Conditions between DSS altitude and Orbit

I'm curious what sort of conditions are present in the operating altitudes of the ATO. First of all, how much difference is there in gravitational force between those altitudes and the ground? I could try to figure it out myself, but I don't know the altitudes and how to take the mass of the atmosphere into account (does it even matter?). I assume that the gravity is still pretty substantial, but I'm curious just the same.

Second, let's say that that the ATO is at maximum altitude, that is, it is floating at the top of the atmosphere where it is neutrally bouyant. In order to reach orbit, it needs to fire it's ion thrusters. Would you fire them horizontally, to gain orbital velocity, or vertically, to lift above the atmosphere before gaining horizontal velocity? Or would it be a combination? That seems more likely, thrusting vertically at first and gradually moving to horizontal thrust. And a nosy question about thrusters that need not be answered: would you use ambient gasses, or need to bring something exotic up, like xenon? Am I right in suspecting that the ATO would be neutrally bouyant in the exosphere, where hydrogen predominates? I suppose this could have implications for Ekkehard's question about ATO to Mars.

One more thing and I'm done. Is insolation (the wattage of sunlight per square meter) greater at DSS altitude because there is less atmosphere above you there? If so, by how much is the difference?

Thanks in advance for any response,

Luke

Edit: Maybe I should just build a couple of Pongsats and find out!!

Gravity at 140,000 feet is about 99% as strong as on the ground. You need to be over 3,000 miles high before it gets down to half.

Thank you for that, Campbelp! I should have just gone and done the calculation, it would have told me that number readily. Oh well, I am lazy, what can I say.

I suppose the solar energy is somewhat greater at those altitudes, but probably not a significant amount (say less than 10% greater). But I'm sure anything extra for an airship would help, having ten percent less solar panels means that much more cargo capacity.