Search

Extensive Air Showers at large zenith angles θ > 70° (Horizontal Air Showers, HAS) are observed at the EAS-TOP array at Campo Imperatore (Gran Sasso Laboratories). The rate of these events exceeds the one due to primary cosmic rays (at this angles) and therefore these showers have to be generated by penetrating particles. Assuming that they are produced by atmospheric muons we derived the muon flux as Fμ(> 30 TeV) = 1.1 × 10−11cm−2s−1sr−1, in good agreement with the underground measurements. The upper limits for diffuse neutrino radiation from these measurements is Iν(> 105GeV) dI ν e (E 0 )/dE ν e −18 cm −2 s −1 sr −1 GeV −1 , for the resonant ( E 0 = m W 2 2m e = 6.4 × 10 6 GeV ) neutrinos.

UHE cosmic rays are studied by means of the detectors of the different components of secondaries produced by their interactions in the atmosphere (EAS). We describe and discuss the reconstruction techniques and accuracies of the e.m. detector of EAS-TOP. They allow, besides independent high resolution measurements of UHE γ-ray astronomy, good correlation possibilities with the detectors of the different EAS components.

On February 23, 1989, an excess of counts of > 100 TeV Extensive Air Showers has been recorded from the direction of the Crab Nebula by three independent detectors: Baksan (North Caucasus, USSR), KGF (India) and EAS-TOP (Gran Sasso, Italy). The chance probability of the combined detection is ~ 10-7. We discuss the EAS-TOP observations from the statistical point of view (chance probability ≤ 10-2) and present some phenomenological characteristics of the events.

A search for transient point sources of ultra-high-energy (UHE) γ-rays has been performed, based on the correlation of two extensive air shower arrays, BAKSAN (North Caucasus, 1700 m a.s.l., BAKSAN Neutrino Observatory, Russia) and EAS-TOP (Campo Imperatore, 2005 a.s.l., Laboratori Nazionali del Gran Sasso, Italy), which are located at very similar latitudes (φ≈43° N), and separated in longitude by Δλ≈33.7°. The search has been conducted at primary energy Eγ⩾5×1013 eV on the time scale of a daily source transit over the sky, observable in the northern hemisphere (19° The stability of the single detectors has been studied and verified up to high significance level. No coincident excess, not compatible with the expectations from statistical fluctuations, has been observed. The obtained upper limits to the rate of transient events, nγ, at 90% c.l., are • concerning the sky survey, in the declination band corresponding to the zenith, n γ /(Ωt) /(yr sr) with Φγ(Eγ>5×1013 eV)>2.0×10−11 cm −2 s−1 and duration Δt • concerning the candidate sources, e.g., for Markarian 421: nγ/t 5×1013 eV)>1.3×10−11 cm −2 s−1 and Δt A coincident episode from Markarian 421, observed on 15 January, 1994, with expected chance imitation rate nch= 0.01 is discussed.

The amplitude and phase of the first harmonic of the sidereal anisotropy of cosmic rays at E 0 = 2·10 14 eV have been measured by the EAS-TOP experiment (Campo Imperatore, 2005 m a.s.l., LNGS, Italy) and are respectively (at the equatorial plane): A sid = (3.73±0.57)·10 −4 ; φ sid = 1.82±0.49 h lst, with significance 6.5 s.d. The result is supported by different consistency checks. From the comparison with lower energy data, it follows that the amplitude and the phase of the cosmic ray anisotropy are constant over the energy range 10 12 – 2·10 14 eV. This implies at these energies a constant influence of the local interstellar medium on the propagation of cosmic rays.

