My understanding for this is that electrons weigh a lot less than anything else so they have higher velocities (at the same temperature, because of thermodynamics). For matter to escape the earth requires a minimum velocity. So the result is that the earth loses more electrons than positive ions (or negative ions). The result is that the earth is positively charged and this is the simplest (intuitive) explanation for why the atmosphere has a voltage of around 120 volts per meter. See "sky voltage" at wikipedia:
http://en.wikipedia.org/wiki/Sky_voltage
I didn't find a link giving the polarity of the sky voltage, but from the above argument, I presume that the ground is the positive end while the sky is the negative.