UPDATE 4-6-4
Solar Balloons?

Quite a number of people believe these images to be nothing more than "solar balloons" of the type shown in the photos below. While it is possible that the 'mysterious' UFO is some kind of solar balloon, it doesn't appear to be any of the commercially available ones we've been able to find. They are all extremely fat, not thin like the images we're dealing with -- or, they are both fat and very long, to the point that when airborne they curve considerably. Also, such a balloon would very likely catch the eye and probably be in the field of view for some time, whereas in every case of these photos, either the object was never noticed by the photographer or other witnesses at all. Solar balloons need to be fairly thick or fat to really get any lift and altitutde from the super-heated air trapped inside. All the commercial solar baloons we've located have very broad ends, nothing like the images above, which taper down to being very thin. Such a balloon having a large surface area would need to trap enough heat to balance the heat loss to the surroundings. That would mean that the objects in question are extremely huge solar balloons, perhaps 40 or 100 feet long or more, to account for their extremely thin appearance, and it would also put them at a good distance from the photographers, respectively. Again, such a thing would easily have been spotted by the photographer or other onlookers. But that is not what has happened in these cases.

Note the broad ends on this extremely long solar balloon. The shape is
uniform (unlike the UFO images) and the ends, even if some slack were left
and were tied off, would not look like the UFO images at all, but would abruptly
drop off at the end, with the slack ends hanging off.