Changes In Cosmic Ray Intensity Research Articles

Spectral Features of Forbush Decreases during Geomagnetic Storms

Geomagnetic storms and Forbush decreases (FD) on Earth are primarily caused by interplanetary coronal mass ejections (ICMEs) and stream/corotating interaction regions (SIRs/CIRs) originating in the Sun, which are propagated as a low-energy plasma disturbance through the interplanetary magnetic field (IMF). In this paper, we study the variations of the solar-wind parameters (solar wind velocity, plasma density, and IMF-Bz component) and the Earth's disturbance storm-time index (Dst) in relation to cosmic ray flux measurements from 8 neutron monitor stations distributed over Canada, Russia, Finland, and Greenland, during 3 intense geomagnetic storms occurred during the 24th solar cycle (March 16–18, 2015, June 21–23, 2015, and September 7–9, 2017). The wavelet analysis of the cosmic ray intensity reveals the clear evolution of the classical two-step FD with a peak period of approximately 2.1 h. The correlation-delay analysis shows a very strong correlation (∼0.9) between the relative count rate changes in cosmic ray intensity and the indices of solar wind speed and Dst. We obtain similar time-delay responses to the solar wind speed for all the cases (∼4 h), but large discrepancies are seen for the Dst index between the storms. We, therefore, recommend not using the Dst index for predicting Forbush decreases. Finally, we employ the resulting delay times to parameterise the Forbush decreases in terms of the solar wind, and we obtain a predictive model with R2 parameter of an approximate value of 0.8. Moreover, we observe a possible dependence on solar wind proton density which modulates the magnitude of Forbush decreases under similar solar wind speed conditions. Our results verify the suitability of using solar wind parameters to predict Forbush decreases in the cosmic ray flux.

Dynamic complexity based analysis on the relationship between solar activity and cosmic ray intensity

AbstractThe Earth’s atmosphere is incessantly bombarded by energetic charged particles called cosmic rays (CR) which are having either solar or non-solar origin. Analysis based on information theoretic estimators can be effectively employed as a potential technique to analyze the dynamical changes in cosmic ray intensity during different solar cycles. In the present study, dynamical complexity based analysis using Jensen-Shannon divergence (JSD) has been employed which reveals the existence of some peculiar fluctuation properties in CRI flux at Jung neutron monitor station. JSD based dynamical complexity analyses confirm the existence of difference in dynamical properties of CR flux during solar cycles 20-21 and 22-23.

Simple coincidence technique for cosmic-ray intensity exploration via low-energy photon detection

Changes in cosmic-ray intensity can significantly influence the search for rare events or processes in nuclear and astroparticle physics through corresponding variations in detector background count rate. In this work, we present an approach to explore cosmic-ray intensity and corresponding cascade production of secondary particles in the detector vicinity using low-energy photon background spectra induced by cosmic rays at the earth’s surface. The coincidence system based on a plastic scintillator and an extended range HPGe detector, including a multiparameter device, was used for the acquisition of low-energy photon spectra. This system was also simulated by the GEANT4 toolkit, and the simulated and experimental spectra were compared. Single aperiodic events, as well as possible periodic behavior of low-energy photon emission were searched for.

Causes and importance of new particle formation in the present‐day and preindustrial atmospheres

AbstractNew particle formation has been estimated to produce around half of cloud‐forming particles in the present‐day atmosphere, via gas‐to‐particle conversion. Here we assess the importance of new particle formation (NPF) for both the present‐day and the preindustrial atmospheres. We use a global aerosol model with parametrizations of NPF from previously published CLOUD chamber experiments involving sulfuric acid, ammonia, organic molecules, and ions. We find that NPF produces around 67% of cloud condensation nuclei at 0.2% supersaturation (CCN0.2%) at the level of low clouds in the preindustrial atmosphere (estimated uncertainty range 45–84%) and 54% in the present day (estimated uncertainty range 38–66%). Concerning causes, we find that the importance of biogenic volatile organic compounds (BVOCs) in NPF and CCN formation is greater than previously thought. Removing BVOCs and hence all secondary organic aerosol from our model reduces low‐cloud‐level CCN concentrations at 0.2% supersaturation by 26% in the present‐day atmosphere and 41% in the preindustrial. Around three quarters of this reduction is due to the tiny fraction of the oxidation products of BVOCs that have sufficiently low volatility to be involved in NPF and early growth. Furthermore, we estimate that 40% of preindustrial CCN0.2% are formed via ion‐induced NPF, compared with 27% in the present day, although we caution that the ion‐induced fraction of NPF involving BVOCs is poorly measured at present. Our model suggests that the effect of changes in cosmic ray intensity on CCN is small and unlikely to be comparable to the effect of large variations in natural primary aerosol emissions.

