Variation Of Galactic Cosmic Ray Intensity Research Articles

A Study of Variations of Galactic Cosmic-Ray Intensity Based on a Hybrid Data-processing Method

Abstract The low-energy cosmic-ray (CR) fluxes measured by space-borne instruments are generally considered to consist of gradually changing galactic CRs (GCRs) and short-lived solar energetic particles (SEPs). The SEP events cause sharp and ephemeral increases in the time profile of CR observations, with a higher occurrence rate in solar maximum. It is necessary to eliminate such spikes and obtain the pure GCR component while studying the modulation of GCRs in both short and long timescales. A hybrid data-processing method based on spike detection and time series analysis techniques is developed to remove the spikes and decompose the GCR data observed by the Interplanetary Monitoring Platform 8 into the long-term variation trend and the 27 day variation components. With the hybrid data-processing method, the 11 yr and 27 day variations in the intensity of low-energy GCRs can be studied systematically. Using the fitted trend component, the time lag in the solar modulation of low-energy GCRs is studied, and the results show that the time lag is both epoch and energy dependent. The obtained 27 day variation component is anticorrelated with the changes in solar wind velocity even during solar maximum. Implementing the running Fourier series fit procedure, the 27 day variation amplitude of the proton flux is computed. It is found that the yearly averaged values clearly show 11 and 22 yr variation cycles. In addition, the energy spectrum of the 27 day variation amplitude is softer in the A < 0 solar minimum than that in the A > 0 solar minimum.

Hale cycle and long-term trend in variation of galactic cosmic rays related to solar rotation

Context. Galactic cosmic ray (GCR) intensities around solar minimum times are modulated by magnetic drifts that depend on the overall solar polarity. GCR intensities reach a higher but more narrow peak during negative minima than during positive minima. However, despite these higher intensities, the variation of GCRs over timescales of solar rotation is smaller during negative minima than during positive minima. Aims. We study the variation of GCR intensity over the 27-day synodic solar rotation and over the 14-day half-rotation, in particular the long-term trend and cyclic pattern of this variation, and propose a unifying explanation for the observations. Methods. We used two high-latitude neutron monitors, Oulu and Apatity, which are most sensitive to the low-energy part of the GCR spectrum and thereby more strongly affected by the changes in the conditions of the local heliosphere. We calculated the yearly mean amplitudes of the GCR intensity variation during the full solar rotation ( A 27 ) and half-rotation ( A 14 ) in 1964–2016. Results. We verify that the A 27 and A 14 amplitudes exhibit a clear 22-yr Hale cycle during solar minima at both stations, with larger amplitudes in positive minima. We find that the mean amplitude of the Hale cycle is about 30–45% of the mean amplitude for A 14 , while is only about 15–30% for A 27 . We also find that all amplitudes depict a declining long-term trend, which we suggest is due to the weakening of solar polar magnetic fields during the last four solar cycles and the ensuing latitudinal widening of the heliospheric current sheet (HCS) region. An exceptionally wide HCS region during the last solar minimum, when A 14 reached its all-time minimum, is demonstrated by Ulysses probe observations. Conclusions. Our results emphasize the effect of polarity-dependent drift and the properties of the HCS in modulating the variation of GCR intensity during solar rotation in solar minimum times. The second rotation harmonic yields a larger Hale amplitude than the first because it is more probable for the Earth to be outside the HCS only once during the rotation than twice or more, which more strongly reduces A 14 during negative polarity times than A 27 . With the HCS region widening from minimum to minimum, the decrease in A 14 is relatively faster than in A 27 .

Cosmophysical situation in the epoch of the Cambrian explosion

Based on available published paleodata, cosmophysical environment of the Earth during the Cambrian evolutionary explosion is considered. Some astrophysical data demonstrate that, about 500 Ma, the Sun entered into the Perseus arm with the enhanced density of star population and spent there several dozen million years. According to some meteorite data, the average level of the galactic cosmic ray (GCR) intensity during that period was considerably higher than previously, when the Sun passed the space between the galactic arms. The GCR flux varied from 25 to 135%, as the Solar System consequently crossed other galactic arms. Some correlation of the GCR intensity variations with periods of global warming and cooling has been found. However, there was no unambiguous relation between climatic data and the GCR intensity. Unfortunately, an accuracy of estimation of the GCR intensity through meteorite data varies to within 0.30–1.5, which does not allow making definite conclusions. For more reliable conclusions, additional astrophysical data obtaining is needed, and new approaches to modeling of the GCR propagation, which take into account their characteristic lifetime of ∼10–100 Ma, should be applied. A possibility of impact of Supernova outbursts and superincreases of solar cosmic ray (SCR) flux on climatic changes are discussed. A possible bioeffective role of the geomagnetic field reversals, oscillations of the position of the Sun in the Galaxy, and other unidentified yet factors is also considered.

