Planetary Magnetic Fields

Much of the following overview is adapted from the website http://www.science.nasa.gov/ssl/pad/sppb/edu/lionroar/

• At the start of the 20th century, after the development of telephones, people in England heard unidentified sounds during periods when the northern lights were present.  The sounds they heard were unexplained at the time, but they sounded similar to the following:

• Some 20 years later, during World War I, soldiers using electronic listening devices heard not only enemy conversations, but also strange whistling sounds, akin to bombs flying overhead.   The sounds they heard, called "whistlers" were similar to the following:

• When the space age began in the late 1950s, satellites with scientific equipment on board began investigating the near-Earth environment of space, and an understanding of some of the sounds heard on Earth began to develop.

• When the solar wind comes to the Earth, it meets the Earth's magnet and most of the tiny particles are pushed around the Earth because of this magnet. They begin their journey around in a curve called the "Bow Shock". Just like water makes a curved wave in front of a boat, the solar wind makes a curve in front of the Earth. The particles then follow a path that goes around the Earth in a sort of cover or sheath. This curve is called the "magnetosheath". 

• Not all the solar wind particles go around the earth by way of the magnetosheath. Some get through the bow shock and get caught between the Earth and the bow shock. These particles mix with other particles that come up from the Earth's ionosphere to fill the magnetosphere. They are pushed sideways so they are close to the North or South Pole. There they cause the sounds on radio and in electronic equipment.

• There are special places in the magnetosphere where the plasma (charged particles) and the magnetic field  of the Earth make light and cause sounds on telephones and radios. The first sounds we heard were caused by the plasma bouncing back and forth in the magnetic field of the Earth.  The whistler sounds were caused in the part of the magnetosphere shown below. The plasma here causes the high pitched sounds to be heard before the lower pitched sounds so that what you hear sounds like a whistle.

• In the region called the "lion roars region" above, the plasma bounces back and forth causing the radio and telephone to sound like a roaring lion. Here the signal lasts about 2 seconds and has a low tone or pitch.   It sounds like the following:

In summary, the detailed structure of the Earth's magnetic field, and its interaction with the Sun, can be studied by listening with the proper instruments, both in the optical:            and in the radio:

Consider the significance of these results

By listening, through radio techniques, one can study various regions of the Earth's magnetosphere.  Similar studies from the ground and from spacecraft reveal information about magnetospheres around other planets!

• The Voyager 1 spacecraft, for example, detected the magnetosphere around Jupiter as it approached the planet, and crossed the bowshock of the Jovian magnetosphere:

• The Voyager 1 spacecraft also detected plasma closer in to Jupiter.  Listen carefully, and see if you can tell where in the Jovian magnetosphere this plasma is located by comparing the sounds to what you heard for the Earth:

• Additionally, the Voyager 1 spacecraft detected sounds, called "chorus", that are also similar to those detected for the Earth.  Again, see if you can hear similarities to the Earth's sounds. 

• What about Saturn, for example?  The Voyager 2 spacecraft detected the following sounds there.   Perhaps they sound familiar?

• Perhaps they are new!  The following sounds are a result of the Voyager spacecraft passing through the ring plane of Saturn.

It is possible to be more quantitative, by plotting the frequency of the sounds as a function of time.  By doing this, a detailed understanding of the physical processes occurring in a planet's magnetosphere can be obtained.