The scaler mode in the Pierre Auger Observatory to study heliospheric modulation of cosmic rays

The impact of the solar activity on the heliosphere has a strong influence on the modulation of the flux of low energy galactic cosmic rays arriving at Earth. Different instruments, such as neutron monitors or muon detectors, have been recording the variability of the cosmic ray flux at ground level for several decades. Although the Pierre Auger Observatory was designed to observe cosmic rays at the highest energies, it also records the count rates of low energy secondary particles (the scaler mode) for the self-calibration of its surface detector array. From observations using the scaler mode at the Pierre Auger Observatory, modulation of galactic cosmic rays due to solar transient activity has been observed (e.g., Forbush decreases). Due to the high total count rate coming from the combined area of its detectors, the Pierre Auger Observatory (its detectors have a total area greater than 16,000 m²) detects a flux of secondary particles of the order of ∼10⁸ counts per minute. Time variations of the cosmic ray flux related to the activity of the heliosphere can be determined with high accuracy. In this paper we briefly describe the scaler mode and analyze a Forbush decrease together with the interplanetary coronal mass ejection that originated it. The Auger scaler data are now publicly available.     

An analytical model, describing dynamics of galactic cosmic ray heavy particles.

The present paper analyses the problems of modeling galactic cosmic ray particle fluxes. A model representation which enables the particle energy spectra for large-scale solar activity induced modulations to be calculated is described.     

Prediction of expected global climate change by forecasting of galactic cosmic ray intensity time variation in near future based on solar magnetic field data

A method of prediction of expected part of global climate change caused by cosmic ray (CR) by forecasting of galactic cosmic ray intensity time variation in near future based on solar activity data prediction and determined parameters of convection-diffusion and drift mechanisms is presented. This gave possibility to make prediction of expected part of global climate change, caused by long-term cosmic ray intensity variation. In this paper, we use the model of cosmic ray modulation in the Heliosphere, which considers a relation between long-term cosmic ray variations with parameters of the solar magnetic field. The later now can be predicted with good accuracy. By using this prediction, the expected cosmic ray variations in the near Earth space also can be estimated with a good accuracy. It is shown that there are two possibilities: (1) to predict cosmic ray intensity for 1–6 months by using a delay of long-term cosmic ray variations relatively to effects of the solar activity and (2) to predict cosmic ray intensity for the next solar cycle. For the second case, the prediction of the global solar magnetic field characteristics is crucial. For both cases, reliable long-term cosmic ray and solar activity data as well as solar magnetic field are necessary. For solar magnetic field, we used results of two magnetographs (from Stanford and Kitt Peak Observatories). The obtained forecasting of long-term cosmic ray intensity variation we use for estimation of the part of global climate change caused by cosmic ray intensity changing (influenced on global cloudiness covering).     

Regional and temporal variability of solar activity and galactic cosmic ray effects on the lower atmosphere circulation

In this work we studied the spatial and temporal structure of long-term effects of solar activity (SA) and galactic cosmic ray (GCR) variations on the lower atmosphere circulation as well as possible reasons for the peculiarities of this structure. The study revealed a strong latitudinal and regional dependence of SA/GCR effects on pressure variations in the lower troposphere which seems to be determined by specific features of baric systems formed in different regions. The temporal structure of SA/GCR effects on the troposphere circulation at high and middle latitudes is characterized by a roughly 60-year periodicity which is apparently due to the epochs of the large-scale atmospheric circulation. It is suggested that a possible mechanism of long-term effects of solar activity and cosmic ray variations on the troposphere circulation involves changes in the evolution of the polar vortex in the stratosphere of high latitudes, as well as planetary frontal zones.     

On statistical relationship of solar, geomagnetic and human activities.

