Why three planets radio the sun.

Why three planets radio the sun

When the solar wind, a continuous outpouring of electrically charged particles from the sun, collides at supersonic speed with the magnetic field of a planet, it creates a shock wave on the sunward side. This in turn triggers the emission of radio waves that radiate back toward the sun. The widths of the arc-shaped shock waves responsible for the radio signals from the solar system's inner three planets -- Mercury, Venus and Earth -- vary widely. The signals themselves, however, are quite similar, according to a new analysis of spacecraft data more than 10 years old.

Earth and Mercury owe their shock waves to magnetic fields generated by the dynamo at each planet's spinning core, says Chris T. Russell of the University of California, Los Angeles. He and two graduate students coauthored the analysis, which appears in the December GEOPHYSICAL RESEARCH LETTERS. The width of Earth's shock wave is about 10 times the planet's diameter. Owing to Mercury's roughly 50 percent smaller size and weaker magnetic field, its shock wave is only 5 percent as big as Earth's.

Venus, nearly Earth's twin in size, derives its weak magnetic field from a far different mechanism: the collision of the solar wind and that planet's upper atmosphere. The resulting shock wave is only about 10 percent as wide as Earth's.

Sunward-radiating radio waves spawned by these vastly different-sized shock waves share a number of common properties, the space physicists found. For instance, all radiate at extremely low frequencies -- about 1 to 3 cycles per second. They also leave the shock wave around each planet at approximately the same range of angles. And the amplitudes or strengths of the radio signals -- which naturally decrease with distance -- appear comparable when measured at about the same distance from their source.

The new analysis also appears to have answered a long-standing question about which type of solar wind particles trigger the radio waves: negatively charged electrons or positively charged particles such as protons (ionized hydrogen atoms).

Previous studies, including one by Russell himself, had suggested that the solar wind ions took a "left-handed" spiral around the planetary magnetic field lines that captured them -- a sign that the ions were positively charged. But in the new report, Russell's team concludes that the spiraling only appeared left-handed owing to the spacecrafts' motion relative to the solar wind. By analyzing the spiral's direction with respect to the flowing solar wind, they have now determined that it's actually right-handed, Russell says. This indicates the radio waves are really spawned by electrons, his team now reports.