Variations of the Galactic Cosmic Rays in the Recent Solar Cycles

Due to the supersonic expansion of the solar wind (SW), the heliosphere is like a giant asymmetric bubble filled with various energetic particles. These particles are the dominant source of space radiation environment, posing a significant threat to the regular operation of spacecraft and high-altitude aircraft, and the health of astronauts and flight personnel (e.g., Mertens & Slaba 2019). Solar energetic particles (SEPs), galactic cosmic rays (GCRs) and anomalous cosmic rays (ACRs) are the three commonly seen energetic particles in the interplanetary space.

SEPs, also known as solar cosmic rays, are the particles originating from the Sun with energies ranging from a few tens of keV to several GeV. They are frequently associated with coronal mass ejections (CMEs) and solar flares. It is widely accepted that CME-driven shock is an essential site for accelerating solar energetic particles (e.g., Hu et al. 2017, 2018; Fu et al. 2019; Ding et al. 2020). GCRs are often regarded as a stable background, and are believed to originate from the supernova remnants (SNRs) and accelerated by supernova blast shocks driven by expanding SNRs. This is confirmed by observations and numerical simulations (Aharonian et al., 2007, 2011; Ptuskin et al., 2010). During the quiet periods, the fluxes of some cosmic ray species with high first ionization potentials, including H, He, C, N, O, Ne and Ar, are significantly higher than the background GCRs, i.e., anomalous cosmic rays (Hovestadt et al., 1973; McDonald et al., 1974; Hasebe et al., 1997). A classical explanation of ACRs is that they originate from the interstellar medium, in which the neutral atoms drift into the heliosphere and become singly ionized by the solar wind or the ultraviolet radiation which are subsequently accelerated to energies above 10 MeV/nuc, mostly occurring at or near the heliospheric termination shock (Mewaldt et al., 1993; Cummings & Stone, 2007; Gloeckler et al., 2009). New mechanisms for ACR acceleration have been put forward (e.g., Giacalone et al., 2012, and references therein), such as, Zhao et al., 2019 suggested that ACR protons are accelerated in the heliosheath via reconnection processes. Studying the nature of energetic particles provides insight into the acceleration mechanisms and the transport processes between the sources and the observers.

When GCR particles from the outer space (the so-called primary cosmic ray) encounter the Earth and penetrate the Earth’s atmosphere, they interact with atmospheric atoms and generate cascades of secondary particles (e.g., Mishev et al., 2014). The secondary particles (predominantly neutrons and muons at the ground level) can be measured by the ground-based neutron monitors (NMs) and muon detectors once arriving at the surface of the Earth. The history of neutron monitors can be traced back to the 1950s (Bieber, 2013). The combination of GCR intensities from in-situ spacecrafts and GCR count rates from NM stations is helpful to sketch a relatively complete picture of cosmic rays (CRs). In addition, the cosmogenic isotope recorded in tree rings and ice cores can extend our understanding of GCRs over a longer time scale (e.g., Owens & Forsyth, 2013).

The transport of cosmic rays throughout the heliosphere is significantly influenced by the large-scale solar wind flow and the turbulent interplanetary magnetic field embedded in it, collectively referred to as “solar modulation”. Four major modulation processes have been well described by the Parker transport equation (Parker, 1965), including gradient and curvature drifts, diffusion through the irregular interplanetary magnetic field (IMF), convection in the radial expanding solar wind, and adiabatic deceleration (or adiabatic energy loss) (e.g., Sabbah, 2000; Zhao & Zhang, 2015; Ihongo & Wang, 2016; Zhao et al., 2018). Secular measurements provide direct evidence for evaluating their relative role in affecting cosmic ray propagation over the last few decades (Kóta, 2013), but the numerical simulation of cosmic ray transport is still a highly challenging task. The solar magnetic polarity (conventionally described as [$qA$], where [$q$] denotes a positively-charged particle) is an indispensable factor in modulating cosmic ray intensities. Commonly, the positively charged particles drift inward along the heliospheric current sheet (HCS) when the heliospheric magnetic field at the north pole points inward (negative polarity, $qA<0$) and the charged particles drift outward along the HCS when the heliospheric magnetic field points outward at the north pole (positive polarity, $qA>0$) (e.g., Jokipii & Thomas, 1981; Belov, 2000; Thomas et al., 2014). The 22-year heliomagnetic cycle (also called Hale cycle) results in the alternating sharp (negative polarity, $qA<0$) and flat-topped (positive polarity, $qA>0$) shape of cosmic ray intensity (McDonald et al., 2010).

