Cosmic rays radiate radio waves

May 27, 2005
Astrophysicists using a radio antenna array have detected radio flashes associated with ultrahigh-energy cosmic rays (UHECRs). These flashes are the brightest and fastest radio flashes ever seen in the sky, and this marks the first time radio flashes associated with UHECRs have been imaged. The international group's findings were published in the May 19 issue of Nature.

The radio antenna array used, called LOPES, is a prototype experiment of the future Low-Frequency Array (LOFAR). LOPES is located in Karlsruhe, Germany, on the same site as the Karlsruhe Shower Core and Array Detector (KASCADE), an astroparticle detector. Because LOPES and KASCADE are at the same location, they can study different aspects of a cosmic-ray shower.

By combining data from both radio and particle detectors, astrophysicists expect to learn more about the nature and composition of UHECRs — cosmic rays with energies above 10²⁰ eV — which remain a mystery.

Cosmic rays are high-energy particles that bombard Earth at a rate of about 1 particle per centimeter-squared each second. These particles — the "primary" cosmic rays — are mostly protons, but also can be heavier nuclei or electrons. (UHECRs could be "less common" particles.) When the primary cosmic rays collide with atoms in the atmosphere they produce showers of particles called "secondary" cosmic rays. These secondaries are mostly pi-mesons — neutral and charged pions. The neutral pions decay into pairs of gamma rays, which may create an electron and a positron. The charged pions eventually decay into muons and muon neutrinos.

These particles hit Earth's surface and are detected by particle experiments. Cosmic-ray detectors also measure these particles' energies, and therefore can calculate the primary particle's energy.

As electrons and positrons speed through the atmosphere, they are deflected by Earth's magnetic field. This process gives off synchrotron radiation — as the electron changes direction something must be emitted to conserve momentum — with some of this radiation in the radio end of the electromagnetic spectrum.

LOPES detected radio pulses that were coincident with the direction of shower particles. The astrophysicists write in their Nature paper: "The close association between radio flashes and cosmic ray air showers in direction and time shows that the radio emission is directly associated with the shower itself."

LOPES imaged this radio flash. The movie is not in "real time" but spans 0.1 microsecond. The flash itself lasts roughly 30 nanoseconds. LOPES collaboration