Characteristics and Origins of the Solar System

Addendum to Lecture 23

April 3, 2004

At the end of Lecture 23 (the last picture in the lecture notes) I discussed measurements made at the very edge of the solar system with the University of Iowa radio wave experiment on the Voyager spacecraft. This instrument was built by the research group of Professor Donald Gurnett, and major discoveries have been made in the remote solar system by Professor Gurnett and Dr. William Kurth.

The picture shown is very eye-catching, but I wanted to add a few words, particularly for our EXW students who didn’t hear the remarks I made in lecture.

The University of Iowa experiment receives radio waves that are naturally generated in space. The frequencies of these waves are lower than AM radio transmissions. They are sufficiently low as to be in the audio range; that is, if you could convert them directly to sound waves, you could hear them.

The figure I includes, reproduced below, shows the spectrum of these waves over a long period of time.

From top to bottom of this plot is displayed the frequency of the radio wave, from a low of 1 kHz, to a high of 4 kHz. The intensity of radio emission is color-coded; dark blue indicates no emission above the noise level (like “hiss” when you are tuning your car radio). Red indicates very intense emission. The straight red line at about 2.4 kHz is internally-generated signal in the Voyager spacecraft. Finally from left to right represents time, starting in 1982, and going up to the the present time. So this is over 20 years of listening to the radio noise in the outer solar system. During this time, the Voyager spacecraft has moved out through space from a distance of 15 astronomical units from the Sun to more than 90.

The brightly colored “clouds” in this figure represent times when the level of radio emission in the outer solar system has substantially picked up. If you had been on the Voyager spacecraft, listening over headphones like Jodie Foster in “Contact” you would have heard loud, strange-sounding noises. Scientists believe these radio waves are generated at the “Heliopause” , or boundary between the solar system and space between the stars, which we call the “Interstellar Medium”.

These data are available in the form on an audio file as well. That gives you the real sensation of what you would have heard if you had been on the Voyager spacecraft, listening on the low frequency radio during this twenty year period. Listen to the “play audio” feature on the URL below, and try and match up the loud noises you hear with the data above.

http://www-pw.physics.uiowa.edu/space-audio/vgr-helio.html