Plasma waves and cosmic rays

With energies exceeding 10²⁰ eV, the highest-energy cosmic-ray protons are as energetic as well-hit tennis balls. How does a proton become so energetic? Recent cosmic-ray data disfavor the notion that these ultra-energetic protons have exotic origins such as the decay of very massive particles as yet unidentified. So one must seek the proton acceleration mechanism in familiar astrophysical environments. The conventional suggestions—acceleration by relativistic shocks, spinning black holes, or flares on hypermagnetized neutron stars—each have problems accounting for the highest observed energies. Shock acceleration, for example, becomes increasingly inefficient at high energy because the inevitable trajectory bending causes severe synchrotron energy loss. Now theorist Pisin Chen (SLAC and National Taiwan University) and coworkers have demonstrated analytically and by computer simulation that so-called magnetowaves—electromagnetic waves with unusually strong magnetic components in magnetized plasmas—can drive plasma waves in their wake much as laser pulses in the laboratory drive plasma wakefields in...