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For years, humanity and nephilim have battled (more or less) together to staunch the unending flow of demons and angels wrecking havoc on Earth in their unending wars.

Now, we are finally in position to strike back. By creating the Collegium, we shared our knowledge in arcana and xenobiology and feel confident in our capacity to repel those extraplanar warmongers.

Our Great Marshal tasked (among other things) the Collegium to design a spell to let our allies over the world know when to launch the attack. Through inner politics and games of influence, the task was finally assigned to my team of arcanists.

The thing is, I would have preferred if the GM had a little less flair for the dramatic, as he described the signal as "the day the sky will burn". The main reason, aside the psychological effect, is that the angels monitor Earth from the sky, and impeding their intel could prove crucial in our effort. We are free to come up with any solutions we want, but the prerequisites are:

  • It has to be visible from far away. If we can not cover the Earth in a single go, we can arrange for other arcanists to relay the signal by casting the same spell. Around 10 teams of arcanist, ourselves included, could participate in that effort (the fewer, the better).
  • For tactical reasons, this attack will be launched during the night from where the order is given. Admittedly, the light pollution on the sunny side will probably deteriorate the visual signal, but we need to achieve maximum coverage and will make do with sunny areas not seeing the signal.
  • The sky does not have to be "ablaze". We concluded that putting the sky on fire could prove harmful to our own survival. Any kind of visual effect, like auroras, that can fill the bill are allowed (color is irrelevant).
  • My team and I are quite competent, and with enough preparation, could achieve the same energy output as the famed blast of Hiroshima (around 15kT). But the less energy we spend, the better we will fare for the battle that will follow. Our effective range is about 1000km, as long as we can achieve line of sight (no physical obstacle between the emission point and the target). 1
  • The effect has to last as long as possible, as it will hide Earth's surface to angels eyes, but we need first and foremost to send a signal. Any duration our sentry around the world will be able to reliably perceive will be good. A few dozens of seconds could go a long way in striking the enemy before they can coordinate.

This is why I gathered you, our specialists, to browse the archives. We need you to look through our tomes about physics and chemistry, as they were passed to us by our ancestors of the 21st millennium. The solution requiring the fewer arcanists and power spent shall be favoured. If it also could be as harmless as possible to Earth, it would be preferable. (Certainly nothing that would kill us all shortly afterwards).

Order of importance is: number of arcanists team > power > harmful to Earth's habitability by humans > duration of the effect.

1: Any kind of reaction we know how to theoretically achieve today, in the energy limit stated above, can be used.
PS: Don't hesitate to ask for details about limitations of magic, as my answer will be highly dependent of what you need to know.

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    $\begingroup$ How long does the sky have to burn; a few seconds, a few minutes, a few hours? Can your spell operate outside the Earth's atmosphere? Is the Earth flat or does it follow 21st century physics? $\endgroup$ – chasly from UK Mar 1 at 12:41
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    $\begingroup$ The longer the better, has we will obfuscate earth to angels eyes. The spell can create any physical effect within the distance range of 1000km and the limited energy input of 15kT, given the right environmental conditions (so if you wanted for exemple to trigger a fusion reaction and it fits those parameters, and you manage to do it outside of athmosphere, you could). $\endgroup$ – Nyakouai Mar 1 at 12:46
  • $\begingroup$ Please can you provide a simple diagram showing the relative positions of the Sun, the Earth, the burning part of the sky, etc. Thanks. $\endgroup$ – chasly from UK Mar 1 at 13:01
  • $\begingroup$ Does it look like this i.stack.imgur.com/WtEBy.png ? $\endgroup$ – chasly from UK Mar 1 at 13:06
  • $\begingroup$ I'll try to add one as soon as I can, but yes, this is what it would look like. If more coverage on the sunny side can be achieved, then great $\endgroup$ – Nyakouai Mar 1 at 13:13
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Since you said auroras are acceptable, I'd go with that. An aurora is caused by charged particles which originate from the sun. The Earth's magnetic field deflects those particles, protecting us from them. A lot of them get concentrated in the van-Allen belt out in space. However near the poles, the magnetic field lines enter the atmosphere (and finally the Earth), which means at that point, the charged particles are actually directed into the atmosphere. There they collide with air molecules and cause the auroras.

