Session E9: Galactic Cosmic Rays

Asymmetric diffusion of Cosmic Rays

We study propagation of Cosmic Rays (CR) in turbulent magnetized ISM in the presence of a gradient of the mean magnetic field. We discovered that CR propagate via asymmetric diffusion: the generalization of the conventional random walk to that with unequal probabilities. We presented a toy model of CR propagation in the Galaxy as a 1D Markov chain and demonstrate that the particle density distribution drastically differs from the linear gradient set by the standard diffusion process. We discuss implications of our findings.   

Cosmic-Ray Anisotropy with the HAWC Observatory

The High-Altitude Water Cherenkov (HAWC) Observatory is a TeV gamma-ray and cosmic-ray detector operating at an altitude of 4100 meters in Mexico. HAWC is an extensive air-shower array. Upon completion in 2014, it will comprise 300 optically-isolated water-Cherenkov detectors. While the observatory is only partially deployed, with $\sim$ 100 Cherenkov detectors in data acquisition since summer 2013, statistics are sufficient to perform studies of cosmic-ray anisotropy. We discuss the status and performance of the detector, including the pointing accuracy and angular resolution as inferred from the observation of the moon shadow and simulations, and present new results on small-scale cosmic-ray anisotropy from our ever-growing detector and dataset.   

Cosmic-ray anisotropy studies with IceCube

The IceCube neutrino observatory detects tens of billions of energetic muons per year produced by cosmic-ray interactions with the atmosphere. The size of this sample has allowed IceCube to observe a significant anisotropy in arrival direction for cosmic rays with median energies between 20 and 400 TeV. This anisotropy is characterized by a large scale structure of per-mille amplitude accompanied by structures with smaller amplitudes and with typical angular sizes between $10^{\circ}$ and $20^{\circ}$. IceTop, the surface component of IceCube, has observed a similar anisotropy in the arrival direction distribution of cosmic rays, extending the study to PeV energies. The better energy resolution of IceTop allows for additional studies of the anisotropy, for example a comparison of the energy spectrum in regions of a cosmic-ray excess or deficit to the rest of the sky. We present an update on the cosmic-ray anisotropy observed with IceCube and IceTop and the results of first studies of the energy spectrum at locations of cosmic-ray excess or deficit.   

Constraints on Galactic Cosmic-Ray Origins from Elemental Composition Measurements

We present measurements of the elemental abundances of ultra-heavy (Z$>$29) cosmic rays made by the Cosmic Ray Isotope Spectrometer (CRIS) on NASA's Advanced Composition Explorer (ACE) satellite. The data correspond to more than 5000 days of data collection beginning December 4, 1997. The resolution in charge that we obtain is excellent, exhibiting essentially complete separation of adjacent charges in the Z$>$28 range. We detected 166 events over the charge range of 30$<$Z$<$41, slightly more than the corresponding number of events from the combination of two earlier balloon flights of the TIGER instrument. Our data agree well with the TIGER results. They show that the ordering of refractory and volatile elements with atomic mass is greatly improved when compared to a mix of massive star outflow and SN ejecta with normal ISM, rather than pure ISM, that the refractory and volatile elements have similar slopes, and that refractory elements are preferentially accelerated by a factor of $\sim$4. We conclude that these data are consistent with an OB association origin of GCRs.   

Energy Spectrum Measurements of Cosmic-Ray Hydrogen and Helium Isotopes with the BESS-Polar II Instrument

The Balloon-Borne Experiment with a Superconducting Spectrometer (BESS-Polar II) flew successfully over Antarctica during 24.5 days in December 2007 through January 2008 during a period of minimum solar activity. The long duration of the flight, and the good stability of the detectors, improved by a factor of 5 the number of cosmic-ray events previously recorded with BESS-Polar I, reaching about 4.7 billion collected particles. Energy spectrum of cosmic-ray hydrogen and helium isotopes have been measured with the instrument from 0.2 to about 1.5 GeV/n, with unprecedented accuracy. These new flux and ratio measurements provide important information to better understand the propagation history of cosmic rays in the Galaxy. The results obtained with the BESS-Polar II instrument will be presented and compared with different propagation models.   

Elemental Abundances of Ultra-Heavy Nuclei Measured by SuperTIGER: Preliminary Results

The SuperTIGER (Trans-Iron Galactic Element Recorder) experiment was launched on a long-duration balloon flight from Williams Field, Antarctica, on December 8, 2012 and flew for a total of 55 days at a mean atmospheric depth of 4.4 g/cm$^{2}$. The instrument is designed to measure the abundances of galactic cosmic rays in the charge (Z) range from 10 $\leq Z \leq 40$ with high statistical precision and excellent charge resolution, with exploratory measurements into the 40 $ < Z \leq$ 60 range. The instrument, the methods of charge identification that are employed, and the SuperTIGER balloon flight will be described. The data that will be presented contain more than 600 events with charge 30 $< Z \leq$ 40, $\sim$ 4.5x that collected by the two TIGER flights combined. The charge resolution obtained for iron is $<$0.18 cu. We will discuss the OB association model for the origin of galactic cosmic rays and show how SuperTIGER measurements will be used as a stringent test of this model.   

CALET -- The CALorimetric Electron Telescope for the International Space~Station

The CALET space experiment, currently under development by a Japanese-Italian-US collaboration, will measure the flux of cosmic ray electrons and positrons to 20 TeV, gamma rays to 10 TeV, and nuclei with Z$=$1 to 40 up to 1,000 TeV during a five year mission. CALET will provide information on possible nearby astrophysical sources of high energy electrons, study the details of galactic particle propagation and search for dark matter signatures. The instrument consists of a particle charge identification module, a thin (3 radiation lengths) imaging calorimeter with tungsten plates and interleaved scintillating fiber planes, and a thick (27 radiation lengths) lead tungstate calorimeter. CALET has the depth, imaging capability, and energy resolution necessary for excellent separation of hadrons, electrons and gamma rays. The instrument is currently being prepared for launch and installation by the end of 2014 on the Japanese Experiment Module - Exposed~~Facility (JEM-EF) on the International Space Station (ISS).   

Predicted Effect of Geomagnetic Field on CALET Measurements

The CALorimetric Electron Telescope (CALET), comprised of the main calorimeter (CAL) and Gamma-ray Burst Monitor (CGBM) subsystem, is under construction for launch to the ISS. CAL consists of a scintillator Charge Detector (CHD), a 3 radiation length ($X_{0}$) deep scintillating fiber Imaging Calorimeter (IMC), and a 27 $X_{0}$ deep PWO Total Absorption Calorimeter (TASC). The primary objectives of CAL are to measure energy spectra of electrons from 1GeV to 20 TeV and nuclei through iron up to 1,000 TeV, and to detect gamma-rays above 10 GeV. Earth's geomagnetic field in the 51.6$^{\circ}$ inclination ISS orbit will affect the observed fluxes of charged particles. Rigidity cutoffs based on geomagnetic latitude and East-West angle will introduce structure to the charged particle energy spectra. They can also be exploited to facilitate the measurement of distinct positron and electron fluxes between $\sim$3-20 GeV, and the relative abundances of the rare ultra-heavy (UH) nuclei ($30 \le Z \le 40$) by using the cutoffs to select nuclei near and above the CHD minimum ionization threshold so that they can be identified using the CHD and top IMC layers without requiring energy determination in the TASC. In 5-years CAL would collect $\sim$2$\times$ the UH statistics of TIGER.