The cosmic ray energy spectrum in the range E-o = 10(15)-10(16) eV (including the region of the steepening, "knee") is studied by means of the EAS-TOP array (Campo Imperatore, Gran Sasso Laboratories, atmospheric depth 820 g cm(-2)). Measurements of the electromagnetic size (N-e = total number of charged particles at the observation level) are performed as a function of zenith angle with statistical accuracies of a few percent. The change of slope of the spectrum is observed in each bin of zenith angle at size values decreasing with increasing atmospheric depth. Its attenuation is compatible with the one of shower particles (Lambda(e) = 219 +/- 3 g cm(-2)). This observation provides a consistency check, supporting a normal behaviour of showers at the break, that make plausible astrophysical interpretations based on an effect on primaries occurring at a given primary energy. The break has a "sharp" shape (i.e., within experimental errors is compatible with two intersecting power laws) that represents a constraint with which any interpretation has to match. The change of slope of the power law index reproducing the size spectrum is Delta gamma = 0.40 +/- 0.09. The derived all particle energy spectrum is in good agreement with the extrapolation of the direct measurements at low energies and with other EAS data at and above the knee. Power laws fits to the energy spectrum below and above the knee give tin units of m(-2) s(-1) sr(-1) TeV-1) S(E-o) = (3.48 +/- 0.06) x 10(-10) (E-o/2300)(-2.76+/-0.03) for 900 TeV < E-o < 2300 TeV and S(E-o) = (3.77 +/- 0.08) x 10(-11) (E-o/5000)(-3.19+/-0.06) for 5000 TeV < E-o < 10(4) TeV. The systematic uncertainties connected to the interaction model and the primary composition are discussed. (C) 1999 Elsevier Science B.V.

We have previously shown that the very low activity of the cosmogenic 44Ti in meteorites records the galactic cosmic ray (GCR) flux over a century time-scale (Bonino et al., 1995). The heliospheric magnetic field (HMF) variations are the principal source of the GCR modulation in the heliosphere. We present here new measurements which extend our investigation over the last two centuries. The 44Ti activity at the time of fall has been obtained on the basis of the recently revised value of its half-life of 59.2 years. We basically confirm our earlier findings (Bonino et al., 1997). The 44Ti profile, with the time of fall of the meteorites, shows variations in phase in agreement with those expected. The magnitude of oscillations is ∼ 4 times higher than expected (∼ 6%) on the basis of the GCR flux over the past centuries deduced from sunspot number since 1700 and by the neutron monitor and balloon measurements in the last decades. The higher 44Ti production rate during prolonged solar quiet periods (Gleissberg minima) may imply that during these minima the HMF was weaker than during the recent minima of the 11-year solar cycle.

Production of cosmogenic radioisotopes in meteorites depends on the galactic cosmic ray flux, which is controlled by the heliospheric magnetic field (HMF), which, in turn, depends on the solar activity. The radioisotopes can, therefore, serve as proxy records of the behaviour of the heliosphere in the past, in particular over prolonged solar quiet periods, such as during Gleissberg or Maunder minima, when the production rate of radioisotopes is expected to change significantly. For this purpose we have measured the 44 Ti (T 1/2 =66.6 yr) in ten stony meteorites, which fell between 1840–1992. The results indicate that the production of 44 Ti was significantly higher when the solar activity was persistently low, indicating that the HMF was weak during such periods, enabling higher flux of cosmic rays to enter the inner heliosphere.

A search for GRBs of energy E> 10 GeV has been performed by studying the single particle counting rate of the EAS array EASTOP. No candidates have been observed in coincidence with 292 BATSE GRBs and the typical upper limits obtained on the energy fluence are F (10 GeV 1 TeV) 10 4 10 3 erg cm 2 for the events with zenith angle 10 GeV can reach the detector level; although the number of sec- ondary particles is too small to reconstruct the primary direction, GRBs could be detected as a short time increase in the single particle counting rate. The background con- sists of e and generated by low energy cosmic rays. All sources of background modulation have time scales (hours) much larger than the typical GRB duration (sec- onds), hence they do not aect the GRBs search. The EASTOP experiment is running since 1991 at Campo Imperatore in Italy, at 2000 m a.s.l. The ar- ray is devoted to the measurement of the various com- ponents of extensive air showers. In the GRBs search

The Large Volume Detector (LVD) in the Gran Sasso underground Laboratory is a multipurpose detector consisting of a large volume of liquid scintillator interleaved with limited streamer tubes. In this paper we discuss its power to study low-energy cosmic neutrinos. The results show that the first LVD tower (368 tons of liquid scintillator) is well suited to detect neutrinos from gravitational stellar collapses within all of our Galaxy over a wide range of burst duration (up to a few hundred seconds). No burst candidates have been observed in the first two months of data taking.

