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Inside Earth's Magnetic Shield

Inside Earth's Magnetic Shield

Solar A shock wave forms as the solar A flow of charged subatomic particles Sun Inside Earth’s wind encounters Earth’s magneto- () streams from the Sun at sphere. The oncoming plasma speeds up to 2 million mph (3 million abruptly slows down and heats up. Most of the hot, turbulent km/h). It extends the Sun’s magnetic plasma behind the bow field throughout the solar system. magnetic shield shock deflects around the space controlled by Earth’s magnetic field. Corona An invisible structure protects Earth from This is the Sun’s outermost atmosphere and the solar all but the Sun’s worst outbursts. Scientists wind’s source. are starting to understand how it works. This is the border of Earth’s , the boundary by Francis Reddy; illustration by Roen Kelly that separates the from plasma within our planet’s magnetic control. Under typical conditions, its nose magnetic shield envelops our planet, but the only vis- THEMIS lies 40,000 miles (64,000 km) toward the Sun, but a strong ACE ible evidence it exists are the rays and curtains of the Launched in 2007, NASA’s Time History of CME can push it closer than geosynchronous . Events and Macroscale Interactions during nightside . Such a structure, which scientists call SOHO A Substorms (THEMIS) mission consists of five a magnetosphere, forms wherever the Sun’s outflow encoun- identical probes in at different distances. ters a strong planetary magnetic field. Coronal ejection (CME) In 1958, America’s first , Explorer 1, began direct A billion-ton cloud of plasma shot Wind study of the magnetosphere when it discovered the Van Allen from the Sun, a CME can race through the inner solar system at more than 5 Early warning system radiation belts — two regions of charged particles. Since then, million mph (8 million km/h). The NASA/ESA Solar and Heliospheric scientists have worked to understand the magnetosphere’s Observatory (SOHO) and NASA’s structure and the complex interactions occurring within it. Advanced Composition Explorer (ACE) and Wind spacecraft monitor solar The most recent exploration involves fleets from gusts and gales 930,000 miles (1.5 NASA and the European Space Agency (ESA). NASA’s entry Flux rope Solar wind magnetic fields can million km) upwind of Earth. is called THEMIS, for Time History of Events and Macroscale organize into a short-lived braided Interactions during Substorms. Five probes pursue orbits in bundle that directly channels which the spacecrafts’ highest altitudes periodically align plasma into the magnetosphere. above North American ground stations. In 2007, THEMIS found a temporary hole in Earth’s shield. A magnetic conduit Geosynchronous orbit called a flux rope formed and decayed over the course of a 22,240 miles (35,790 km) few hours, channeling solar wind energy inside. high complete one orbit each day and ESA’s Cluster mission flies four probes in a pyramid-shaped remain above a fixed point on Earth. Earth Many weather and communications formation. Findings include giant plasma swirls that form in satellites operate here. much the same way as the ripples of a flag in the wind. Earlier this year, scientists announced that Cluster had This probe, launched in 1992, represents a located the source of auroral kilometric radiation. The intense collaborative effort by NASA and Japan. It 50-to-500-kilohertz radio emission beams into space thou- explored the magnetotail more than 790,000 sands of miles above auroral regions. miles (1.3 million km) behind Earth. Such broadcasts appear to be common features of all plan- etary , says Robert Mutel, a Cluster scientist Auroral oval at the . Radio observatories now under con- This glowing oval band, which forms around each magnetic pole, struction may one day hear this signal from far-flung alien is where structured aurorae occur. Cluster worlds protected by their own magnetic shields. Launched in 2000, four identical probes make up this European Space Agency and radiation belts mission. The probes fly orbits but Francis Reddy is a senior editor of Astronomy. maintain a pyramid-shaped formation. The plasmasphere contains relatively dense, low-energy plasma. It includes the famous Van Reconnection Allen radiation belts and can be considered an The solar wind magnetic field can briefly extension of Earth’s . merge with Earth’s. These so-called Hot flow anomaly reconnection events blast magnetotail A fast stream embedded in the solar wind strikes the bow shock plasma toward Earth. THEMIS data and explodes, creating pressures that briefly push back the proved these events trigger auroral magnetopause. THEMIS detected such an event July 4, 2007. activity cycles (substorms). This extensive area of low-energy ionized Kelvin-Helmholz vortices Magnetotail gases in the magnetotail undergoes considerable change during solar storms. In 2001, Cluster detected swirls of plasma 25,000 The solar wind stretches the nightside miles across (40,000 km) at the magnetopause. magnetosphere into a tail hundreds of The swirls form when fluids move at different Earth diameters long. speeds across an interface. The same process causes a flag to ripple in the wind.