C4. Commission on Cosmic Rays

Report to the 1999 General Assembly for 1996-99

Officers 1996-1999:

Chairman: J. Arafune Japan
Vice-Chairman: L.O'C. Drury, Ireland
Secretary: T.K. Gaisser, USA

Members:

R. Cowsik, India
M. Giller, Poland
P. Goret, France
G.B. Khristiansen, Russia
M.S. Potgieter, South Africa
R.J. Protheroe, Australia
O. Saavedra, Italy
R. Schlickeiser, Germany
A.Jr. Turtelli, Brazil
K.-P. Wenzel, Netherlands

Associate Members:

B. Barish, USA (C11)
C.E. Fichtel, USA

ACTIVITIES:

The 25th International Cosmic Ray Conference was held in Durban, South Africa from 30th July to 6th August 1997. Four hundred, ninety-five delegates from thirty-five countries attended the conference. The length of this conference was seven working days (as compared to ten for previous conferences). The IUPAP grant was used for partial support of scientists from Russia (33), India (8), Egypt (3), Eastern Europe (3), China (2), Latin America (2) and Thailand (1) to attend the conference. The registration fee was waived for 52 scientists, 27 of whom were also awarded free accommodation. Information related to cosmic-ray physics and the Commission is posted on the C4 Web page, the address of which is http://www.dias.ie/c4.html.

IUPAP, with the endorsement of C4, was also a sponsor of the XVIII International Conference on Neutrino Physics and Astrophysics (Neutrino '98) at Takayama, Japan, 4-9 June 1998. There was a meeting of the ad hoc committee to establish the PANAGIC Committee, which is to report through C4 to IUPAP and to have representation from C11, C12 and C19. During this meeting a charge was drafted and an initial membership proposed to IUPAP.

The 26th International Cosmic Ray Conference is scheduled for 17-25th August, 1999 in Salt Lake City Utah, and the 27th is set for Hamburg, Germany, 8-15 August, 2001. Several groups have expressed interest in hosting the 28th ICRC for 2003.

NEW DEVELOPMENTS:

Major discoveries occurred in all three of the sub-disciplines covered by C4, solar and heliospheric physics (SH), origin (OG) and high energy (HE):

The report of evidence for neutrino oscillations by the Super-Kamiokande Collaboration made during the Neutrino'98 meeting;

Coordinated observations with detectors around the globe of bursts of TeV gamma-radiation from the active galaxy Markarian 501;

Further evidence that the cosmic-ray spectrum extends beyond the cutoff energy around 5 x 1019 eV anticipated from energy losses of protons from cosmological distances in the cosmic microwave background radiation.

An improved three-dimensional picture of the heliosphere from analysis of data of the Ulysses satellite;

Measurement of radioactive isotopes which reveal characteristic features of cosmic ray sources and accelerators;

Observations of anti-protons with sufficient sensitivity to probe models of cosmic-ray propagation and modulation and to begin to search for "primary" antiprotons.

Discovery of X-ray afterglows of gamma-ray bursts with the Italian-Dutch satellite Beppo-SAX.

1. The very large underground detector Super-Kamiokande began operation in April of 1996. It consists of 50 kilotonnes of pure water viewed by more than 11,000 big (50 cm) photo-multiplier tubes. Measurement of the ratio of electron to muon events in the detector confirms with high statistics the previously observed anomaly in the ratio of electron neutrinos to muon neutrinos-the ratio is significantly higher than value of approximately one-half expected around 1 GeV. More importantly, clear evidence of an angular dependence has been observed that has the characteristic signature of neutrino flavor oscillations. The data constitute strong evidence for neutrino mass, and are therefore potentially of the greatest significance for theory of elementary particles. In addition, the detector is giving a large amount of new data on solar neutrinos. Both these results generated great excitement when they were presented at the Neutrino'98 meeting.

