Search

The paper presents data obtained on the production rates of cosmogenic radionuclides of different half-life in 42 chondrites that fell in 1959–2016. A quantitative approach is developed for using cosmogenic radionuclides as natural detectors of galactic cosmic rays (GCR) for different time periods and for different heliocentric distances. A long set of homogeneous data is compiled on 54Mn, 22Na, and 26Al production rates along meteorite orbits (at 2–4 AU) in 1959–2016. Its correlative analysis with the corresponding sets of data on some major parameters of magneto hydrodynamic processes in the heliosphere allows us to identify principal relationships and trends in the temporal and spatial variations in GCR (E > 100 MeV) in the inner heliosphere (≤5 AU) over a long time scale. The monitoring of this analysis makes it possible both to study these processes in the past and to predict their characteristics in the future, which still cannot be done by means of direct measurements of the GCR intensity in interplanetary space. A project is suggested to demonstrate the expedience of establishing an international meteorite-patrol survey to study spatiotemporal variations in GCR in the inner heliosphere by means of measuring and analyzing the production rates of cosmogenic radionuclides in chondrites falling to Earth. No alternative projects have been put forth as of yet. [ABSTRACT FROM AUTHOR]

The results of multiyear investigations of cosmogenic radionuclide production rates along the orbits of 42 chondrites that fell to the Earth between 1959 and 2016 are presented. They constitute a long set of homogeneous data, statistical smoothing of which demonstrates some main regularities of the distribution and variation of the galactic cosmic rays (GCR) with energy >100 MeV in the internal (<5 AU) heliosphere. This set is exclusive, and it has timeless importance for all the future investigations of the magneto-hydrodynamic peculiarities of the GCR solar modulation mechanism. Analysis of the 26Al contents in ten chondrites of known orbits allowed us to find the spatial profile of the GCR intensity (E > 100 MeV), averaged over a 1 My in the internal heliosphere, testifying to continual development and dissipation of a layer of the solar wind magnetic irregularities, which efficiently modulates cosmic radiation. It suggests constancy of the magneto-hydrodynamic environment in the Solar System for at least the last million years. [ABSTRACT FROM AUTHOR]

Cosmogenic radionuclides with half-life periods T1/2 ranging from several days to a million years, produced in the nuclear reactions of galactic cosmic rays (GCR) with meteoritic matter, provide valuable information on the GCR intensity variations on a long time scale (∼1 million years) within meteorite orbits 2–4 AU from the Sun. Information on the variations of GCR gradients in the inner heliosphere was obtained by comparing the measured contents of 54Mn and 22Na in stone meteorites (chondrites) with known orbits at the time of their fall to Earth with the calculated production rates of these radionuclides in them using balloon intensity measurements of GCR (E > 100 MeV) in the stratosphere at appropriate times. Although individual gradient values show significant uncertainties, the important information is that cosmogenic radionuclides in chondrites predict low gradients (0–10% per 1 AU) for all periods of minimum solar activity in 1957–2013, according to direct measurements in interplanetary space. High gradients (50–100% per 1 AU) are predicted for periods of maximum solar activity, especially in 1992 and 2012 (up to ∼200% per 1 AU). Average values of gradients are (20 ± 10)% per 1 AU for modern solar cycles (according to the production rate of 22Na), similar to the average values for the last ∼1 million years (according to the production rate of 26Al), which indicates the constancy of the solar modulation of the GCR, at least for the last million years. [ABSTRACT FROM AUTHOR]

