C4. Commission on Cosmic Rays (1947)

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Report to the 2005 General Assembly for 2002-2005
Cape Town, South Africa
26 – 28 October 2005


C4 is the standing international advisory committee for the International Cosmic Ray Conference (ICRC) series. The 28th ICRC was held at Tsukuba, Japan, 31 July - 7 August 2003. There were 760 attendees, including 160 students. The O'Ceallaigh Award was presented to Frank McDonald (USA) for his major contributions to cosmic-ray physics over many years. The Yodh Prize for important and pioneering contributions to the field of cosmic-ray physics went to B.V. Sreekantan (India). Pasquale Blasi (Italy) won the Shakti P. Duggal Award for outstanding young scientists in the field of cosmic-ray physics.

The 29th ICRC was held in Pune, India, 3-10 August 2005. It was attended by 580 scientists. Tom Gaisser (USA) received the O'Ceallaigh Medal. The Yodh Prize went to Michael Hillas (UK). Jim Hinton (UK) was presented with the Duggal Award.

The 30th ICRC is scheduled from 3 - 11 July 2007 in Mérida, the capital of the Mexican state of Yucatán. 

Web site:

A unique aspect of the ICRC series is the opportunity for interdisciplinary interactions between particle physics and cosmic-ray physics on the one hand and between space physics and high-energy astrophysics on the other. There have been major new results in both areas that are driving major new endeavours (see below).

C4 met during both conferences. The Commission recommended IUPAP support for the following topical conferences: 8 th International Workshop on Topics on Astroparticle and Underground Physics, TAUP2003 (Seattle, USA, 5 - 9 September 2003); 9 th International Conference on Topics on Astroparticle and Underground Physics, TAUP2005 (Zaragoza, Spain,11 -14 September 2005) ; the 20 th European Cosmic Ray Symposium (Lisbon, Portugal, 5 – 8 September 2006).

C4 is also the IUPAP liaison (along with C11, C12 and C19) to IUPAP Working Group 4, the Particle and Nuclear Astrophysics and Gravitation International Committee (PaNAGIC), Working Group 4 of IUPAP. The PaNAGIC report will be presented separately.

C4 maintains its homepage and newsletter COSNEWS on the IUPAP web site. C4 also provided a chapter to ‘Physics Now’, a compilation by C14 of reviews by scientists from various IUPAP Commissions (IUPAP-39, 2004).

The proceedings of the ICRC series can be found via the Astronomical Data System (ADS).

Web site:


The Voyager 1 spacecraft, after more than 28 years in space and at a distance of 94 AU from the Sun, crossed the solar-wind termination shock and is now exploring the final layer of the solar plasma environment, the heliosheath. The strongest evidence that Voyager 1 passed through the termination shock was the enhanced magnetic field, as expected in the subsonic flow in the heliosheath. Contrary to predictions, the low-energy anomalous cosmic rays (ACR) were not observed, indicating that their source region is remote from the location of Voyager 1.

The highest energy cosmic rays may reveal their secrets over the next few years. These particles, at energies in excess of 10 20eV, are expected to be extra-galactic in origin. These particles should show a “cutoff” due to interactions with the microwave background radiation, as originally proposed by Greisen, Zatsepin and Kuzmin (GZK). The progress of the Piere Auger Observatory in Argentina has made a high statistics observation of this feature possible within the next few years. The establishment of an extra-galactic contribution to the observed cosmic rays through observation of the GZK effect will be a major scientific result. The absence of this effect will be puzzling indeed, since a source within our galaxy producing these enormous energies has yet to be identified.

In the last two years the new generation of TeV (10 12 eV) gamma-ray telescopes have begun to operate. The production of new results is presently led by the H.E.S.S. group which has had a telescope array operating in Namibia since 2003. In the near future the CANGAROO, MAGIC and VERITAS groups are expected to become similarly productive as these telescopes become fully operational. These new results have been spectacular; for the first time images of high-energy gamma-ray sources have been produced, including shells of supernovae explosions. If it can be demonstrated that these gamma-rays are produced through hadronic processes, the sources of most of the cosmic rays in our galaxy will finally be identified through these images. The increased sensitivity of the new measurements revealed many new TeV sources both galactic and extragalactic in nature. Some of the sources are unknown in other wavelength regions. This field is expected to provide many new scientific results over the next few years.