The EAS-TOP Extensive Air Shower array, on top of the under-ground Gran Sasso Laboratory, is fully operating as a γ-ray astronomy observatory since the beginning of 1989. After showing the measured angular resolution of the detector, we present the results obtained from the analysis of 280 days of measurements in 1989–1990, with the purpose of investigating possible emission (d.c. and sporadic) from the candidate sources in the Northern hemisphere at an energy, >200 TeV.

A search for UHE gamma-rays from 13 candidate point sources observable in the northern hemisphere (the Crab Nebula and Pulsar, Cygnus X-3, Hercules X-1, Geminga and others) has been performed by the EAS-TOP array during four years of operation since January 1989 to December 1993, at different energy thresholds (E(0) = 30-300 TeV). DC, periodic and sporadic emissions have been studied, and no evidence for significant excesses has been found from any of these searches. The derived 90% c.l. upper limits to the d.c. flux, for a source culminating at the zenith such as Cygnus X-3, are Phi(E > 230 TeV) < 2 x 10(-14) cm(-2) s(-1), Phi(E > 90 TeV) < 4.1 x 10(-14) cm(-2) s(-1) and Phi(E > 25 TeV) < 1.9 x 10(-13) cm(-2) s(-1). The excess from the Crab Nebula of February 23, 1989, as reported by the EAS-TOP, Baksan and KGF arrays, remains the only sporadic excess detected with statistically significant confidence level (probability of background imitation approximate to 10(-5)).

A study of the primary cosmic ray composition in the energy range 5ṡ1014−5ṡ1015 eV is performed through the analysis of the deep underground muons and of the e.m. component of Extensive Air Showers detected in coincidence by the MACRO and EAS-TOP experiments at the Gran Sasso Laboratory. We conclude that: a) pure proton and Iron primary beams are excluded by the experimental data; b) a fraction of nuclei heavier than Helium is necessary to explain the experimental data both below and above the knee; c) a mixed composition, obtained a priori from extrapolations of the spectra directly measured below 100 TeV, yields a reasonable account of the observations up to the knee of the primary spectrum.

The cosmic ray primary composition in the energy range between 10(15) and 10(16) eV, i.e., around the "knee" of the primary spectrum, has been studied through the combined measurements of the EAS-TOP air shower array (2005 m a. s.l., 10(5) m(2) collecting area) and the MACRO underground detector (963 m.a.s.l., 3100 m w.e. of minimum rock overburden, 920 m(2) effective area) at the National Gran Sasso Laboratories. The used observables are the air shower size (N-c) measured by EAS-TOP and the muon number (N-mu) recorded by MACRO. The two detectors are separated on average by 1200 m of rock, and located at a respective zenith angle of about 30degrees. The energy threshold at the surface for muons reaching the MACRO depth is approximately 1.3 TeV. Such muons are produced in the early stages of the shower development and in a kinematic region quite different from the one relevant for the usual N-mu - N-e studies. The measurement leads to a primary composition becoming heavier at the knee of the primary spectrum, the knee itself resulting from the steepening of the spectrum of a primary light component (p, He) of Deltay = 0.7 +/- 0.4 at E-0 similar to 4 x 10(15) eV. The result confirms the ones reported from the observation of the low energy muons at the surface (typically in the GeV energy range), showing that the conclusions do not depend on the production region kinematics. Thus, the hadronic interaction model used (CORSIKA/QGSJET) provides consistent composition results from data related to secondaries produced in a rapidity region exceeding the central one. Such an evolution of the composition in the knee region supports the "standard" galactic acceleration/propagation models that imply rigidity dependent breaks of the different components.. and therefore breaks occurring at lower energies in the spectra of the light nuclei. (C) 2003 Elsevier B.V. All rights reserved.

