These rarefied upper reaches are more electrically conductive than those below, enabling the electrons charged and accelerated by the solar wind to travel from 100,000 km out in space to as close as 100 km from the Earth's surface. At this point the atmosphere becomes suddenly denser, causing the electrons to lose energy upon impact. This energy illuminates the atmosphere as the aurora. We can conduct an experiment to see for ourselves how electricity flows more freely in rarefied air, and how impact with denser air causes the luminescence of the aurora. This is a discharge tube .
Voltage is applied at both ends of the tube to generate an electric current.
Next, turn the vacuum pump on to evacuate the air from inside the tube. At first, when the air inside the tube is dense like the air we live in, at most a few flashes like lightning appear. As the air becomes gradually thinner, however, the luminescence effect grows increasingly clear.