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Characteristics of air-shower-triggered atmospheric families detected in the hybrid experiments together with emulsion chamber and AS-array at high mountains are compared with those of simulations. The analysis of families accompanied by air-showers with N e ≥ 10 7 shows that the experimental family-energy distribution favors a proton-dominant chemical composition of primary cosmic-rays but the lateral distribution favors a heavy-dominant composition. Thus no simulations can describe the overall characteristics of the data of hybrid experiments.

Relativistic neutrons were observed by the neutron monitors at Mt. Chacaltaya and Mexico City and by the solar neutron telescopes at Chacaltaya and Mt. Sierra Negra in association with an X17.0 flare on 2005 September 7. The neutron signal continued for more than 20 min with high statistical significance. Intense emissions of γ -rays were also registered by INTEGRAL , and during the decay phase by RHESSI . We analyzed these data using the solar-flare magnetic-loop transport and interaction model of Hua et al. [Hua, X.-M., Kozlovsky, B., Lingenfelter, R.E. et al. Angular and energy-dependent neutron emission from solar flare magnetic loops, Astrophys. J. Suppl. Ser. 140, 563–579, 2002], and found that the model could successfully fit the data with intermediate values of loop magnetic convergence and pitch-angle scattering parameters. These results indicate that solar neutrons were produced at the same time as the γ -ray line emission and that ions were continuously accelerated at the emission site.

Longitudinal and lateral characteristics of the families detected by emulsion chambers in the hybrid experiment with AS-array at Mt. Chacaltaya are studied in detail. Although many groups discuss about an increase of the heavy component in primary cosmic-rays beyond the knee region, it is shown that the observed characteristics of the families accompanied by large shower size, N e > 10 6 , can not be explained by an increase of heavy primaries alone. It is necessary to assume some changes of nuclear interaction in order to explain the observed characteristics of the air-showers accompanied by families.

We have successfully detected solar neutrons at ground level in association with the X17.0 solar flare that occurred on 2005 September 7. Observations were made with the solar neutron telescopes and neutron monitors located in Bolivia and Mexico. In this flare, large fluxes of hard X-rays and γ-rays were observed by the GEOTAIL and the INTEGRAL satellites. The INTEGRAL observations include the 4.4 MeV line γ-rays of 12 C. The data suggest that solar neutrons were produced at the same time as these hard electromagnetic radiations. We have however found an apparent discrepancy between the observed and the expected time profiles. This fact suggests a possible extended neutron emission.

Strong signals of neutral emissions were detected in association with a solar flare that occurred on 2005 September 7. They were produced by both relativistic ions and electrons. In particular, relativistic neutrons were observed with the solar neutron telescopes (SNTs) located at Mount Chacaltaya in Bolivia and Mount Sierra Negra in Mexico and with neutron monitors (NMs) at Chacaltaya and Mexico City with high statistical significances. At the same time, hard X-rays and γ-rays, which were predominantly emitted by high-energy electrons, were detected by the Geotail and the INTEGRAL satellites. We found that a model of the impulsive neutron emission at the time of the X-ray/γ-ray peak can explain the main peaks of all the detected neutron signals, but failed to explain the long tailed decaying phase. An alternative model, in which the neutron emission follows the X-ray/γ-ray profile, also failed to explain the long tail. These results indicate that the acceleration of ions began at the same time as the electrons but that ions were continuously accelerated or trapped longer than the electrons in the emission site. We also demonstrate that the neutron data observed by multienergy channels of SNTs put constraints on the neutron spectrum.

An experiment using an air shower array, a hadron calorimeter and an emulsion chamber is under way at Mt. Chacaltaya (5200 m, Bolivia). One of the highest energy events, having a halo (∼ 1 cm) in the centre of the family together with many γ -ray and hadron showers, is analyzed in detail. Available data for the event are on the halo ( E h a l o = 750 TeV) and on the high energy particles of electron/photon components by the emulsion chamber, and on the characteristics of the accompanied air shower ( N e = 7.0 × 10 7 , s = 0.59 ) by the air shower array. The diagram of the air shower size and the total energy of electron/photon components in the family, which shows discrepancy between the experimental data and simulated events (QGSJET code for nuclear collisions) in our previous reports, is discussed including the present event.

