Which waves are responsible? Both of them. "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," says Qiugang Zong from Peking University (China) and University of Massachusetts Lowell (USA), lead author of the paper describing this result.
The data show 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.
Although the analysis has been a long one, the results have been worth the wait. Now astronomers know how killer electrons are accelerated. "Data from the four Cluster satellites allowed the identification of ULF waves able to accelerate electrons," says Malcolm Dunlop, Rutherford Appleton Laboratory, Didcot (UK) and co-author of this study.
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," says Philippe Escoubet, ESA's Cluster mission manager.
Data from Cluster, and other spacecraft monitoring the magnetosphere, have shown that interplanetary shocks -- caused by coronal mass ejections from the Sun -- can create "killer electrons" in the near-Earth space environment within 15 minutes of the shock reaching the Earth's protective magnetic bubble. Killer electrons are highly energetic particles trapped in the Earth's outer radiation belt.
(Photo Credit: ESA)
ESA's mission Cluster consists of four identical spacecraft flying in formation between 19 000 and 119 000 km above the Earth. They study the interaction between the solar wind and Earth?s magnetosphere, or the Sun-Earth connection in 3-D.
(Photo Credit: ESA)
Source: European Space Agency