Data of galactic cosmic rays, solar and geomagnetic activities and solar wind parameters on the one side and car accident events (CAE) in Poland on the other have been analyzed in order to reveal the statistical relationships among them for the period of 1990-2001. Cross correlation and cross spectrum of the galactic cosmic ray intensity, the solar wind (SW) velocity, Kp index of geomagnetic activity and CAE in Poland have been carried out. It is shown that in some epochs of the above-mentioned period there is found a reliable relationship between CAE and solar and geomagnetic activities parameters in the range of the different periodicities, especially, 7 days. The periodicity of 7 days revealed in the data of the CAE has the maximum on Friday without any exception for the minimum and maximum epochs of solar activity. However, the periodicity of 7 days is reliably revealed in other parameters characterizing galactic cosmic rays, SW, solar and geomagnetic activities, especially for the minimum epoch of solar activity. The periodicity of 3.5 days found in the series of CAE data more or less can be completely ascribed to the social effects, while the periodicity of 7 days can be ascribed to the social effect or/to the processes on the Sun, in the interplanetary space and in the Earth's magnetosphere and atmosphere.     

Study of the 27-day variations of the galactic cosmic ray intensity and anisotropy

We demonstrate that the general features of the radial and azimuthal components of the anisotropy of galactic cosmic rays can be studied by the harmonic analysis method using data from an individual neutron monitor with cut off rigidity <5 GV. In particular, we study the characteristics of the 27-day (solar rotation period) variations of the galactic cosmic ray intensity and anisotropy, solar wind velocity, interplanetary magnetic field strength and sunspot number. The amplitudes of the 27-day variations of the galactic cosmic ray anisotropy are greater, and the phases more clearly established, in A > 0 polarity periods than in A < 0 polarity periods at times of minimum solar activity. The phases of the 27-day variations of the galactic cosmic rays intensity and anisotropy are opposite with respect to the similar changes of the solar wind velocity in A > 0 polarity periods. No significant dependence of the amplitude of the 27-day variation of the galactic cosmic ray anisotropy on the tilt angle of the heliospheric neutral sheet is found. Daily epicyclegrams obtained by Chree’s method show that the 27-day variations of the galactic cosmic ray anisotropy during A > 0 polarity periods follow elliptical paths with the major axes oriented approximately along the interplanetary magnetic field. The paths are more irregular during A < 0 polarity periods.     

Evolution of Dst index,cosmic rays and global temperature during solar cycles 20–23

We have studied conditions in interplanetary space, which can have an influence on galactic cosmic ray (CR) and climate change. In this connection the solar wind and interplanetary magnetic field parameters and cosmic ray variations have been compared with geomagnetic activity represented by the equatorial Dst index from the beginning 1965 to the end of 2012. Dst index is commonly used as the solar wind–magnetosphere–ionosphere interaction characteristic. The important drivers in interplanetary medium which have effect on cosmic rays as CMEs (coronal mass ejections) and CIRs (corotating interaction regions) undergo very strong changes during their propagation to the Earth. Because of this CMEs, coronal holes and the solar spot numbers (SSN) do not adequately reflect peculiarities concerned with the solar wind arrival to 1 AU. Therefore, the geomagnetic indices have some inestimable advantage as continuous series other the irregular solar wind measurements. We have compared the yearly average variations of Dst index and the solar wind parameters with cosmic ray data from Moscow, Climax, and Haleakala neutron monitors during the solar cycles 20–23. The descending phases of these solar cycles (CSs) had the long-lasting solar wind high speed streams occurred frequently and were the primary contributors to the recurrent Dst variations. They also had effects on cosmic rays variations. We show that long-term Dst variations in these solar cycles were correlated with the cosmic ray count rate and can be used for study of CR variations. Global temperature variations in connection with evolution of Dst index and CR variations is discussed.     