In late 2009, both the GCR intensities measured from near-Earth spacecrafts and the GCR count rates measured from ground-based NM stations reached their then all-time maximum levels (e.g., McDonald et al. 2010; Leske et al. 2013). At that time, the heliospheric environment was unusual in several aspects, including weakened interplanetary magnetic field and turbulence level, reduced solar wind dynamic pressure, and prolonged solar minimum (Mewaldt et al., 2010). The small magnitude of the IMF causes higher particle drift velocities, and the weakened IMF turbulence leads to larger particle mean free paths (MFPs) in the solar minimum 2009 than those in its previous solar minima. These anomalous interplanetary behaviours would reduce the level of GCR modulation and contribute to the unexpected high GCR intensities in 2009. As reported, the solar cycle 24 was the weakest in magnitude in the space age (Hajra, 2021), and it also exhibited some abnormal signs, such as, decreased sunspots (especially large spots) (Chapman et al., 2014), reduced coronal mass ejection events (Wang & Colaninno, 2014), and very flattened HCS. The solar activity seems to be going through a period of “grand minimum” (Jiang & Cao, 2018; Upton & Hathaway, 2018; Gonçalves et al., 2020). Former studies have predicted that the peak of the GCR intensities in the solar minimum 24/25 (hereafter solar minimum $P_{24/25}$) would exceed or close to the level recorded in 2009–2010 (e.g., Strauss & Potgieter, 2014; Kuznetsov et al., 2017; Fu et al., 2020).

The main motivation of this paper is to provide a close re-examination of the GCR variations during the successive solar cycles 23 and 24, with particular interest to the maximum GCR intensities observed at 1 AU during the solar minimum $P_{24/25}$ epoch. The paper is constructed as follows. In Section 2, we describe the sources of the dataset utilized in this work. In Section 3, we present the observational results of GCRs and investigate the possible reasons for the record-breaking GCR intensities in 2019–2020 observed at 1 AU. We summarize this work in Section 4.

Solar energetic particles and anomalous cosmic rays do not contribute to the intensities measured by ACE/CRIS, and hence they do not participate in the following analysis.

GCR intensities at 1 AU are observed by the in-situ Cosmic Ray Isotope Spectrometer (CRIS) instrument on board the NASA’s Advanced Composition Explorer (ACE) spacecraft, and GCR count rates at the ground level are monitored by the five neutron monitor stations listed in Table 1.

The ACE spacecraft was launched on 25 August 1997 and has been continuously monitoring solar wind plasma, interplanetary magnetic fields and energetic particles (including both solar energetic particles and cosmic rays) at the Sun-Earth $L$1 Lagrange point (Stone et al., 1998) for nearly 24 years, spanning the solar cycles 23 and 24. The CRIS instrument was designed to measure GCR intensities of 24 heavy species (from boron to nickel, within the energy range 50–500 MeV/nuc). Despite the low abundance ($\sim$1%), the high atomic number and energy nuclei carry abundant information of the origin of the cosmic rays, and are of special significance to space radiation (e.g., Zhao & Qin, 2013; Fu et al., 2020). With the large geometric acceptance and high charge and mass resolution, the CRIS instrument records the most detailed and statistically significant GCR data to date (Stone et al., 1998). Historical ACE/CRIS observations are well documented and publicly accessible from the ACE Science Center (ASC) via http://www.srl.caltech.edu/ACE/ASC/index.html. Here we apply the re-evaluated level-2 CRIS products, and these data are organized into 27-day Bartels rotation averages for the period from 25 August 1997 through 31 October 2020.

Pressure-corrected and 27-day averaged GCR count rates are derived from five neutron monitor stations (HRMS, JUNG, NEWK, OULU, THUL) over the 1968–2020 time period. The detailed information of the five NM stations is given in Table 1, where $P_{c}$ is the local geomagnetic cutoff rigidity of each NM station, the characteristic energy (i.e., the median energy) $E_{M}$ is defined so that cosmic rays with energy higher (or lower) than $E_{M}$ contribute to half of the detector’s counting rate (Usoskin et al., 2008; Zhao & Zhang, 2016). All data are acquired from the Neutron Monitor Data Base (NMDB, http://www01.nmdb.eu/).

The monthly mean sunspot number (SSN) is obtained from the Solar Influences Data Analysis Center (WDC-SILSO, http://www.sidc.be/). The SW speed ($V_{sw}$), the SW dynamic pressure ($P_{d}$), the IMF magnitude ($B$) and the root mean square variation in the vector IMF ($\delta B$) are acquired from the OMNIWeb databasde (https://omniweb.gsfc.nasa.gov/ow.html). The HCS tilt angle and the mean solar polar magnetic field strength are obtained from the Wilcox Solar Observatory database (http://wso.stanford.edu/). The mean polar field is defined as $(N-S)/2$, where $N$ is the polar field strength at the northern pole, $S$ is the polar field strength at the southern pole. The halo CME lists are collected from the $SOHO$/LASCO HALO CME catalog (https://cdaw.gsfc.nasa.gov/CME_list/halo/halo.html), and the annual CME rates are from CACTus CME catalog (http://www.sidc.be/cactus/). Besides, the GCR radiation dose rates near the lunar surface are measured by the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument on board the Lunar Reconnaissance Orbiter (LRO) and are available at https://crater-web.sr.unh.edu/. Note that unless explicitly stated otherwise, all of the above-mentioned data are processed as 27-day average.