Now my suggestion would be to temporarily modify the Earth's magnetic field so that the van-Allen belt is lowered into the atmosphere (but still high enough to not cause serious radiation problems to humans living on Earth's surface), causing a global aurora. Since the Earth's magnetic field is rather weak, this should not require too much energy; note that the energy of the actual glow is provided by the sun.

Given that this is a temporary change, there also shouldn't be significant damage of the atmosphere; the extra amount of atmosphere blown away by the solar wind should be rather small. The only downside is if you have satellites in low Earth orbit; given that they are most likely not designed for passing through the van-Allen belt (well, technically, the van-Allen belt will pass through them), they will likely go toast.

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Perhaps releasing large amounts of charged particles into the atmosphere via statically charged dust. If you could also form a large amount of clouds, you would get lightning over selective areas. Since magic exists in your world, I'd imagine creating large amounts of steam shouldn't be difficult (in different positions of course, by evaporating water from lakes, rivers, seas, oceans). Clouds are a natural occurrence, so it wouldn't be too obvious to the eyes of an observer, either on the ground or outside the planet.

Cumulonimbus clouds can be seen from as far as 400km away, so you would only need to cover a total of 1% of the Earth (off the top of my head, didn't actually calculate) for every person on Earth to be able to see a cloud somewhere along the horizon. Assuming you only want to cover land activities, and you want complete coverage, just 30% would be enough, as the rest would be open ocean with nothing going on there.

So you would have a few days to gather enough clouds, and once you feel the coverage is enough, release the dust. Thunder and lightning appears across the globe in unnatural amounts, and it could last anywhere from a few hours to a few days, depending on the size of the clouds and the amount of charged particles within them.

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If it doesn't have to be the entire sky then we go back to things that do burn in the sky, which is meteors. We (today, 2019) know which size meteor is needed to it burns up entirely in the atmosphere so we don't hit some cows or farmhouses below.

You surely have either "meteor storm" or some gateway, teleportation or other spell in your arsenal.

Open up either one huge portal or a large number of portals and throw lots of rocks of the right size into it. That can easily be accomplished by collecting the rocks in one place and then creating the portal right under them. Spread it over a large area. The best result would be if your exit portal could be larger than the entrance portal so that distances between the rocks are enlarged.

For example, you collect a 100x100m area and fill it with rocks of the right size. Distance between rocks: 1-10 cm. Now you open your portal into the sky, but that portal is 100x100 km large (1000 times as large). Distance between rocks: 10-100 m. Looking at it from the ground, it will still be a hell of a burn (the visual fireball of each rock will be much larger than the rock itself).

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The idea of lightning is a genius one. After reading a few articles on this matter I found a few interesting things. Nitrogen (78% of the atmosphere) is not combustible on its own. so i looked into what would cause it to combust. I found that massive amounts of electricity can bind nitrogen and oxygen, creating a combustible gas. however, I read further into your question and you mentioned a fear for its affect on your own survival and I realized that, indeed, combusting the nitrogen and oxygen that makes our breathable air on such a large scale would be devastating and nearly irreversable, if at all. so i looked into a slightly less devastating method that can be reversed afterward:

You could release balloons of helium carrying hydrocarbons, a naturally occuring combustible that can be in gas form. The hydrocarbon containers (whatever you choose to use for this) are magically enchanted to open at a certain altitude with an electric zap. the desired altitude? well, the ozone layer. ozone is highly combustible but as an oxidizer needs to have a fuel source in order to combust (https://en.wikipedia.org/wiki/Fire_triangle#/media/File:Fire_triangle.svg). In this case, the fuel is hydrocarbon, the heat is the sun/the spark, and ozone is our oxygen. this will start the chain reaction of the atmosphere combusting. granted the people of earth would die without ozone but you could replace it with some sort of rune magic that protects the world from ultraviolet until you manually release ozone back into the atmosphere.

https://www.google.com/search?rlz=1C1CHBF_enUS826US826&ei=PZd5XNm5F4j4-gTe45PYBQ&q=o3+combustion&oq=o3+combust&gs_l=psy-ab.3.0.0i22i30.1276.8048..9541...3.0..0.93.765.13....2..0....1..gws-wiz.......35i39j0i67j0i131j0j0i131i67j0i131i20i263j0i20i263j38j0i30.MXLRHMM9exQ&safe=active&ssui=on

https://delphian.com/chc.htm

https://sciencing.com/nitrogen-combustible-5397514.html

https://en.wikipedia.org/wiki/Hydrocarbon

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