A hybrid experiment to operate simultaneously an air shower array, a hadron calorimeter and an emulsion chamber is under way at Mt Chacaltaya (5200 m, Bolivia). An event with a halo, a blackened area of ~1 cm on x-ray film of the emulsion chamber, was observed with the experiment. Information about the halo (Ehalo = 850 TeV) and on high energy particles of electromagnetic and hadronic components outside the halo (?E? = 632.5 TeV and ?E(?)h = 278.8 TeV with a detection threshold of 2 TeV) was obtained from emulsion chamber data. Details about low energy hadrons were determined from the hadron calorimeter data, and characteristics of the accompanying air shower (Ne = 7.0 ? 107, s = 0.59) were determined by the air shower array. We reconstruct the event, based on the observed data. The event is compared with simulated events, which supports the conclusion that nuclear interactions change their characteristics in the high energy region so as to result in stronger energy subdivision.

Solar neutrons have been detected using the neutron monitor located at Mt. Chacaltaya, Bolivia, in association with a large solar flare on November 24, 2000. This is the first detection of solar neutrons by the neutron monitor that have been reported so far in solar cycle 23. The statistical significance of the detection is 5.5 sigma. In this flare, the intense emission of hard X-rays and gamma-rays has been observed by the Yohkoh Hard X-ray Telescope (HXT) and Gamma Ray Spectrometer (GRS), respectively. The production time of solar neutrons is better correlated with those of hard X-rays and gamma-rays than with the production time of soft X-rays. The observations of the solar neutrons on the ground have been limited to solar flares with soft X-ray class greater than X8 in former solar cycles. In this cycle, however, neutrons were detected associated with an X2.3 solar flare on November 24, 2000. This is the first report of the detection of solar neutrons on the ground associated with a solar flare with its X-ray class smaller than X8.

An experiment of an air shower array, a hadron calorimeter and an emulsion chamber is under way at Mt. Chacaltaya (5,200 m). Analysis of a halo event, detected by hybrid detectors, is presented. The data available for the event are on the halo ( E halo = 750 TeV), on the height of the interaction, the energy of the electron/photon and hadronic components from the emulsion chamber, on low energy hadrons from the hadron calorimeter, and on the characteristics of the accompanied air shower ( Ne = 7.0 × 10 7 , s = 0.59) using the air shower array. The structure and origin of the event is discussed based on the observed data of various components obtained by the hybrid detectors. The event is compared to simulated events, which support the conclusion that nuclear interactions change their characteristics in the high energy region resulting in a stronger energy subdivision.

Using EAS data from the EAS-EXC group (EAS plus hadronic calorimeter and nuclear emulsion-X chamber hybrid experiment) on Mount Chacaltaya during the 1990's the fractal properties of the energetic cosmic ray fluxes was investigated. A discrimination in the hadron content of the energetic primaries furnishes two different data samples, the total EAS fluxes and the hadron-less fluxes. The sidereal diurnal variation of the hadron-less EAS time variation shows values of 0.5% with a phase at about 2.29 hr sidereal time. With the help of the Crassberger Procaccia algorithm the fractal dimensions have been investigated of, on the one hand, the differences in arrival times of the hadron-less showers and on the other hand the EAS fluxes time series with an integration time of five min. The obtained fractal dimensions show inconclusive evidence of continuous chaotic component in the data samples investigated.

An experiment of an air shower array, a hadron calorimeter (8 m 2 ) and an emulsion chamber (8 m 2 , 15 cm Pb) is under way at Mt. Chacaltaya (5200 m above sea level, Bolivia), in order to study the hadron interactions and the primary cosmic rays in the energy region exceeding 10 15 eV. The number of particles in the hadronic component in the air shower, which is detected by the hadron calorimeter, is not compatible with that obtained by simulations, indicating that violation of the Feynman scaling law is stronger at 10 16 eV than the one assumed in the simulations. The average mass number of the primary cosmic rays, estimated from the distribution of the number of hadrons in the air shower, is A >= 2.8 ± 0.5 at 10 16 eV.

