Cluster Satellites Catch the Waves That Catch the Solar Wind

Press Release From: Goddard Space Flight Center
Posted: Thursday, August 12, 2004

image Researchers using the Cluster satellites have for the first time observed vortices that trap plasma and energy from the solar wind in the Earth's magnetic field, or magnetosphere. The result, to be published in the August 12 issue of Nature, may help explain the origin of much of the hot, magnetically charged gas (plasma) that is stored inside the tail of Earth's magnetic field.

The solar wind is a thin stream of electrified and magnetized gas (plasma) continuously discharged from the Sun. As the solar wind blows past Earth, the planet's magnetosphere flaps like a windsock, creating ripples and waves along its flanks in the tail. Like cresting ocean waves that peak, curl, and mix with the air as they crash, these ripples and waves roll up into large vortices that surround the solar wind and capture it into the plasma of Earth's magnetic tail.

The vortices -- which look like the water "pipelines" of waves that human surfers like to ride -- form because the solar wind and the magnetosphere behave like fluids moving at different speeds, creating friction along their edges and intersections. This phenomenon -- known to both space scientists and Earth scientists as a Kelvin-Helmholtz instability -- appears to be a major process for transport and mixing between the solar wind and Earth's magnetic field.

"These multi-point, high time-resolution observations open a new window into understanding the connection of the solar wind to the Earth's magnetosphere," said Dr. William Peterson, NASA's geospace program scientist.

Reviewing data from the four matching satellites of the Cluster mission, space physicist Dr. Hiroshi Hasegawa of Dartmouth College and colleagues found that the satellites had flown through a region where the magnetosphere had curled around the solar wind and absorbed it into the tail. With satellites positioned on several sides of the wave in space, the researchers were able to convincingly resolve the whole structure.

"This is the first time 'rolled-up' Kelvin-Helmholtz vortices have been detected unambiguously," said Hasegawa. "Past observations, which were based on single-spacecraft measurements, could not tell with certainty whether the waves along the flanks of the magnetosphere were large rolled-up vortices or just small ripples that do not trap the solar wind."

Discussions of space weather often focus on the connections between the Sun and Earth that happen on the sunward or forward facing side of Earth's magnetic field. When the Sun emits wind or a coronal mass ejection with a southward magnetic polarity, the solar cloud "reconnects" magnetically with Earth's northward-pointing magnetic field. This connection allows energy and plasma from the Sun to energize the space around Earth and cause disturbances such as auroras, magnetic storms, and radiation belt storms. (See for an explanation of reconnection.)

But these magnetic reconnections have never been able to account for all of the solar wind plasma that seems to enter and fill up Earth's magnetosphere when the solar wind has the "wrong" -- that is, northward -- polarity. Researchers have long been puzzled that Earth's magnetosphere contains three to five times more particles when the solar wind has a northward magnetic orientation, a time when the edges of Earth's magnetic field should be acting like a barrier to the solar wind. Something had to be allowing solar plasma to fill up the magnetosphere at a time when the main theories could not account for the growth. This new result from Cluster helps explain that phenomenon.

"The Kelvin-Helmholtz instability has often been ignored as an important solar wind entry process," said Dr. Tai Phan, a space physicist at the University of California at Berkeley and a co-author of the paper. "Thanks to its multi-spacecraft measurements, Cluster has now proven the existence of these large-scale vortices that could lead to substantial entry of solar wind to populate the Earth's magnetosphere."

The Cluster satellites, built by the European Space Agency with significant participation from NASA, were launched in the summer of 2000. The Cluster mission investigates three-dimensional structures throughout the Earth's magnetosphere and solar wind. NASA supports U.S.-based researchers associated with the mission. For an image and more information, refer to:

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