Neutrino physics:

One of the most important contributions of cosmic-ray physics to particle physics in the last decades has been the discovery, using natural fluxes of neutrinos, of the phenomenon of neutrino oscillations (flavour transitions), i.e. the transformation of a type (or flavour) of neutrinos into another. The neutrino flavour transitions have been observed for solar neutrinos, produced in nuclear reactions in the centre of the Sun, and for atmospheric neutrinos, produced in cosmic ray showers in the Earth’s atmosphere. The results obtained from both observations are compatible with each other and give important constraints on the neutrino masses and the family structure of elementary particles.

The experimental study of solar neutrinos began in the late 1960’s with the Homestake detector that measured the highest energy part of the solar neutrino flux, obtaining a flux approximately one third of the expectations. After this result several other experiments, most notably the water Cherenkov detector Super-Kamiokande in Japan, the heavy water Sudbury Neutrino Observatory (SNO) experiment in Canada, GALLEX-GNO in Italy and SAGE in Russia, also measured the flux of solar neutrinos with greater accuracy. These experiments established that while all solar neutrinos are created in the core of the Sun with the electron flavour, only approximately one third of them reach the Earth in this original state, while the remaining fraction arrives transformed into muon and/or tau neutrinos. The long standing solar neutrino problem no longer exists.

The experimental study of atmospheric neutrinos started in the 1980’s with large mass underground detectors, originally built to search for the existence of proton decay. These studies demonstrated that approximately one half of the muon neutrinos that travel on trajectories that cross the Earth transform into tau neutrinos.

Both results on the flavour transitions of solar and atmospheric neutrinos have been confirmed in studies using man-made neutrinos: the solar neutrino results by studying the propagation of anti-electron neutrinos generated in nuclear reactors over a mean distance of 180 km with the Japanese KamLAND experiment, and the atmospheric neutrino results by studying the propagation of accelerator neutrinos over a distance of 250 km with the Japanese K2K experiment. Several other “long-baseline” neutrino experiments are under construction for future more detailed studies.

It is rewarding for this field that two pioneers of neutrino detection, Raymond Davis and Masatoshi Koshiba, were amongst the 2002 Physics Nobel Laureates.

Sudbury Neutrino Observatory (SNO):


The solar wind, a turbulent, magnetized plasma emanating from the Sun with a velocity of 400-800 km/s, dominates the region around our Sun out to approximately 100 AU. A global picture of the solar wind and other activity in the heliosphere has emerged from data collected from a variety of spacecraft, including Ulysses that pioneered the exploration of the regions over the solar poles, and the two Voyager spacecraft in the outer heliosphere. Various kinds of shocks driven by solar activity, including huge coronal mass ejections, accelerate particles to velocities orders of magnitude greater than that of the ambient plasma. These events have been studied in situ by spacecraft that can associate specific transient populations of energetic particles with specific shocks. The heliosphere thus serves as a laboratory for cosmic-ray acceleration on larger scales by distant galactic and extra-galactic sources not accessible to direct observation.

On 16 December 2004 Voyager 1, at a distance of 94 AU from the Sun, crossed the solar-wind termination shock and entered the solar system's final frontier, the heliosheath. The strongest evidence that Voyager 1 has moved into the slower, denser wind beyond the weak shock is the measured increase (a factor of two and a half) in the strength of the magnetic field carried by the solar wind and the inferred decrease in its speed. The energetic particle measurements revealed new surprises: the spectrum of the anomalous cosmic rays (ACR) did not unroll at the shock, indicating that the ACR source is a remote region of the shock not connected to Voyager 1 along the magnetic field. Episodes of enhanced intensities of termination shock particles (TSP) of energies below the ACR have been observed upstream of the shock and in the heliosheath. The future evolution of these particle populations should reveal new aspects of the acceleration processes and their sources. With the termination shock moving inward as the solar wind pressure declines, Voyager 2, now at 75 AU, may also encounter the shock within a few years.

