Underground Muon Telescopes Research Articles

Investigation of Energy Spectrum of Semidiurnal Variations of Cosmic Rays

An energy spectrum of semidiurnal variations of cosmic rays observed at the Earth is investigated. The data of registration of cosmic rays by the ground and underground multidirectional muon telescopes (stations Yakutsk, Nagoya, Hobart, Kuwait, Sao Martinho da Serra, Misato, Sakashita, Matsushiro), which covers the time period from 1971 to 2018 and the median energy range from 55 to 686 GeV, are used. Average annual values of semidiurnal variations of cosmic rays observed by the mentioned stations and free from atmospheric and geomagnetic effects are obtained. Based on the analysis of the obtained data, a form and temporal dynamics of their energy spectrum are investigated. It is established that the energy spectrum reveals dependence on the solar activity cycle.

Solar Polarity Dependence of Cosmic Ray Power Spectra Observed with Mawson Underground Muon Telescopes

Power spectral density (PSD) of cosmic rays has been calculated from hourly averaged counts observed by underground muon telescopes located at Mawson over the low-frequency range 2.7×10−7 – 1.4×10−4 Hz. The first two harmonics of the solar daily variation are well defined for even cycles (20 and 22) whereas only the first harmonic is defined in cycle 21. The amplitude of the diurnal variation is lower for even cycles than for the odd cycle. The spectral power of the odd cycle exceeds those of the even cycles. The spectra are flatter and have lower power when the interplanetary magnetic field (IMF) is directed away from the Sun above the current sheet (A>0) than when the IMF is directed toward the Sun above the current sheet (A<0). The spectra imply that heliospheric magnetic turbulence may be more variable on time scales of several years than previously suspected.

Coexistence of cosmic-ray sidereal anisotropies originating in galactic space and at the heliomagnetospheric nose and tail boundaries, observed with muon detectors in the energy region of 60∼100 GeV

Abstract The coexistence of two kinds of cosmic-ray sidereal anisotropy was found by observations with underground muon telescopes in the energy region (&gt; ∼200 GeV) in 1995: one is the galactic anisotropy with a deficit flux in the direction with right ascension α G = 12 hr and declination δ G = 20°. The other is the excess flux from the heliomagnetospheric tail direction (α T ≃ 6 hr) and would be produced on the heliotail boundary where it is considered that the interaction between the galactic and solar magnetic fields could produce the cosmic-ray acceleration. On the other hand, another anisotropy of helioboundary origin from the helionose direction (α ≃ 18 hr), being accompanied by the heliotail-in anisotropy, was found through the observations with neutron monitors in the low energy region (∼20 GeV) in 2005. These observations, however, lack information in the mid-energy region (20∼200 GeV). In order to bridge the absence of information, the cosmic-ray sidereal daily variations in the energy regions (60∼100 GeV) have been derived from the observations with muon telescopes and ion chambers on the ground in the period 1936–2003. It is shown that all the three anisotropies coexist in this energy region and are subject to their respective solar modulations. On the basis of these modulations, the characteristics of the anisotropies are determined through intercomparison with the observations in the high and low energy regions.

Antarctic Cosmic Ray Astronomy

AbstractCosmic ray observations related to Antarctica commenced in the austral summer of 1947-48 from sub-Antarctic Heard and Macquarie Islands and from the HMAS Wyatt Earp. Muon telescope observations from Mawson station, Antarctica, followed from 1955. The International Geophysical Year was the impetus for the installation of a number of neutron monitors around Antarctica, observing the lowest energy cosmic rays accessible by ground based instruments. In 1971 a new observatory was built at Mawson including the only underground muon telescope system at polar latitudes in either hemisphere. Over more than half a century, cosmic ray astronomy has been undertaken from Antarctica and its surrounding regions and these observations have been critical to our growing understanding of the heliosphere.

Gaussian analysis of two hemisphere observations of galactic cosmic ray sidereal anisotropies