PRESSURE PULSES AT VOYAGER 2: DRIVERS OF INTERSTELLAR TRANSIENTS?

ABSTRACT Voyager 1 (V1) crossed the heliopause into the local interstellar medium (LISM) in 2012. The LISM is a dynamic region periodically disturbed by solar transients with outward-propagating shocks, cosmic-ray intensity changes and anisotropies, and plasma wave oscillations. Voyager 2 (V2) trails V1 and thus may observe the solar transients that are later observed at V1. V2 crossed the termination shock in 2007 and is now in the heliosheath. Starting in 2012, when solar maximum conditions reached V2, five possible merged interaction regions (MIRs) have been observed by V2 in the heliosheath. The timing is consistent with these MIRs driving the transients observed by V1 in the LISM. The largest heliosheath MIR was observed by V2 in late 2015 and should reach V1 in 2018.

Early 18th century cosmic ray flux inferred from 44Ti in Agen meteorite

We report the measurement of radioactivity of cosmogenic 44Ti in Agen meteorite, a H5 chondrite that fell in 1814. The 44Ti activity in meteorites is related to centennial-scale changes in cosmic ray intensity caused by heliospheric magnetic field modulation in the interplanetary space between heliocentric distances of 1 and 3 AU. The measured low 44Ti activity in Agen suggests a strong modulation of galactic cosmic rays at the turn of the 18th century, resulting in a low cosmic ray flux and is consistent with the linearly decreasing trend of GCR flux, modulated by the Gleissberg solar cycle during the past 250 years, as previously suggested by us.

Voyager 1 observes sudden changes in cosmic ray intensity

The Voyager 1 spacecraft, which was launched in 1977, has reached a new region of space near the edge of the heliosphere, the bubble of charged particles surrounding our solar system. At the edge of the heliosphere is the heliosheath, the region where the solar wind slows down as it hits incoming particles from the interstellar medium. As it traveled through the heliosheath, Voyager 1 recently encountered a dramatic, unprecedented change in the intensity of cosmic rays.

The contribution of cosmic rays to global warming

A search has been made for a contribution of the changing cosmic ray intensity to the global warming observed in the last century. The cosmic ray intensity shows a strong 11 year cycle due to solar modulation and the overall rate has decreased since 1900. These changes in cosmic ray intensity are compared to those of the mean global surface temperature to attempt to quantify any link between the two. It is shown that, if such a link exists, the changing cosmic ray intensity contributes less than 8% to the increase in the mean global surface temperature observed since 1900.

Solar, geomagnetic and cosmic ray intensity changes, preceding the cyclone appearances around Mexico

Recently it has been suggested that there exist specific changes in the cosmic ray intensity and some solar and geomagnetic parameters during the days, preceding the hurricane appearances over the North Atlantic Ocean. To understand better these phenomena, data for all hurricanes born not only over the Atlantic but also over the Pacific waters in the last 55 years that hit the Mexican borders were elaborated. As basic hurricane parameters the maximum rotational velocity and the estimated total energy were used. To avoid any interference all hurricanes, overlapping the preceding ones with more than 20 days were not included. Then the behavior of the cosmic ray (CR) intensity, the sunspot (SS) numbers, and the geomagnetic parameters (AP) and (KP) in 35 days prior and 20 days after the cyclone start were investigated. The CR, SS, AP and KP showed much more intensive disturbances in the periods preceding and following the hurricane appearance. For SS this disturbance gradually increase with the hurricane strength. A characteristic peak in the CR intensity appears before the hurricane start. But its place varies between 5 and 20 days before that start. Specific changes were observed in the SS. For major hurricanes they begins sometimes more than 20 days in advance. The AP and the KP show series of bursts, spread over the whole period of 30 preceding days. The obtained results from the performed correlational analysis are enough interesting to motivate a further statistical analysis with more precise techniques: in particular a common periodicity of 30 years found in the number of tropical storms landing into Mexico, the averaged rotational wind velocity and the ACE must be studied in connection with the solar Hale cycle. Using coherence wavelet spectral analysis we present a comparative study between one terrestrial and one cosmophysical phenomena that presumable influence hurricanes development: African dust outbreaks versus cosmic rays for all North Atlantic tropical cyclones. It is shown that the cosmophysical influence cannot be considered as a negligible effect.

