Through a combination of good luck and shrewd data analysis, researchers
using NASA’s Wind spacecraft have made rare direct observations of the
mysterious process that allows the solar wind to connect to Earth’s magnetic
field (the magnetosphere). Known as reconnection, this process allows the
magnetic field of the Sun – as carried in the solar wind – to connect to
Earth’s field, allowing energy and matter to flow from one to the other.
Like water rushing through a pipe and spigot, the solar wind flows into
Earth’s magnetosphere through a narrow valve-like region a quarter of a
million miles downwind of the planet, in a region known as the magnetic
tail. Depending upon the orientation of the solar wind and Earth’s magnetic
field, that valve opens and closes to allow plasma and energy from the Sun
to enter Earth’s space. In April 1999, Wind passed right through that valve
as solar wind poured in.
The research findings were made by Marit Oieroset and colleagues at the
University of California-Berkeley, with scientists from the Tokyo Institute
of Technology and NASA’s Goddard Space Flight Center, Greenbelt, Md. They
will appear in the July 26 issue of Nature.
The solar wind is a tenuous, ionized gas (called plasma) that flows out
constantly from the Sun in all directions. The wind ebbs and flows, shaping
the magnetosphere of Earth and carrying energy, magnetic fields, and matter
from the Sun. On the sunlit or “day” side of Earth, the solar wind pushes,
stretches, and energizes Earth’s magnetic field, but the plasma barely
penetrates Earth’s magnetic shell. It is in the distant reconnection region
of the tail, on the night side of Earth, where the solar wind enters the
magnetosphere. The flow of solar wind into the tail of the magnetosphere
fills Earth’s space with plasma and energy. This energy is stored like a
battery until it is eventually released in bursts that cause auroras and
other space weather phenomena. Space weather can affect radio
communications, satellite operations, and the control of electric power
systems on Earth.
Magnetic reconnection was proposed more than forty years ago as the key
process that allows this flow of solar wind into the magnetosphere. During
reconnection, magnetic fields that are heading in opposite directions —
having opposite north or south polarities — break and connect to each
other. Previous studies have observed the consequences of reconnection after
it had occurred, mainly the flow of plasma and energy toward or away from
Earth. But eyewitness observations of the process in action have been
elusive.
In April 1999, Wind flew right through the reconnection region as the
process was occurring. While flying tailward through the magnetosphere (away
from Earth), Wind’s instruments detected jets of plasma racing toward Earth.
Part-way through the journey, the jets stopped and Wind detected unusual
electric currents and magnetic signatures that were predicted to occur at
the point of reconnection. Some time later, as Wind kept flying away from
Earth, the spacecraft detected that jets of plasma flow in the exact
opposite direction of the previous flows, racing away from the planet. The
event was comparable to a plane flying through the eerie, calm eye of a
hurricane.
“Reconnection is one of the fundamental physical processes in the universe,”
says Oieroset. “It occurs in so many places in the universe, yet the only
place we can observe it directly in a natural environment is in our
magnetosphere. It is absolutely crucial for understanding how the Sun
connects to the Earth. This is how you populate the magnetosphere with
plasma.”
The direct observation of reconnection has implications for many fields of
physics. Reconnection on the Sun likely plays a role in the development of
solar flares and of coronal mass ejections. Reconnection likely plays a role
in the interaction of neighboring stars. And observations of reconnection in
nature may aid the study of nuclear fusion and other plasma processes in the
laboratory. The magnetosphere is the only place where reconnection has been
observed first-hand as it occurs naturally.
The rare observation of reconnection was made possible due to a decision to
move Wind out of its originally planned orbit and into a new pattern that
took it on petal-like loops around the Earth and Moon. “When Wind was
launched it was not supposed to study the deep tail at all,” says Keith
Ogilvie, project scientist for the Wind mission and a researcher at NASA
Goddard. “But with a new orbit and region comes new discoveries.”
Launched in 1994, Wind studies the solar wind both inside and outside of
Earth’s magnetosphere. The spacecraft is a key component of the
International Solar-Terrestrial Physics program, is a joint scientific study
of the Sun-Earth system conducted by NASA, Japan’s Institute for Space and
Astronautical Studies, and the European Space Agency (ESA).
