Recent Solar Cycle Research Articles

Comparison of Solar Multifrequency Microwave Data with Other Solar Indices for Understanding Solar and Stellar Microwave Data

Thermal microwave emissions detected from stellar atmospheres contain information on stellar activity. However, even for the Sun, the relationship between multifrequency microwave data and other activity indices remains unclear. We investigated the relationships among the thermal microwave fluxes with 1, 2, 3.75, and 9.4 GHz, their circular polarizations, and several activity indices recorded during recent solar cycles and observed that these relationships can be categorized into two groups. In the first group, the relationship between the microwave fluxes and solar indices, which are strongly related to the active regions, can be well-fitted by using a linear function. In the second group, the fitting function is dependent on frequency. Specifically, the microwave fluxes at 1 and 2 GHz can be well-fitted to the total unsigned magnetic and extreme ultraviolet fluxes by employing a power-law function. The trend changes around 3.75 GHz, and the trend for the 9.4 GHz fluxes can be fitted by using a linear function. For the first time, we present the relationship between circular polarization and solar indices. Moreover, we extrapolated these relationships of the solar microwave fluxes to higher values and compared them with the solar-type stars. We found that ϵ Eri, whose microwave emission originates from thermal plasma, follows the extrapolated relationship. However, to date, only one star’s emission at 1–10 GHz has been confirmed as thermal emission. More solar-type stars should be observed with future radio interferometers to confirm that relationships based on solar data can be applied to stellar microwave data.

Estimation of Radiation Exposure for Various Flights from Athens International Airport

In recent decades, the exposure of aviation crews and passengers to cosmic radiation has been progressively increasing due to the growing need of flights. Therefore, there is a need for radiation assessment software tools to act as a complement to other radiation protection techniques and countermeasures. In this work, the exposure to cosmic radiation is estimated for flights from Athens International Airport to various international destinations, by performing Monte Carlo simulations with the validated tool DYASTIMA/DYASTIMA-R. The results of the estimated total ambient dose equivalent, as well as the ambient dose equivalent rate for different flights, applying a typical flying level and constant atmospheric conditions, are presented for the first time. This study is carried out for different phases of solar activity for the time period from 1996 to 2019, which includes the two recent Solar Cycles 23 and 24.

Greenhouse gas effects on the solar cycle response of water vapour and noctilucent clouds

Abstract. The responses of water vapour (H2O) and noctilucent clouds (NLCs) to the solar cycle are studied using the Leibniz Institute for Middle Atmosphere (LIMA) model and the Mesospheric Ice Microphysics And tranSport (MIMAS) model. NLCs are sensitive to the solar cycle because their formation depends on background temperature and the H2O concentration. The solar cycle affects the H2O concentration in the upper mesosphere mainly in two ways: directly through the photolysis and, at the time and place of NLC formation, indirectly through temperature changes. We found that H2O concentration correlates positively with the temperature changes due to the solar cycle at altitudes above about 82 km, where NLCs form. The photolysis effect leads to an anti-correlation of H2O concentration and solar Lyman-α radiation, which gets even more pronounced at altitudes below ∼ 83 km when NLCs are present. We studied the H2O response to Lyman-α variability for the period 1992 to 2018, including the two most recent solar cycles. The amplitude of Lyman-α variation decreased by about 40 % in the period 2005 to 2018 compared to the preceding solar cycle, resulting in a lower H2O response in the late period. We investigated the effect of increasing greenhouse gases (GHGs) on the H2O response throughout the solar cycle by performing model runs with and without increases in carbon dioxide (CO2) and methane (CH4). The increase of methane and carbon dioxide amplifies the response of water vapour to the solar variability. Applying the geometry of satellite observations, we find a missing response when averaging over altitudes of 80 to 85 km, where H2O has a positive response and a negative response (depending on altitude), which largely cancel each other out. One main finding is that, during NLCs, the solar cycle response of H2O strongly depends on altitude.

Performance of the IRI-2016 and IRI-Plas 2020 considering Mg II as EUV solar proxy

To assess upper atmosphere parameters, current models use different variables to account for solar activity levels. Studies that evaluate the most recent solar cycles have shown that when dealing with the ionospheric F2 layer, and in particular with foF2, the Mg II index is a better choice than other solar activity proxies to account for solar EUV variation. The International Reference Ionosphere, version 2016 (IRI-2016), a model widely used by the scientific community, uses the Ionosonde Global (IG) index to assess foF2. A most recent and expanded version, IRI-Plas, allows selecting a solar activity proxy among 7 options including IG and Mg II. In the present work, the performance of IRI considering Mg II to assess foF2 is analyzed, comparing three modeling cases: IRI-2016 with its default solar activity index (IG), IRI-2016 with Mg II instead of IG, and IRI-Plas choosing Mg II as the solar proxy option. This model comparison was evaluated in terms of their agreement with measured foF2 data from four stations located in Japan (Northern Hemisphere), and four in Australia (Southern Hemisphere). Monthly medians for the 24 h, along two periods of three years were considered: 2000–2002 and 2008–2010, which correspond to epochs of maximum and minimum solar activity levels, respectively. Based on goodness-of-fit statistics, our results show that in the case of IRI-2016, the Mg II implementation does not reveal a significant improvement for foF2 assessment, and the IRI-Plas performance with the Mg II option is only slightly better than the other two options. Dispersion diagrams between measured and modeled foF2 also evince a similar performance between the three models. All models show the same over- and underestimation patterns, except in the nighttime case during the minimum activity level period for both Australian and Japanese stations where IRI-Plas outperforms the other two model options. The comparable performances of the three options might be attributed to a stronger variation in the evaluated periods which could be related to seasonal changes rather than to solar EUV proxies. Future studies considering time series over several years, covering at least a complete solar activity cycle, are required to determine stronger model performance differences.

