What is the polarity of the Schumann resonances?

Hi. The resonances are initiated by lightning strikes which cause an electromagnetic wave to travel omni-directionally and get trapped by the ionosphere. Since there are also other sources of waves, in particular the 60cps North American power grid and the 50cps European power grid, you would need to isolate a particular lightning impulse to determine its characteristics. I do not know any details of this being different from amateur radio waves, and these wave can be modified by antenna type and orientation. In short, I do not know.