The inner heliosphere from solar minimum to solar maximum: Short- and long-term variations in the energetic particle population

Between 1975 and 1983 HELIOS 1 scanned the interplanetary medium between 0.3 and 1 AU 31 times. The observed variations in the differential and integral flux of protons and helium nuclei in the energy range from 4 to >50 MeV/n are characterized by large temporal changes in the intensities, moderate changes in the energy spectrum and changes in the gradient below the detection level (60%). During solar minimum conditions recurrent disturbances are caused mainly by corotating interaction regions. The onset of solar activity near the end of 1977, characterized by a large number of solar events, is accompanied by a monotonous decrease of galactic cosmic radiation. The successive reduction of the cosmic ray intensity to the level of solar maximum is discussed in view of the role of large transient disturbances as compared to processes as diffusion, convection, adiabatic energy losses and drifts.     

New insights from cross-correlation studies between solar activity indices and cosmic-ray flux during Forbush decrease events

Observed galactic cosmic ray intensity can be subjected to a transient decrease. These so-called Forbush decreases are driven by coronal mass ejection induced shockwaves in the heliosphere. By combining in situ measurements by space borne instruments with ground-based cosmic ray observations, we investigate the relationship between solar energetic particle flux, various solar activity indices, and intensity measurements of cosmic rays during such an event. We present cross-correlation study done using proton flux data from the SOHO/ERNE instrument, as well as data collected during some of the strongest Forbush decreases over the last two completed solar cycles by the network of neutron monitor detectors and different solar observatories. We have demonstrated connection between the shape of solar energetic particles fluence spectra and selected coronal mass ejection and Forbush decrease parameters, indicating that power exponents used to model these fluence spectra could be valuable new parameters in similar analysis of mentioned phenomena. They appear to be better predictor variables of Forbush decrease magnitude in interplanetary magnetic field than coronal mass ejection velocities.     

The dependence of solar energetic particle fluxes in the Earth-Mars-Earth route on solar activity period.

This report presents the results of analyzing the relative importance of particle fluxes of different origin in the Earth-Mars-Earth route during different solar activity periods. The analysis has been made in terms of the galactic cosmic ray and solar energetic particle flux models developed at Moscow State University. The results demonstrate the extreme importance of the high-energy solar particle fluxes in interplanetary space even during the years of "quiet" Sun.     

Estimation of long-term cosmic ray intensity variation in near future and prediction of their contribution in expected global climate change

On the basis of results obtained in our paper [Dorman, L.I. Long-term cosmic ray intensity variation and part of global climate change, controlled by solar activity through cosmic rays, Paper D2.1/C2.2/E3.1-0097-04. Adv. Space Res., 2004 (accepted)], we determine: the dimension of the Heliosphere (modulation region), radial diffusion coefficient and other parameters of convection–diffusion; drift mechanisms of long-term variations of cosmic ray (CR) dependence on particle energy; level of solar activity (SA); and generally, the solar magnetic field. We obtain this important information on the basis of CR and SA data in the past, taking into account the theory of convection–diffusion and global drift modulation of galactic CR in the Heliosphere. By using these results and other regularly published predictions of expected SA variation in the near future, as well as predictions of the next SA cycle, we may make predictions of long-term cosmic ray intensity variation expected in the near future (up to 10–12 years). In [Dorman, L.I. Long-term cosmic ray intensity variation and part of global climate change, controlled by solar activity through cosmic rays, Paper D2.1/C2.2/E3.1-0097-04. Adv. Space Res., 2004 (accepted)], properties of connections between long-term variation in CR intensity and some part of a global climate change were estimated, controlled by solar activity through CR. We show that in this way we may make predictions of some part of a global climate change expected in the near future (up to 10–12 years and maybe more, depending upon the period during which definite predictions of SA can be made), controlled by solar activity through CR. In this case, estimations of expected long-term changes in the planetary distribution of cutoff rigidities, which also influence CR intensity, as well as CR-influenced effects on global climate variation, become important.     

