A PURE SINE WAVE IN THE MAGNETOSPHERE

In a quiet room, you can hear a pin drop. Norwegian citizen scientist Rob Stammes just heard a pin drop on Earth's magnetic field.

"It was very quiet when it happened," says Stammes, who runs a space weather observatory in Lofoten, Norway. On Oct. 17th, his magnetometer was monitoring Earth's magnetic field as it does every night, and the instrument's needle had settled itself into a straight line, indicating very low geomagnetic activity. Suddenly, Earth's magnetic field began to ring.

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"A very stable ~25 second magnetic oscillation appeared in my recordings, and lasted for more than 20 minutes," he says. "It was fantastic to see the magnetic field swing back and forth by about 0.1 degrees, peak to peak."

This kind of pure tone is rare, but it has happened before. Researchers call it a "pulsation continuous" -- or "Pc" for short. Pc waves are classified into 5 typesdepending on their period. The waves Stammes caught fall into category Pc3.

The "pin dropping" was a gentle gust of solar wind. Imagine blowing across a piece of paper, making it flutter with your breath. The solar wind can have a similar effect Earth's magnetic field. Pc3 waves are essentially flutters propagating down the flanks of our planet's magnetosphere excited by the breath of the sun.

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Stammes is a longtime observer of Pc waves. Usually he catches them during Solar Minimum when "the room is quiet" for months at a time. "Recording one now so close to Solar Max is unexpected," he says. "Lately, my magnetometer traces have been too noisy for such delicate waves--so it came a surprise!"

Pc3 waves, which can only be heard in moments of quiet, can also bring the quiet to an end. The oscillations sometimes flow all the way around Earth's magnetic field and cause a "tearing instability" in our planet's magnetic tail. This, in turn, sets the stage for magnetic reconnection and geomagnetic storms.

That didn't happen on Oct. 17th, though. The pin dropped, the magnetosphere rang, and quiet resumed. Stammes is already listening for more. Stay tuned!

See: https://spaceweather.com

Recent low-orbiting observations at satellites with high-accuracy magnetometers onboard (Oersted, CHAMP, and ST5) have provided a detailed picture of the Pc3 wave structure in the topside ionosphere. Pc3 waves were detected very clearly in the compressional component of the satellite magnetic field data, whereas on the ground their signature was found in the H component. The occurrence of a significant compressional component in Pc3 pulsations in the topside ionosphere was also evidenced by radio-sounding measurements of ionospheric plasma oscillations. The following possibilities of ULF compressional disturbance excitation are considered: (1) an incident Alfvén wave generates an evanescent fast mode as a result of its interaction with the anisotropically conducting ionosphere; (2) transport of ULF wave energy from a distant source toward the ionosphere predominantly occurs by a fast mode. We estimate quantitatively the expected relationships between the Pc3 wave magnetic components above the ionosphere and on the ground produced by these different mechanisms and have derived simple analytical relationships between the compressional and ground signals for both mechanisms. Numerical modeling with the use of exact formulas has shown that these approximations work well over a wide range of wave scales. This model has been applied to the interpretation of Pc 3 waves observed by CHAMP in the upper ionosphere and by ground stations at midlatitudes. In general, the observed ratio between the compressional component in space and the ground signal corresponds better to the scenario of direct fast mode transmission to the ground.

See: https://www.researchgate.net/publication/251428252_Structure_of_ULF_Pc3_waves_at_low_altitudes

When interpreted in terms of MHD wave modes, the oscillations in the fields are consistent with fast magnetosonic waves propagating Earthward. These results lend strong support to the view that ULF waves generated near the quasi-parallel portion of the Earth's bow shock wave propagate into the magnetosphere and are observed as compressional Pc 3 pulsations in the dayside magnetosphere.
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