published in air
The sky appears to be crossed by rays, spots, luminescent flames the colour of which ranges from green, to red, to white. The band within which such phenomena take place ranges from 30 to 300 km from the earth’s surface. According to the hemisphere in which they take place, they are called aurora borealis or australis. Although they are typical of the polar skies, in the past they were visible also in Italy. It was proved that the Roman emperor Tiberius dispatched his firemen to the Ostia harbour since, from Rome, he had noticed a red light in the sky which proved to be not a fire but an aurora borealis. The auroras are the result of the interaction between the Earth’s magnetic field and the solar wind. The latter is a flow of electrically charged particles emitted by the Sun into the Universe. When they reach the Earth, owing to the shape of its magnetic field, the charged particles penetrate into the polar atmosphere where they stimulate the gases present in the air which, in their turn, emit red or green light. Approximately one hundred polar auroras take place each year.