We have analyzed the yearly averaged sidereal daily variations in the count rates of 46 underground muon telescopes by fitting Gaussian functions to the data. These functions represent the loss cone and tail‐in anisotropies of the sidereal anisotropies model proposed by Nagashima et al. [l995a, b]. The underground muon telescopes cover the median rigidity range 143–1400 GV and the viewing latitude range 73°N–76°S. From the Gaussian amplitudes and positions we have confirmed that the tail‐in anisotropy is more prominent in the southern hemisphere with its reference axis located at declination (δ) ∼14°S and right ascension (α) ∼4.7 sidereal hours. The tail‐in anisotropy is asymmetric about its reference axis, and the observed time of maximum intensity depends on the viewing latitude of the underground muon telescopes. We also find that the declination of the reference axis may be related to the rigidity of the cosmic rays. We show that the loss cone anisotropy is symmetric and has a reference axis located on the celestial equator (δ ∼ 0°) and α ∼ 13 sidereal hours. We have used the parameters of the Gaussian fits to devise an empirical model of the sidereal anisotropies. The model implies that the above characteristics of the anisotropies can explain the observed north‐south asymmetry in the amplitude of the sidereal diurnal variation. Furthermore, we find that the anisotropies should cause the phase of the sidereal semidiurnal variation of cosmic rays to be observed at later times from the northern hemisphere compared to observations from the southern hemisphere. We present these results and discuss them in relation to current models of the heliosphere.

Sidereal daily variation of galactic cosmic rays

We have analyzed the yearly averaged sidereal daily variations in the count rates of 44 underground muon telescopes by fitting Gaussian functions to the data. These functions represent the Loss-cone (LC) and Tail-in (TI) anisotropies proposed by Nagashima et al.. The telescopes cover the median rigidity range 143GV–1400GV and the viewing latitude range 73°N–76°S. We find that the TI anisotropy has its reference axis located at declination ( δ) ≈ 14°S and right ascension ( α) ≈ 4.7 sidereal hours. We show that the TI anisotropy is asymmetric about the reference axis and its observed α depends on the viewing latitude of the telescopes. We also show that the LC anisotropy is symmetric and has a referenceaxis located at the celestial equator and α ≈ 13 sidereal hours. From the parameters of the Gaussian fits we devise an empirical model of the sidereal anisotropies which implies that the above characteristics of the anisotropies can explain the north-south asymmetry in the amplitude of the sidereal diurnal variation. Furthermore, we find that the phase of the sidereal semi-diurnal variation of cosmic rays should be recorded at later times when measured from the northern hemisphere compared to observations made from the southern hemisphere.

An extended analysis of the September 1989 cosmic ray ground level enhancement

The September 29, 1989, cosmic ray ground level enhancement had a complex intensity‐time profile with two distinct peaks observed at some neutron monitors. The event was detected by surface and some underground muon telescopes indicating the presence of particles up to ∼30 GV. We have modeled the response of neutron monitors and surface muon telescopes at the times of both intensity peaks and during the decay phase. We have determined particle rigidity spectra and pitch angle distributions centered on apparent arrival directions. Some evidence of bidirectional flow is present during the second peak. This may be due to backscattering of particles from beyond Earth. There was a high degree of scattering from all directions resulting in a significant isotropic component. Spectra derived from ground level responses are consistent with satellite observations. The spectrum between a few MeV and a few tens of GeV is consistent with shock acceleration.

Preliminary analysis of two‐hemisphere observations of sidereal anisotropies of galactic cosmic rays

By using the two‐hemisphere network of underground muon telescopes we have examined the average sidereal daily variations in the count rates recorded by 48‐component muon telescopes. The telescopes respond to primary cosmic rays with rigidities between ∼140 and 1700 GV and view almost the entire celestial sphere. We have modeled the data by using Gaussian functions, and we have related the Gaussian parameters to the recent tail‐in and loss cone anisotropy model proposed by Nagashima et al. [1995a, b] to explain the sidereal daily variations. We have used the model parameters to derive the rigidity and latitude spectra of the galactic anisotropies and find them to be qualitatively in agreement with Nagashima et al.'s predictions. The results indicate, however, that the tail‐in anisotropy is asymmetric about its reference axis, whereas the loss cone anisotropy is more symmetric. We show that these characteristics of the galactic anisotropies may explain the north–south asymmetry observed in the amplitude of the sidereal diurnal variation derived from Fourier analysis techniques.

On the cosmic ray convection-diffusion and drift anisotropies

We separate the convection-diffusion and drift cosmic ray anisotropy effects on the base of 25 years data of underground muon telescopes and ground based neutron supermonitors by using space observational data of interplanetary magnetic field (IMF) and information on the Earth's position relative to the solar equator. It is found that rigidity spectrums of these types of anisotropy are sufficiently different.