The SSN observations reveal that the solar activity has been persistently declining since the early 1980s. As shown in Figures 1 and 2(a), the maximum smoothed SSNs of solar cycles 21 through 24 are 232.9, 212.5, 180.3, and 116.4, respectively, showing a prominent reduction in the solar cycle amplitude. GCR transport in the heliosphere is sensitive to varying solar activity. An intense solar activity (such as CME and solar flare) can trigger a series of space weather effects and effectively prevent GCR particles from entering the solar system (strong solar modulation), while a quiet Sun is conductive to enhance GCR fluxes as a consequence of the weakened modulation level. Next, we review the variations of the solar wind and interplanetary parameters from solar cycles 20 to 24 in Subsection 3.1; GCR observational results, including GCR intensities in interplanetary space and GCR counts at the ground level, are displayed in Subsection 3.2; in Subsection 3.3, we investigate how the inner heliospheric environment influences the GCR intensity, with an emphasis on the record-breaking fluxes during the solar minimum $P_{24/25}$; in Subsection 3.4, we point out a sudden dip in the GCR intensity during the descending phase of the solar cycle 24 and give a brief analysis of the possible causes; the measured galactic cosmic radiation doses on the lunar surface are exhibited in Subsection 3.5.

The Sun was extraordinarily quiet during the recently complete solar cycle 24, including very low sunspot numbers, extremely flat heliospheric current sheet, reduced CME eruption rate, weak interplanetary magnetic field and turbulence level. The observed galactic cosmic ray intensities (both in interplanetary space and at the ground level) are sensitive to the heliospheric conditions, the so-called solar modulation. In this work, we focus on the variations of the GCR intensities for the period from 1997 through 2020 with measurements from the ACE/CRIS instrument, and investigate the influence of inner heliospheric environments on the GCR intensities. Besides, the long-term variations of the ground-based NM count rates are also studied. The conclusions are summarized as follows.

(1) The GCR intensities observed at 1 AU reach the highest levels in the solar minimum $P_{24/25}$ since the launch of ACE spacecraft, which is $\sim$25% higher than that in the solar minimum $P_{22/23}$ and $\sim$6% higher than that in the solar minimum $P_{23/24}$. With the extension of IMP-8 measurements and BON2020 numerical simulation results, we find that the GCR intensities during the recent solar minimum are at the highest levels since the space age, having the particle energy from tens to several hundred MeV/nuc. The record-breaking GCR intensities at 1 AU increase the radiation dose rates near the lunar surface by $\sim$5% in comparison to the solar minimum $P_{23/24}$, from the measurements of LRO/CRaTER instrument.

(2) The peak value of NM count rates in the solar minimum $P_{24/25}$ is lower than that in late 2009. The difference between GCR intensities in interplanetary space and NM count rates seems to be relevant to the different modulation processes of the high and low energy particles. We think that the high-energy GCR particles are not heavily modulated as the low-energy ones, which means the low-energy GCR particles are more likely to be influenced by varying solar modulation. An alternate or additional possibility is that the NM count rates are sensitive to the conditions of the Earth’s magnetosphere and atmosphere. It remains to be further studied in a future work.

(3) We find that during the solar minimum $P_{24/25}$, the mean solar polar field strength remains weak and is close to the value in the 2009 solar minimum; the HCS tilt angle reaches a minimum value of $\sim$2.1^∘ in April 2020, which is $\sim$22% lower than that in the solar minimum $P_{22/23}$ and $\sim$53% lower than that in the solar minimum $P_{23/24}$; the CME eruption rate is very small, which is less than half of the CME rate during the solar minima $P_{22/23}$ and $P_{23/24}$; the strength and turbulence of IMF keep at relatively low levels, which causes an $\sim$10% increase in the estimated cosmic ray MFP ($\lambda_{\parallel}$) compared to that in late 2009. All these factors collectively contribute to less solar modulation and unusual increase in the GCR intensity.

This study is a pure statistical analysis of the GCR variations in the relatively low energy band (50–500 MeV/nuc). In the meanwhile, we discuss the unusual heliospheric conditions in the inner heliosphere in order to explain our findings of the record-breaking GCR intensities at 1 AU. How much of the heliospheric changes influence the GCR intensities needs to be studied. Further work includes the quantitative diagnosis of the effects of drifts, diffusion, and convection on the cosmic-ray modulation from the perspective of numerical simulation (e.g., Strauss & Potgieter, 2014; Zhao et al., 2014; Shen & Qin, 2018; Shen et al., 2019).