  We propose a new method to identify flavors of ultra high energy cosmic
neutrinos. Energy loss of leptons in matter provides important informations for
the detection of neutrinos originated from high energy astrophysical sources.
50 years ago, Askaryan proposed to detect Cherenkov signals by radio wave from
the negative charge excess of particle showers. The theory of Cherenkov pulses
with Fraunhofer approximation was widely studied in the past two decades.
However, at high energies or for high density materials, electromagnetic shower
should be elongated due to the Landau-Pomeranchuck-Migdal (LPM) effect. As such
the standard Fraunhofer approximation ceases to be valid when the distance
between the shower and the detector becomes comparable with the shower length.
We have performed Monte Carlo simulations recently to investigate this regime
based on the finite-difference time-domain (FDTD) method, and modified time
domain integration method. In this work, we adopt the deduced relationship
between the radio signal and the cascade development profile to investigate its
implication to lepton signatures. Our method provides a straightforward
technique to identify the neutrino flavor through the detected Cherenkov
signals.
