Gamma ray signatures of ultra high energy cosmic ray accelerators: electromagnetic cascade versus synchrotron radiation of secondary electrons

We discuss the possibility of observing ultra high energy cosmic ray sources in high energy gamma rays. Protons propagating away from their accelerators produce secondary electrons during interactions with cosmic microwave background photons. These electrons start an electromagnetic cascade that results in a broad band gamma ray emission. We show that in a magnetized Universe (B≳10⁻¹² G) such emission is likely to be too extended to be detected above the diffuse background. A more promising possibility comes from the detection of synchrotron photons from the extremely energetic secondary electrons. Although this emission is produced in a rather extended region of size ∼10 Mpc, it is expected to be point-like and detectable at GeV energies if the intergalactic magnetic field is at the nanogauss level.      

Observing cosmic microwave background polarization through ice

Ice crystal clouds in the upper troposphere can generate polarization signals at the μK level. This signal can seriously affect very sensitive ground-based searches for E and B modes of cosmic microwave background polarization. In this paper, we estimate this effect within the C_ℓOVER experiment observing bands (97, 150 and 220 GHz) for the selected observing site (Llano de Chajnantor, Atacama desert, Chile). The results show that the polarization signal from the clouds can be of the order of or even bigger than the cosmic microwave background expected polarization. Climatological data suggest that this signal is fairly constant over the whole year in Antarctica. On the other hand, the stronger seasonal variability in Atacama allows for a 50 per cent of clean observations during the dry season.     

Microstructural characterization of firn

In this paper, we use a scanning electron microscope (SEM) coupled with X‐ray spectroscopy and electron back‐scattered diffraction patterns to examine firn in cores retrieved by the United States International Trans‐Antarctic Scientific Expedition. From grain boundary grooves we were able to see where the previously existing snow crystals were joined, and can determine grain sizes. From the SEM images, the porosity and the surface area per unit volume of the pores were measured. Finally, we have shown that we can determine the microchemistry of impurities in firn and demonstrated that we can determine the orientations of the firn crystals. Copyright © 2007 John Wiley & Sons, Ltd.     

The Latitudinal Effect of Corotating Interaction Regions on Galactic Cosmic Rays

The response of galactic cosmic rays (GCRs) to an isolated enhancement of the non-axisymmetric component of the solar open magnetic field between June and November 1996 is investigated by using a combination of solar observations and numerical modelling of the interplanetary medium. The most obvious coronal hole visible from Earth at the time had little shielding effect on the flux of GCRs, as measured at Earth by neutron monitors. It is found that the evolution of the corotating interaction regions generated by a less obvious coronal hole was the principal controlling factor. Moreover, we demonstrate the imprint of the latitudinal and longitudinal evolution of that coronal hole on the variation of GCRs. The latitudinal extent of this solar minimum corotating interaction region had a determining, but local, shielding effect on GCRs, confirming previous modelling results.