*Okay while approaching Earth, aliens have bombarded three of our largest cities into smoking craters. Obviously they mean us harm. So when their large motherships start to enter our atmosphere our leaders make a desperate choice. They fire the majority of Earth's nuclear arsenal at those ships.* 

Now my question is, what would happen to the planet. High atmosphere detonations (500-600km) don't produce enough (dust) particles to create a nuclear winter. Fallout will be widely dispersed but much lower because it doesn't bind with said particles I think? Further more the ionized radiation will react with the magnetic field of the planet. Creating a radiation belt. 

What I've found difficult is estimating the strength of the radiation belt, how long the radiation particles remain trapped in the magnetic field and the amount of fallout. Will the fallout be strong enough to kill most life on Earth?

**So with the detonation of the majority of Earth's nuclear arsenal at high altitude, how would that effect the Earth and life on it?**

Obviously pretty much the entire planet will be enveloped by EMPs. On that I can find data, it's the rest I struggle with.

Some additional data that might help. There 9 motherships landing. They're en route to:
 - Houston
 - Orlando
 - Beijing
 - Mumbai
 - Moscow
 - Johannesburg
 - São Paulo
 - Canberra
 - Frankfurt