Clouds of particles floating in space may affect the strength of magnetic fields protecting the Earth from cosmic rays, according to research by Dartmouth professor Hans-Reinhard Mueller. Mueller's research into understanding the relationship between space clouds and particles on Earth may eventually allow for a three-dimensional map of interstellar dust clouds, he said.
Cosmic rays are high-energy particles created by "violent and energetic" cosmic events such as supernovas, according to University of Chicago professor Priscilla Frisch, who co-authored the study. When cosmic rays penetrate the Earth's atmosphere, they collide with nuclei to create isotopes of elements, such as beryllium-10 and carbon-14, which are seldom found otherwise, Frisch said.
Most of these rays are deflected by two magnetic fields surrounding Earth before they reach us, Mueller said. One magnetic field, the sun's heliosphere, stretches far beyond the solar system, while the Earth's magnetosphere provides a smaller range of protection.
Both spheres vary in their strength, according to Mueller. The magnetosphere weakens and flips in polarity approximately every 30,000 years, while the heliosphere waxes and wanes in strength every 11 years with the sunspot cycle, Mueller said.
Mueller and Frisch's research focused on whether these magnetic variations were correlated with the abundance of various elements created by cosmic rays, such as beryllium-10. Using Arctic ice-core samples, Mueller and Frisch tested the correlation and observed several anomalies between the observed and predicted levels of these various elements present on Earth.
The presence of solar clouds interstellar masses of particles left over from star formations and supernovas may explain these anomalies, Mueller said. As the sun rotates around the center of the Milky Way galaxy in a 220 million-year cycle, it passes through a number of clouds of varying temperatures and densities.
"In the big picture, everything is supposed to be in orbit around the center of the galaxy," Mueller said. "In contrast, gas is much more mixed."
If the sun passes through a particularly dense or fast-moving cloud, the heliosphere shrinks, Mueller said. This could explain the anomalies in the ice-core record, he said.
"When the sun moves in and out of interstellar clouds, the galactic flux of cosmic rays changes as well," Frisch said.
Mueller said most of the changes come from the sun passing through the edge of a cloud. When two clouds are nearby, they interact, heating the surfaces of the two clouds. The space cloud's influence on the heliosphere is therefore a "surface phenomenon," Mueller said.
The solar system is currently inside a space cloud, according to Mueller. The solar system, which has been in the cloud for about 40,000 years, will exit in about 5,000 years, he said.
Mueller and Frisch said they are still unsure precisely how space clouds may be causing these anomalies in the element record, and that further research is needed. If there is a connection between space clouds and the data anomalies, they said, the presence of exotic elements in old ice cores could be used to create a three-dimensional map of the interstellar dust clouds.
Using data from the Hubble Space Telescope, Mueller and Frisch said they hope to gather information about the current cloud, such as its density and velocity. Mueller said that this knowledge could be used to "calculate what the interstellar environment was at [past times in the element record]."
"Once we know the physical distribution, then we will be able to produce reliable indications of when the sun enters and leaves the clouds," Frisch said.
Mueller said the process would likely be difficult because the connections between the many variables are not yet fully understood.
"Many details need to be taken into account," he said.
