A CHANCE sighting of a tear in the Earth鈥檚 magnetic shield has led to what
astronomers are claiming is the first empirical proof to end a 50-year-old
debate about how space weather causes polar auroras and disrupts satellite
communication systems.
In 1996, a NASA spacecraft named Polar appeared to travel directly through a
tear in the cocoon. One of Polar鈥檚 goals was to study how energy from charged
particles streaming out from the Sun鈥攖he solar wind鈥攊s transferred
into the Earth鈥檚 magnetosphere, the cocoon of charged particles travelling along
the Earth鈥檚 magnetic field lines. The problem till now has been that most
scientists thought the tears, known as 鈥渕agnetic reconnections鈥�, were too small
to be seen directly.
Since the initial pass, however, Polar has buzzed through some 40 more tears,
and NASA researchers have spent the past few years studying and confirming what
they have seen. 鈥淚 think everybody is surprised,鈥� says Jack Scudder of the
University of Iowa in Iowa City, who headed the analysis. Jim Drake, a
theoretical physicist at the University of Maryland in College Park, agrees:
鈥淔rankly, a lot of us theorists looked at their claims and said, `How is this
possible?'鈥� But a close look convinced scientists that it really had happened,
he says.
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The Earth鈥檚 magnetosphere normally shrugs off the magnetic field lines from
the Sun that are generated by the solar wind. However, they exert continuous
pressure on our magnetic cocoon, and from time to time鈥攑erhaps as often as
twice per hour鈥攖he solar wind is strong enough to stretch the
magnetosphere, breaking its field lines. Earth鈥檚 field lines then reconnect with
solar field lines, creating two new lines, each anchored at the Sun on one side
and Earth on the other.
Physicists have debated for half a century whether the tears form because the
magnetosphere is stretched so thin it can no longer hold the charged particles
that define magnetic field lines, or whether some other effect is at work. By
measuring the thickness of the tears Polar has now helped solved the problem. At
just 1 kilometre thick, the magnetosphere is thin enough to tear.
When field lines on the side of the Earth facing the Sun connect with the
solar field lines, the solar wind鈥檚 continued march stretches these new lines
around the Earth like rubber bands, forming a trailing tail on the night side.
Eventually the two ends reconnect to each other, reforming a line anchored at
both ends to Earth, plus a second anchored to the Sun.
When this happens, the release in the tension throws a ball of charged
particles towards the Earth like a slingshot. It is this that creates the aurora
and disrupts satellite telecoms links. This mechanism is what transports energy
from the Sun into the Earth鈥檚 atmosphere.
