I bought a solar powered light from Clippersharp for my yard. It wasn't cheap, over £200 but it's been up for at least 3 years and is still good as new. If you look at their website they do deals on multiples and tell you how many you might need, size of arena, expected use; jumping needs more light than flatwork etc.
Before investing in lights make sure the arena doesn't have a planning condition banning floodlights. Many do.
I used to stable at a place that had an arena with the ban and I used to watch for moonlight and also knew that my horse can see better in the dark than I could.
He used to spook at shadows under not very good lights in the dark but never did when ridden under moonlight - horses are strange beasts at times!
Car headlights if you can park in front of the arena and it’s safe to do this. I used to use my neighbours’ security lights to ride by in my old field.
Always just ridden in the dark. Even more so if have a nice arena service rather than just a field ?
There’s definitely more good portable lighting options around now though
I bought some builders lights for my stables with no power a few years ago. They weren’t very expensive & go on a tripod (it wasn’t massively tall maybe wither height) and are rechargeable, they still hold charge well. Maybe 3 spaced along each of the long sides would work. They were in jewsons or screwfix I think though lots of places online stock similar… wouldn’t be anywhere near floodlights but pretty cheap and cheerful!
I just ride in the dark. Unless the lights are above your eyeline i found they were just blinding. Every horse has been 100% fine. On brighter nights we have white poles so its good enough to do polework too. I just dont jumo.
I often ride without the arena lights because they are blinding. We have a good arena surface, so on a clear, moonlit night, I often ride in the dark. Horses have good night vision.