Earth is surrounded by an ocean of plasma—an electrically charged “fourth state” of matter—and researchers have discovered tides rippling across its surface. The shifting field of plasma surrounding the planet may affect Earth’s radiation belts, which can damage spacecraft or astronauts in orbit or traveling to or from the Moon. Similar tides could race around planets in other star systems, the researchers suggested.
The plasmasphere is a donut of cold plasma centered over Earth’s magnetic equator within the magnetosphere—the region encompassing our planet’s magnetic field. The plasma is supplied by the ionosphere, the electrically charged layer of the upper atmosphere.
The outer boundary of the plasmasphere—known as the plasmapause—is typically found 20,000–38,000 kilometers from Earth’s center at the magnetic equator, although its location can vary with the seasons, solar activity, and other factors, Quanqi Shi and Chao Xiao, physicists at China’s Shandong University and the study’s lead authors, wrote in an email.
“In the past, lunar tides were mainly found to affect the first three states [of matter]: solid Earth tides, liquid ocean, and neutral gas–dominated atmospheric tides,” Shi and Xiao wrote. “Whether lunar tides can influence the plasma-dominated regions had not yet been explored.”
To fill in that gap, Shi and Xiao and their colleagues combed through a database of more than 50,000 plasmapause crossings recorded from 1977 through 2015 by NASA’s twin Van Allen Probes, the five-spacecraft THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission, Europe’s four-spacecraft Cluster mission, and others.
Because solar flares and related phenomena can cause substantial changes in the plasmasphere, the researchers eliminated readings taken when the Sun was active, leaving almost 36,000 observations.
Their analysis revealed tides in the plasmapause that ranged from about 0.12 times Earth’s radius (300 kilometers) above average at high tide to 0.14 times Earth’s radius (350 kilometers) below average at low tide—a difference of just 3% in the “smoothness” of the plasmapause. The researchers published their findings in Nature Physics.
Ocean and land tides are highest and lowest close to new and full Moon, when the Moon aligns with Earth and the Sun, and lunar and solar gravity pull along the same line. Unlike those tides, however, the plasma tides are offset from the Moon’s position in the sky by 90°.
High plasmapause tide occurs when the Moon is at first quarter, and low tide occurs 2 weeks later, at last quarter. The tidal waves in the plasmasphere occur once per day and once per month, versus the twice-per-day and twice-per-month cycles for ocean and other tides.
It is reasonable that the Moon has a small but noticeable effect on the plasmasphere, said Jerry Goldstein, a space physicist at Southwest Research Institute in San Antonio who studies the plasmasphere but was not involved in this project. “The most surprising thing is that the largest effect is 90° away from the lunar direction. Intuitively, one might expect the largest effect to line up with the Moon.”
The plasmapause tide itself is caused by the Moon’s gravitational pull, according to the researchers, but the 90° offset is more difficult to explain. It doesn’t vary with the lunar phase, the Earth-Moon distance, changes in seasons, or any other known factors. That means it’s not caused by gravity alone, but by gravity and electromagnetic forces working together, the researchers said.
One possibility is that electrically neutral winds in the ionosphere, which vary with the Moon’s phase, modify electric currents that thread along magnetic field lines through the ionosphere and into the plasmasphere. The currents might disturb the magnetic field, pulling the plasmasphere out of sync with the motions of the Moon. The researchers noted that this mechanism is poorly understood, however, and “is a subject of ongoing research.”
The plasma tides “may indicate a fundamental interaction mechanism in the Earth-Moon system that has not been previously considered,” Shi and Xiao wrote in their email. “That is to say, lunar tides may not be ignored in the study of the magnetosphere.”
Diving into Plasma Physics
The tides in the plasmasphere could have implications for space travel and studies of other planets in our solar system and beyond.
Magnetospheres have been detected at other planets, the study’s authors noted, suggesting that plasma tides “may be observed universally throughout the cosmos,” opening new avenues of research into planetary systems and other bodies with strong magnetic fields.
Earth’s magnetic field plays an important role in spaceflight as well. It deflects cosmic rays and particles of the solar wind, offering some protection to craft within the magnetosphere. However, it funnels other particles toward Earth’s surface or traps them in the radiation belts, potentially threatening spacecraft and astronauts, particularly when they travel to the Moon, outside the protection of the magnetic field.
“We suspect that the observed plasma tide may subtly affect the distribution of energetic radiation belt particles, which are a well-known hazard to space-based infrastructure and human activities in space,” the authors noted. “It is therefore worthwhile to look for evidence of this effect in future studies.”
“Understanding this new result is going to require a fun dive into some basic space plasma physics,” Goldstein said.
—Damond Benningfield, Science Writer