Galactic cosmic ray flux decline and periodicities in the interplanetary space during the last 3 centuries revealed by 44Ti in meteorites

44Ti and 26Al activities and heavy nuclei tracks produced by cosmic rays have been measured in 19 stone meteorites that fell during the period 1766 to 2001. The gamma activity measurements of cosmogenic isotopes were performed using a highly specific and selective large volume Ge‐NaI (Tl) spectrometer. The 44Ti activity, corrected for the target element abundances and shielding effects, shows a decrease of about 43% over the past 235 years. Superimposed on this declining trend, the 44Ti activity shows a centennial oscillation. The variations in 44Ti activity are related to changes in cosmic ray intensity caused by heliospheric magnetic field modulation in the interplanetary space between heliocentric distances of 1 and 3 AU. Using the relations between solar open magnetic flux, modulation parameter, and galactic cosmic ray flux, we have calculated the variation in production rates of 44Ti in meteorites back to about 1700 AD. We show that the measured 44Ti activity in meteorites over the past 235 years and the calculated production rates are in agreement. Our results are consistent in phase and magnitude with doubling of the solar open magnetic field intensity over the past century. The data also imply that the centennial scale oscillation deduced by 44Ti is in phase with Gleissberg solar cycle, but its amplitude is larger than expected from the calculated cosmic ray flux.

Intensities of galactic cosmic rays of ∼1.5 GV rigidity at Earth versus the heliospheric current sheet tilt

In this paper we have compared the cosmic ray intensity variations at ∼1.5 GV rigidity with the corresponding time variations of the heliospheric current sheet tilt (HCST). These studies extend earlier work comparing neutron monitor variations with the HCST. In addition to the results of this earlier work we have found that (1) the delay of the cosmic ray intensity changes relative to the HCST changes is much less in 11‐year cycles starting in a negative solar magnetic cycles. (2) During the specific periods of actual changeover of the solar magnetic field from positive to negative polarity, the cosmic ray intensity decrease is ∼50% of the total 11‐year cycle intensity decrease. During the changeover from negative to positive polarity, the entire cosmic ray 11‐year decrease has almost fully occurred before the polarity changeover. (3) In 11‐year modulation cycles starting in a negative polarity period, the regression coefficient between the cosmic ray intensity and tilt is ∼0.9% per degree of tilt during both the decreasing and increasing phases. In the 11‐year modulation cycles starting in a positive polarity cycle, the regression coefficient is much smaller, only ∼0.4% per degree during much of the cycle; then it becomes much steeper toward the end of the cycle. These features are discussed in terms of current models for cosmic ray modulation that include diffusion, convection, drifts, and the wavy current sheet.

The heliospheric magnetic field from 850 to 2000 AD inferred from 10Be records

10Be found in ice cores is an indicator of cosmic ray intensity in the past. We use this isotope to study cosmic ray transport and the heliospheric magnetic field before the advent of instrumental cosmic‐ray measurements in the modern space era. The galactic cosmic ray intensity is governed by scattering, convection, and drift of the charged particles in the heliospheric magnetic field, which leads to a modulation in their intensity. We model these cosmic ray intensity changes observed at Earth during the space era with solutions of the cosmic ray transport equation. This gives a set of diffusion mean free paths during the past few solar activity cycles. A relationship is then determined between these diffusion mean free paths and satellite observations of the heliospheric magnetic field during the same period, yielding a relationship between the observed cosmic ray intensity and the heliospheric magnetic field. We then calculate the diffusion mean free paths that explain the variations in the 10Be concentration during the last millennium and use the space‐era calibration to infer heliospheric magnetic field since 850 AD. It is shown how this inversion of the 10Be data depends on the strength of the heliospheric magnetic field and variations in its turbulence, both of which are quite uncertain. Nevertheless, the results show that for a wide range of parameters, there was a significant heliospheric magnetic field with a strength of 2 to 5 nT at Earth during the so‐called Grand Minima of solar activity. It is also shown that the strength of this field has attained six maxima in the past 1150 years, all approximating the present‐day field strength, and we speculate that a limiting mechanism may be in operation. On several occasions the strength of the field has switched rapidly from ≈2 nT to ≈6 nT within 40 years. During the Grand Minima the total field derived from the 10Be data differs significantly from the open solar magnetic field calculated from the models of Solanki et al. [2002] and Schrijver et al. [2002].

