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13 Nov 2018 15:36

Hey, has anyone on the forum ever saw and/or took a picture of the so-called STEVE (Strong Thermal Emission Velocity Enhancement) effect in the atmosphere at night? It is a little-known aurora-like phenomena that takes the shape of a purple ribbon in northern hemisphere skies that has, as of 2017, been officially discovered by Albertan Canadian astronomers.

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Contrary to what you might think, this is not  an aurora. No-one really knows WHAT it is. The most recent scientific opinion of what it might be is a sort of 'sky-glow', one not caused by artificial light-pollution, but rather some sort of natural plasma-based process.

Just curious to know if anyone here ever saw this before, during their astronomy nights.
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14 Nov 2018 00:16

It looks very similar to what I've seen described as a Proton Arc by the astronomers on APOD and SpaceWeather.  A proton arc is basically the same thing as aurora, but caused by the protons of the solar wind hitting the atmosphere, rather than electrons.

Edit:  Oh, I'm out of date!  The proton aurora is a real thing and well studied, but the STEVE phenomenon is unrelated and was often incorrectly called that.  That's very interesting.

Speaking of the color, I have seen and photographed purple aurora many times (for some reason they almost always appear purple here), but I am pretty sure it's the normal aurora and I have not seen it in such a tight ribbon.  Something to look out for, for sure, especially if these typically appear south of the main aurora band.
 
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14 Nov 2018 03:15

Watsisname wrote:
Source of the post Speaking of the color, I have seen and photographed purple aurora many times (for some reason they almost always appear purple here), but I am pretty sure it's the normal aurora and I have not seen it in such a tight ribbon.

Powerful aurora sometimes extends into a redlike colour at higher altitudes, and at your latitude this might often be the aurora you see because the it's quite far away.
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14 Nov 2018 05:00

midtskogen wrote:
Source of the post Powerful aurora sometimes extends into a redlike colour at higher altitudes, and at your latitude this might often be the aurora you see because the it's quite far away.

I can't think of anything that would turn a red spectral color into purple or violet over distance, though, except by adding something new.  I have also seen red aurora from a great distance (actually the only time I've seen it), and there were some separate regions of purple along with it.  More often the colors are green and purple, and sometimes purple is visible quite high above the horizon.  So I think it must have a different origin.

I decided to look into the physics of it.  The red is due to atomic oxygen.  As you say, it occurs at high altitude (200-400km), because the transition is quite slow (~100s lifetime), and at lower altitudes collisions between atoms are too frequent to allow the emission to happen.

Green (oxygen transition with ~0.7s lifetime) aurora reach down to around 100km before getting cutoff.  The short lifetime is also why the green curtains can shift around so quickly, while red tends to be slow and diffuse.  

Sometimes an intense pinkish-purplish aurora appears below the green curtain, when the electrons come in fast enough to penetrate deeper and excite even faster transitions in nitrogen.  I remember seeing this effect in one of your videos and it was very neat. :)

So why does purple appear higher up?  Apparently, it is because the auroral curtain is being illuminated by sunlight!  The ionized nitrogen has a strong tendency to scatter blue light.  This is very interesting to me, and I think I can make sense of what I observe now.  For example in this shot, the lower aurora is green, shifting to purple higher up, and then blue.  The blue top must be the pure scattering of sunlight from the ionized nitrogen, while the purple is a combination of the blue scattering and the red from the oxygen.  I guess in most cases what I'm seeing as purple is really blue and red together, from completely different causes.  And every time I've seen it, it has been either close to twilight or else in summer when twilight never ends.
 
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14 Nov 2018 20:12

Watsisname wrote:
Source of the post So why does purple appear higher up?  Apparently, it is because the auroral curtain is being illuminated by sunlight!

Yes, they thought that was the original cause for STEVE (what a majestic name!), but that was eventually ruled out as a prime cause. Anyway, I have never seen an aurora in my life - very funny since I live further north then Watsisname. I wish I could though. Where exactly did you see the aurora Wats?
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14 Nov 2018 20:31

Stellarator wrote:
Source of the post Anyway, I have never seen an aurora in my life - very funny since I live further north then Watsisname. I wish I could though. Where exactly did you see the aurora Wats?