The observation of families and air showers is carried out at Mt. Chacaltaya (5200m, 540 g/cm2 atmospheric depth, Bolivia) by operating the emulsion chamber and the air shower array simultaneously. We summarize in this paper the results of the experiment, leaving the description of experimental apparatus, experimental procedure, etc. elsewhere (Kawasumi N. et al., AIP Conf. Proc.,276 (1992) 88;Proc. XXIV ICRC, Rome,1 (1995) 201). The analysis relies on the data of 47 families of ΣE γ≥10 TeV with the accompaniment of air showers.

Simultaneous observations of families and accompanied air showers with emulsion chambers and the air shower array of electronic equipment at Mt. Chacaltaya (5200 m, 540 g/${\mathrm{cm}}^{2}$) reveal that families bear the data of nuclear interactions generated deep in the atmosphere. 47 outstanding families with \ensuremath{\Sigma}${\mathit{E}}_{\ensuremath{\gamma}}$\ensuremath{\ge}10 TeV are correlated with the accompanied air showers of the size ${10}^{5}$--${10}^{8}$. A scatter plot of the average family energy versus the size of the relative air shower requires further energy fractionizing process(es) in the propagation of high energy cosmic rays in the atmosphere, such as a larger dissipative mechanism in nuclear interaction, heavier chemical composition of the primary cosmic rays, etc. We reach the conclusion that nuclear interaction changes its features in the energy region ${\mathit{E}}_{0}$\ensuremath{\gtrsim}${10}^{16}$ eV, because the heavier composition, proposed so far, is not sufficient for the required dissipative process. A comparison with the data from the HADRON experiment at a similar altitude with a similar technique shows that no larger deviations are present between both experiments. \textcopyright{} 1996 The American Physical Society.

At mountain altitudes, high energy γ -ray bundles ( γ -rays in parallel trajectories and with the same direction of an air shower) can be observed in the central part of air showers. These γ -ray bundles (≥ 1 TeV ) and their associated air showers are observed simultaneously at Mt. Chacaltaya (5200 m ) by our detectors. The γ -ray bundle in the air shower can give information on the primary composition and the characteristics of proton interaction. In this paper, we report on experimental results, simulations calculated for the present experiments, and also problems in simulations.

Observation to detect VHE gamma-ray emissions has been carried out at Mt.Chacaltaya with the SYS air shower array since Feb.1986. A significant excess of hadron-less air showers with energies above 1014eV from the direction of Scorpius X-1 has been found during the last fifteen days in May 1986. However, no excess as the continuous emissions could be observed for several X-ray binary objects in the southern hemisphere. Small abundance of hadron-less air showers could be seen from the diffused region of galactic centre.

Simultaneous observation of families and accompanied air showers is carried out at Mt. Chacaltaya with emulsion chambers and a air shower array. The present analysis is based on the data of experiment from 1969 to 1986. 47 families with ΣEγ ≥ 10 TeV are coupled to the accompanied air showers of size 105 to 108. There is discrepancy between the experimental data and the simulation calculation, which indicate a need of more rapid energy dissipating process in the atmosphere in high energy region of E0 > 1016 eV.

An experimental setup of an air shower array, hadron calorimeter, and emulsion chamber is being carried out at Mt. Chacaltaya (5200 m, Bolivia), in order to study the hadron interaction and the primary cosmic rays in the energy region exceeding ${10}^{15}$ eV. The number of hadrons in the air shower, detected by the hadron calorimeter, is discussed mainly in this paper. A comparison with the simulation shows that the number of hadrons in the air shower is not compatible with that of the simulation, indicating that the Feynman scaling law is violated more strongly than the one assumed in the simulation at ${10}^{16}$ eV. The average mass number of the primary cosmic rays, estimated from the distribution of the number of hadrons, is $〈\mathrm{ln}A〉=2.8\ifmmode\pm\else\textpm\fi{}0.5$ at ${10}^{16}$ eV.

We have observed high energy γ families and associated air showers simultaneously at Mt. Chacaltaya. Monte Carlo simulations were calculated for the present experimental setup. From comparisons with both results, mechanism of γ family production, change of interaction model, study of the chemical composition are discussed.