Low-energy Galactic Cosmic Rays Research Articles

Influence of magnetosphere disturbances on particle fluxes measured by ground-based detectors

This study examines how the Earth's surface particle fluxes are modulated by the interplanetary magnetic field (IMF) carried by coronal mass ejections (ICME). Our findings underscore the role of magnetic reconnection in allowing low-energy galactic cosmic rays (GCRs) to penetrate the magnetosphere, leading to enhanced secondary particle fluxes through reduced cutoff rigidity —a phenomenon known as the magnetospheric effect (ME). In contrast, the Forbush decrease (FD) driven by the scalar magnetic field strength results in significant particle flux reductions. On May 10–11, 2024, the FD, directly linked to the enormous geomagnetic storm (GMS), was complicated by the simultaneous registration of secondary particles from the solar energetic particle (SEP) event, which was energetic enough to generate secondary particles in space and on the ground, leading to increases in detector count rates, known as ground-level enhancements (GLEs). Using new experimental facilities, we reveal that secondary particles during ME events release up to 10 MeV energy (maximum energy of approximately 10 MeV), whereas, during FD and GLE events, the energy release extends to 100 MeV (maximum energy of approximately 100 MeV). These insights contribute to refining event classification schemes and predictive models of space weather.

Measurements of anomalous cosmic rays from the WIND spacecraft over 1994–2021

ABSTRACT Observations provide direct evidence for a sustained decline in solar activity since the 1980s, with the minimum between solar cycles 24 and 25 (P24/25) reaching its deepest trough in the past hundred years. In response to the reduced solar modulation, low-energy (<GeV) galactic cosmic rays (GCRs) at 1 au attain historical new highs in P24/25, while anomalous cosmic rays (ACRs) show very inconsistent variations with GCRs. To better understand the long-term ACR variations, we revisit the 6.0–13.4 MeV nuc−1 oxygen measurements from the WIND/LEMT instrument over 1994–2021, with particular interests to the recent three solar minima: P22/23 (solar magnetic polarity A > 0), P23/24 (A < 0), and P24/25 (A > 0). We find that the peak ACR intensities in P24/25 are ∼20 per cent lower than those in P22/23 but ∼48 per cent above those in P23/24. Additionally, for a specified heliospheric current sheet (HCS) tilt angle, the GCR intensities in P24/25 are inferred to be significantly higher than those in P22/23 (both are in A > 0 cycles), whereas the ACR intensities in P24/25 are close to those in P22/23. These results indicate that large variability in ACRs exists not only between opposite-polarity cycles, but also between different cycles of the same polarity, and ACRs may be more sensitive to varying HCS compared to GCRs. We speculate that variations in the ACR source intensity may be a key to understanding the ACR–GCR discrepancies. This is the first time for such a long-term study being possible with the same instrument.

The Galactic cosmic ray intensity at the evolving Earth and young exoplanets

ABSTRACT Cosmic rays may have contributed to the start of life on the Earth. Here, we investigate the evolution of the Galactic cosmic ray spectrum at the Earth from ages t = 0.6−6.0 Gyr. We use a 1D cosmic ray transport model and a 1.5D stellar wind model to derive the evolving wind properties of a solar-type star. At $t=1\,$ Gyr, approximately when life is thought to have begun on the Earth, we find that the intensity of ∼GeV Galactic cosmic rays would have been ∼10 times smaller than the present-day value. At lower kinetic energies, Galactic cosmic ray modulation would have been even more severe. More generally, we find that the differential intensity of low-energy Galactic cosmic rays decreases at younger ages and is well described by a broken power law in solar rotation rate. We provide an analytic formula of our Galactic cosmic ray spectra at the Earth’s orbit for different ages. Our model is also applicable to other solar-type stars with exoplanets orbiting at different radii. Specifically, we use our Galactic cosmic ray spectrum at 20 au for $t=600\,$ Myr to estimate the penetration of cosmic rays in the atmosphere of HR 2562b, a directly imaged exoplanet orbiting a young solar-type star. We find that the majority of particles <0.1 GeV are attenuated at pressures ≳10−5 bar and thus do not reach altitudes below ∼100 km. Observationally constraining the Galactic cosmic ray spectrum in the atmosphere of a warm Jupiter would in turn help constrain the flux of cosmic rays reaching young Earth-like exoplanets.

Search for Albedo Tritium with PAMELA Experiment

Particles of albedo radiation are the particles born in interactions of primary cosmic rays penetrating inside Earth’s atmosphere and magnetosphere and the atmosphere nuclei. These interactions result in so-called particle showers and the detection of these showers is the essence of work for ground-based cosmic-ray detectors such as neutron monitors, muon hodoscopes and EAS arrays. Some products of these interactions undergoing by scattering and having the curve paths in geomagnetic field propagate upwards to the boundary of Earth’s magnetosphere. These particles are called albedo particles. The principle of their identification is simple. They are registered in geomagnetic zones where the penetration of low-energy galactic or solar cosmic rays is restricted due to Earth’s magnetic field. The results of measurements of albedo protons, deuterons, electrons and positrons has been already published by PAMELA collaboration. In this work the first approach to search for the albedo tritium nuclei with energies above 100 MeV/nucleon is described.