The heliospheric magnetic field (HMF) is controlled by solar activity, as established by measurements over the last few decades, but its characteristics when the sun was quiet for prolonged periods, such as during Gleissberg or Maunder minima, are not known. Titanium-44, produced in meteorites, provides a monitor of the galactic cosmic ray (GCR) flux and allows estimation of the modulation effect of the sun for the period 1883 to 1992. The titanium-44 activity is consistent with the expected value, but the increase, due to the last Gleissberg minimum, is four times greater than expected for a GCR modulation based solely on sunspot numbers. This result implies that the HMF was weaker than at present and as a result the GCR flux (for energy greater than 1 gigaelectron volt) was higher between 2.2 to 3.6 protons per square centimeter per second per 4π steradians at 1 to 3 astronomical units in solar cycles 12 to 15.

The recent data on the EAS size spectrum, and on the rate of ‘Horizontal Air Showers’ (HAS), as detected by the EAST-TOP experiment at Campo Imperatore (LNGS), are presented. Their implications for the interpretation of underground muon data, and for UHE neutrino astronomy are discussed.

The flux of cosmic ray hadrons at the atmospheric depth of 820 g/cm^2 has been measured by means of the EAS-TOP hadron calorimeter (Campo Imperatore, National Gran Sasso Laboratories, 2005 m a.s.l.). The hadron spectrum is well described by a single power law : S(E_h) = (2.25 +- 0.21 +- 0.34(sys)) 10^(-7)(E_h/1000)^(-2.79 +- 0.05) m^(-2) s^(-1) sr^(-1) GeV^(-1) over the energy range 30 GeV-30 TeV. The procedure and the accuracy of the measurement are discussed. The primary proton spectrum is derived from the data by using the CORSIKA/QGSJET code to compute the local hadron flux as a function of the primary proton spectrum and to calculate and subtract the heavy nuclei contribution (basing on direct measurements). Over a wide energy range E_0 = 0.5-50 TeV its best fit is given by a single power law : S(E_0) = (9.8 +- 1.1 +- 1.6(sys)) 10^(-5) (E_0/1000)^(-2.80 +- 0.06) m^(-2) s^(-1) sr^(-1) GeV^(-1). The validity of the CORSIKA/QGSJET code for such application has been checked using the EAS-TOP and KASCADE experimental data by reproducing the ratio of the measured hadron fluxes at the two experimental depths (820 and 1030 g/cm^2 respectively) at better than 10% in the considered energy range., Comment: 16 pages, 9 figures, accepted for publication in Astroparticle Physics

Extensive Air Shower (EAS) experiments with high granularity have reported the detection of multicore events. On the other hand, multicluster and double-core events have been observed with emulsion chambers (EC) at mountain altitudes since the late '70s. A striking characteristic of these events is the large transverse momentum of the clusters. At accelerator energy region, it is known that jet production in deep inelastic hadronic processes is associated with the emission of large-P T particles. In this paper we analyse the possibility that both EAS and EC events represent the same physical process, discussing the physical consequences of this fact and the possibilities offered by both techniques in integrating the accelerators data.

The Large Volume Detector (LVD) in the Gran Sasso underground Laboratory is a multipurpose detector consisting of a large volume of liquid scintillator interleaved with limited streamer tubes. In this paper we discuss its power to study low energy cosmic neutrinos. The results show that the first LVD tower (368 tons of liquid scintillator) is well suited to detect neutrinos from gravitational stellar collapses within all of our Galaxy over a wide range of burst duration (up to a few hundred seconds). No burst candidates have been observed in the first two months of data taking.

