Part of a lecture about the aurora, the natural light display also

Section 4. You will hear part of a lecture about the aurora, the natural light display also known as the Northern Lights. First, you have some time to look at questions 31 to 35. Now listen carefully and answer questions 31 to 35. WOMAN: Good afternoon. Today we're looking at one of nature's great spectacles, the shimmering curtains of light in the night sky that we call the aurora, or, in the northern hemisphere, the Northern Lights. For centuries people explained them with myths, but the real story, which we've only fully understood recently, is arguably more astonishing. WOMAN: The story begins not on Earth but on the sun. The sun constantly streams out a flow of tiny charged particles in every direction, and this stream is known as the solar wind. When the sun is especially active, it hurls out great bursts of these particles, and some of that material travels the ninety-odd million miles to Earth. WOMAN: Now, you might expect these particles simply to hit us, but they don't, because the Earth is protected. Our planet behaves like a giant magnet, surrounded by an invisible magnetic field, and this field acts as a shield, deflecting most of the incoming particles safely around the planet. Without it, life on the surface would be impossible. Before you hear the rest of the lecture, you have some time to look at questions 36 to 40. WOMAN: However, the shield is not uniform. It is weakest at the two poles, the north and the south, and it's there that some particles are funnelled down and allowed to enter the upper atmosphere. When those particles collide with the gases high above us, they transfer their energy to the gas, and the gas releases that energy as light. That glow is the aurora. WOMAN: This also explains the colours, which is a lovely detail. The colour depends on which gas is struck and at what height. The most common colour, a vivid green, is produced by oxygen at lower altitudes. Oxygen much higher up gives a rarer red, while nitrogen tends to produce blues and purples. So the sky is, in effect, showing you its chemistry. WOMAN: Finally, where and when to see it. Because the effect is concentrated at the poles, the aurora is best seen from places at high latitude, close to the Arctic, northern Norway, Iceland, northern Canada. And you need darkness and clear skies, so the displays are seen in the winter months, never in the bright summer nights of the far north. It's a reminder that the sky above us is not empty, but the meeting point of the Earth and the sun. We'll stop there. That is the end of Section 4. You now have some time to check your answers.