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The FORTE VHF instrument as a high-energy cosmic ray detector

The FORTE satellite records bursts of electromagnetic waves arising from near the Earth's surface in the radio frequency (RF) range of 30 to 300 MHz with a polarization-selective antenna. We investigate possible RF signatures of ultra-high energy cosmic rays (UHECR), including Cherenkov radiation in ice, UHECR-triggered lightning emission, incoherent bremsstrahlung of the ionization trail, and direct geomagnetic synchrotron radiation from the high-energy particles in the shower. The FORTE database consists of over 4 million recorded events to date, and may include a significant number associated with cosmic rays near or beyond the Greisen-Zatsemin-Kuzmin (GZK) cutoff. As a first stage of investigation, we search for FORTE events in the period from September 1997 to December 1999 which can have been produced by Cherenkov VHF radiation from a UHE neutrino shower in the Greenland ice sheet. After application of several background rejection methods, one event is left that requires further investigation.