The turbulent solar wind modulates the intensity of galactic cosmic rays (GCR) with an 11-year periodicity as they diffuse upstream against the outward flowing wind to reach the inner heliosphere. The 11-year cycle of solar activity is characterized by a reversal of the solar magnetic field associated with each solar maximum. Thus there is a 22-year cycle, with alternating decade-long intervals of relatively quiet positive and negative solar magnetic fields.

2007 will see an international program of scientific collaboration: the International Heliophysical Year (IHY). Its goal is to advance the understanding of the universal heliophysical processes that govern the Sun, Earth and heliosphere as a system.


International Heliophysical Year (IHY):

GCR spectra, composition and antimatter :

A new generation of measurements with magnetic spectrometers has greatly improved our knowledge of the fundamental observables in cosmic-ray physics: the energy spectra of proton, helium and light nuclei, the isotope abundances of light elements, including the radioactive 10Be isotope, and the abundances of antiprotons and positrons. While the discovery of heavy antimatter would have a major impact on our understanding of the origin of matter in our universe, the accurate studies of cosmic ray abundances and their variations with energy are the key science motivations for these observations.

These measurements were to a large part performed with balloon-borne experiments such as BESS, CAPRICE and others. A test flight of the Alpha Magnetic Spectrometer (AMS 01) on the Space Shuttle in 1998 also contributed to this effort. The cosmic ray spectra of protons and helium up to some hundreds of GeVs are now known to a precision of about 10%. The observed flux of antiprotons as measured in the energy range from a few hundred of MeV up to 40 GeV is consistent with a secondary origin from collisions between high energy  cosmic-ray protons and nuclei and the interstellar medium. There is yet no sign in the antiproton data for the existence of exotic particles, such as hypothetical weakly interacting massive particles that are believed to be dark matter candidates. There is also no sign for primodial antideuterons and antihelium.

A series of balloon flights of the BESS experiment through 1993 to 2002 (covering nearly a solar activity cycle) provided a unique set of observations which unambiguously
indicate a charge sign effect which can help to clarify the interplay of magnetic drifts and diffusion in the process of solar modulation. Recently a BESS-Polar long-duration balloon flight of 9 days was successfully performed to extend the antiproton spectrum down to 100 MeV with better statistics.

The wealth of information contained in cosmic-ray isotope abundances makes it possible to study aspects of acceleration, propagation and lifetime in the interstellar medium. Based on isotope data from the Cosmic Ray Isotope Spectrometer (CRIS) on ACE and the ISOMAX experiment the time between nucleosynthesis and acceleration of 10 5 yrs and a mean confinement time in the galaxy of about 15 Myrs have been determined.

Significant new data on the cosmic ray composition have also been recently acquired in the poorly known energy interval around and above the TeV (10 15 eV) region. The spectra and composition of cosmic rays up to tens of TeV were measured by ATIC (Advanced Thin Interaction Calorimeter) as well as by CREAM (Cosmic Ray Energetics And Mass) on long-duration balloon flights around the south pole, the latter with a recent record breaking 42-day flight. The TIGER (Trans-Iron Galactic Element Recorder) experiment had two successful flights around the south pole making a measurement of trans-iron elements in the GeV/amu energy range. The TRACER (Transition Radiation Array for Cosmic Energy Rays) had a successful balloon flight measuring spectra and composition from oxygen to iron in the 0.5 - 10 TeV/amu range. Another major experimental challenge is to study directly the composition of cosmic-ray nuclei at energies above 100 TeV, where the conventional acceleration models (based on shock acceleration in supernova remnants) fail.

Future long-duration exposure in space of the PAMELA instrument and of the large AMS 02 on the International Space Station (ISS) will extend the measurements of antiprotons and positrons into the region of some hundreds of GeVs with high precision and will increase the sensitivity to detect antimatter. PAMELA, to be flown aboard the Russian Resurs DK-1 satellite with tentative launch in December 2005, combines a permanent magnet with other detectors to measure primarily antiprotons and positrons in the range 10 8-2×10 11eV and a search for antihelium with sensitivity for antiHe/He < 10 -7. (PAMELA will also be able to measure cosmic ray spectra of light elements up to some hundreds of GeVs.)
AMS 02, with a planned launch in 2008, uses a large superconducting magnet spectrometer. Its emphasis is on the search of antimatter aiming for a sensitivity of antiHe/He to about 10 -9. It also will measure antiprotons and positrons with very high statistics in an energy regime comparable to PAMELA and further improve the measurements of cosmic ray spectra of elements up to iron in the energy range from some GeVs up to 1 TeV. AMS 02 is also capable of measuring individual light isotopes. A good measurement of 10Be in the energy regime from 100 MeV up to 10 GeVs as planned in AMS 02 can help to distinguish between different propagation models.

