from ScienceNASA Website
NASA's five THEMIS spacecraft have discovered a breach in Earth's magnetic field ten times larger than anything previously thought to exist. Solar wind can flow in through the opening to "load up" the magnetosphere for powerful geomagnetic storms. But the breach itself is not the biggest surprise.
Researchers are even more amazed at the strange and unexpected way it forms, overturning long-held ideas of space physics.
The magnetosphere is a bubble of magnetism that surrounds Earth and protects us from solar wind. Exploring the bubble is a key goal of the THEMIS mission, launched in February 2007. The big discovery came on June 3, 2007, when the five probes serendipitously flew through the breach just as it was opening.
Onboard sensors recorded a torrent of solar wind particles streaming into the magnetosphere, signaling an event of unexpected size and importance.
Li's colleague Jimmy Raeder, also of New Hampshire, says,
The event began with little warning when a gentle gust of solar wind delivered a bundle of magnetic fields from the Sun to Earth.
Like an octopus wrapping its tentacles around a big clam, solar magnetic fields draped themselves around the magnetosphere and cracked it open. The cracking was accomplished by means of a process called "magnetic reconnection."
High above Earth's poles, solar and terrestrial magnetic fields linked up (reconnected) to form conduits for solar wind.
Conduits over the Arctic and Antarctic quickly expanded; within minutes they overlapped over Earth's equator to create the biggest magnetic breach ever recorded by Earth-orbiting spacecraft.
A computer model of solar wind flowing around Earth's magnetic field on June 3, 2007.
Background colors represent solar wind density; red is high density, blue is low.
Solid black lines trace the outer boundaries of Earth's magnetic field.
Note the layer of relatively dense material beneath the tips of the white arrows; that is solar wind entering Earth's magnetic field through the breach.
Credit: Jimmy Raeder/UNH.
The size of the breach took researchers by surprise.
The circumstances were even more surprising. Space physicists have long believed that holes in Earth's magnetosphere open only in response to solar magnetic fields that point south.
The great breach of June 2007, however, opened in response to a solar magnetic field that pointed north.
Here is why they can't believe their ears: The solar wind presses against Earth's magnetosphere almost directly above the equator where our planet's magnetic field points north.
Suppose a bundle of solar magnetism comes along, and it points north, too. The two fields should reinforce one another, strengthening Earth's magnetic defenses and slamming the door shut on the solar wind.
In the language of space physics, a north-pointing solar magnetic field is called a "northern IMF" and it is synonymous with shields up!
Northern IMF events don't actually
trigger geomagnetic storms, notes Raeder, but they do set the stage
for storms by loading the magnetosphere with plasma. A loaded
magnetosphere is primed for auroras, power outages, and other
disturbances that can result when, say, a CME (coronal
mass ejection) hits.
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