In the Lofoten Islands of Norway, Spaceweather.com reader Rob Stammes operates a magnetic observatory. 24 hours a day, he measures the strength and direction of the local magnetic field as well electrical currents running through the ground. During geomagnetic storms, his chart recordings go haywire. On Sept. 12th, something different happened. They rang like a bell:
"During the morning and especially around noon, sinusoidal pulsations appeared on my instruments," says Stammes. "The period was close to 115 seconds."
These are natural ultra-low frequency oscillations known to researchers as "pulsations continuous" (Pc). The physics is familiar to anyone who has studied bells or resonant cavities. Earth's magnetic field carves out a cavity in the surrounding solar wind. Pressure fluctuations in the solar wind can excite wave modes in this cavity much like Stammes observed. References: #1, #2, #3.
The Sept. 12th oscillations are Pc4 waves; in other words, their frequencies fall in the range 6.7–22 mHz. Pc4 waves, and their even lower frequency cousins Pc5 waves (1.7–6.7 mHz), can have an energizing influence on particles in Earth's inner magnetosphere because the waves resonate with the natural motion of particles around the geomagnetic field. Perhaps it is no surprise, then, that bright auroras were observed on both Sept. 11th and 12th.
Ole Salomonsen photographed these from the window of an airplane just before sunrise in Norway:
These are natural ultra-low frequency oscillations known to researchers as "pulsations continuous" (Pc). The physics is familiar to anyone who has studied bells or resonant cavities. Earth's magnetic field carves out a cavity in the surrounding solar wind. Pressure fluctuations in the solar wind can excite wave modes in this cavity much like Stammes observed. References: #1, #2, #3.
The Sept. 12th oscillations are Pc4 waves; in other words, their frequencies fall in the range 6.7–22 mHz. Pc4 waves, and their even lower frequency cousins Pc5 waves (1.7–6.7 mHz), can have an energizing influence on particles in Earth's inner magnetosphere because the waves resonate with the natural motion of particles around the geomagnetic field. Perhaps it is no surprise, then, that bright auroras were observed on both Sept. 11th and 12th.
Ole Salomonsen photographed these from the window of an airplane just before sunrise in Norway:
"To get the shot was not easy," says Salomonsen. "I had to use a handheld camera in a moving plane! To create some darkness, I draped one of my jackets to over me and the window to prevent cabin-light reflections ruining the shot. The people sitting next to me probably must have had a good laugh :) Anyway I think it was worth it. Observing the auroras from air is definitely special."
Rob Stammes has been monitoring magnetic pulsations in Norway for years. "They seem to occur most often around the equinoxes," he says. That means we should stay tuned for more. It is aurora season, after all.
www.spaceweather.com
Rob Stammes has been monitoring magnetic pulsations in Norway for years. "They seem to occur most often around the equinoxes," he says. That means we should stay tuned for more. It is aurora season, after all.
www.spaceweather.com
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