Modulation of galactic cosmic rays in solar cycles 22–24: Analysis and physical interpretation

This work represents a physical interpretation of cosmic ray modulation in the 22nd–24th solar cycles, including an interpretation of an unusual behavior of their intensity in the last minimum of the solar activity (2008–2010). In terms of the Parker modulation model, which deals with regularly measured heliospheric characteristics, it is shown that the determining factor of the increased intensity of the galactic cosmic rays in the minimum of the 24th solar cycle is an anomalous reduction of the heliospheric magnetic field strength during this time interval under the additional influence of the solar wind velocity and the tilt angle of the heliospheric current sheet. We have used in the calculations the dependence of the diffusion tensor on the rigidity in the form K _ij ∝ R ^2?μ with μ = 1.2 in the sector zones of the heliospheric magnetic field and with μ = 0.8 outside the sector zones, which leads to an additional amplification of the diffusion mechanism of cosmic ray modulation. The proposed approach allows us to describe quite satisfactorily the integral intensity of protons with an energy above 0.1 GeV and the energy spectra in the minima of the 22nd–24th solar cycles at the same value of the free parameter. The determining factor of the anomalously high level of the galactic cosmic ray intensity in the minimum of the 24th solar cycle is the significant reduction of the heliospheric magnetic field strength during this time interval. The forecast of the intensity level in the minimum of the 25th solar cycle is provided.     

Galactic Cosmic Ray Intensity in the Upcoming Minimum of the Solar Activity Cycle

During the prolonged and deep minimum of solar activity between cycles 23 and 24, an unusual behavior of the heliospheric characteristics and increased intensity of galactic cosmic rays (GCRs) near the Earth’s orbit were observed. The maximum of the current solar cycle 24 is lower than the previous one, and the decline in solar and, therefore, heliospheric activity is expected to continue in the next cycle. In these conditions, it is important for an understanding of the process of GCR modulation in the heliosphere, as well as for applied purposes (evaluation of the radiation safety of planned space flights, etc.), to estimate quantitatively the possible GCR characteristics near the Earth in the upcoming solar minimum (~2019–2020). Our estimation is based on the prediction of the heliospheric characteristics that are important for cosmic ray modulation, as well as on numeric calculations of GCR intensity. Additionally, we consider the distribution of the intensity and other GCR characteristics in the heliosphere and discuss the intercycle variations in the GCR characteristics that are integral for the whole heliosphere (total energy, mean energy, and charge).     

Specific features of the rigidity spectrum of Forbush effects

We analyze variations in the rigidity spectrum of primary cosmic rays during Forbush effects recorded in cycles 20–24 of solar activity on the basis of data from the global network of neutron monitor stations processed by global survey. We investigate variations in the rigidity spectrum index of Forbush effects as a function of the solar activity level, phases of the effect, polarity the total magnetic field of the Sun, type and parameters of the source of cosmic ray modulation, etc. Comprehensive analysis of our results revealed regularities in the dynamics of the energy spectrum of galactic cosmic rays that reflect the dynamic processes occurring in the interplanetary space.     

Long-term and transient time variation of cosmic ray fluxes detected in Argentina by CARPET cosmic ray detector

We present results obtained at El Leoncito (CASLEO, San Juan, Argentina) with the CARPET charged particles detector installed in April 2006. The observed modulation of the cosmic ray flux is discussed as a function of its time variability and it is related to longer solar activity variations and to shorter variations during solar and geomagnetic transient activity. Short period (few minutes, few hours) cosmic ray modulation events are observed during rain time (precipitation) and significant variations of the atmospheric electric field. Complementary observations of the atmospheric electric field indicate that its time variations play an important role in the detected cosmic ray event.     

Effect of active longitudes in cosmic ray flux modulation

Galactic cosmic rays, registered by ground-based neutron monitors, are strongly affected by the heliosphere, i.e., being subjected to solar modulation. Cosmic ray variations are closely related to different solar activity indices and IMF parameters. The longitudinal inhomogeneity of the general solar magnetic field as a star and the manifestation of this inhomogeneity in the magnetic field are considered in the work. It has been established that the longitudinal inhomogeneity of this field, with the dipole distribution of polarities along heliolongitude, mainly contributes to 27-day modulation of galactic cosmic rays.     

Extrema of long-term modulation of the cosmic ray intensity in the last five solar cycles

Modulation of galactic cosmic rays in cycles 19–23 of solar activity has been determined based on observations of their long-term variations on the ground and in the near-Earth space. The extreme values of long-term variations in cosmic rays, obtained from the data of continuous cosmic radiation monitoring on the ground and in the near-Earth space in the last five solar cycles, have been analyzed. The results are compared with the extrema in the characteristics of solar magnetic fields and the sunspot numbers in these cycles. The similarities and differences in cosmic ray modulation between different cycles are discussed.     