I have only seen aurora from here in Washington State.  Most often I see it when up in the mountains where the skies are quite dark, but once we had a nice display after a solar storm that I could see from my backyard.  I walked down to the beach and made this timelapse of it.  It was pretty marvelous, even with the light pollution.

Image

 
If you are farther north than me (to be more precise it's your geomagnetic latitude that really matters), there surely must be times when the aurora is visible -- sufficiently dark skies permitting.  Even people much farther south can sometimes see it.  It just depends on the activity level.  You can keep track of what the sun and auroral activity are doing if you follow spaceweather.com, or the Space Weather Prediction Center.

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Stellarator wrote:
Source of the post Yes, they thought that was the original cause for STEVE (what a majestic name!)

LOL.  I hope they come up with a more fanciful name/acronym for it.  But usually once a name circulates around enough, it is stuck.

I like the names they gave for the various upper atmospheric lightnings, for instance. :)
 
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14 Nov 2018 22:14

Wow these are so beautiful!  I wish we could simulate things like proton storms, STEVE, sprites and elves in SE! I would love to see some purple shimmery lights and arcs of multicolored light lol.

Wouldn't mind seeing ball lightning either, which is another rare, not well understood phenomenon.  Little balls of electricity have been made in the lab but they only last a split second and no one knows how natural ball lightning lasts so long and how it can come through walls and roll down power lines and spin.
 
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15 Nov 2018 01:42

A-L-E-X wrote:
Source of the post  I wish we could simulate things like proton storms, STEVE, sprites and elves in SE!

Well, we're half-way there with auroras in SE, but it's not quite the same I'll agree.

Watsisname wrote:
Source of the post If you are farther north than me (to be more precise it's your geomagnetic latitude that really matters), there surely must be times when the aurora is visible -- sufficiently dark skies permitting.

Hmmm, I don't have too much technical knowledge on auroras, and no doubt this next bit will sound dumb, but does mountainous terrain obscure aurora visibility at all? I live right in the middle of the Kootenay Mountains, so peaks loom all around me, especially to the north.
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15 Nov 2018 04:39

Stellarator wrote:
Source of the post does mountainous terrain obscure aurora visibility at all? I live right in the middle of the Kootenay Mountains, so peaks loom all around me, especially to the north.

At your latitude the more common aurora might be obscured if you can't see below, say, 20 degrees from the horizon, but even then some occasional stronger displays should reach well above that.  I've once seen them reach zenith where I live.  An exceptional display may fill your whole sky.

Right now we are also near the minimum in the solar cycle, so good aurora shows are not as common.  But they can still happen.  Activity should be a lot more common closer to the next solar max in the mid 2020s.
 
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15 Nov 2018 22:57

Thats interesting I wonder how the low solar will impact weather.  We have been having very late and very early heavy snowfalls here and the city is paralyzed right now and the predictions are for a very cold and very snow winter.
 
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16 Nov 2018 00:18

Watsisname wrote:
Source of the post Right now we are also near the minimum in the solar cycle, so good aurora shows are not as common.  But they can still happen.

Absolutely.  The claim that aurora is not that common during solar minimum is somewhat latitude biased.  It may be true at low latitudes, but under the aurora oval things are pretty much as usual during the minimum.  The displays are common, though the big explosions might be rarer.

It's been often claimed that aurora completely disappeared during the Maunder minimum on the basis that a Norwegian poet who lived in northern Norway in the late 17th century and wrote a lot of poetry about life and nature in this part of Norway, never ever mentioned auroras.  But I think this conclusion is weakly founded.

A-L-E-X wrote:
Source of the post I wonder how the low solar will impact weather.  We have been having very late and very early heavy snowfalls here and the city is paralyzed right now and the predictions are for a very cold and very snow winter.

Whatever influence the solar activity has, it's likely dwarfed by other factors.  Perhaps there is a link between the Maunder minimum and the climate at the time, but it's debated and it lasted for a long time unlike regular minimums.

Over here October had its share of snow, but November so far has been warm and rainy, even pretty far up in the mountains.
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16 Nov 2018 00:25

Watsisname wrote:
Source of the post Activity should be a lot more common closer to the next solar max in the mid 2020s.

I'll keep my eyes to the skies then.
midtskogen wrote:
Source of the post Whatever influence the solar activity has, it's likely dwarfed by other factors.