Coronal Mass Ejections, the Tail of the Solar Wind Magnetic Field Distribution, and 11 Year Cosmic‐Ray Modulation at 1 AU

Using a recent classification of the solar wind at 1 AU into its principal components (slow solar wind, high-speed streams, and coronal mass ejections [CMEs]) for 1972-2000, we show that the monthly averaged Galactic cosmic-ray intensity is anticorrelated with the percentage of time that the Earth is embedded in CME flows. We suggest that this anticorrelation results primarily from a CME-related change in the tail of the distribution function of hourly averaged values of the solar wind magnetic field (B) between solar minimum and solar maximum. The number of high B-values (≥10 nT) increases by a factor of ~3 from minimum to maximum (from 5% of all hours to 17%), with about two-thirds of this increase due to CMEs. On an hour-to-hour basis, average changes of cosmic-ray intensity at Earth become negative for solar wind magnetic field values ≥10 nT.

Cosmic ray survey to Antarctica and coupling functions for neutron component near solar minimum (1996– 1997): 3. Geomagnetic effects and coupling functions

We present the results of a ship survey conducted from Italy to Antarctica and back during the 1996–1997 solar minimum measuring the latitude variation in the neutron component of cosmic radiation at sea level. High‐energy atmospheric neutrons were detected by a 3NM‐64 and thermalized atmospheric neutrons were detected by two bare BF3 counters. Discussions of the internal consistency of the data and the stability of the detectors, investigations of meteorological effects, and data corrections are presented in two companion papers. In this paper we compute updated vertical cutoff rigidities corrected for the penumbra effect, and we estimate apparent cutoff rigidities, which take into account the contribution of nonvertically incident particles to the counting rate. When comparing cosmic ray intensities observed in the same place, a small forward‐backward effect is found and explained as the effect of an asymmetric shielding structure around the monitor. Latitude dependencies (i.e., neutron intensities versus cutoff rigidity) and associated coupling functions are computed for both monitors and compared. The NM latitude dependence obtained for the 1996–1997 solar minimum is found to be almost identical to that obtained by other authors in the previous solar minimum. The absence of the so‐called “crossover” effect when comparing coupling functions of subsequent solar minima is discussed also on the basis of cosmic ray intensity changes observed by neutron monitor stations.

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.

Modulation of cosmic ray precipitation related to climate

High energy cosmic rays may influence the formation of clouds and thus impact weather and climate. Due to systematic solar wind changes, the intensity of cosmic rays incident on the magnetopause has decreased markedly during this century. The pattern of cosmic ray precipitation through the magnetosphere to the upper troposphere has also changed. Early in the century, the part of the troposphere open to cosmic rays of all energies was typically confined to a relatively small high‐latitude region. As the century progressed the size of this region increased by over 25% and there was a 6.5° equatorward shift in the yearly averaged latitudinal position of the subauroral region in which cloud cover has been shown to be cosmic ray flux dependent. We suggest these changes in cosmic ray intensity and latitude distribution may have influenced climate change during the last 100 years.

Cosmic ray modulation during solar cycle 22: Solar maximum loops at earth and global transients in the heliosphere

Neutron monitor data from two pairs of cosmic ray stations, Kiel/Tsumeb and Climax/Huancayo, are used to study the rigidity dependence of solar modulation during the solar activity cycle 22. The long‐term decrease of cosmic ray intensity during the ascending phase of cycle 22 is characterized by the same rigidity dependence as for the long‐term recovery during the descending phase of cycle 21. This is equivalent to a lack of any significant spectral hysteresis in cosmic ray modulation for both half cycles surrounding the 1987 cosmic ray maximum. The above is true at least for 27‐day averages of cosmic ray intensity and at neutron monitor rigidities. In December 1988 the first large Forbush decrease of cycle 22 marked the beginning of increased solar activity which resulted in several prominent interplanetary shocks in 1989‐1991. These strong heliospheric disturbances produced global merged interaction regions (GMIRs) which were responsible for large step decreases in cosmic ray intensity seen at all spacecraft throughout the heliosphere, including Voyager 2, Pioneer 10 and 11, as well as Voyager 1. It was found that in the neutron monitor rigidity range, such decreases were followed by rigidity‐dependent recoveries. The rigidity‐dependent recovery manifests itself by formation of hysteresis loops on correlation plots for low‐ versus high‐rigidity cosmic ray intensity changes. The loops, if completed, close approximately at the modulation level from which the corresponding step decrease started. On the other hand, the current long‐term cosmic ray recovery (starting from Bartels solar rotation 2162) follows the "normal" rigidity dependence characteristic of both the A>0 recoveries and A<0 decreases/recoveries (where A<0 and A>0 are two different phases of the solar magnetic cycle). All this together confirms the bimodal character of cosmic ray modulation for 1985‐1992, the same as found previously for cycles 19‐21; the transient modulation (T ≅ 0.5 to ∼1 year) related to GMIRs is accompanied on recovery by the phenomenon of hysteresis, while the long‐term modulation (T>1 year) is hysteresis free. The two explanations of the phenomenon of spectral hysteresis proposed in the literature, i.e., a time‐dependent modulation via convection, diffusion, and drift effects and an acceleration of a normal cosmic ray spectrum in multiple shocks in the outer heliosphere (as proposed by Stoker and Moraal (1986)), are discussed in the context of the presented cosmic ray data.

