet index (measured in nanotesla) is regularly derived from ground data and serves as a general measure of auroral activity. Kristian Birkeland[36] deduced that the currents flowed in the east-west directions along the auroral arc, and such currents, flowing from the dayside toward (approximately) midnight were later named “auroral electrojets” (see also Birkeland currents).

https://en.wikipedia.org/wiki/Aurora#Causes_of_auroras

Believe it or not, the first and only time I've ever seen them was in New York's Central Park. I had been an amateur astronomy for over 10 years previously and heard and read a day or so before that they might be visible as a result of an especially energetic flare (or CME?) from as far down as the New York City area. So I went out to try from a relatively dark area of CP, the path that surrounds the Great Lawn baseball fields, an area where we, I and a few friends, at the time, regulatory held free astronomy viewing sessions for the public.

And there it was, a faint shimmering red curtain of light! You could even make out Earth's magnetic field lines! From Manhattan!! Couldn't detect any other colors, though. Still, I will never forget it.

The photo below is the closest thing I could find to what I saw...

As I was looking at this and others photos of the aurora, I just now realized that the red light seems to always appear at the top. This may explain why I only saw red light from the comparatively low latitude of NYC. The green and other light was probably below the (northern) horizon.

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