In the video included in this article [https://arstechnica.com/science/2018/11/small-drone-soars-on-an-ionic-wind-with-no-moving-parts/ https://arstechnica.com/

In the video included in this article [https://arstechnica.com/science/2018/11/small-drone-soars-on-an-ionic-wind-with-no-moving-parts/ https://arstechnica.com/science/2018/11/small-drone-soars-on...], the subject discusses that ion thrusters are less effective at higher altitudes as the air is rarified.