C4. Commission on Cosmic Rays

Report to the 2002 General Assembly for 1999-2002
Berlin, Germany
October 7-12, 2002

ACTIVITIES:

The 26th International Cosmic Ray Conference (ICRC) was held at Salt Lake City, Utah, 17-25 August 1999. There were 550 registered participants. Martin Pohl (Germany) won the Duggal Prize for outstanding young scientist, and the O'Ceallaigh Award was shared by John Simpson (USA) and George Zatsepin (Russia) for their major contributions to Cosmic-ray physics over many years.

The 27th International Cosmic Ray Conference was in Hamburg, Germany, 7-15 August, 2001. It was attended by 800 scientists. Teresa Montaruli (Italy) won the Duggal Award, and V. Ginzburg (Russia) received the O'Ceallaigh Award for his seminal contributions to the field. A new award presented for the first time at this conference, the Yodh Prize, went to Reuven Ramaty (US) posthumously. C4 met during both conferences.

Among other business, the Commission recommended IUPAP support for the following topical conferences: Neutrino-2000 (Sudbury, Canada, 16-21 June 2000); Topics on Astroparticle and Underground Physics, TAUP2001 (Gran Sasso, Italy 7-12 Sept, 2001); Neutrino-2002 (Munich, Germany, 25-30 May, 2002). The Commission also recommended IUPAP endorsement of Gamma-2000 (Heidelberg, Germany, 26-30 June, 2000).

C4 is also the liaison to IUPAP (along with C11, C12 and C19) for the Particle Astrophysics and Gravitational-wave International Committee (PaNAGIC), a working group of IUPAP. The report of PaNAGIC is presented separately.
PANAGIC: http://www.lngs.infn.it/site/exppro/panagic/

The 28th International Cosmic Ray Conference is scheduled for 31 July-7 August, 2003 at Tsukuba, Japan. C4 is the standing international advisory committee for the ICRC series.
Next International Conference: http://www-rccn.icrr.u-tokyo.ac.jp/icrc2003/

NEW DEVELOPMENTS:

A unique aspect of the series of International Cosmic Ray Conferences 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 results in both areas that are driving major new endeavors.

Measurements of atmospheric neutrinos produced by interactions of cosmic rays have given strong evidence for neutrino oscillations, with fundamental implications for neutrino masses and the family structure of elementary particles. The large and precisely measured sample of neutrino interactions accumulated over five years from 1996 to 2001 with the 50 kiloton, deep underground Super-Kamiokande detector in Japan led to the discovery, which had been hinted at by previous measurements with smaller detectors. The whole picture of neutrino oscillations was extended with the confirmation by the Sudbury Neutrino Observatory that the longstanding solar neutrino problem is another manifestation of neutrino oscillations. Underground science is growing in importance, largely driven by the desire to understand in detail the properties of neutrinos-through double beta-decay, by further studies of solar and atmospheric neutrinos, and with long-baseline experiments that use neutrino beams from accelerators and neutrinos from nuclear reactors to determine more precisely the oscillation parameters. The deep environment is needed to shield against cosmic-ray induced backgrounds in order to study the weakly interacting neutrinos.
Super-Kamiokande: http://www-sk.icrr.u-tokyo.ac.jp/doc/sk/
Sudbury Neutrino observatory: http://www.sno.phy.queensu.ca/

The solar wind, a turbulent, magnetized plasma emanating from the Sun with a velocity of 400-800 km/s, dominates the region called the heliosphere out to approximately 100 AU. A global picture of the solar wind and other activity in the heliosphere is emerging from data collected from a variety of spacecraft, from Ulysses, exploring the regions over the solar poles for the first time, to the two Voyager spacecraft, now approaching the termination shock after some twenty-five years in space. Various kinds of shocks driven by solar activity (including huge coronal mass ejections, currently imaged in detail by the SOHO spacecraft) accelerate particles to velocities orders of magnitude greater than that of the ambient plasma. These events can be 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. The recent maximum of solar activity provide a wealth of new data, exemplified by the huge Bastille day 2000 event as studied with the Advanced Composition Explorer (ACE) spacecraft.
http://ulysses.jpl.nasa.gov/, http://voyager.jpl.nasa.gov/, http://www.srl.caltech.edu/ACE/,
http://sohowww.nasacom.nasa.gov