The Japanese CALET (CALometric Electron Telescope) mission being developed for the ISS is a large electromagnetic calorimeter primarily aimed at detecting electrons and gamma rays into the 10 12eV energy range. The spectrum of high-energy cosmic-ray electrons is expected to “cutoff” in this energy range because of radiative losses in the interstellar medium.

NASA balloon program:

Advanced Composition Explorer (ACE):

Alpha Magnetic Spectrometer (AMS):
Payload for AntiMatter Exploration and Light-nuclei Astrophysics (PAMELA):

Above 100 TeV the flux is too low to be accessible to current direct measurements above the atmosphere. This has long been the province of large arrays of detectors on the ground that measure the extensive air shower (EAS) cascades of secondary particles from the initial interaction in the atmosphere of a high energy primary cosmic-ray nucleus. In this situation it is a challenge to measure the total energy and even more difficult to determine the mass of the incident nucleus. Systematic uncertainties in the models used to interpret the data may soon become the limiting factor. The well-instrumented KASCADE air shower array at Karlsruhe, Germany recently succeeded in separating on a statistical basis the spectra of several groups of nuclei in the region of the "knee" of the spectrum above 10 15 eV. The measurements show for the first time the pattern of spectral steepening ordered by the masses of the primary cosmic-ray nuclei.


Ultra-high energy cosmic rays:

Ultra-high energy (UHE) cosmic rays are the highest energy messengers of the present universe. Their origin is one of the most profound mysteries in high-energy astrophysics. Between 10 18 and 10 19 eV the composition appears to shift to lighter elements. At the highest end of the cosmic-ray spectrum, several different experiments report events around 10 20 eV and above. In particular the AGASA (Akeno Giant Air Shower Array) reported about a dozen of events with energies above 10(20) eV. This is remarkable because it had been expected that the energy spectrum would become steeper above 5 x 10 19 eV as a consequence of energy loss by inverse photo-pion production as UHE protons propagate through the microwave background radiation from sources at cosmological distances. Due to the GZK effect, there should be fewer such events. These few UHE events have generated great excitement because their explanation would require novel physics. Measurements from the HiRES (High Resolution Fly’s Eye cosmic ray observatory) atmospheric fluorescence detectors (in monocular mode) indicate smaller fluxes above 10 20 eV which appear to be consistent with the expected steepening of the spectrum. Events at the high energy end of the spectrum are extremely rare, and more data are needed to resolve the problem. Measurements with Stereo HiRes are underway. The Pierre Auger Observatory in Argentina, consisting of giant air shower arrays, water Cherenkov detectors and fluorescence telescopes with larger acceptance, is completed to 50% (area of 1500 km 2). A first estimate of the UHE spectrum has been presented. The Extreme Universe Space Observatory (EUSO) is a fluorescence detector that is envisaged to look down onto the Earth’s atmosphere with a wide angle telescope from the ISS. Its implementation is uncertain.


High-energy gamma-rays:

High-energy gamma-rays probe the workings of active galactic nuclei, supernova remnants, pulsar wind nebulae, gamma-ray bursts and other energetic astrophysical objects. Gamma-ray astronomy has long been associated with cosmic-ray physics because the gamma-rays imply the existence of energetic particles (such as electrons or protons) from which they are radiated. Gamma-rays are therefore also probes of potential sources of cosmic rays.

Major discoveries about such gamma-ray sources began about twenty-five years ago. Space-borne instruments, most notably EGRET on the Compton Gamma Ray Observatory (CGRO), detected sources up to a GeV. Ground-based optical telescopes to register the Cherenkov light produced by the cascades generated when high-energy gamma-rays interact in the atmosphere, extended the energy range into the TeV region. The imaging technique, introduced by the Whipple Observatory group, allowed the separation of hadronic showers due to charged particles from those due to gamma-rays. As a result the Whipple telescope could detect the Crab Nebula with high significance. These discoveries motivated construction of a new generation of detectors.

