NIGHTGLOW: an instrument to measure the Earth’s nighttime ultraviolet glow—results from the first engineering flight

We have designed and built an instrument to measure and monitor the “nightglow” of the Earth’s atmosphere in the near ultraviolet (NUV). In this paper we describe the design of this instrument, called NIGHTGLOW. NIGHTGLOW is designed to be flown from a high altitude research balloon, and circumnavigate the globe. NIGHTGLOW is a NASA, University of Utah, and New Mexico State University project. A test flight took place from Palestine, Texas on July 5, 2000, lasting about 8 h. The instrument performed well and landed safely in Stiles, Texas with little damage. The resulting measurements of the NUV nightglow are compared with previous measurements from sounding rockets and balloons.     

An interferometric analysis method for radio impulses from ultra-high energy particle showers

We present an interferometric technique for the reconstruction of ultra-wide band impulsive signals from point sources. This highly sensitive method was developed for the search for ultra-high energy neutrinos with the ANITA experiment but is fully generalizable to any antenna array detecting radio impulsive events. Applications of the interferometric method include event reconstruction, thermal noise and anthropogenic background rejection, and solar imaging for calibrations. We illustrate this technique with applications from the analysis of the ANITA-I and ANITA-II data in the 200–1200 MHz band.     

Cosmic-ray induced neutrino fluxes at the Moon: A semi-analytic approach

Cosmic-ray induced neutrino backgrounds at the Moon are estimated using a semi-analytic approach. The analytic expressions are derived, flux estimates for and are given, and comparisons with the analogous backgrounds generated in the Earth’s atmosphere are presented. Suppression of the lunar fluxes relative to the terrestrial fluxes is found. At energies >10 GeV, the suppression approaches a maximum of order 10⁻⁴. The lower background environment suggests that the Moon may be advantageous for future particle astrophysics endeavors.     

The pionic contribution to diffuse γ-rays: upper limits

Diffuse γ-rays probe the highest-energy processes at the largest scales. Here we derive model-independent constraints on the hadronic contribution to the Galactic and extragalactic γ-ray spectra at in the energy range 50 MeVE_γ10 GeV. The hadronic component is dominated by emission from neutral pions, with a characteristic spectrum symmetric about m_π⁰/2. We exploit the well-defined properties of the pion decay spectrum to quantify the maximum pionic fraction of the observed γ-ray intensity. We find that the Galactic spectrum above 30 MeV can be atmost about 50% pionic. The maximum pionic contribution to the extragalactic spectrum is energy dependent; it also depends on the redshift range over which the sources are distributed, ranging from as low as about 20% for pions generated very recently, to as much as 90% if the pions are generated around redshift 10. The implications of these constraints for models of γ-ray and neutrino emission are briefly discussed.     

The ExaVolt Antenna: A large-aperture, balloon-embedded antenna for ultra-high energy particle detection

We describe the scientific motivation, experimental basis, design methodology, and simulated performance of the ExaVolt Antenna (EVA) mission, and planned ultra-high energy (UHE) particle observatory under development for NASA’s suborbital super-pressure balloon program in Antarctica. EVA will improve over ANITA’s integrated totals – the current state-of-the-art in UHE suborbital payloads – by 1–2 orders of magnitude in a single flight. The design is based on a novel application of toroidal reflector optics which utilizes a super-pressure balloon surface, along with a feed-array mounted on an inner membrane, to create an ultra-large radio antenna system with a synoptic view of the Antarctic ice sheet below it. Radio impulses arise via the Askaryan effect when UHE neutrinos interact within the ice, or via geosynchrotron emission when UHE cosmic rays interact in the atmosphere above the continent. EVA’s instantaneous antenna aperture is estimated to be several hundred m² for detection of these events within a 150–600 MHz band. For standard cosmogenic UHE neutrino models, EVA should detect of order 30 events per flight in the EeV energy regime. For UHE cosmic rays, of order 15,000 geosynchrotron events would be detected in total, several hundred above 10 EeV, and of order 60 above the GZK cutoff energy.     

Galactic magnetic turbulence from radio data

Fluctuations in the Galactic synchrotron emission can be traced by the angular power spectrum of radio maps at low multipoles. At frequencies below few GHz, large-scale anisotropies are mainly induced by magnetic field turbulence, since non-thermal electrons radiating at these frequencies are uniformly distributed over the scales of magnetic field inhomogeneities. By performing an analysis of five radio maps, we extract constraints on turbulence spectral index and halo scale. Results favour a power spectrum significantly flatter than for 3D Kolmogorov-like turbulence, and a thin halo. This can be interpreted as an indication supporting non-conventional models of propagation of cosmic-ray particles in the Galaxy, or as a suggestion of a spectral-index break in the observed magnetic turbulence power spectrum.     

In-flight measurement of the absolute energy scale of the Fermi Large Area Telescope

The Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope is a pair-conversion telescope designed to survey the gamma-ray sky from 20 MeV to several hundreds of GeV. In this energy band there are no astronomical sources with sufficiently well known and sharp spectral features to allow an absolute calibration of the LAT energy scale. However, the geomagnetic cutoff in the cosmic ray electron-plus-positron (CRE) spectrum in low Earth orbit does provide such a spectral feature. The energy and spectral shape of this cutoff can be calculated with the aid of a numerical code tracing charged particles in the Earth’s magnetic field. By comparing the cutoff value with that measured by the LAT in different geomagnetic positions, we have obtained several calibration points between ∼6 and ∼13 GeV with an estimated uncertainty of ∼2%. An energy calibration with such high accuracy reduces the systematic uncertainty in LAT measurements of, for example, the spectral cutoff in the emission from gamma ray pulsars.     

Extensive air showers and the physics of high energy interactions

Extensive air showers are still the only source of information on primary cosmic-rays and their interactions at energies above PeV. However, this information is hidden inside the multiplicative character of the cascading process. Inspite of the great experimental and theoretical efforts the results of different studies are often ambiguous and even conflicting. These controversies can partly be referred to imperfections of our models of high energy particle interactions.

The first part of the paper is concerned with this problem. The author thinks that the present models should be corrected to give slightly deeper penetration of the cascade into the atmosphere. In this respect the modification suggested by the QGSJET-II model seems to be the step in the right direction. The Sibyll 2.1 model provides a similar penetrating properties. However, this modification is not enough and a small additional transfer of the energy from EAS hadrons to the electromagnetic component is needed too. As a possible candidate for such a process the inelastic charge exchange of pions is discussed.

In the second part of the paper the author discusses the need to account for the interaction of EAS with the stuff of detectors, their environment and the ground in the light of the ‘neutron thunder’ phenomenon, discovered recently.