Proton Fluxes Measured by the PAMELA Experiment from the Minimum to the Maximum Solar Activity for Solar Cycle 24

Abstract Precise measurements of the time-dependent intensity of the low-energy (<50 GeV) galactic cosmic rays (GCRs) are fundamental to test and improve the models that describe their propagation inside the heliosphere. In particular, data spanning different solar activity periods, i.e., from minimum to maximum, are needed to achieve comprehensive understanding of such physical phenomena. The minimum phase between solar cycles 23 and 24 was peculiarly long, extending up to the beginning of 2010 and followed by the maximum phase, reached during early 2014. In this Letter, we present proton differential spectra measured from 2010 January to 2014 February by the PAMELA experiment. For the first time the GCR proton intensity was studied over a wide energy range (0.08–50 GeV) by a single apparatus from a minimum to a maximum period of solar activity. The large statistics allowed the time variation to be investigated on a nearly monthly basis. Data were compared and interpreted in the context of a state-of-the-art three-dimensional model describing the GCRs propagation through the heliosphere.

The descent of the solar Cycle 24 and future space weather

We study the development of the Cycle 24 using the recently revised sunspot number (SSN) time series proposed by Clette et al. (2014) labeled SSN (V2). The smooth SSN for the cycle increased since its onset in December 2008, developing a shoulder in 2012, a plateau in 2013 and a peak in 2014, a behavior similar to the Cycle 15 that reached a plateau and rose to a much higher peak. Present status of the Cycle 24 is described in terms of SSN (V2) and 10.7cm (2800MHz) microwave flux (F10.7) from the sun and its activity is compared to some prior cycles of the nineteenth and twentieth century. SSNs declined at a faster rate than F10.7 starting near the Cycle 21 minimum reaching the peak 2months after the SSN peak. The physical cause(s) for these differences is not known. The decay phase for both indices has set in. The historic evidence suggests that the Cycle 25 may be less than half as active as the Cycle 24 with a longer duration leading to a reduced in size heliosphere and a significant decrease in the phenomena that track SSNs at 1AU, such as: the intensity (B) of the interplanetary magnetic field (IMF), the frequency of the fast coronal mass ejections (CMEs), the storm sudden commencements and the large Forbush decreases accompanied by a significant increase in the low energy galactic cosmic rays (GCRs) and reduced disturbances in the ionosphere; even so one cannot rule out the occurrence of an extreme CME in future.

Solar modulation of low energy galactic cosmic rays in the near-earth space environment

This is an overview of the solar modulation of galactic cosmic rays as seen from the Earth and spacecrafts closeby, where we have put the contributions of Latin-American researchers in the global context in the last five to ten years. It is a broad topic with numerous intriguing aspects so that a research framework has to be chosen to concentrate on, therefore we have put our emphasis on measurements of the cosmic ray flux, without attempting to review all details or every contribution made in this field of research. In consequence, after establishing the basic characteristics of the cosmic radiation such as composition and energy spectrum, we focus on a few selected subjects, almost all within the framework of solar modulation of galactic cosmic rays such as Forbush decreases, periodic variations, space and atmospheric weather cosmic ray relationships, to which we add a general description of ground level enhancement observations. Controversial aspects are discussed where the appropriate results are presented, some of the challenges and prospects of key issues are also pointed out. At the end of the paper, a brief summary of the last decade Latin-American contributions to the subjects treated is given.

Verification of shielding effect by the water-filled materials for space radiation in the International Space Station using passive dosimeters

The dose reduction effects for space radiation by installation of water shielding material (“protective curtain”) of a stack board consisting of the hygienic wipes and towels have been experimentally evaluated in the International Space Station by using passive dosimeters. The averaged water thickness of the protective curtain was 6.3g/cm2. The passive dosimeters consisted of a combination of thermoluminescent detectors (TLDs) and plastic nuclear track detectors (PNTDs). Totally 12 passive dosimeter packages were installed in the Russian Service Module during late 2010. Half of the packages were located at the protective curtain surface and the other half were at the crew cabin wall behind or aside the protective curtain. The mean absorbed dose and dose equivalent rates are measured to be 327μGy/day and 821μSv/day for the unprotected packages and 224μGy/day and 575μSv/day for the protected packages, respectively. The observed dose reduction rate with protective curtain was found to be 37±7% in dose equivalent, which was consistent with the calculation in the spherical water phantom by PHITS. The contributions due to low and high LET particles were found to be comparable in observed dose reduction rate. The protective curtain would be effective shielding material for not only trapped particles (several 10MeV) but also for low energy galactic cosmic rays (several 100MeV/n). The properly utilized protective curtain will effectively reduce the radiation dose for crew living in space station and prolong long-term mission in the future.