Upper limits have been obtained to the diffuse gamma-ray emission from the Galactic disk. The limits I(gamma)/I(p) less than 0.2 percent, i.e., I(gamma)(greater than 130 TeV) less than 3.2 x 10 exp -13/q cm/s/rad provide the first measurement obtained in the angular window corresponding to the distribution of the ISM. It is still, however, a factor of about 30 higher than the flux calculated from the cosmic ray interactions by Berezinsky and Kudryavtsev (1990). A lower limit of gamma greater than 2.3 is obtained to the power-law index of the differential energy spectrum of Galactic gamma rays from 100 MeV to 100 TeV, setting a limit to the possibility of a hard extrapolation of the Galactic gamma-ray spectrum to the 100 TeV region. 21 refs.

This paper presents data on the sidereal variation of muons recorded at 570 hg/cm 2 , corresponding to primary rigidities around 1.8 TV. The record investigated covers a period of five years, from September 1985, near solar minimum, to September 1990, near solar maximum. The features of the observed sidereal variation histogram are related to the changing heliospheric conditions. Three conclusions emerge: (1) The Fourier reconstruction of the 24-h histogram requires a large number of harmonics, since the first and the second (diurnal and semidiurnal) are often outranked by higher order Fourier harmonics; (2) the anisotropy observed during the period of minimum solar activity has galactic origin; (3) as the level of solar activity increases the information on this anisotropy is altered by solar modulation processes.

The EAS‐TOP array at Gran Sasso (Italy) has been fully operating as a gamma‐ray astronomy observatory since the beginning of 1989 (energy range 1014‐1017 eV). We present the results on search for gamma ray emission from candidate point sources in northern hemisphere obtained by data collected through 1989‐1990. The UHE energy event observed from Crab Nebula on February 23rd, 1989, is also discussed.

The thermoluminescence (TL) profile of the GT14 Ionian Sea core has been recently analyzed in connection with other records of solar activity. Phenomenological similarities among the TL profile, the radiocarbon record in tree-rings and the variations of the mean anuual sunspot number Rz suggest a solar control of the TL signal. In this paper we consider new readings of the TL profile of the GT14 core which were obtained at different glow temperatures. While the main spectral peaks discussed in previous papers are observed in all readings, a few new significant periodic components may now be separated from the noise background. Among these, a strong 22 yr (Hale) cycle is evident, together with a periodicity of 28.5 yr which has already been detected in the spectrum of the sunspot number series. We finally test the temporal persistency of the main TL periodic components by using a cyclogram method and we explore the effects of background noise by considering the TL profile of bleached samples.

This paper presents results on the fractal and statistical behavior of cosmic ray time series detected in an air shower experiment located at 2000-m altitude above the underground Gran Sasso Laboratory, Italy. We consider single particles (muons), corresponding to primary energies of ≥10 GeV, and air showers, corresponding to primary energies of ≥80 TeV. For all time series the analysis indicates a clear stochastic monofractal, non-Gaussian character; comparing these results with those obtained for underground muons and for neutron monitors, we conclude that these properties likely belong in general to cosmic ray time series, irrespective of the nature of the particles and the energies of their progenitors. In particular, the air shower time series from high-energy primaries have a fractal dimension larger than the single-muon time series originating from low-energy primaries.

The authors investigate the fractal and statistical properties of underground muons recorded at 570 hg/cm{sup 2} and study their response to solar activity variations in the second part of solar cycle 21. A constant feature detected throughout the whole period considered is the self-affine nature of the underground muon fluctuations which display a scaling range extending approximately from a few hours to more than 2 months. An important result of the study is that the scaling and spectral exponents are positively correlated with the sunspot cycle. In addition, the intensity distribution shows a more marked presence of short- and long-term fluctuations near solar maximum. The presence of solar modulation in the muon record suggests dimensions of the heliosphere at least as large as a few scattering mean free paths of the 1.8-TV primaries.