Periodic variations of ionization rate in the low and middle atmosphere

The cosmic ray ionization source functions which were obtained using a simplified extensive air shower model are used to calculate the eleven year cycle, seasonal and diurnal variations of ionization rate in the low and middle atmosphere. The ionization source function, as a function of the penetrating depth and the energy of cosmic ray particles, is the ionization rate per unit depth for a unit flux of incoming cosmic ray particles with certain energy.The calculation of the eleven year cycle variation of ionization rate in the low and middle atmosphere due to the modulation of galactic cosmic ray intensity by solar activity shows that the amplitude is larger at a higher magnetic latitude and is generally larger at higher altitudes. The relative amplitude of fluctuation of the ionization peak value (at altitudes near 15 km) is up to 45% in the magnetic polar region. The ionization rate, due to the seasonal variation of the atmospheric density, varies from several per cent below the ionization peak to several tens per cent above the peak. This seasonal variation of ionization rate reaches 35% at 70 km. The diurnal variation of atmospheric densities caused by atmospheric tidal oscillation can produce a diurnal variation of the ionization rate to an amplitude of several per cent at altitudes above 40 km. The diurnal oscillation is less than 1% below 35 km.     

Rigidity spectrum of the 27-day variation of the galactic cosmic ray intensity in different epochs of solar activity

We study the temporal evolution of the power rigidity spectrum of the first (27 days) and the second (14 days) harmonics of the 27-day variation of the galactic cosmic ray intensity measured by neutron monitors in the period of 1965–2002. The rigidity spectrum of these variations can be approximated by a power law. We show the rigidity spectra of the first and the second harmonics of the 27-day variation of the galactic cosmic ray intensity have similar time profiles. These spectra are hard (γ ≈ 0.5 ± 0.1) and soft (γ ≈ 1.1 ± 0.2) during solar maximum and minimum activity, respectively. We ascribe this to the alternation of the sizes of the modulation regions responsible for the 27-day variation of the galactic cosmic ray intensity in different epochs of solar activity. Especially, the average radial sizes of the modulation regions of the 27-day variation and the heliolatitudinal extension of the heliolongitudinal asymmetry are smaller during solar minimum than during solar maximum. We show also, that the temporal changes of the power rigidity spectra of the first and the second harmonics of the 27-day variation of the galactic cosmic ray intensity are in a negative correlation with the changes of the rigidity spectrum of the corresponding 11-year variation.     

Radial distribution of galactic cosmic rays at solar maximum

In this paper we analyze the spatial distribution of galactic cosmic rays during periods of maximum solar activity of the cycles 21, 22 and 23. We have used a two dimensional model to solve the cosmic ray transport equation. This model includes all relevant physical processes: diffusion, convection, drift and shock effects on cosmic ray propagation inside the heliosphere. We focused on the study of the radial distribution of galactic cosmic rays, and compare our results with the spacecraft observations for two energies (175 MeV H and 265 MeV/n He). Although the radial intensities of galactic cosmic rays can be explained qualitatively with all three local interstellar spectra (LISs) used in this work, we applied a reduced chi-squared analysis to investigate the best LIS that could fit the data.     

Mid 19th century minimum of galactic cosmic ray flux inferred from Ti in Allegan meteorite

Measurements of ⁴⁴Ti activity in meteorites show that the galactic cosmic ray (GCR) intensity has been declining in the interplanetary space during the past three centuries and has a component of cyclic variation, with periodicity of about 87 years [Taricco, C., Bhandari, N., Cane, D., et al. Galactic cosmic ray flux decline and periodicities in the interplanetary space during the last 3 centuries revealed by ⁴⁴Ti in meteorites. J. Geophys. Res. 111, A08102, 2006.]. In order to verify these results, we have measured ⁴⁴Ti activity in Allegan meteorite which fell in 1899 and in some other meteorites with better precision. The measurements confirm low cosmic ray flux and consequently high solar activity near the middle of 19th century.     