Rigidity Dependence of the Long-Term Variations of Galactic Cosmic-Ray Intensity in Relation to the Interplanetary Magnetic-Field Turbulence: 1968 – 2002

We studied the relationship between the power-law exponent γ on the rigidity R of the spectrum of galactic cosmic-ray (GCR) intensity variation (δD(R)/D(R)∝R −γ ) and the exponents ν y and ν z of the power spectral density (PSD) of the B y and B z components of the interplanetary magnetic field (IMF) turbulence (PSD∼f −ν , where f is the frequency). We used the data from neutron monitors and IMF for the period of 1968 – 2002. The exponents ν y and ν z were calculated in the frequency interval Δf=f 2−f 1=3×10−6 Hz of the resonant frequencies (f 1=1×10−6 Hz, f 2=4×10−6 Hz) that are responsible for the scattering of GCR particles with the rigidity range detected by neutron monitors. We found clear inverse correlations between γ and ν y or ν z when the time variations of the resonant frequencies were derived from in situ measurements of the solar wind velocity U sw and IMF strength B during 1968 – 2002. We argue that these inverse relations are a fundamental feature in the GCR modulation that is not restricted to the analyzed years of 1968 – 2002.

Peculiarities of cosmic ray modulation in the solar minimum 23/24

AbstractWe study changes of the galactic cosmic ray (GCR) intensity for the ending period of the solar cycle 23 and the beginning of the solar cycle 24 using neutron monitors experimental data. We show that an increase of the GCR intensity in 2009 is generally related with decrease of the solar wind velocity U, the strength B of the interplanetary magnetic field (IMF), and the drift in negative (A < 0) polarity epoch. We present that temporal changes of rigidity dependence of the GCR intensity variation, before reaching maximum level in 2009 and after it, do not noticeably differ from each other. The rigidity spectrum of the GCR intensity variations calculated based on neutron monitors data (for rigidities > 10 GV) is hard in the minimum and near‐minimum epoch. We do not recognize any nonordinary changes in the physical mechanism of modulation of the GCR intensity in the rigidity range of GCR particles to which neutron monitors respond. We compose 2‐D nonstationary model of transport equation to describe variations of the GCR intensity for 1996–2012 including the A > 0 (1996–2001) and the A < 0 (2002–2012) periods; diffusion coefficient of cosmic rays for rigidity 10–15 GV is increased by ~ 30% in 2009 (A < 0) comparing with 1996 (A > 0). We believe that the proposed model is relatively realistic, and obtained results are satisfactorily compatible with neutron monitors data.

Dependence of the 27-day variation of cosmic rays on the global magnetic field of the Sun

We show that the higher range of the heliolongitudinal asymmetry of the solar wind speed in the positive polarity period (A>0) than in the negative polarity period (A<0) is one of the important reasons of the larger amplitudes of the 27-day variation of the galactic cosmic ray (GCR) intensity in the period of 1995–1997 (A>0) than in 1985–1987 (A<0). Subsequently, different ranges of the heliolongitudinal asymmetry of the solar wind speed jointly with equally important corresponding drift effect are general causes of the polarity dependence of the amplitudes of the 27-day variation of the GCR intensity. At the same time, we show that the polarity dependence is feeble for the last unusual minimum epoch of solar activity 2007–2009 (A<0); the amplitude of the 27-day variation of the GCR intensity shows only a tendency of the polarity dependence. We present a three dimensional (3-D) model of the 27-day variation of GCR based on the Parker’s transport equation. In the 3-D model is implemented a longitudinal variation of the solar wind speed reproducing in situ measurements and corresponding divergence-free interplanetary magnetic field (IMF) derived from the Maxwell’s equations. We show that results of the proposed 3-D modeling of the 27-day variation of GCR intensity for different polarities of the solar magnetic cycle are in good agreement with the neutron monitors experimental data. To reach a compatibility of the theoretical modeling with observations for the last minimum epoch of solar activity 2007–2009 (A<0) a parallel diffusion coefficient was increased by ∼40%.

Modeling and experimental study of the 27-day variation of galactic cosmic-ray intensity for a solar wind velocity depending on heliolongitude

We develop a three-dimensional (3-D) model of the 27-day variation of galactic cosmic-ray (GCR) intensity with a spatial variation of the solar wind velocity. A consistent, divergence-free interplanetary magnetic field is derived by solving the corresponding Maxwell equations with a variable solar wind speed, which reproduces in situ observed experimental data for the time interval to be analyzed (24 August 2007–28 February 2008). We perform model calculations for the GCR intensity using the variable solar wind and the corresponding magnetic field. Results are compatible with experimental data; the correlation coefficient between our model predictions and observed 27-day GCR variation is 0.80 ± 0.05.