A meteorite shower fell at Torino, Italy on May 18, 1988. Petrographic studies indicate that the stone is an H6 chondrite having features of moderate to severe shock. Chemical analyses of the meteorite are reported. Cosmic ray produced He-3, Ne-21 and Xe-126 yield an exposure age of about 48 Ma. The cosmic ray track densities in three fragments range between 1.8 to 5 x 10 to the 5th/sq cm suggesting about 99 percent mass ablation in the atmosphere,. Twelve radioisotopes with half lives ranging between 5.6 days to 7.3 x 10 to the 5th years have been measured with high precision. Marginal signals were observed for several short-lived nuclides and upper limits were obtained for the activity levels of eight radionuclides, some of which have not been hitherto detected in fresh falls. The data are generally consistent with the nuclide production by galactic cosmic rays when modulation due to the solar cycle is taken into consideration. 23 refs.

Thermoluminescence from the different layers of a recent core drilled in the Tyrrhenian Sea has been measured. The sedimentation rate of 0.115 cm y−1 previously determined by the210Pb−226Ra method allows the profile of TL to be obtained in the last 1500 years. We notice that sharp peaks in the TL intensity appear during the years in which records of hystorical supernovae explosions are given. A working model of the process based on the irradiation of dust in the upper atmosphere by γ-rays produced in the SN burst allows a satisfactory energy estimate of the observations to be done. Moreover, the profile shows the modulation of the irradiations of the Earth environment produced by the Sun in the past.

The Zurich sunspot relative number R z series has been analysed by the cyclogram method. The amplitude and the frequency variations of the Fourier 11 yr component between 1700–1983 A.D., were determined in a continuous way. Four distinct time intervals with significantly different characteristics of the periodicities are observed and discussed. Their second harmonics are also considered. The periodicity changes are contemporary to those of the 11 yr cycles. Around the year 1903 it seems that an important event has happened in the Sun. In fact the 11.4 yr cycle periodicity, that was very stable since at least 1825 started to change gradually to smaller values and similarly it happened to the second harmonic which also stopped and abruptly changed of phase of 90°.

We present the results of a spectral analysis in the frequency domain of the fluctuations of cosmic rays extended to primary rigidity ∼250 GV. The experimental power spectrum is compared with that expected on the basis of the theory of interplanetary scintillations, Forbush effect and the effects of CM passages of solar recurrent active regions. The agreement confirms the influence of heliospheric phenomena up to 250 GV and the presence of nonlinear interactions between particles and interplanetary magnetic fields.

The diurnal variations of underground cosmic ray intensities are analyzed on a day-to-day basis. At the depths of the London and Torino stations an anisotropy in the direction of the interplanetary magnetic field is observed in addition to the corotation effect, which varies in relation to the perturbations produced by the sun. This effect is not recorded in the Hobart data during the same period of observation.

We have correlated intensity decreases in the galactic component of cosmic rays observed at the Earth with solar phenomena. We have found that there exist intensity decreases detected underground, which are probably associated with long-lived interplanetary streams and which are not directly associated with single flares. We expect to detect the streams with underground monitors before the arrival of the streams at the Earth. The early detection will be in the northern or in the southern hemisphere, depending on the sign of the interplanetary field.

Cosmic-ray intensity measurements have been carried out during the last year at a depth of 70 m.w.e. in the Monte dei Cappuccini laboratory in Torino, using solid vertical semicubical scintillator telescopes with a total area of 2 m2. Hourly data for 245 days corrected for barometric changes have been analyzed for the solar, apparent sidereal, and antisidereal daily variations whose harmonics are as follows:[Formula: see text]The true sidereal diurnal variation is estimated to have an amplitude of 0.019% with a time of maximum at 1720 h local sidereal time. The solar diurnal variation at different depths underground follows the energy dependence calculated with Axford's theory. The solar semidiurnal variation shows instead a fairly constant value at different μ energies. Its order of magnitude agrees with that expected as a result of Fermi acceleration in collisions of primaries moving in roughly solar and antisolar directions with solar wind inhomogeneities.

