Lightning's Reach

"One one-thousand, two one-thousand…" Counting the time between a flash of lightning has long been a popular way to figure out how far away a lightning flash is. Even today, some people still recommend this method to determine if you're safe from a direct strike, though this is something of a fallacy–thunder isn't audible very far, and a lightning bolt can reach eight miles in front of a storm.

But what's even more interesting is that the effects of a single lightning bolt can cause a chain reaction of events over hundreds of thousands of square miles, hundreds of miles away. Before we talk about why, we'll have to talk about some of the things that go on high above the earth, in the Van Allen radiation belts.

The earth has a magnetic field. Everyone knows this; if you've ever used a magnetic compass, you know that the earth's magnetic field is what makes the needle point north. This NASA model of the magnetic field shows how complex it is, and how the magnetic lines of force loop from the north pole to the south pole.

The magnetic field does more than just make compass needles move. It also helps make life on earth possible. It deflects charged particles streaming from the sun, protecting us from radiation that would otherwise make conditions on earth inhospitable to life. This stream of charged particles is called the "solar wind," and where it meets the magnetic field, all kinds of fun and vigorous things happen.

This illustration (click for a larger, beautiful version) shows the way the solar wind interacts with the earth's magnetic field. Most of the stream of charged particles (protons and electrons, primarily) is deflected away from the earth; but some of it becomes trapped by the magnetic force. These trapped particles create regions of charged ionizing radiation above the earth, called the Van Allen radiation belts. The auroras over the north and south poles are the result of these charged particles interacting with the upper atmosphere.

So what does this have to do with lightning? Bear with us; we're getting to that.

The Van Allen radiation belts are actually divided into two discrete sections. The outer layer is composed primarily of highly energetic electrons, which spiral around the lines of magnetic force in the earth's magnetic field and bounce back and forth between the north and south poles. These electrons come from the solar wind; as time goes on, more and more electrons become trapped here until finally an event causes them to break free of the magnetic field and stream down into the upper atmosphere. (The inner layer is primarily made up of energetic protons.)

Often, that event is lightning.

Researchers have recently discovered that lightning strikes at certain latitudes can cause electrons to rain out into the upper atmosphere.

Every bolt of lightning creates a characteristic electromagnetic pulse. This pulse can travel great distances, and can reach high into the outermost layers of Earth's atmosphere. At this altitude, the electromagnetic pulse interacts with the Earth's magnetic field; if the electromagnetic pulse is oriented correctly, it will scatter the electrons trapped high above the earth, breaking them free from the magnetic field and causing them to precipitate down into the upper atmosphere. The EMP can affect an area of tens or even hundreds of thousands of square miles, causing electrons in a very wide area to descend into the ionosphere.