Solar Flares: Research

As radiation and charged particles from the fall 2003 solar flares buffeted communications satellites, a spacecraft called Cassini (see photo) was safely speeding towards a 2004 rendezvous with Saturn. Cassini was designed to make a variety of measurements in the near-Saturn environment, including radio emissions, so it was well equipped to observe radio bursts that were produced by these two solar flares.

 

The Cassini spacecraft (image courtesy of NASA).

 

 

The Van Allen radiation belts, with Earth in the center. The belts consist of charged particles spiraling around the lines of Earth's magnetic field, bouncing repeatedly from pole to pole and back again. An electron in the inner belt can make this round trip in about one second. (The source of this material is Windows to the Universe, at http://www.windows.ucar.edu/ at the University Corporation for Atmospheric Research (UCAR). ©1995-1999, 2000 The Regents of the University of Michigan; ©2000-01 University Corporation for Atmospheric Research. All Rights Reserved.) 

These radio bursts were not part of the flare itself, but rather were generated when a beam of electrons ejected by the flare interacted with the solar wind. The beam excited oscillations in the solar wind at the “electron plasma frequency,” which decreases rapidly with increasing distance from the sun. Once excited, these electrons emit radio waves at that same frequency, and these were the signals that Cassini observed.

Part of the Cassini data for the October 28 flare is shown in the graph, which plots frequency vs. time. The labels show that the data covered roughly the frequency range of AM radio. The leading edge of the burst arrived 69 minutes after the solar flare, since that was the radio travel-time from the sun to the spacecraft, then located about 8.7 A.U. from the sun (A.U. is Astronomical Unit, the distance from Earth to the sun, about 8 light-minutes). In the graph, notice how the frequency of the radio waves diminished with time after the burst arrived. As the electron beam propagated farther from the sun, it excited radio waves with a lower frequency, corresponding to the decrease of frequency with time that Cassini observed.

The data from this radio burst have been turned into sound by electronic manipulation. To hear this sound, click and then click on each of the colored graphs you will see. Notice how the drop in pitch of the sound corresponds to the rapid decrease of the frequency shown in the graph.

The principal investigator of the radio wave instrument on the Cassini Project is Don Gurnett, James Van Allen Professor of Physics at the University of Iowa, which has long been a center of space research. At Iowa, in 1958, Van Allen built the detector that found the first evidence for the regions of energetic electrons around Earth, known as the Van Allen radiation belts (see drawing below). In his own career at Iowa, Gurnett has contributed to many probes, including serving as principal investigator for the plasma wave investigation on Galileo, whose mission included observations of the magnetic field of Jupiter and the plasma that surrounds it, and principal investigator for the plasma wave instrument on the two Voyager spacecraft, which flew by Jupiter, Saturn, Uranus, and Neptune, and are now proceeding outward into interstellar space.

 

A plot of radio wave frequency vs. time for the October 28 solar flare, as observed by the Cassini spacecraft on its way to Saturn. The y-axis shows the frequency of the radio waves, which is approximately in the AM band. The x-axis shows time. (image courtesy of Ron Gurnett, University of Iowa)