Planetary Sciences Research Spotlight

What Causes the Strange Pulses in Saturn's Magnetosphere?

A new model shows that a spiral wave may explain why many phenomena in the gas giant's magnetosphere undergo periodic cycles.

Source: Geophysical Research Letters

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In addition to its signature rings and many moons, Saturn is surrounded by its own magnetosphere—the bubble of space influenced by the planet’s magnetic field. The behaviors of various phenomena within the magnetosphere, including charged particles, magnetic fields, the aurora, radio emissions, and even the rings themselves, undergo periodic cycles that last about 10.7 hours.

For the past decade, scientists have puzzled over what could cause these pulses. The periodic cycles of the magnetospheres of Earth and Jupiter are caused by the misalignment of each planet’s spin axis and magnetic field axis. However, Saturn has no such mismatch.

Saturn’s spiral as seen from the north pole of the planet looking down on the equatorial plane, with the Sun to the right. The orbits of Saturn’s principal satellites are shown, along with the Earth-Moon system drawn to scale. The dotted area around the disk of Saturn indicates the well-known rings. The dash-dotted curve marks the magnetopause, which separates the magnetosphere from the solar wind. A dotted curve indicates one orbit of the Cassini spacecraft. The spiral pattern itself rotates in a counterclockwise sense with a period of 10.7 hours. Credit: J. Carbary and the Cassini/MIMI team
Saturn’s spiral as seen from the north pole of the planet looking down on the equatorial plane, with the Sun to the right. The orbits of Saturn’s principal satellites are shown, along with the Earth-Moon system drawn to scale. The dotted area around the disk of Saturn indicates the well-known rings. The dash-dotted curve marks the magnetopause, which separates the magnetosphere from the solar wind. A dotted curve indicates one orbit of the Cassini spacecraft. The spiral pattern itself rotates in a counterclockwise sense with a period of 10.7 hours. Credit: J. Carbary and the Cassini/MIMI team

Now, a new model by Carbary provides new evidence for a potential mechanism that has been previously explored by several scientists: a rotating spiral wave of increased particle density that travels outward from the planet. As the spiral turns, regions of higher plasma density and magnetic field strength periodically sweep through Saturn’s magnetosphere, giving rise to the observed cyclical behavior.

To construct the new model, Carbary used observations made by NASA’s Cassini spacecraft during the first 200 days of 2010. As it swung around Saturn—well within the magnetosphere—Cassini measured magnetic field strength and plasma density. Carbary used these data to calculate the wave speed of the hypothetical spiral.

An observer looking down at the north pole of Saturn from above would see that the spiral travels outward from the planet and rotates counterclockwise. It appears stiff near the planet where the wave speed is high, like the spokes of a wheel. However, the spiral takes shape as the wave speed slows toward the outer magnetosphere.

Carbary tested the new model’s predictions against other Cassini observations from 2010. The model accurately predicted the 10.7-hour periodic signature. However, it also predicted smaller secondary signals approximately 20 minutes before and after the main signal, which were not observed by Cassini. Future studies are needed to refine the model and identify the underlying mechanism that drives the spiral. (Geophysical Research Letters, doi:10.1002/2015GL067292, 2016)

—Sarah Stanley, Freelance Writer

Citation: Stanley, S. (2016), What causes the strange pulses in Saturn’s magnetosphere?, Eos, 97, doi:10.1029/2016EO047311. Published on 4 March 2016.

© 2016. The authors. CC BY-NC 3.0