'Killer Electrons' Get Super-Charged Above Earth
The solar wind pushes Earth's protective magnetosphere away from the sun, forming a magnetotail. At full moon, the moon passes through this tail.
Credit: NASA

When a shock wave from a solar storm hits the Earth?s protective magnetic bubble, it creates highly energetic particles dubbed ?killer electrons? that can be dangerous to satellites.

During solar storms, the number of killer electrons in the radiation belt grows at least 10 times. The European Space Agency's Cluster mission has helped figure out how these killer particles are created, which could help to better protect Earth's satellites and astronauts.

Killer electrons ? this is what the scientists call them ? are highly energetic charged particles that are trapped in the Earth's outer radiation belt, which extends from about 7,500 miles to 40,000 miles (12,000 km to 64,000 km) above our planet. As their name suggests, these particles are energetic enough to penetrate satellite shielding, potentially damaging them.

Wave impact

On Nov. 7, 2004, the sun blasted one of its many solar storms in Earth's direction. The storm was composed of an interplanetary shock wave followed by a large magnetic cloud.

The shock wave was detected by the joint ESA-NASA solar satellite SOHO. When it passed by SOHO, the speed of the solar wind (the constant flow of solar particles moving out from the sun), suddenly jumped from 300 miles per second (500 km per second) to nearly 450 miles per second (700 km per second).

Shortly afterward, the shock wave hit the Earth's magnetosphere, the magnetic shield that surrounds the planet and generally protects it from energetic solar particles. The impact from the shock wave created a wave front that propagated inside the magnetosphere at more than 7,500 miles per second (12,000 km per second) at geostationary orbit (22,400 miles or 36,000 km above the ground) around Earth.

When the shock wave hit, the number of killer electrons in the outer radiation belt started to increase too, as detected by instruments on Cluster's four satellites, which sweep in an elliptical orbit around the Earth.

Cluster's measurements have helped scientists choose between possible explanations for how killer electrons are created. The two proposed possibilities include electrons being accelerated by waves, with one method relying on very low frequency (VLF) waves, which are in the 3 to 30 kHz range, the other on ultra low frequency (ULF waves), in the 0.001 to 1 Hz range.

Two types of waves

It turns out that both types of waves are to blame.

"Both VLF and ULF waves accelerate electrons in Earth's radiation belts, but with different timescales. The ULF waves are much faster than the VLF, due to their much larger amplitudes," said Qiugang Zong from Peking University (China) and University of Massachusetts Lowell, lead author of the study describing the process. The study is detailed in a recent issue of the Journal of Geophysical Research.

The Cluster data shows that a two-step process causes the substantial rise of killer electrons: The initial acceleration is due to the strong shock-related magnetic field compression. Immediately after the impact of the interplanetary shock, Earth's magnetic field lines began wobbling at ultra low frequencies. In turn, these ULF waves were found to effectively accelerate the seed electrons provided by the first step, to become killer electrons.

Thanks to this analysis of Cluster data, if the killer electrons happen to be ejected towards Earth, we now know that they can strike the atmosphere within just 15 minutes.

"These new findings help us to improve the models predicting the radiation environment in which satellites and astronauts operate. With solar activity now ramping up, we expect more of these shocks to impact our magnetosphere over the months and years to come," said Philippe Escoubet, ESA's Cluster mission manager.

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