Cosmic ray transverse gradient for a Hale Cycle

We have used neutron monitor data obtained at Deep River and Huancayo as well as the vertical underground muon telescope data obtained at Embudo to compute the cosmic ray transverse gradient (Gθ) for the 1965 to 1990 period. The computed mean value of Gθ, for a given qA epoch, changes sign with the change in the solar magnetic polarity in accordance with the predictions of some contemporary hypotheses. Also, we find that the magnitude of Gθ undergoes a significant time variation during a given polarity epoch. It has the largest value near the solar polar field reversal epoch boundaries, when magnetic polarity in the solar northern hemisphere is positive (qA&gt;0). For each of the three solar polar field reversals, the change in the magnitude of Gθ is steeper across the epoch boundaries. There is a good correlation between Gθ and the tilt angle of the heliospheric neutral current sheet (HNCS). Our results are in agreement with the reported piecemeal measurements of the latitudinal gradient made by spacecrafts, at different times, at lower primary energies, at far off distances from Earth's orbit. As such, their global significance is appreciable. Even so, it is not clear yet how these findings reflect upon the drift hypothesis in its present form. However, they do affirm the important role played by the tilt angle of HNCS in the modulation processes.

Modulation of high‐energy cosmic rays in the heliosphere

We have used sidereal diurnal variations observed by neutron monitors and surface and underground muon telescopes to examine the north‐south anisotropy and radial density gradient of high‐energy galactic cosmic rays between 1957 and 1985. We have derived the average amplitude and rigidity spectrum of the anisotropy for this period and estimate the radial gradient to be best represented as a function of rigidity P by 1.02 × (P/10)−0.5% AU−1 for rigidities less than ≈450 GV. The temporal variation of both the north‐south anisotropy and the radial density gradient show a solar cycle variation. Possible dependence of the anisotropy on the solar magnetic polarity state has been examined under the assumptions of a static and a nonstatic rigidity spectrum, with conflicting results. We show that the analysis is affected by a further sidereal variation and that the responsible anisotropy is related to the asymmetric heliospheric modulation of a galactic anisotropy.

Radial and Latitudinal Gradients in Galactic Cosmic Rays

AbstractWe have deduced the yearly averaged value of the solar diurnal variation as observed by a surface muon telescope and three underground muon telescopes over the years 1957 to 1985. This has allowed us to examine the temporal variation in both the latitudinal gradient Gz and the product of the parallel mean free path and the radial gradient of galactic cosmic rays during three consecutive solar cycles. The median rigidities of the primary particles being detected by the telescopes are 50 GV in the case of the surface muon telescope and greater than 150 GV in the case of the underground muon telescopes. We have compared our results with those of a similar study made from observations of the solar diurnal variation by neutron monitors and an ion chamber, which have median rigidities of response between 17 and 70 GV (Bieber and Chen 1991a). The product has a solar magnetic cycle dependence and our values are lower than those observed by neutron monitors, in agreement with the Bieber and Chen observation that reverses after a solar magnetic field reversal, in accordance with drift theories.

Cosmic Ray Transient Variations Observed from the Earth

AbstractCosmic ray transient variations are signatures of the underlying solar processes which affect the heliomagnetic structure. They can be found in the data from surface systems, such as neutron monitors and muon telescopes, and from shallow underground muon telescopes. Although sometimes observable at all latitudes, the global distribution of the effects is important in determining the structures and causes. Three transient variation types are known, namely Forbush decreases, ground level enhancements, and quasi-periodic fluctuations. The latter category includes some variations which are, perhaps, too long-lived to be considered truly transient. In this review, the detection techniques and background of cosmic ray research are followed by a summary of some observations in each transient category. The heliomagnetic structures and dynamics inferred from such transients are discussed together with the possible impact such events can have on human activity.

Local-Time-Dependent Pre-IMF-Shock Decrease of Cosmic Rays, Produced by Their IMF-Collimated Outward Flow across the Shock from the Inside of Forbush Decrease.

In the previous paper (NAGASHIMA et al., 1992), the authors pointed out the existence of the local-time-dependent precursory decrease of cosmic rays in front of the shock wave of the interplanetary magnetic field (IMF), and interpreted it as being due to their IMF-collimated outward flow across the shock from the inside of the Forbush decrease (Fd). In the present paper, they report its typical example recently observed in front of the Fd on September 9, 1992, as it bears the following dominant characteristics. The precursory decrease was not only exceedingly significant but also observed twice at the same station for two days running, which could not be clearly recognized in the previous analysis without taking the average of many events, and it was observed also with the underground muon telescope at Sakashita (depth: 80Hg/cm2), indicating its harder rigidity spectrum than the previous estimate. Based on these observations, the characteristics of the collimated flow responsible for the decrease is discussed.