Ha! Very funny, as during work today the subject of auroras came up. A co-worker said that because global-warming was elevating temperatures, we wouldn't get auroras anymore because it was too warm for them to form. One the one hand I congratulated him for his recognition of global warming. On the other hand I had to facepalm, then carefully explain that auroral displays have nothing to do with temperature, and that they don't form in the north and south poles because its 'cold' there :lol:.
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16 Nov 2018 00:48

I think there's an uncle Scrooge story in which Scrooge tells that the weather during his gold digging days was so bad that the aurora blew away.  More seriously, aurora is associated with cold weather, but that's because it's generally much colder during clear nights.

Solar activity is a bit of an unknown so far.  On one side some people argue that the recent warming is simply a recovery from the little ice age (popularly explained by low solar activity).  And if the warming seriously fails to follow climate models, don't be surprised if you hear people on the other side suggest that the expected warming has been masked by low solar activity.  Attribution is a messy business.
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16 Nov 2018 04:54

midtskogen wrote:
Source of the post More seriously, aurora is associated with cold weather, but that's because it's generally much colder during clear nights.

I guess that would be visibility of the aurora, since the aurora is still present even if it is cloudy. :)

As for climate, the link is fairly well understood, with lower sunspot number related to decreased solar irradiance which cools the planet.  There are some complicating factors that appear over longer timescales though, such as how it modifies the formation of clouds.  But in geologic history these do not appear to be very important, and the majority of climate history is well understood without invoking changes in the Sun.  (Unless you go to very long timescales -- millions to billions of years -- over which the Sun's luminosity has been increasing, but the rock-weathering cycle acts as a natural thermostat to balance it.)

The Little Ice Age is interesting.  It was caused by a combination of lower solar insolation and a period of volcanic eruptions, both of which are negative forcings (prolonged low sunspot activity correlates with decreased flux, and volcanic eruptions produce sulfur aerosols that block sunlight).  So in a sense the last few centuries of warming has been in part a recovery from the LIA, but humans are adding an additional greenhouse forcing on top of that.  The magnitude of that greenhouse forcing is a lot larger, especially going into the 21st century.

midtskogen wrote:
Source of the post And if the warming seriously fails to follow climate models, don't be surprised if you hear people on the other side suggest that the expected warming has been masked by low solar activity.  Attribution is a messy business.

Solar activity is measurable, so if that happens they must show that it is consistent with the observed change in forcings.  Historically, high vs. low solar activity can modify climate by a few tenths of a degree.  But the magnitude of projected warming (what CO2 concentration curve we choose to follow) matters more by about an order of magnitude.  If greenhouse gas concentrations go up a lot and the temperature doesn't, then it won't be possible to blame the Sun unless the Sun does something very observably crazy.  

More likely people will point to internal climate variability (especially, what was the ocean doing?), but even then they must show it is consistent with observations of what those systems have been doing.  Several degrees of temperature, globally averaged over the surface, is a lot of energy, and it is hard to hide that much energy without leaving clues.
 
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16 Nov 2018 05:36

Hmm.  I wonder now if a change in global temperature could actually have an effect on aurora. 

Yes, seriously.  As we saw a little bit ago, the altitude of the different aurora colors is determined by the average time between collisions of the excited atoms or molecules.  If the collisions happen too frequently, then colors associated with slower transitions won't have a chance to be emitted.

The greenhouse effect increases the temperature of the Earth's surface and lower atmosphere, but also decreases the temperature of the upper atmosphere.  This is fairly un-intuitive, and I describe the physics behind it briefly here on the old forum, under the greenhouse effect tag.  If the upper atmosphere cools, molecules at a given altitude will move more slowly, and also the pressure height will decrease.  The average time between collisions at a given altitude will increase.  Basically, the whole upper atmosphere including the thermosphere where aurora occur, collapses down slightly.  (This also poses a problem for space debris -- in a warmer world they take longer to de-orbit.)  Another phenomenon that could affect them is Sudden Stratospheric Warming (no relation to global warming), which does have a documented effect on the whole atmosphere including ion densities in the thermosphere.

So perhaps global warming will not scare the aurora away, but instead make them come down a little closer to us.  I haven't seen a study on it, but it would be interesting if this is correct and measurable.

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