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 http://pwg.gsfc.nasa.gov/istp/news/0005/fullpr.html 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:
http://www.esa.int/science/media