Trains of diurnal waves in the muonic component at 70 MWE have been found during the preliminary phases of some cosmic-ray decreases. A predecrease the day immediately preceding a Forbush decrease has repeatedly been reported from neutron monitor observations. Our analysis indicated that underground data reveal this effect earlier and for a longer period. This is consistent with the interpretation of a Forbush decay as the result of the advance of a shock front.

Evidence has recently been put forward that Jupiter might be modulating the intensity of cosmic radiation observed at the earth (Pizzella and Venditti 1973; Cini-Castagnoli et al. 1973). It therefore seemed worthwhile to analyse underground meson data in Jovian time for any evidence of an effect at the higher energies over a complete orbital period.

The EAS-TOP is an air shower array situated on the vertical of the Underground Gran Sasso Laboratory, in Italy. Its performances in Ultra High Energy (UHE) gamma-ray astronomy are described both in the search for point sources (Eo > 5.1013 eV) and in the identification of primary gamma-rays.

The existence of the 11-year cycle in solar activity before the Maunder minimum is clearly demonstrated with cosmogenic /sup 10/Be in polar ice during 1180--1500 A.D. For that interval a periodicity of 11.4 +- 0.2 years is found with a high significance level. Indication of a cyclicity that resembles the Hale magnetic cycle is also observed at a lower significance level. A highly variable cyclicity in the band 9.5--11.5 years is also found in the record of historical aurorae which appears to be well correlated with the /sup 10/Be cyclicity for the same time interval. It is concluded that the Schwabe, or 11-year, cycle and the Hale magnetic cycle were present before and after the Maunder minimum, even though it is not possible to understand the variability of the cycle. copyright American Geophysical Union 1988

We study a correlation between the solar magnetic dipole moment and the amplitude of the diurnal variation of underground muons from cosmic ray primaries of mean rigidity approx. = 1800 GV. The time series investigated was recorded at Artyomovsk, in the Soviet Union, at a vertical depth of 570 hg/cm/sup 2/. Our results indicate that both diurnal and semidiurnal variations in the muon intensities correlate well with the variations of the solar magnetic dipole. At the time of the maximum amplitude of the heliomagnetic dipole moment (summer 1982) the vector of the diurnal variation shows an increase in amplitude by a factor of /similar to/3.5 relative to the average value for the remaining period, and a phase shift from the direction of /similar to/18 hours toward /similar to/12 hours. These results suggest that during the enhancement of the magnetic dipole field of the Sun the 1800-GV cosmic rays stream preferably along the radially outward direction, and consequently the mechanism of solar convection dominates over that of diffusion. copyright American Geophysical Union 1989

We present results of the spectral analysis of the fluctuations of cosmic rays recorded by different underground muon detectors (h=7--76 meters water equivalent). The experimental power spectra, compared with the predictions of diffusion theory in the nonlinear closure approximation, suggest the persistence of solar modulation up to rigidities of approx.300 GV and the existence of nonlinear interactions between particles and the interplanetary magnetic field.

We study the self-affine properties of intensity fluctuations of underground muons generated by primary cosmic rays of rigidity ≥1600 GV. The muon time series were recorded at a vertical depth of 570 Hg/cm² during the years 1981–1983. This time interval includes a period of intense solar activity in the summer of 1982 which was characterized by the creation of large solar wind shocks and associated production of major interplanetary disturbances. The purpose of our analysis is to obtain from the behavior of the muon intensity some insight into the large-scale behavior of interplanetary magnetic field fluctuations and their effects on the propagation of high-rigidity galactic cosmic rays. The results give good evidence for fractal behavior of large-scale muon fluctuations in the range from a few hours to more than 10 days, with the scaling exponent H varying according to the time interval analyzed. We find that the large time scale muon fluctuations evolve from a preshock condition in late 1981 to early 1982 characterized by a scaling exponent H = 0.28 and power spectrum f−1.6 to a condition with H = 0.07 and an f−1.1 power spectrum during the most intense solar activity in 1982, followed by a slow return in 1983 to the original preshock conditions (H = 0.14, f−1.3).