Galactic gamma rays from the inverse Compton process

The extent to which gamma rays from inverse Compton (IC) collisions contribute to the measured gamma-ray flux in the satellite energy band (30-4000 MeV) is examined. The problems inherent in the estimate are discussed, not least the uncertainty in the scale height of cosmic-ray (CR) electrons and in the extent to which the local cosmic-ray intensity is spatially variable. Most of the paper refers to the derivation of the effective IC contribution, effective in the sense of being such as to account for the bulk of the large-scale gamma-ray intensity variations. If the cosmic-ray intensity is constant over the region of space within approximately 1 kpc of the Sun, the IC contribution to the total Galactic diffuse intensity is 32+or-10% for the Galactic latitude range 10-20 degrees , increasing to 44+or-17% at the Galactic poles. The corresponding scale height of the electron component is z12/=(2.5+or-1) kpc. There are still CR intensity changes needed locally but these are on comparatively small scales; if later work should show that the CR intensity changes are, in fact, larger then the IC contribution will have to be reduced.

富士山における宇宙線強度の高度変化 球形ＮａＩ（Ｔｌ）シンチレータによる電離成分の測定

Variation of cosmic ray intensity with altitude was measured at Mt. Fuji located at about 26°N on geomagnetic latitude on August 1994. The measurement was carried out with a spectrometer equipped with a 76.4 mm (3″) diameter spherical NaI (Tl) scintillator. Analysis was made for absorbed energy spectra above 3 Me V by the peeling-off method, and the cosmic ray intensity was calculated as the exposure rate from the ionizing component of the cosmic rays. The cosmic ray intensity increased with increasing altitude. The maximum (30.7×10-10 C/kg/h) and minimum (8.5×10-10 C/kg/h) values were observed at summit area of Mt. Fuji (3740 m) and near the sea level (about 20 m), respectively. Above 2500 m, the rate of change of cosmic ray intensity with altitude was estimated to be about 0.99×10-12 (C/kg) /h/m. The rate from near the sea level to about 2 500 m was also estimated to be about 0.39×10-12 (C/kg) /h/m.

Cosmogenic 36Cl accumulation in unstable landforms: 2. Simulations and measurements on eroding moraines

Cosmogenic 36Cl surface exposure ages obtained for multiple boulders from single landforms are usually characterized by a variance larger than that of the analytical methods employed. This excessive boulder‐to‐boulder variability, progressively more profound with increasing age of landforms, is due to removal of soil and gradual exposure of boulders at the surface. In our gradual exposure model, boulders are initially buried in moraine matrix. With time, erosion lowers the moraine surface and the boulders are gradually exposed to cosmic rays. Because the cosmic ray intensity changes with depth, the boulders are subjected to variable production rates of the cosmogenic 36Cl. Initial depth of boulders and their chemical composition are variable, which results in different amounts of the accumulated cosmogenic 36Cl and thus different apparent ages of boulders. The shape of the resulting distribution of the apparent ages and the coefficient of variation depend on the erosion depth, while the first moment is a function of the true surface age and the erosion depth. These properties of the apparent age distributions permit calculation of the surface age, the erosion depth, and also the average erosion rate. We tested the model calculations using 26 boulders from a late Pleistocene moraine at Bishop Creek, Sierra Nevada, California. The set exhibited a bimodal distribution of the 36Cl surface exposure ages. We interpreted the older mode as the result of gradual exposure and the younger one as the result of surficial processes other than soil removal. The 10 samples that constitute the older mode produced a distribution which closely matches the modeled distribution calculated using an age of 85 kyr and erosion depth of 570 g cm−2. This age is the same as an independent estimate obtained from cation ratio studies, and the calculated erosion depth is very close to the erosion depth of 600 g cm−2 based on a simple analytical model of soil erosion. These results indicate that our statistical model adequately describes effects of soil erosion on accumulation of cosmogenic 36Cl. The approach can be used to simultaneously obtain the true landform age and the erosion rate from apparent 36Cl ages and thereifore may help in evaluation of surface exposure ages of eroding landforms.