Based on the analysis of published data on exposure ages of iron meteorites determined with the K/K method ( T ) and ages calculated using short-lived cosmogenic radionuclides (with the half-life T < 1 Myr) in combination with stable cosmogenic isotopes of noble gases (T), the following results have been obtained. (1) The distribution of T ages (106 values) has an exponential shape, similar to that for ordinary chondrites, but different from the distribution of T ages (80 values). The difference is most likely due to small amounts of data for meteorites with low T ages (less than ~200-300 Myr). The latter can be ascribed to the difficulty of measurement of small concentrations of cosmogenic potassium isotopes. This circumstance makes the selection of meteorites with K/K ages nonrepresentative and casts doubt on the correctness of conclusions about the variations of the intensity of galactic cosmic rays (GCR) based on the analysis of distribution of these ages. (2) The magnitude of the known effect (systematic overestimation of T ages in comparison with T ages) has been refined. The value k = T / T = 1.51 ± 0.03 is acquired for the whole population of data. We have shown the inefficiency of the explanation of this effect on account of an exponential change in the GCR intensity ( I ) with time ( T) according to the relation I = I exp(-γ T) over the whole range of ages of iron meteorites. (3) In order to explain the overestimation of T ages in comparison with T ages, a model has been proposed, according to which the GCR intensity has exponentially increased in the interval of 0-1500 Myr governed by the relation: I = I (1 + αexp(-βT)). For one of the variants of this model, the GCR intensity has exponentially increased by a factor of two only over the recent ~300 Myr, remaining approximately constant for the rest of the time. The data acquired with the use of this model indicate that the measured T ages are close to the actual time that the meteorites existed in space; the data are in agreement with the observed exponential distribution of T ages. [ABSTRACT FROM AUTHOR]

The distribution of the cosmic-ray exposure ages (T) of iron meteorites was analyzed to establish the possible variations in the intensity of the galactic cosmic ray (GCR) over the last billion years. The analysis was made for the entire data set containing ~80 age values from the literature (Voshage et al., 1983) and the corrected set after the exclusions of paired meteorites (using the Akaike information criterion). The dependence of the criterion χ in the distribution of the phase values Ph = T/t-int(T/t) on the values of the assumed period (t) of GCR variations was analyzed for both sets of meteorites. The significant deviations of these parameters from the respective average values were found for t ~ 400-500 Myr and, in part, for t ~ 150 Myr. These deviations were interpreted by numerical modeling using the values of ages randomly distributed in the range of 0-1000 Ma. It was found that for variations with a period of 450 Myr, the distribution of the phase values and cosmic-ray exposure ages in the model data set is similar to that of iron meteorites. These results testify to the existence of the GCR variations with a period of ~400-500 Myr during the last 1 Gyr. The variations in the GCR flux can be explained by periodic galactic spiral arm crossings of the solar system. The GCR variations with a period of ~150 Myr discussed in the previous studies (Shaviv, 2002; 2003; Scherer et al., 2006) appears to be less certain. [ABSTRACT FROM AUTHOR]

To ascertain probable variations of the intensity of galactic cosmic rays (GCR) for the recent billion years, the distribution of exposure ages T of iron meteorites has been analyzed. We considered all ~80 values of ages from the data by Voshage and Feldmann (1979), Voshage et al. (1983), and Voshage (1984), as well as a set of values obtained from the correction for eliminating the meteorites formed in a single collision. To correct the data, the Akaike information criterion was used. For the distributions of the phase values Ph = T/t-int( T/t), the dependence of the criterion χ on the presumable period t in the exposure age variations was analyzed. For t ~ 400-500 Myr and, partly, for t ~ 150 Myr, the significant deviations of this criterion from the corresponding mean values were found. To clear up the influence of the GCR intensity variations on the age distribution, the numerical models were calculated with an account of the set of ages randomly distributed in the interval of 0-1000 Myr with the presumptive mean lifetime of iron meteorites in outer space τ = 700 Myr. It has been ascertained that, for variations with a period of t = 450 Myr, the distribution of exposure ages of the model set is similar to that found for iron meteorites. The obtained data suggest that the GCR intensity variations with a period of approximately 400-500 Myr have probably existed during the recent billion years. These variations may be caused by periodic passages of the Solar System through spiral arms of the Galaxy. It has been shown that the earlier discussed changes in the GCR intensity with a period of ~150 Myr (Shaviv, 2002; 2003; Scherer et al., 2006) are less defined. [ABSTRACT FROM AUTHOR]

The article focuses on a study which analyzed the character of the solar magnetic field restructuring, as well as its effect on the modulation depth of galactic cosmic rays (GCR) in the heliosphere. It notes on the correlations between the GCR gradients and solar activity (SA). It illustrates the secular SA cycles from 1700-2001.

Examines the distribution and variation of galactic cosmic rays in the heliosphere. Investigation of heliosphere; Manifestation of the solar cycle; Modulation of the galactic cosmic rays.