According to this video, the sylphs work in concert with the scout ships - the smaller ships that enter the atmosphere to collect data at various levels and activities. He doesn't refer to them as scout ships here, but that's what they are. It's pretty common.
So, a big mystery has been solved. I can look to the sky now and know that the sylph-angels are playing a big part in diminishing the ill effects of air pollution and negative weather patterns. Another indication of how the higher beings selflessly serve Gaia and humanity. ~Sharda~