Voyager 1 Observes Low-Energy Galactic Cosmic Rays in a Region Depleted of Heliospheric Ions

On 25 August 2012, Voyager 1 was at 122 astronomical units when the steady intensity of low-energy ions it had observed for the previous 6 years suddenly dropped for a third time and soon completely disappeared as the ions streamed away into interstellar space. Although the magnetic field observations indicate that Voyager 1 remained inside the heliosphere, the intensity of cosmic ray nuclei from outside the heliosphere abruptly increased. We report the spectra of galactic cosmic rays down to ~3 × 10(6) electron volts per nucleon, revealing H and He energy spectra with broad peaks from 10 × 10(6) to 40 × 10(6) electron volts per nucleon and an increasing galactic cosmic-ray electron intensity down to ~10 × 10(6) electron volts.

Search for the Exit: Voyager 1 at Heliosphere’s Border with the Galaxy

We report measurements of energetic (>40 kiloelectron volts) charged particles on Voyager 1 from the interface region between the heliosheath, dominated by heated solar plasma, and the local interstellar medium, which is expected to contain cold nonsolar plasma and the galactic magnetic field. Particles of solar origin at Voyager 1, located at 18.5 billion kilometers (123 astronomical units) from the Sun, decreased by a factor of >10(3) on 25 August 2012, while those of galactic origin (cosmic rays) increased by 9.3% at the same time. Intensity changes appeared first for particles moving in the azimuthal direction and were followed by those moving in the radial and antiradial directions with respect to the solar radius vector. This unexpected heliospheric "depletion region" may form part of the interface between solar plasma and the galaxy.

Magnetic Field Observations as Voyager 1 Entered the Heliosheath Depletion Region

Magnetic fields measured by Voyager 1 (V1) show that the spacecraft crossed the boundary of an unexpected region five times between days 210 and ~238 in 2012. The magnetic field strength B increased across this boundary from ≈0.2 to ≈0.4 nanotesla, and B remained near 0.4 nanotesla until at least day 270, 2012. The strong magnetic fields were associated with unusually low counting rates of >0.5 mega-electron volt per nuclear particle. The direction of B did not change significantly across any of the five boundary crossings; it was very uniform and very close to the spiral magnetic field direction, which was observed throughout the heliosheath. The observations indicate that V1 entered a region of the heliosheath (the heliosheath depletion region), rather than the interstellar medium.

Spectrum and ionization rate of low-energy Galactic cosmic rays

Abstract We consider the rate of ionization of diffuse and molecular clouds in the interstellar medium by Galactic cosmic rays (GCRs) in order to constrain its low-energy spectrum. We extrapolate the GCR spectrum obtained from PAMELA at high energies (≥200 GeV nucleon−1) and a recently derived GCR proton flux at 1–200 GeV from observations of gamma-rays from molecular clouds, and find that the observed average Galactic ionization rate can be reconciled with this GCR spectrum if there is a low-energy cut-off for protons at 10–100 MeV. We also identify the flattening below a few GeV as being due to (a) decrease of the diffusion coefficient and dominance of convective loss at low energy and (b) the expected break in energy spectrum for a constant spectral index in momentum. We show that the inferred CR proton spectrum of Φ ∝ E−1.7±0.2kin for Ekin ≤ few GeV is consistent with a power-law spectrum in momentum p−2.45± 0.4, which we identify as the spectrum at source. Diffusion loss at higher energies then introduces a steepening by E−α with α ∼ 1/3, making it consistent with high-energy measurements.

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 16,000m2) 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.

Are Anomalous Cosmic Rays the Main Contribution to the Low-Energy Galactic Cosmic Ray Spectrum?

While the high-energy part of the Galactic cosmic ray spectrum is well observed, its nature at energies below about 1 GeV nucleon-1 is still not known well. Recent in situ measurements made with the Voyager 1 spacecraft in the heliosheath between the solar wind termination shock and the heliopause have added further constraints on the local interstellar spectrum of Galactic cosmic rays at low energies. We show here that they also suggest how the low-energy proton part is formed locally in the heliosphere and globally in the Galaxy. The measured flux of anomalous cosmic rays in the heliosheath is unexpectedly high compared to expectations before Voyager 1 reached the shock, which might be a temporal effect or due to an additional acceleration beyond the termination shock. Combining this finding with recent model results for astrospheres immersed in different interstellar environments shows that the astrospheric anomalous cosmic ray fluxes of solar-type stars can be a hundred times higher than thought earlier and, consequently, their total contribution to the lower end of the interstellar spectrum can be significant.

