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The ANITA (ANtarctic Impulsive Transient Antenna) experiment is a balloon-borne neutrino telescope which consists of an array of 32 broad-band horn antennas. It successfully completed a 35 day flight over Antarctica during the 2006-2007 austral summer. The primary goal of ANITA is to search for astrophysical neutrinos with energies E > 10¹⁹eV by detecting radio Cherenkov signals from neutrino induced showers in the Antarctic ice. We present preliminary results from ongoing analyses of ANITA data.

Extended Air Showers produced by cosmic rays impinging on the earth atmosphere irradiate radio frequency radiation through different mechanisms. Upon certain conditions, the emission has a coherent nature, with the consequence that the emitted power is not proportional to the energy of the primary cosmic rays, but to the energy squared. The effect was predicted in 1962 by Askaryan and it is nowadays experimentally well established and exploited for the detection of ultra high energy cosmic rays.

In this paper, we discuss in detail the conditions for coherence, which in literature have been too often taken for granted, and calculate them analytically, finding a formulation which comprehends both the coherent and the incoherent emissions. We apply the result to the Cherenkov effect, obtaining the same conclusions derived by Askaryan, and to the geosynchrotron radiation.

Detecting ultra-high energy neutrinos (UHECNs) with energies above 10¹⁷eV, or the GZK neutrinos, is a fundamental problem in neutrino astronomy. By finding GZK neutrinos, not only the GZK process can be verified, but also provides valuable insights of the ultra-high energy cosmic rays. Equally important is to identify the flavor of the GZK neutrinos. The flavor composition of these neutrinos provides information about the source of these UHECNs. With these UHECNs travelling astronomical distances, it is also possible to measure neutrino oscillation parameters with extremely long baseline. Here we present the current status of searching UHECNs with Askaryan Radio Array (ARA) in South pole, and the potential of flavor identification of UHECNs with ARA experiment.