The last decade has seen a remarkable technological breakthrough with the introduction of sophisticated imaging systems on Cherenkov telescopes, which have so improved the background rejection that real observational astronomy is now possible in the TeV region. As usual, when a new observational window is opened, something unexpected was found. In this case the Whipple, and subsequently other groups, discovered strong and rapidly varying emission from several nearby Blazars. These are thought to be galaxies with active nuclei (powered by accretion onto a central, super-massive black hole) where we happen to be looking almost straight down the axis of the relativistic jet of material emitted by the nucleus. During outbursts the power registered in the TeV region from these objects exceeds that at all other wavelengths! By now these observations are placing important limits on the intergalactic optical/infrared background radiation field and on possible quantum gravity effects on the propagation of high-energy photons.

In addition to extragalactic sources, several galactic sources have been detected apart from the Crab Nebula. Most importantly, TeV emission has been detected from at least eight shell-type supernova remnants (including SN1006, RX J1713-3946, and Cassiopeia A). Both the Inverse Compton radiation from electrons of energies up to about 100 TeV and the neutral pion decay emission due to collisions of high energy protons with gas atoms inside supernova remnants could be responsible for this TeV emission. Their successful separation is expected from synchrotron measurements at radio and X-ray wavelengths, with the hope to find out whether supernova remnants are indeed the sources of the bulk of the cosmic rays.

TeV astronomy has now reached a sound level of maturity. A most exiting aspect of the recent results is the diversity of objects that are proving to be sources of TeV gamma-ray sources; many of them were not detected by EGRET. Most new results have come from the H.E.S.S. (High Energy Stereoscopic System) group using an array of four imaging atmospheric Cherenkov telescopes situated in Namibia since 2003. Together with other telescopes (CANGAROO-III, MAGIC, VERITAS) that will soon be fully operational, these ground-based systems will dominate the TeV observational arena for the next decade. With the improved sensitivity of the GLAST ( Gamma Large Area Space Telescope ) the 100 MeV components of these sources are expected to be detected.

Compton Gamma Ray Observatory: http://cossc.gsfc.nasa.g ov /cgro/
Atmospheric Cherenkov Telescopes:

Fred Lawrence Whipple Observatory:



Very Energetic Radiation Imaging Telescope Array System (VERITAS):

Neutrino telescopes:

A qualitatively different probe of potential cosmic-ray sources will be provided by neutrino telescopes capable of detecting high-energy neutrinos from deep inside cosmic accelerators. Whereas photons are radiated prolifically by electrons, as well as from decays of neutral pions, observation of neutrinos would require the presence of higher energy protons to produce the charged pions from which neutrinos originate. There are currently two working neutrino telescopes, the Baikal detector in Lake Baikal and the AMANDA detector in Antarctica. Both have detected upward-moving muons produced by atmospheric neutrinos that have penetrated the Earth. What is measured is the Cherenkov light generated as the muon passes through the detector. These measurements provide a proof of principle that both clear water and clear ice are feasible as the detection medium for neutrino astronomy. Estimates of signals that might be expected from sources such as gamma-ray bursts, flares of active galaxies and other cosmic accelerators, show that larger, kilometer-scale detectors are needed to have a good expectation of seeing a signal. Two observatories (km 3 size) will be necessary to see the two hemispheres and to fully exploit the complementarity of water and ice as detecting media. Major efforts are underway to reach the kilometer scale, both in Antarctic ice (IceCube) and in the Northern hemisphere. Joining of the European efforts (NESTOR in Greece, Antares in France and NEMO in Italy) aims towards the construction of a single kilometer-cube neutrino detector (KM3NeT) in the Mediterranean Sea. More details are described in the PANAGIC report. New techniques (radio, acoustic, and EAS measurements) are also being developed to detect the highest energy neutrinos (E above 10 17 eV). The ANITA balloon experiment is planned for long-duration flights in Antarctica to detect the radio emission of UHE neutrinos interacting in ice.

For links to neutrino telescope projects see:

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