Yes, it is possible to ionize air using microwaves, even at atmospheric pressure. In the paper titled "Jet propulsion by microwave air plasma in the atmosphere," published by the team at Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, they describe a method to create a plasma jet in the atmosphere using microwaves.
The process involves using intense microwave radiation to ionize the air molecules, creating a plasma state. This plasma, consisting of charged particles, is then expelled as a plasma jet, which can be used for jet propulsion. By manipulating the properties of the plasma, the researchers were able to generate a directional force that propels the plasma forward, leading to a jet-like effect.
The paper does not mention the specific pressure at which the experiment was conducted, but it is safe to assume that the experiment was performed at atmospheric pressure, as the goal was to create a plasma jet in the atmosphere.
Creating and controlling plasma jets using microwaves has potential applications in various fields, including jet propulsion and aerospace engineering. However, it's essential to note that the technology is still in its early stages of development, and further research is needed to explore its practical applications and potential limitations.