What caused the Wall of Fire?

In the movie Independence Day, when the Alien Ships begin to arrive they seem to be covered in fire as seen when a Recon Plane that was sent to inveterate passes through the clouds.

Now at first i thought it was because they entered the atmosphere however they seems to be covered in fire for far too long and when we first see one of the ships emerge (i think it was over New York) the ship seems to be flying horizontally rather than at an angle.

So i am wondering what was the cause of the Wall of Fire which engulfed the ships and way it hung around for so long.