The rigidity spectrum of the harmonics of the 27-day variation of the galactic cosmic ray intensity in different epochs of solar activity: 1965–2002

We study temporal changes of the rigidity (R) spectrum of the harmonics of the 27-day variation of the galactic cosmic ray (GCR) intensity using neutron monitors (NM) data for the period 1965–2002. We show that the rigidity spectrum of the third harmonic (9 days) of the 27-day variation of the GCR intensity changes in a similar way as the spectra of the first and second harmonics, being hard in the maximum epochs and soft in the minimum epochs of solar activity. We ascribe this finding to the alternation of the sizes of the modulation regions of the 27-day variation of the GCR intensity in different epochs of solar activity. The average size of the vicinity of the corotating interaction regions, causing the 27-day variation of the GCR intensity, is less in the minimum epochs than in the maximum epochs of solar activity. A vicinity of the corotating interaction regions of larger size involves in modulation higher rigidity particles of GCR than the vicinity of smaller size; thus, this statement can be considered as one of the reasons leading to the hardening of the rigidity spectrum of the harmonics of the 27-day variation of the GCR intensity in maximum epochs compared with minimum epochs of solar activity.We also show that the temporal changes of the power rigidity spectrum of the third harmonic of the 27-day variation of the GCR intensity are negatively correlated with the rigidity spectrum of the 11-year variation of the galactic cosmic ray intensity.We found a recurrence in the temporal changes of the amplitudes of the first harmonic of the 27-day variation of the GCR intensity and in some parameters of solar activity and solar wind.     

Cosmic ray modulation during the solar activity growth phase of cycle 24

Recent years allowed us to study long-term variations in the cosmic ray (CR) intensity at an unusually deep solar activity (SA) minimum between cycles 23 and 24 and during the SA growth phase in cycle 24, which was the cycle when SA was the lowest for the epoch of regular ground-based CR observations since 1951. The intensity maximum, the value of which depends on the particle energy, was observed in CR variations during the period of an unusually prolonged SA minimum: the CR density during the aformentioned period (2009) is higher than this density at previous CR maxima in cycles 19–23 for low-energy particles (observed on spacecraft and in the stratosphere) and medium-energy particles (observed with neutron monitors). After 2009 CR modulation at the SA growth phase was much weaker over three years (2010–2012) than during the corresponding SA growth periods in the previous cycles. The possible causes of this anomaly in CR variations, which are related to the CR residual modulation value at a minimum between cycles 23 and 24 and to variations in SA characteristics during this period, were examined. The contribution of different solar magnetic field characteristics and indices, taking into account sporadic solar activity, has been estimated.     

A 22 yr hurricane cycle and its relation with geomagnetic activity

Applying spectral analysis to the Atlantic and Pacific hurricane time series, we found periodicities that coincide with the main sunspot and magnetic solar cycles. To assess the possibility that these periodicities could be associated with solar activity, we obtain correlations between hurricane occurrence and several solar activity-related phenomena, such as the total solar irradiance, the cosmic ray flux and the Dst index of geomagnetic activity. Our results indicate that the highest significant correlations are found between the Atlantic and Pacific hurricanes and the Dst index. Most importantly, both oceans present the highest hurricane–Dst correlations during the ascending part of odd solar cycles and the descending phase of even solar cycles. This shows not only the existence of a 22 yr cycle but also the nature of such periodicity. Furthermore, we found that the Atlantic hurricanes behave differently from the Pacific hurricanes in relation to the solar activity-related disturbances considered.     

Galactic cosmic ray fluctuation parameter as an indicator of the degree of magnetic field inhomogeneity

The parameter of cosmic ray fluctuations, which indicates the degree of IMF inhomogeneity, was introduced in order to quantitatively describe the dynamics of the galactic cosmic ray (GCR) intensity fluctuations during the geoeffective phases of the 11-year cycle. The 5-min data of the high-latitude neutron monitor at Oulu station (Finland) during cycles 20–23 was used in the calculations. The nonrandom non-Gaussian character of the GCR fluctuation parameter is caused by the nonstationary semiannual variation reflecting the transient nonstationary oscillatory process of sign reversal of the general solar magnetic field. This transient oscillatory process is responsible for the maximal geoeffectiveness and duration of the phase of polarity reversal, which manifests itself in a sharp and deep GCR intensity minimum during the final stage of the field sign reversal. The invariant of the 11-year “amplitude-duration” cycle was confirmed on a new basis: the LF drift of the “low” cycle period was detected, which was observed in an increase in the duration of cycle 23 we anticipated.     

