نطق سوم

Lecture 3

Listen to part of a lecture in an astronomy class. The professor is discussing auroras.

Okay. The aurora. The aurora refers to the rays of bright colors in the night sky near the North and South Poles. In the Northern Hemisphere, it’s called the aurora borealis. And in the Southern Hemisphere, it’s called the aurora australis. You’ve probably seen pictures of it. It’s quite beautiful. lt took centuries to figure out what’s behind these beautiful colors in the night skies.

In the early 1700s, scientists proposed that there was an electric current that stretched between the North and South Poles, and that if this electric current was disturbed, an aurora would form. Others postulated that the phenomenon was caused by light that refracted off glaciers and snow in the Arctic.

Then in the 1800s, scientific interest in Earth’s magnetic field and strange variations in Earth’s magnetic field led to the observation that the biggest magnetic disturbances coincided with dramatic auroras, and also with the timing of the most intense sunspot activity. Sunspots were first observed centuries earlier, temporary dark spots on the face of the sun. They are gaseous, highly magnetic regions that move across the sun’s surface. Sunspots’ cycles are at their height every eleven years, and so are aurora cycles. They peak together.

By the early 20th century, it was found that Earth’s magnetic field is constantly being bombarded by charged particles streaming from the sun. We call it solar wind. And do I need to tell you when the solar wind is especially strong?Yup!

Every eleven years! When the magnetic activity of sunspots is peaking. The charged particles interact with Earth’s magnetic field and they are pulled toward the North and South Poles. Some of them make it into our upper atmosphere where they collide with atoms, with oxygen and nitrogen atoms. This collision causes the atoms to light up, to glow. Different types of atoms glow in different colors. And this is what’s happening when we are seeing an aurora.

Now, let’s jump ahead to the early 1970s, at a discovery made using a device called a coronagraph.

A coronagraph attaches to a telescope and acts like a disc that blocks out the sun. lt creates an artificial solar eclipse, you could say, when you are looking through the telescope. This makes the sun’s corona, or outer atmosphere, much easier to observe.

Now, it’s true that whenever there is a total eclipse of the sun you can see the corona, that outer white circle surrounding the sun. But how long does a total solar eclipse last? Less than ten minutes. And they occur maybe once a year.

With the coronagraph, you can observe the corona continuously anytime you want.

And during the early 1970s, using a coronagraph mounted on an orbiting satellite we witnessed what are called coronal mass ejections, or CMEs for short.

So, coronal mass ejections, what are those?

Well, they are huge, magnetized gas clouds that are thrown from the sun during a big atmospheric storm. They erupt from the sun over the course of several hours.

These huge clouds are made of billions of tons of those charged particles that rush toward the Earth at incredibly high speeds. This mass reaches our planet’s magnetic field in anywhere from just several hours to a few days.

So, we found that during CMEs, with their enormous ejections of particles from the sun, auroras are particularly intense.

Now, as we’ve said, we can predict peaks in sunspot activity, but so far we can’t say the same for CMEs. We don’t know when they’ll occur or how large they’ll be.

But what would be the advantage in knowing that?

Well, throughout history, we’ve noticed correlations between aurora intensity and technical problems, disruptions. First with compasses going awry. Then when we developed telegraph systems, they were affected. And then telephone systems and shortwave radio systems. Today, even whole electrical power stations. For example, in 1989 there was a really intense magnetic storm initiated by a flare up on the sun, and it caused electricity to go out for 12 hours in Quebec, Canada.