The turbulent solar wind also modulates the intensity of galactic cosmic rays 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. High-altitude balloon flights of the BESS detector in the years up to and including the most recent solar maximum provide a unique series of measurements of cosmic-ray protons and antiprotons. Together with similar studies of electrons and positively charged nuclei, these data are clarifying the interplay of magnetic drifts and diffusion in the process of solar modulation. Moreover, the situation is sufficiently well understood to conclude that the observed flux of antiprotons is entirely consistent with a secondary origin in collisions between higher energy cosmic-ray protons and nuclei with interstellar gas. There is as yet no sign in the data of exotic sources of antiprotons such as annihilation of hypothetical weakly interacting massive particles.

A new generation of measurements with magnetic spectrometers has greatly improved our knowledge of the fundamental observables of cosmic-ray physics--the energy spectra of protons, helium and other nuclei. A test flight of the Alpha Magnetic Spectrometer (AMS) on the Space Shuttle contributed to this effort along with several balloon-borne spectrometers. Future long duration exposures in space of the PAMELA detector and of AMS on the Space Station will extend these measurements to the TeV range, where the spectrum is currently very poorly known. The capability for long-duration ballooning is also playing a role in cosmic-ray measurements. The ATIC thin calorimeter was up for 15 days during a balloon-borne circumnavigation of Antarctica in January 2001 to measure the composition and spectrum of cosmic rays to 100 TeV. The TIGER experiment stayed aloft for 31 days during two circles around the continent making a measurement of trans-iron elements in the cosmic radiation.
AMS: http://ams.cern.ch/AMS/ams_homepage.html
PAMELA: http://wizard.roma2.infn.it/pamela/pam_home.htm
NASA balloon program: http://www.wff.nasa.gov/~code820/index.shtml

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 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. 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. http://ik1au1.fzk.de/KASCADE_home.html

At the high end of the cosmic-ray spectrum, several different experiments report events around 10(20) eV and above. In particular the Akeno Giant Air Shower Array reports some 10 such events. 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 ultra-high energy protons propagate through the microwave background from sources at cosmological distances. If so there would be fewer such ultra-high energy events. These few ultra-high energy events have generated great excitement because their explanation would require novel physics. The latest development is that new measurements from the HiRes atmospheric fluorescence detectors (in monocular mode) now 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, and the Auger detector, with larger acceptance, is under construction in Argentina and beginning to take data using two complementary techniques.
AGASA: http://www-akeno.icrr.u-tokyo.ac.jp/AGASA/
HiRes: http://www.cosmic-ray.org/
Auger: http://www.auger.org/

High-energy gamma rays probe the workings of gamma-ray bursts, active galactic nuclei, supernova remnants 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 occurred in the past decade, notably with the EGRET and BATSE detectors on the Compton Gamma Ray Observatory up to a GeV and with several ground based air Cherenkov telescopes in the TeV energy range. These discoveries motivated construction of a new generation of detectors that is currently in progress. These include the GLAST detector in space and several new ground-based telescopes that will image the Cherenkov light produced by the cascades generated when high-energy gamma-rays interact in the atmosphere. The efforts are coordinated, so that their coverage in energy will overlap each other.
Compton Gamma Ray Observatory: http://cossc.gsfc.nasa.gov/cgro/
Gamma Large Area Space Telescope: http://www-glast.stanford.edu/
Atmospheric Cherenkov Telescopes: http://icrhp9.icrr.u-tokyo.ac.jp/c-experiments.html

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. The Antares experiment, under construction for the Mediterranean Sea, will be comparable in size to AMANDA. 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. Major efforts are underway to reach the kilometer scale, both in Antarctic ice and in the Mediterranean Sea. Their status is described in the report of the HENAP sub-panel of PANAGIC. For links to neutrino telescope projects see: http://neutrinooscillation.org/neutrino_telescopes.html