APOD: Flowing Auroras Over Norway (2013 Jun 09)

[BDanielMayfield]

... And also how bright can auroras get?

This is a harder question, but I believe I have an answer you can relate to.

Like other natural phenomenon, auroral brightness does have a standard, and it has a logarithmic scale. It is the International Brightness Coefficient (IBC) standard. (You might find S&T article interesting: An Aurora Watcher's Guide where brightness and other characteristics are discussed)
The levels range from 0 - 4, on a logarithmic scale:
0 ≡ subvisual, detected only with instruments,
1 ≡ comparable to the Milky Way; whitish with no discernible color,
2 ≡ comparable to moonlit cirrus clouds; color is barely identifiable (usually yellow-green),
3 ≡ like brightly lit cirrus or moonlit cumulus clouds; colors are evident,
4 ≡ much brighter than three; may cast discernible shadows; good colors, many of them fleeting

The unit of auroral brightness is the Rayleigh intensity unit. This allows a quantitative assessment of sky brightness, and referencing this paper: Auroral contribution to sky brightness for optical astronomy on the Antarctic Plateau, a calculation of the brightness for an IBC 4 aurora using a V-Band (Green) filter, you get ≈+14 Magntudes/arcsec² sky brightness.

So, what does this mean?
The equivalent brightness of the full moon ≈ 3.5 Magntudes/arcsec², so the full moon brightness per unit area ≈10.5 magnitudes brighter than the brightest aurora! Asked another way, how big would a uniformly bright, IBC 4 aurora need to be such that its integrated brightness equals a full moon (mag = -12.5)? If you were directly under a uniformly bright, circular IBC 4 aurora with a diameter about 65°, you would experience a full-moon lighting condition!

By the way, this same aurora would not be visible with the naked-eye during the daytime. As a thought experiment, consider increasing the moons diameter by 130x without changing the integrated brightness - it would not be visible with the bare eye.

Thanks alter-ego. And nice skill too; locating a known star and measuring the elapsed time from its rotation angle.

Thank you! I love solving problems by using old tools in new ways

Thanks for that very complete answer alter-ego. You've helped myself and many others I'm sure.

[neufer]

sounds like whales ! Wonder if they can hear those sounds ?

Well... keep in mind that the videos Art posted are not demonstrating the sound of the auroras. The sounds in those videos are just the audio output of radio receivers. Auroras can produce actual sounds, and those don't sound like whales, but more like sizzling bacon, and occasionally pings or pops. A Finnish study last year produced audio recordings (and at least in their case, the evidence suggests the sounds were produced a hundred meters or so above the ground).

If the aurora radio waves are heard accidentally in the headphones of telegraph operators or on purpose with a barbed wire antenna and phonograph amplifier how do we know that the Finns simply aren't picking up the same radio signal in their "audio" recordings?

And if the Finns are picking up real sounds (and/or actually hearing "sizzling bacon, and occasionally pings or pops") how do we know that these have anything whatever to do with auroras?

Ice whispering with Sennheiser in Siberia
4 March 2013

<<It all began with a documentary on the TV arts channel 'arte' about the world's coldest inhabited place, Oymyakon in the far east of Russia. The programme also mentioned 'ice whispering', a phenomenon that occurs when people breathe and speak in extremely low temperatures. The breath immediately freezes, generating a crackling noise that follows the speaker like a shadow. On his acoustic hunt to record the rare phenomenon of ice whispering - which the natives call "the whisper of the stars" - Juergen Staack took along an MKH 8060 RF condenser microphone from Sennheiser as part of his equipment.>>

[Chris Peterson]

If the aurora radio waves are heard accidentally in the headphones of telegraph operators or on purpose with a barbed wire antenna and phonograph amplifier how do we know that the Finns simply aren't picking up the same radio signal in their "audio" recordings?

Because they picked up the sound using a microphone array, and used the phase shift between the different channels to triangulate the source. That would only work if the source was transmitted at the speed of sound, not the speed of light.

And if the Finns are picking up real sounds (and/or actually hearing "sizzling bacon, and occasionally pings or pops") how do we know that these have anything whatever to do with auroras?

The sounds were in sync with auroral activity and originated several hundred feet overhead. Seems like a reasonable hypothesis.

[neufer]

if the Finns are picking up real sounds (and/or actually hearing "sizzling bacon, and occasionally pings or pops") how do we know that these have anything whatever to do with auroras?

The sounds were in sync with auroral activity and originated several hundred feet overhead. Seems like a reasonable hypothesis.

An hypothesis apparently lacking any suggested physical mechanism.

[Chris Peterson]

if the Finns are picking up real sounds (and/or actually hearing "sizzling bacon, and occasionally pings or pops") how do we know that these have anything whatever to do with auroras?

The sounds were in sync with auroral activity and originated several hundred feet overhead. Seems like a reasonable hypothesis.

An hypothesis apparently lacking any suggested physical mechanism.

That's not entirely true... there are well formed ideas about the mechanisms behind aurora-induced electrophonic sounds.

But in any case, science starts with observations. We know that auroras can produce sound from many, many witness reports. Now we have instrumental observations of that sound, along with some information about where it is formed (at least in some cases). From observations it is possible for researchers to start developing testable theories.

[neufer]

if the Finns are picking up real sounds (and/or actually hearing "sizzling bacon, and occasionally pings or pops") how do we know that these have anything whatever to do with auroras?

The sounds were in sync with auroral activity and originated several hundred feet overhead. Seems like a reasonable hypothesis.

An hypothesis apparently lacking any suggested physical mechanism.

[b][color=#0000FF]Huckleberry Finn in The Adventures of Huckleberry Finn (1939) [/color][/b]

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That's not entirely true... there are well formed ideas about the mechanisms behind aurora-induced electrophonic sounds.

But in any case, science starts with observations. We know that auroras can produce sound from many, many witness reports. Now we have instrumental observations of that sound, along with some information about where it is formed (at least in some cases). From observations it is possible for researchers to start developing testable theories.

"Chris Peterson told the truth, mainly. There was things which he stretched, but mainly he told the truth."

"The stars were shining, and the leaves rustled in the woods ever so mournful; and I heard an owl, away off, who-whooing about somebody that was dead, and a whippowill and a dog crying about somebody that was going to die."