6 Inch Plasma Ball
An alternating current, at relatively high voltages (Hey, this is USB remember, so it can't get too crazy), are pumped into the sphere via an electrode in the center. This energy causes electrons to be stripped from the gasses,leaving in its wake some positively charged ions and a gorgeous gaseous and electrically conductive environment (aka Plasma). That high voltage served up through the electrode will now arc up through the plasma to the lower voltage edge of the plastic sphere (starting at the top where the temperature is greater). The arcs glow because they are hotter than the surrounding plasma. You can touch the plastic and the arcs will strongly congregate to your touch because you will be creating an even lower voltage jump from the electrode. All this made possible by the evil genius of Nikola Tesla, oh, and some science.