I am currently designing an alien world and I had the idea that the planet would have constant auroras nearly everywhere due to its host star. At first, I just had some ideas on how that might affect the creatures to evolve and all that but then I had the thought.
Would plants be able to use the light from the Aurora to photosynthesize?
The Aussie space weather forecasters have a page about auroras. One line of interest:
Bright as the full moon. Casts shadows. Very rare.
Now, there are two problems here.
Firstly, the moon is 400000 times less bright than the sun. From this nice answer by Willk to a question about nocturnal photosynthesis, you can see that moonlight at its peak can provide enough light for some photosynthetic bacteria, but not much more than that.
Oho, you might say. Maybe your auroras are much brighter! Well, maybe they are, but that brings us on to the second problem.
We only get auroras that bright on Earth when there's a big solar flare pointed directly at us. This is thankfully a rare event. If it were commonplace, it would simply blow our atmosphere away and we'd be left with nothing, and you'd have bigger problems than photosynthesis. Moreover, if it were commonplace the star must be losing mass at a surprising rate... not something necessarily sustainable for long enough to for a nice planetary system with biospheres.
So:
Would plants be able to use the light from the Aurora to photosynthesize?
Briefly.
Rule of cool being what it is, you should of course feel free to handwave this in to your setting because it is awesome, not because it is realistic. If you want to make scifi rather than fantasy though, maybe have a rethink.
This planet has very powerful aurora borealis.
Until now, the brightest known auroras came from Jupiter, which has the most powerful magnetic field in the solar system. In comparison, these newfound auroras are more than 10,000 times — and maybe 100,000 times — brighter than Jupiter's, Hallinan said. This is because LSR J1835+3259 has a magnetic field perhaps 200 times stronger than Jupiter's, he said.
If you had a monstrously powerful magnetic field on the planet then yes, the aurora borealis would be monstrously strong.
(side note: Auroras are mostly green (557.7 nm wavelength) from oxygen reactions, so chlorophyll might not be the best choice for photosynthesis there, as it reflects green rather than absorbing it. Make sure you understand the mechanisms and wavelengths of your auroras, then make your plants an appropriate color to absorb those wavelengths.)
As has already been suggested by other answers, there are two main variables that govern the brightness of an aurora:
Ramping both up will give you a brighter aurora. But to be worthwhile, you need the light from the aurorae to be significant compared to the sun. That's a hard challenge, but not impossible.
There are some things you can do to the upper atmosphere to make it brighter. Earth's atmosphere's 78% Nitrogen, 21% oxygen, but the majority of the light from the auroras is the green from the oxygen. So increasing the oxygen a bit might help with brightness.
You could also have the atmosphere filled somewhat with dust that could filter out visible wavelengths from the sun while also causing brighter auroras. This is a bit handwavey, though.
Perhaps the most powerful option is a big cloud of dust. This has the advantage of obscuring much of the direct light from the sun, while at the same time allowing for far more charged particles, a more powerful "wind" of electrons for the planet to pass through, and hence a bigger auroral effect, against a dimmer star.
This wouldn't work so well if it's part of the initial cloud that the planets form from: those clear up pretty fast, and to even be classed as a planet, the planet will have needed to clear its path through the dust.
Another option is to bury the whole solar system in an electrically charged dust cloud/nebula - perhaps one that the sun has been passing through or colliding with for billions of years. Nebulae are very vacuous, nothing like you see in movies. No billowing clouds, no lightning. But 10,000,000,000 particles per cubic meter, compared to 1 particle for interstellar space, is still not nothing. I'm not sure it would have a significant dimming effect on a sun, but that, plus a change to solar type and size, plus an increase in distance from the planet, might all make for a dimmer sun.
An emissive nebula might give you a faint background glow (a nice sort of secondary/background effect to the aurora), as well as faintly reflecting the glow of the sun all the time, even in the night-time. Something to do more serious math on before relying on, perhaps, or you could just handwave it, since impossible pea-soup nebulas are a common enough sci-fi trope.
The nebula/dust and aurora might affect the distance for "goldilocks zone", too - giving more incident light to the planet, so raising the temperature. The particles in a nebular are bigger (atoms and dust particles) than the plasma of the solar wind, so it might also cause a brighter aurora. It might also cause some atmospheric warming, but if it's hard sci-fi, then do the math on that before relying on it.
Since it is an alien world, the plants are probably different from the plants on Earth. Maybe the entire eco-system is radically different - possibly silicon-based instead of carbon-based. In those situations, it might be possible to use the dim light for photo-synthesis. Also, you might want to "design" the plants to be really small and to be really leafy - to absorb as much light as possible. So I assume, no cactuses or pine-trees.
Alternatively, the particles (electrons, protons...) on that planet are different than the particles on Earth - therefore physics and chemistry as we know them do not work any more.
