Wind/WAVES radio spectrogram of CME/magnetic cloud event of 6--10 January, 1997


After it was launched from the sun in the direction of Earth, the coronal mass ejection (CME) of Jan 6 was quickly lost from view because it became too diffuse to be seen by the LASCO telescope. Thus for its long journey of 93 million miles to Earth, the CME would have been completely out of sight except for the remote sensing capability of the WAVES instrument on the WIND spacecraft. Here's how it works. As it propagates toward Earth, the CME drives a shock front before it, much like a jet breaking the sound barrier drives a sonic boom into the atmosphere. In the case of the CME shock, however, the sonic boom takes the form of very intense, low-frequency radio emissions known as solar Type II bursts. (The Type II bursts cannot be detected by instruments on Earth because the ionosphere blocks the waves. Only an observatory in space can detect them.) As the shock approaches Earth, these Type II bursts drift down in frequency (see radio spectrogram in figure) because the electron density of the interplanetary medium to which the solar bursts are `tied' is also constantly decreasing. The WAVES instrument on Wind is capable of detecting these radio bursts from a great distance, long before the CME arrives at Earth. WAVES can also determine the direction from which the waves are arriving. Combined with the known propagation speed of the shock, this information allows the WIND investigators to track the CME shock front nearly from the time of its disappearance as seen by LASCO, all the way to Earth --- and beyond. The cartoons above the spectrogram illustrate the derived location of the shock at three times during its traverse from the Sun to Earth.


This information was provided by the Wind WAVES Team, courtesy of M. Kaiser.

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Last updated: January 30, 1997