Shielded by the wind: the influence of the interstellar medium on the environment of Earth

We report on the recent studies on the long-term influence of cosmic rays on the Earth's environment. While on short time-scales solar activity is the driver for atmospheric changes suspected to be due to cosmic rays, for long time-scales the heliosphere, i.e. the circumsolar region occupied by the expanding part of the Sun's atmosphere, has to be considered. The heliosphere is identified as an important shield against interstellar influences and hazards. It has been demonstrated by quantitative modelling that a change of the interstellar medium surrounding the heliosphere as a result of the Sun's quasi-Keplerian motion around the galactic center triggers significant changes of planetary environments caused by enhanced fluxes of neutral atoms as well as by the increased cosmic ray fluxes. We give a compilation of recent space science results of interest to the atmospheric science community.     

22-year cycle in the frequency of aurora occurrence in XIX century: Latitudinal effects

The 22-year variation in the frequency of aurora occurrence is found through an analysis of data of the Russian network of meteorological stations from 1837–1909. This variation is obtained in a form of asymmetry between even and odd solar cycles. We found that the nature of the 22-year variation depends on the latitude of the observation station. The annual number N of midlatitude auroras (geomagnetic latitudes Φ < 56°) for about three years at the end of the descending part of solar cycles is larger for the even cycles than for the odd. For high-latitude auroras (Φ ≥ 56°), the pattern is opposite: at the descending part of the solar cycle, N is larger in the odd cycles than in the even. For the high-latitude sector, asymmetry of the polar sun cycles (the period between two magnetic field reversals) is clearly observed: an increased N is observed during the whole odd polar cycle (which starts approximately at the maximum of the odd Schwabe cycle) as compared to the even cycle. Extrapolation of the modern picture of alternation of the sign of the global solar magnetic field back in time leads to the conclusion that the most geoeffective polar cycles in cycles 8–14 were those in which the polar magnetic field in the northen hemisphere was negative.     

The search for cosmic ray effects on clouds

The cause of the correlation of low cloud cover with the sunspot cycle, and the associated cosmic ray intensity, is still the subject of controversy. Insofar as ‘clouds’ come in different types with, doubtless, different sensitivities to the cloud condensation nuclei (charged or otherwise) it is useful to search for differences in the correlation between cloud types. Here, we examine the major cloud components: stratiform and cumuliform, the latter with its much higher upthrust velocities being expected to be less efficient as cosmic ray induced cloud generation. No difference is found between the two types of cloud, in the sense that there is no dependence on the fraction of cloud of stratiform type for the various parameters studied. This result is true over the Globe as a whole and as a function of cosmic ray cutoff rigidity. Once more there is no support for the cosmic ray hypothesis for cloud modulation. There is no obvious implication for the alternative hypotheses of solar irradiance modulation, but this is probably still the more likely.     

Solar modulation in surface atmospheric electricity

The solar wind modulates the flux of galactic cosmic rays impinging on Earth inversely with solar activity. Cosmic ray ionisation is the major source of air's electrical conductivity over the oceans and well above the continents. Differential solar modulation of the cosmic ray energy spectrum modifies the cosmic ray ionisation at different latitudes, varying the total atmospheric columnar conductance. This redistributes current flow in the global atmospheric electrical circuit, including the local vertical current density and the related surface potential gradient. Surface vertical current density and potential gradient measurements made independently at Lerwick Observatory, Shetland, from 1978 to 1985 are compared with modelled changes in cosmic ray ionisation arising from solar activity changes. Both the lower troposphere atmospheric electricity quantities are significantly increased at cosmic ray maximum (solar minimum), with a proportional change greater than that of the cosmic ray change.     

Manifestations of cyclic variations in the solar magnetic field in long-term modulation of cosmic rays

The possibilities of improving the semiempirical model of cosmic ray (CR) modulation, proposed by us previously, are discussed. The following characteristics have been considered as model parameters in order to describe long-period CR variations using a unified model and to more completely reflect solar cycles in CR modulation as a complex interaction between two systems of fields (large-scale and local): the value and sign of the polar solar field, the average strength of the solar magnetic field (the B _ss integral index), partial indices (zone-even (ZE) and zone-odd (ZO) and sector-even (SE) and sector-odd (SO) indices), the tilt of the heliospheric current sheet, and the special index (F _x ) taking into account X ray flares. The role of each index in CR modulation has been revealed. When we described the long-term CR variations using many parameters and taking into account the integral index or one of four partial indices, the best results of modulation modeling during 1976–1999 were obtained for the B _ss total energetic index and SO index. A difference between the model calculations and observations increases beginning from the middle of 2000; the problem features of the CR behavior and the specific features of modeling this behavior in cycle 23 of solar activity (SA) are discussed. It is assumed that a decrease in the CR density at the last SA minimums (from cycle to cycle) can be related to a decrease in the ZO index and to a recently detected similar decrease in the vertical component of the solar dipole magnetic moment.     