Cosmic-ray sidereal daily variation of galactic origin to be observed with existing underground muon telescopes

Modulation of galactic cosmic-ray anisotropy in the heliomagnetosphere has been theoretically studied by Nagashima et al. (1982, Planet. Space Sci. 30, 879; 1983, Planet. Space Sci. 31, 1259), and sidereal daily variations of galactic origin observable at the Earth's orbit have been obtained. These variations, however, contain only the influence of the heliospheric modulation and do not contain the influences of cosmic-ray's geomagnetic deflection and nuclear interaction with the terrestrial material and also the geometrical configuration of the muon telescope. In the present paper, we take into account these neglected influences and obtain the sidereal daily variations to be compared with those observed with specific telescopes at some specified underground stations.

The September 29, 1989 ground‐level event observed at high rigidity

Ground‐level solar cosmic ray events are usually observed by high latitude neutron monitors at relatively low rigidities (&gt; 1 GV) and are not usually observed by underground cosmic ray detectors because of their much higher threshold rigidity. However, the September 29, 1989 ground‐level event was sufficiently large and unusual that it was recorded by the Embudo, New Mexico underground muon telescope which has a threshold rigidity of 19 GV. The observed increase was 2.2 ± 0.2% in the hour interval from 1100 to 1200 UT. There was no statistically significant increase in the data from the Socorro underground muon telescope which has a threshold rigidity of 45 GV. This is the first detection of a ground‐level solar cosmic ray event by any underground cosmic ray muon telescope where the event has been clearly distinguishable above the background cosmic ray intensity.

The diurnal anisotropy of cosmic rays and the heliospheric transport parameters

We review the data on long-term changes in the solar diurnal variation of cosmic rays, observed at Deep River with a neutron monitor and at Embudo with an underground muon telescope, during the period 1965–1976. We show that the model proposed by Erdos and Kota (1979, Sixteenth ICCR, Kyoto. Conference Papers 4, 45). cannot explain the systematic changes seen in our data during 1971–1976. An insight gained from the diffusion-convection model enables us to obtain a unique solution for K ‖ G r in terms of the parameters applicable to the solar wind and the diurnal anisotropy. This helps us calculate the rigidity ( R) dependence of the diffusion coefficient K ‖. We find that K ‖ = 1.37 × 10 18 R 0.6 m 2 s −1, where R is in gigavolts, and K ⊥ = 1.08 × 10 18 m 2 s −1. Our results are discussed.

Recurrence of periods of enhanced cosmic ray solar diurnal variation associated with a two‐sector structure

Swinson et al. (1981) showed that in 1974 the two‐sector structure of the interplanetary magnetic field gave rise to conditions in which there were extended trains of enhanced diurnal variation in cosmic ray intensity which were organized according to the sector structure. In this paper we show that the heliomagnetosphere, in its continuing evolution, has again produced a distinct two‐sector structure in 1984. We demonstrate, using neutron monitor data and data from underground muon telescopes, that these conditions once more give rise to extended periods of enhanced cosmic ray solar diurnal variation, again organized by the sector structure. These field‐dependent anisotropies are consistent with the observations and expectations described in the earlier paper.

Corrected sidereal anisotropy for underground muons

Data from underground muon telescopes in New Mexico and Bolivia are analyzed in sidereal time and anti-sidereal time to study anisotropies in the rigidity range 20 GV to a few 100s of GV. Using both vertical and North- and South-pointing telescopes in both hemispheres, a latitude range of 70°N–50°S is covered. The distribution of cosmic rays in the inner heliosphere gives rise to a diurnal variation in anti-sidereal time, and also produces a spurious contribution to the anisotropy in sidereal time. It is shown that the anti-sidereal variation is of the P 2 1 type, having opposite phase in the Northern and Southern Hemispheres, and maximum amplitude at mid latitudes. The anti-sidereal data are used to correct the sidereal data, using the Nagashima method (Nagashima, Sakakibara, Fenton and Humble, 1985); the resulting corrected sidereal vectors for Northern Hemisphere telescopes have their sidereal maxima close to 3 h sidereal time, in reasonable agreement with sidereal data at higher energies from small air showers. The Nagashima correction also appears to eliminate effects due to the reversal of the Sun's polar magnetic field which show up in the unconnected sidereal data, and which also remain in corrected data using an alternative correction.

Reversal of the cosmic ray density gradient perpendicular to the ecliptic plane

Annual averages of the diurnal variation in cosmic ray intensity for neutron monitors in Deep River and Oulu and underground muon telescopes in Bolivia and at Embudo and Socorro, New Mexico, have been determined as a function of the sense of the interplanetary magnetic field for the years 1965–1975. These data point to a cosmic ray density gradient, perpendicular to the ecliptic plane, pointing southward prior to 1969 and changing to a northward pointing gradient after the reversal of the sun's polar magnetic field in 1969–1971. This result supports numerical calculations for the prereversal and postreversal field configurations at intermediate and high cosmic ray rigidities.