Low energy galactic cosmic rays in the heliosphere

The flux of galactic cosmic rays (GCR) extends over a wide range of energies (from 10 8 to 10 20 eV); it has a strong dependence on particle energy. Given the large span of energies the detection techniques, transport mechanisms and other characteristics vary as energy increases. In the low energy region (<10 12 eV) the flux of GCR is modulated by the solar activity. Continuous registers are necessary to study intensity variations that must have their origin in the Sun. Detectors were designed and constructed for the purpose, they operate since the middle of the last century providing valuable information to study recurrent periodicities and their relationship to those of solar phenomena, but also to elucidate whose are the relevant transport mechanisms inside the heliosphere. A brief review of the advancement in the comprehension of these phenomena is presented.

The relative transit time of high-rigidity and low-rigidity cosmic rays through the solar system.

A study of neutron monitor count rates with vertical cut-off rigidities from 0.6 to 13 GV indicates that high-energy and low-energy galactic cosmic rays arrive at earth orbit with small time differences compared to a month. This is in contradiction to the theory of the deceleration potential which states that cosmic-ray particles with rigidities greater than 36 V arrive at earth orbit about three months before cosmic-ray particles with lesses rigidines.

Penetration of the galactic cosmic rays into the heliosphere through LISM-solar wind interface

The problem of low energy (E < 1 Gev) galactic cosmic ray (GCR) penetration into the heliosphere through the region of the solar wind interaction with the plasma component of the LISM is investigated. GCR are coupled to the solar wind by scattering with hydromagnetic waves propagating in the plasma flow. GCR are considered to be a hot, low density gas with negligible mass flux, but significant pressure and energy fluxes. The plasma field is taken from the Baranov - Malama model which takes into account the neutral hydrogen. Dependence of GCR penetration into the heliosphere on the diffusion coefficient is analysed. Dependence of GCR modulation on the direction is shown. The possibility of GCR influence on the structure of the interface region is investigated. The present paper is the first step in a full understanding of the problem of the interaction of a three-component (plasma, atoms, cosmic rays) interstellar medium with the solar wind.

The boron isotope ratio in the interstellar medium

OBSERVATIONS of the abundances of elements provide insight into their production and distribution. The production of light elements (in particular, lithium, beryllium and boron) is dominated by spallation reactions1, in which cosmic rays break apart more massive nuclei. Models2,3 suggest that the 11B/10B ratio should be about 2.5, but the observed ratio in the Solar System is about 4 (refs 4,5). This has led to the suggestion5 that the pre-solar nebula was subjected to bombardment by low-energy Galactic cosmic rays, leading to an overproduction of 11B (ref. 6). Until now, it has not been possible to measure the 11B/10B ratio in the interstellar medium, because the lines are very weak. Here we present a spectroscopic measurement of the 11B/10B ratio in the interstellar gas lying between the Earth and the star δ Scorpii, made using the Hubble Space Telescope. Our measured ratio, 3.4+1.3−0.6, is comparable to the meteoritic value5, but somewhat lower. Further measurements will be needed to establish whether the ratio reported here is characteristic of the interstellar medium in general.

Observation and implications of sub-iron and iron abundance ratios in low energy galactic cosmic rays

The Spacelab-3 cosmic ray experiment Anuradha was used to measure the sub-iron (ScCr) to iron abundance ratios in the low energy galactic cosmic rays. Measurements made in four different depth of the detector yielded the (Sc-Cr) Fe ratios of 0.8 to 1.2 in 30 to 300 MeV/N. These are in agreement with results from Skylab and Soyuz-6 experiments and establishes that this abundance ratio is about 1.0 inside the magnetosphere. It is seen that this abundance ratio is about a factor of two higher than values of about 0.5 measured in space crafts in interplanetary space. It is shown that the enhancement of the ratio is probably due to geomagnetic transmission effect and the degree of ionization of the low energy Sc to Cr and Fe ions in galactic cosmic rays. Further studies are needed to fully understand the phenomena and their implications.

Corotating Ion Events Associated with Cosmic Ray Modulation.

Interplanetary ions in a few MeV/n energies were observed by the GEOTAIL spacecraft at 1 AU during the period from December 1993 to April 1994. The measurements were performed with the HEP-particle telescopes onboard the GEOTAIL. We have examinedthe correlation between the interplanetary ion events and cosmic ray modulation. It is found that the variation of counting rates of neutron monitor is well correlated with the flux variation of low energy ions. The corotating interaction regions formed in the interplanetary space significantly affect the intensities not only of low energy galactic cosmic rays (GCRs) but also of high energy GCRs. It implies that corotating streams cause the decrease of GCR flux because they act as barriers that impede the flows of GCRs toward the Sun.