We report on four radio-detected cosmic-ray (CR) or CR-like events observed with the Antarctic Impulsive Transient Antenna (ANITA), a NASA-sponsored long-duration balloon payload. Two of the four were previously identified as stratospheric CR air showers during the ANITA-I flight. A third stratospheric CR was detected during the ANITA-II flight. Here, we report on characteristics of these three unusual CR events, which develop nearly horizontally, 20-30 km above the surface of Earth. In addition, we report on a fourth steeply upward-pointing ANITA-I CR-like radio event which has characteristics consistent with a primary that emerged from the surface of the ice. This suggests a possible τ-lepton decay as the origin of this event, but such an interpretation would require significant suppression of the standard model τ-neutrino cross section.

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We report the observation of sixteen cosmic ray events of mean energy of 1.5 × 10 19 eV, via radio pulses originating from the interaction of the cosmic ray air shower with the Antarctic geomagnetic field, a process known as geosynchrotron emission. We present the first ultra-wideband, far-field measurements of the radio spectral density of geosynchrotron emission in the range from 300-1000 MHz. The emission is 100% linearly polarized in the plane perpendicular to the projected geomagnetic field. Fourteen of our observed events are seen to have a phase-inversion due to reflection of the radio beam off the ice surface, and two additional events are seen directly from above the horizon.The origin of ultra-high energy cosmic rays (UHECR) remains a mystery decades after their discovery [1,2]. Key to the solution will be increased statistics on events of high enough energy (≥ 3 × 10 19 eV) to elucidate the endpoint of the UHECR energy spectrum as seen at Earth. The primary difficulty is the extreme rarity of events at these energies. Despite steady progress with experiments such as the Pierre Auger Observatory, there remains room for new methodologies. Cosmic rays have been detected for decades via impulsive radio geosynchrotron emission [3,[5][6][7][8][9][10][11][12][13][14][15][16]] but until now not in this crucial energy range, which offers the possibility of pointing the UHECRs back to their sources. We present data from the Antarctic Impulsive Transient Antenna (ANITA) [21] which represents the first entry of radio techniques into this energy range. We find 16 UHECR events, at least 40% of which are above 10 19 eV, and we show compelling evidence of their origin as geosynchrotron emission from cosmic-ray showers. Our results indicate degree-scale precision for reconstruction of the UHECR arrival direction, lending strong credence to efforts to develop radio geosynchrotron detection as a competitive method of UHECR particle astronomy.Geosynchrotron emission arises when the electron-positron particle cascade initiated by a primary cosmic ray encounters the Lorentz force in the geomagnetic field. The resulting acceleration deflects the electrons and positrons and they begin to spiral in opposite directions around the field lines [17,18]. In air, the particles' radiation length is of order 40 g cm −2 , a kilometer or less at the altitudes of air shower maximum development. Particle trajectories form partial arcs around the field lines before they lose enough energy to drop out of the shower. The meter-scale longitudinal thickness of the shower particle 'pancake' is comparable to radio wavelengths below several hundred MHz; thus the ensemble behavior of all of the cascade particles yields forward-beamed synchrotron emission which is partially or fully coherent in the radio regime. Therefore, the resulting radio impulse power grows quadratically with primary particle energy, and at the highest energies, yields radio pulses that are detectable at large distances. Current systems under development for detection of thes...

We report on the first observations of the Askaryan effect in ice: coherent impulsive radio Cherenkov radiation from the charge asymmetry in an electromagnetic (EM) shower. Such radiation has been observed in silica sand and rock salt, but this is the first direct observation from an EM shower in ice. These measurements are important since the majority of experiments to date that rely on the effect for ultra-high energy neutrino detection are being performed using ice as the target medium. As part of the complete validation process for the Antarctic Impulsive Transient Antenna (ANITA) experiment, we performed an experiment at the Stanford Linear Accelerator Center (SLAC) in June 2006 using a 7.5 metric ton ice target, yielding results fully consistent with theoretical expectations.Very large scale optical Cherenkov detectors such as the Antarctic Muon and Neutrino Detector Array (AMANDA) and its successor IceCube have demonstrated the excellent utility of Cherenkov radiation in detection of neutrino interactions at >TeV energies [1, 2] with ice as a target medium. However, at neutrino energies above 100 PeV, the cubic-km scale of such detectors is inadequate to detect more than a handful of events from the predicted cosmogenic neutrino fluxes [3] which represent the most compelling models at these energies. The relevant detector volume for convincing detection and characterization of these neutrinos is in the range of hundreds to thousands of cubic km of water equivalent mass, and the economic constraints of scaling up the optical Cherenkov technique almost certainly preclude extending it much beyond the size of the current IceCube detector, which will be completed early in the next decade.Given the need for an alternative technique with a more tractable economy of scale to reach into the EeV (=1000 PeV) energy regime, a new method which we denote the radio Cherenkov technique, has emerged within the last decade. This method relies on properties of electromagnetic cascades in a dielectric medium. It was first hypothesized by Askaryan [4] and confirmed in 2001 at SLAC [5]. High energy processes such as Compton, Bhabha, and Moller scattering, along with positron annihilation rapidly lead to a ∼ 20% negative charge asymmetry in the electron-photon part of a cascade. In dense media the shower charge bunch is compact, largely contained within a several cm radius. At wavelengths of 10 cm or more, much larger than the characteristic shower bunch size, the relativistic shower bunch appears as a single charge moving through the dielectric over a distance of several meters or more. As an example, a typical shower with mean Bjorken inelasticity y = 0.2, initiated by a E ν = 100 PeV neutrino will create a total number of charged particles at shower maximum of order n e+ +n e− = y E ν /1 GeV ∼ 2 × 10 7 . The net charge is thus n e+ − n e− − ∼ 4 × 10 6 e. Since the radiated power for Cherenkov emission grows quadratically with the charge of the emitter, the coherent power in the cm-to-m wavelength regime is ∼ 10 13 times gre...

The first flight of the Antarctic Impulsive Transient Antenna (ANITA) experiment recorded 16 radio signals that were emitted by cosmic-ray induced air showers. The dominant contribution to the radiation comes from the deflection of positrons and electrons in the geomagnetic field, which is beamed in the direction of motion of the air shower. For 14 of these events, this radiation is reflected from the ice and subsequently detected by the ANITA experiment at a flight altitude of ∼36 km. In this paper, we estimate the energy of the 14 individual events and find that the mean energy of the cosmic-ray sample is 2.9 × 10 18 eV, which is significantly lower than the previous estimate. By simulating the ANITA flight, we calculate its exposure for ultra-high energy cosmic rays. We estimate for the first time the cosmic-ray flux derived only from radio observations and find agreement with measurements performed at other observatories. In addition, we find that the ANITA data set is consistent with Monte-Carlo simulations for the total number of observed events and with the properties of those events.