Long-term modulation of galactic cosmic rays at solar activity minimums

Based on observations of long-term variations in galactic cosmic rays (CRs) on Earth and in the near-Earth space, we have determined, using our own semiempirical model, modulation of galactic CRs during solar cycles 19–23. The modulation model relates CR variations to the characteristics of the solar magnetic field obtained for the surface of the solar wind source at distances of 2.50 and 3.25 solar radii. The main focus is CR behavior at the minimums of cycles 19–23 and specific features of CR modulation at a prolonged (as compared to previous cycles) minimum of cycle 23, which is still ongoing. CR modulation at minimums related to a change in the solar field dipole component during this period of the cycle has been considered. It is indicated that the long-term variations in CRs are better described if the last two years (2007 and 2008) of cycle 23 with anomalously low solar activity (SA) are included in the model. The role and value of the contribution of the cyclic variations in each index used in the proposed CR modulation model to the observed CR modulation have been estimated.     

Imprint of Climate Variability on Mesozoic Fossil Tree Rings: Evidences of Solar Activity Signals on Environmental Records Around 200 Million Years Ago?

Evidence of the solar activity modulation of the Earth’s climate has been observed on several parameters, from decadal to millennial time scales. Several proxies have been used to reconstruct the paleoclimate as well as the solar activity. The paleoclimate reconstructions are based on direct and/or indirect effects of global and regional climate conditions. The solar activity reconstructions are based on the production of the ¹⁴C isotope due to the interaction of cosmic ray flux and the Earth’s atmosphere. Because trees respond to climate conditions and store ¹⁴C, they have been used as proxies for both for climate and solar activity reconstructions. The imprints of solar activity cycles dating back to 10,000 years ago have been observed on tree-ring samples using ¹⁴C data, and those dating back to 20 million years ago have been analyzed using fossil tree-growth rings. All this corresponds to the Cenozoic era. However, solar activity imprints on tree rings from earlier than that era have not been investigated yet. In this work, we showed that tree rings from the Mesozoic Era (of ~200 million years ago) recorded 11- and 22-year cycles, which may be related to solar activity cycles, and that were statistically significant at the 95 % confidence level. The fossil wood was collected in the southern region of Brazil. Our analysis of the fossils' tree-ring width series power spectra showed characteristics similar to the modern araucaria tree, with a noticeable decadal periodicity. Assuming that the Earth’s climate responds to solar variability and that responses did not vary significantly over the last ~200 million years, we conclude that the solar–climate connection was likely present during the Mesozoic era.     

The solar magnetic flux mid-term periodicities and the solar dynamo

We investigate here the fluctuations in the total, open and closed solar magnetic flux (SMF) for the period 1971–1999 by means of the maximum entropy method in the frequency range 5×10⁻⁹–10⁻⁷ Hz (6 yr to 120 days). We use monthly data for the total, open and closed magnetic solar fluxes. Periodicities found in the series are similar showing that there is some relationship between the fluxes. The most important finding of this work is the existence of fluctuations at around 1.3 and 1.7 yr in the SMF with alternating importance during consecutive even and odd solar cycles. These fluctuations are directly related with variations present in cosmic rays, solar wind parameters and geomagnetic activity indexes. A quasi-triennial periodicity previously found in sunspots and other solar phenomena is also of importance. The SMF is generated by the action of the solar dynamo; therefore, it is through the magnetic flux that the solar dynamo influences several heliospheric phenomena.     

Long-term variations in the muon flux angular distribution

The intensity of the atmospheric muon flux depends on many factors: the energy spectrum of primary cosmic rays and the state of the Earth’s heliosphere, magnetosphere, and atmosphere. The wide-aperture URAGAN muon hodoscope (Moscow, Russia, 55.7° N, 37.7° E, 173 m a.s.l.) makes it possible to investigate not only variations in the muon flux intensity but also temporal changes in the parameters of its angular distribution. These changes are analyzed using the vector of local anisotropy and its projections, which have different sensitivities to the parameters of modulation of both primary cosmic rays in the heliosphere and the Earth’s magnetosphere and secondary cosmic rays as they pass through the Earth’s atmosphere. The vector of local anisotropy is the sum of unit vectors (directions of the reconstructed muon tracks) normalized to the number of tracks. The results of an analysis of long-term variations in mean hourly projections of the vector of local anisotropy obtained from the 2007–2011 URAGAN hodoscope data are presented.