The structure of the solar cycle maximum phase in the galactic cosmic ray 11-year variation

Two phenomena connected with the maximum phase of the 11-year solar cycle in the galactic cosmic ray intensity – the change in the energy dependence of the intensity variations and the double-peak structure in the intensity modulation time profile – are considered for the last five solar cycles (Nos. 19–23). The distinct 22-year cycle in the magnitude of the so called energy hysteresis is observed.The periods of the solar cycle maximum phase in the galactic cosmic ray intensity, characterized by the specific energy dependence of the intensity, are estimated. It is found that the double-peak structures belonging to the solar cycle maximum phase and those around it are very similar both in the amplitude and in its energy dependence.     

Stratospheric polar vortex as a possible reason for temporal variations of solar activity and galactic cosmic ray effects on the lower atmosphere circulation

Possible reasons for the temporal instability of long-term effects of solar activity (SA) and galactic cosmic ray (GCR) variations on the lower atmosphere circulation were studied. It was shown that the detected earlier ∼60-year oscillations of the amplitude and sign of SA/GCR effects on the troposphere pressure at high and middle latitudes (Veretenenko and Ogurtsov, Adv.Space Res., 2012) are closely related to the state of a cyclonic vortex forming in the polar stratosphere. The intensity of the vortex was found to reveal a roughly 60-year periodicity affecting the evolution of the large-scale atmospheric circulation and the character of SA/GCR effects. An intensification of both Arctic anticyclones and mid-latitudinal cyclones associated with an increase of GCR fluxes at minima of the 11-year solar cycles is observed in the epochs of a strong polar vortex. In the epochs of a weak polar vortex SA/GCR effects on the development of baric systems at middle and high latitudes were found to change the sign. The results obtained provide evidence that the mechanism of solar activity and cosmic ray influences on the lower atmosphere circulation involves changes in the evolution of the stratospheric polar vortex.     

Cosmic ray flux at the Earth in a variable heliosphere

In recent years the variability of the cosmic ray flux has become one of the main issues not only for the interpretation of the abundances of cosmogenic isotopes in cosmochronic archives like, e.g., ice cores, but also for its potential impact on the terrestrial climate. It has been re-emphasized that the cosmic ray flux is not only varying due to the solar activity-induced changes of the solar wind but also in response to the changing state of the interstellar medium surrounding the heliosphere. We demonstrate the significance of these external boundary condition changes along the galactic orbit of the Sun for the flux as well as spectra of cosmic rays. Such interstellar–terrestrial relations are a major topic of the International Heliophysical Year 2007.     

Radiation environment due to galactic and solar cosmic rays during manned mission to Mars in the periods between maximum and minimum solar activity cycles.

A possibility of a manned mission to Mars without exceeding the current radiation standards is very doubtful during the periods of minimum solar activity since the dose equivalent due to galactic cosmic rays exceeds currently recommended standards even inside a radiation shelter with an equivalent of 30 g cm-2 aluminum. The radiation situation at the time of maximum solar activity is determined by the occurrence of major solar proton events which are exceedingly difficult to forecast. This paper discusses the radiation environment during a manned mission to Mars in the years between minimum and maximum solar activity when the galactic cosmic ray intensity is considerably reduced, but the solar flare activity has not yet maximized.     

Effects of solar modulation on the cosmic ray positron fraction

We implemented a 2D Monte Carlo model to simulate the solar modulation of galactic cosmic rays. The model is based on the Parker’s transport equation which contains diffusion, convection, particle drift and energy loss. Following the evolution in time of the solar activity, we are able to modulate a local interstellar spectrum (LIS), that we assumed isotropic beyond the termination shock, down to the Earth position inside the heliosphere. In this work we focused our attention to the cosmic ray positron fraction at energy below ∼10 GeV, showing how the particle drift processes could explain different results for AMS-01 and PAMELA. We compare our modulated spectra with observations at Earth, and then make a prediction of the cosmic ray positron fraction for the AMS-02 experiment.