2. A highlight of the 25th International Cosmic Ray Conference was the presentation of coordinated observations of very intense bursts of TeV gamma-radiation from the blazar Markarian 501. This is an active galactic nucleus with a relativistic jet illuminating the Earth with high energy photons that exhibit extreme variability on a short time scale, indicating a very compact emission region. The engine is believed to be powered by a super-massive black hole. Similar behavior has been seen from the blazar Mrk 421. The Egret detector on the Compton Gamma-ray observatory has discovered a large number of blazars. Measurements with ground-based detectors are complementary to detectors in space because the former have acceptances large enough to explore the TeV energy range. These observations have stimulated multi-wavelength campaigns to clarify the nature of these powerful engines, which may also accelerate cosmic rays to ultra-high energy. In addition to these extragalactic sources, TeV emission has also been detected from several galactic sources including the Crab nebula and Supernova 1006. X-ray observations of the latter source resolve non-thermal radiation from the shocked shell indicating acceleration of electrons to energies of order 100TeV.

3. Evidence that the cosmic-ray spectrum extends beyond the Greisen-Zatsepin-Kuz'min (GZK) cutoff continue to accumulate, most recently from the report of the AGASA experiment, the largest giant air shower detector currently operating. The High-Resolution Fly's Eye is due to turn on very soon, and the giant Auger Project is getting underway. Even more visionary detectors to view the atmosphere from space are being developed. The goal is to identify the sources of these most energetic particles in nature. Among the possibilities are gamma-ray burst sources, jets of active galactic nuclei and decay of massive relicts from the big bang. The absence of a GZK cutoff from interactions in the microwave background would indicate that at least some of the sources are relatively nearby on cosmological scales.

4. The Ulysses Mission continues to provide improved understanding of the three-dimensional structure and dynamics of the solar wind and the heliosphere made possible by its orbit over the solar poles. Important results are that the solar wind speed increases from 400 km/s near the solar equator to 800 km/s over the poles, and turbulence properties change significantly over the poles so that (contrary to expectation) galactic cosmic rays do not have easy access onto the poles. In addition, important questions have emerged related to the scattering and transport of low energy cosmic rays and pickup ions. The particle scattering mean free paths predicted by theory are not supported by observations. This has led to the re-examination of the global configuration of the interplanetary magnetic field as well of the scattering and transport processes themselves. Finally, the very important observations of pickup ions and neutrals by Ulysses has contributed significantly to the development of new models for the global structure of the heliosphere which take into account the coupling of neutral interstellar hydrogen and solar wind plasma. The implications of these results have yet to be incorporated fully into cosmic ray modulation studies.

5. Since its launch in August 1997 the Advanced Composition Explorer has been returning data on composition of the solar wind, solar energetic particles, magnetic fields and the sun-earth environment as well as galactic cosmic rays and sampling the interstellar medium through studies of anomalous cosmic rays. From its vantage point a million miles upstream from earth in the solar wind it is providing real time monitoring of solar activity. Measurements of Ni and Co in galactic cosmic rays show that there has been a long time delay (>105 years) between nucleosynthesis and acceleration, indicating that cosmic rays are accelerated from relatively old material rather than newly synthesized material such as fresh supernova ejecta. This is a key measurement that has been waiting 20 years for an answer. Measurements of the isotopic composition of Neon isotopes in anomalous cosmic rays, which represent a sample of the nearby interstellar medium, show that the 22Ne/20Ne ratio in the ISM is ~0.1, similar to that in the solar system, rather than ~0.4, as in galactic cosmic ray source material. This result (which comes from both SAMPEX and ACE) shows that cosmic rays cannot just be a sample of the ISM, they must include contributions from sources rich in 22Ne, such as massive Wolf-Rayet stars.

6. A new generation of detectors on balloons and in space is now collecting sufficiently large samples of cosmic-ray antiprotons to begin to use the kinematic features of this rare component to gain new insights into propagation of cosmic radiation in the interstellar medium and modulation by the solar wind. In addition will be possible to make improved searches for primary antiprotons (e.g. from dark matter annihilation in the galactic center region) above the background of secondary antiprotons produced during cosmic ray propagation in the interstellar medium.

7. Gamma-ray bursts have been a major mystery of astrophysics for 25 years, mainly because of the difficulty of associating them with known objects. Observations with BeppoSAX allowed directions of some GRBs to be determined with sufficient accuracy to associate them with objects at red shifts of order one, implying a tremendous release of energy. There is a possibility that these are also cosmic accelerators.

T.K. Gaisser ,Secretary of C4