The solar flare was among the four most intense ever observed. It delivered to Earth a host of mighty emissions, initially X-rays and visible light, then a shower of protons, and later hot gas that raced outward at some 5 million mph (2,235 kilometers per second).
Creepy
Cassini picked up radio emissions from the outburst, waves that had made their presence known at our planet and were on their way to Saturn and beyond.
University of Iowa physicist Donald Gurnett and his colleagues scaled the radio recording down to audible sound, reducing the frequency of the data.
"That's an admitted distortion, Gurnett told SPACE.com, "but it's nothing different than what your car radio does."
The scientists also compressed the four hours of noise into 15 seconds, so impatient humans could enjoy the whole cosmic utterance. It sounds like a military jet winging low and fast overhead, yet with a creepy techno hiss that smacks of some alien invasion.
The otherworldly sound can be heard on Gurnett's web site.
Behind the sound
"It's the most intense by far we've ever seen," Gurnett said of the radio burst, whose signature was also converted into a visual form.
The radio burst, he said, is an indicator of the intensity of a solar flare. He explained how the unique emission is generated.
The mechanism behind eruptions on the surface of the Sun remains largely mysterious, but it involves twisted magnetic fields near sunspots. Heat and material welling up from below is capped, and at some point it explodes. Magnetic energy is converted into radiation and electrical output. Clouds of hot gas, called plasma, bubble into space.
A solar flare is the first visible sign of an outburst and is noted by the visible light and X-rays that reach Earth in about 8 minutes, travelling at the speed of light. A much slower cloud of charged particles, called a coronal mass ejection, can buffet Earth anywhere from 16 hours to three days later.
In between, a beam of electrons is shot out from the site of the outburst. It moves at about one-third to one-half the speed of light, Gurnett said.
The electron beam interacts with plasma that was already around the Sun, a so-called "solar wind" of charged particles that streams constantly from the surface. The interaction -- an oscillation, as Gurnett describes it -- creates radio waves, which travel at the speed of light (faster than the electron beam).
"It's kind of like blowing in a whistle," Gurnett explained.
Think of your breath as the electron beam, the whistle as the plasma medium, and the shrill sound you'd make as the radio waves Cassini recorded.
Space music
Because the density of the solar wind plasma drops as the distance from the Sun increases, the frequency of the whistle drops, too. In fact, Gurnett said, radio receivers on Earth long ago used this phenomenon to measure the density of the solar wind and learn some of its properties.
Cassini is on its way to explore Saturn, where it will arrive next year. The robotic craft is 8.7 times as far from the Sun as is Earth. So it heard the storm several minutes after radio receivers on Earth detected it.
Even travelling at light-speed, it took the radio waves a little more than an hour to reach Cassini.
Gurnett has been fiddling with space sounds for 40 years and has collected others from many sources and by various spacecraft. His recordings inspired a music and visual composition called "Sun Rings," which was performed around the world by the Kronos Quartet.
Other scientists have detected all manner of sounds in space, from the distant drone of one black hole to the symphony of another and Earth's own jumble of "sounds," said to be among the more complex known. |
