I have a more or less primitive (They can not talk to other races nor was any encoding or morsing or smoke signals invented) alien race the MLPARs (10000 individuals), which was parted in half and one half was deported to an earthlike (5000 ind.) planet and the others were kept on their home planet (5000 ind.) by a further developed alien race the FDARs for an experiment.
The FDARs were eliminated by decadence which caused climatic changes and civil wars on their home planets and their experiment got forgotten.
Once a year the MLPARs still on the earthlike planet find together and try to contact any spectant to bring them home to their loved ones. (The MLPARs have a very high live expectancy and the females have their fertile days very rarely so the ones who where deported are still living except 10 or 20 and 10 or 20 have been born on their new "home")
To contact all the space traffic, they form a huge aurora which would be visible from space and different from the natural auroras, in hope that anyone realizes it and comes to bring them back home.
Now my question is, is it possible for a live form, to have evolved to be able to create auroras in a group of around 5000 individuals?
They were able to do that before they were parted but it was not used, because there was no need to and it is very exhausting as it needs a huge amount of energy.
I search for a way without technologies where the aurora is made by the organisms, but here some inspiration:
As I'm not a physicist I can't evaluate how truthful these links are.