New index of long-term variations of galactic cosmic ray intensity

We find that the soft rigidity spectrum of the Galactic Cosmic Ray (GCR) intensity variations for the maximum epoch and the hard rigidity spectrum for the minimum epoch calculated based on the neutron monitors experimental data (1960–2002) are related with the various dependence of the diffusion coefficient on the GCR particle’s rigidity for different epoch of solar activity. This dependence is stronger in the maximum epoch than in the minimum epoch of solar activity, and is provided by the essential temporal rearrangements of the structure of the Interplanetary Magnetic Field (IMF) turbulence from the maxima to minima epoch of solar activity. We also show that the rigidity spectrum of GCR intensity variations is harder for the effective rigidities ∼(10–15) GV (by neutron monitors data), than for the effective rigidities ∼(25–30) GV (by neutron monitors and muon telescopes data). A general scenario of GCR modulation versus solar activity is settled on the essential temporal rearrangements of the structure of the IMF turbulence. Therefore, the temporal changes of the power law rigidity spectrum exponent can be considered as a vital (new) index to explain the 11-year variations of the GCR intensity. We assume that ∼(70–80)% of the changes of the amplitudes of the 11-year variations of GCR intensity is related with the changes of the IMF turbulence versus solar activity.

THEORETICAL AND EXPERIMENTAL STUDIES OF THE 11-YEAR AND 27-DAY VARIATIONS OF THE GALACTIC COSMIC RAYS INTENSITY AND ANISOTROPY

The changes of the structure in the energy range of the interplanetary magnetic field (IMF) turbulence versus solar activity can be considered as one of the important reasons of the long period (11-year) modulation of galactic cosmic ray (GCR) intensity; the amplitude of the 27-day variation of GCR anisotropy is greater in the qA &gt; 0 periods than in the qA &lt; 0 periods of the solar magnetic cycles in a good correlation with the similar changes of the 27-day variation of GCR intensity.

Long-Term changes of cosmic ray intensity: Spectral behaviour and 27-day variations

The amplitude of galactic cosmic ray intensity variations with characteristic times of days and months changes in phase with solar activity. However, during periods close to the maximum of solar activity the amplitudes of such variations undergo the temporal depressions (so called Gnevyshev gaps) which are considered for solar activity cycles 19–22 using the techniques of localised spectral analysis on the base of the wavelet transform. It is confirmed that yearly averaged power meanings of 27-day cosmic ray variations demonstrate Gnevyshev gaps in each solar cycle, but the fine time resolution reveals multiplicity of gaps during active phase of a solar cycle. The gaps can often be observed in variations with characteristic times up to several months.

Sidereal daily variation of ∼10 TeV cosmic-ray intensity observed by the Tibet air shower array

We analyze the sidereal daily variation of galactic cosmic-ray intensity observed by the Tibet air-shower (AS) array in one year from January to December 1992. The Tibet AS array can detect the incident direction of each AS and allows us to examine the daily variations in different declination bands on the celestial sphere. The average hourly AS count is 26,372 and the total observation period is 5,651 hours. We analyze the hourly count rates of AS classified into nine cells according to the incident direction of each AS. The hourly count rates are corrected for the atmospheric pressure effect and also for the spurious variation due to the change in the AS size-distribution. We find that the resultant daily variations in sidereal, solar and antisidereal time are influenced by a significant variation (∼0.5%) which is common for the viewing directions. It is also found that such common variation is well eliminated in the “difference” variations between the eastern and western cells. From the harmonic analysis of the “difference” variations, it is concluded that there is no significant variation observed in one year of data.

Cosmic ray variations and magnetic field fluctuations in the outer heliosphere

We have formally confirmed that galactic cosmic ray intensity variations measured by Voyager 2 during recovery from solar maximum are caused by traveling compressions and rarefactions in the mean interplanetary magnetic field. We used Voyager's magnetic field data as input to a time‐independent, spherically symmetric, cosmic ray transport equation in the force field approximation. The solutions closely followed the count rate of cosmic rays greater than 75 MeV/nucleon over 4 years, during the recovery phase of the 11‐year solar‐driven cosmic ray cycle. This strongly supports prior theoretical assertions that turbulent interaction regions traveling with the solar wind are the major cause of the solar cycle variation of galactic cosmic rays in the ecliptic region.

Relativistic cosmic rays and corotating interaction regions

Analyses of relativistic galactic cosmic ray intensity variations have been conducted to determine the nature of the modulations that are related to the presence of corotating interaction regions (CIR) in interplanetary space. The corotating interaction regions have been identified from the plasma and field observations recorded by Pioneer 10 and 11 spacecrafts during the period 1972–1974. This investigation has established that the nucleonic intensity recorded at the polar stations Thule and McMurdo decreases as the streams associated with the CIR overtake the earth. However, this modulation occurs only for those CIR‐associated streams in which a neutral sheet is imbedded. In contrast, geomagnetic storms are related to CIR streams with or without neutral sheets. Taken together, these results suggest that the ostensible effect of CIR's on the intensity of high‐energy particles is primarily a consequence of drifts related to neutral sheets, although diffusion effects cannot be ruled out at this stage.

Two classes of cosmic ray decrease

In a study of galactic cosmic ray intensity variations it had been found that there are two distinct classes of transient intensity decrease, which have been called type I and type II. Type I events are flare-associated Forbush decreases. Type II events appear to be associated with 27-day recurrences. It is suggested, on the basis of several characteristics of the decreases, that type II events are simply the subsequent evolution of type I events or quasi-stationary 'corotating' structures loosely associated with active regions.