Preliminary Modelling of Solar Quiet Geomagnetic Field Average using Non-Linear Autoregressive with Exogeneous Input (NARX)

This paper discusses geomagnetic field attempt modelling using an Artificial Neural Network (ANN). The local horizontal component of geomagnetic field data was collected on April 2011 (equinox) during a solar quiet day at recent solar cycle inclination-24 using the Magnetic Data Acquisition System (MAGDAS) in Langkawi, Malaysia, in the low latitude region. The calculated average values (mean) of the H component geomagnetic field variation during Equinox 2011 characterised the dominant geomagnetic field during that particular solar cycle. The difference in amplitude of maximum and minimum values shows a regular diurnal variation of the geomagnetic field during Sq inthe low latitude region. The output training utilised these calculated mean values during the modelling attempt. Meanwhile, the input training utilised proton density, solar wind plasma speed, plasma flow pressure, and Interplanetary Magnetic Field (IMF) space data using Non-Linear Auto Regressive Input (NARX). The best modelling outputs were 3, 2, and 3 for input delay (𝑛𝑢), output delay (𝑛y), and hidden layer size (h), respectively. Residual test and model fit analysis show an unbiased and high overlap between predicted and actual geomagnetic field average values, suggesting the model can potentially anticipate the geomagnetic field average during solar quiet in November (month of Equinox) on solar cycle inclination.

Solar cycle and seasonal variability of H in the upper atmosphere of Mars

The complex seasonal, dust, and atmospheric circulation patterns at Mars affect the abundance and spatial extent of water and its photo-dissociated atomic byproduct H. Recent observations by the Mars Atmosphere and Volatile Evolution (MAVEN) mission have been used to constrain the H abundance at the exobase (200 km altitude) as well as its thermal and global escape rate. The MAVEN observations span the last seven years and encompass the effects of the declining phase of Solar Cycle 24 and activating phase of Solar Cycle 25 from mid-perihelion of Mars Year (MY) 32 through most of aphelion of MY36. Results show that there are seasonal as well as solar cycle variations in H exobase temperatures, densities, thermal escape rates, and global escape rates. During perihelion of MYs 32–35, the H exobase density fluctuates with a cadence that corresponds to the approach to perihelion as well as the onset and peak of regional dust storms. Additionally, the abundance of H atoms did not show exceptional variability during the MY34 Global Dust Storm, when compared with trends from adjacent years. Seasonal averages of H exobase densities, thermal escape rates, and global escape rates indicated an order of magnitude difference between perihelion and aphelion, and suggest a water global equivalent layer range of 0.02–13 m lost over 4 billon years. H properties are also found to vary with solar cycle to a lesser extent than with seasons across the MAVEN timeline. Trends show that the exobase density varies inversely with solar activity, while both the thermal and global escape rates vary directly with solar activity. It remains to be seen if the effects of a stronger than recent solar cycle on the upper atmospheric H properties would become more pronounced.

Estimation of Cosmic-Ray-Induced Atmospheric Ionization and Radiation at Commercial Aviation Flight Altitudes

The main source of the ionization of the Earth’s atmosphere is the cosmic radiation that depends on solar activity as well as geomagnetic activity. Galactic cosmic rays constitute a permanent radiation background and contribute significantly to the radiation exposure inside the atmosphere. In this work, the cosmic-ray-induced ionization of the Earth’s atmosphere, due to both solar and galactic cosmic radiation during the recent solar cycles 23 (1996–2008) and 24 (2008–2019), was studied globally. Estimations of the ionization were based on the CRAC:CRII model by the University of Oulu. The use of this model allowed for extensive calculations from the Earth’s surface (atmospheric depth 1033 g/cm2) to the upper limit of the atmosphere (atmospheric depth 0 g/cm2). Monte Carlo simulations were performed for the estimation quantities of radiobiological interest with the validated software DYASTIMA/DYASTIMA-R. This study was focused on specific altitudes of interest, such as the common flight levels used by commercial aviation.

Sunspot numbers and proton events in solar cycles 19 to 24

In this study we compare the mean annual sunspot numbers for the six most recent solar cycles (19 to 24) with the number of solar energetic proton events occurring in those years. Though most of the individual cycles exhibit quite strong correlations, when they are combined into a generic solar cycle the result is very significant (ρ = 0.98). For cycles 21 to 24 (when spacecraft observations have been regularly available), we also investigate the variation in the source location of the solar energetic proton events in relation to four peak flux thresholds in orders of magnitude from ≥10 to ≥10,000 pfu. For helio-latitudes within ±40° (the range within which active regions usually occur) there is negligible variation in the helio-longitude of the source regardless of the peak flux threshold. However, the effect on the helio-longitude of varying the peak flux threshold is very significant: the higher the threshold the closer is the median source longitude to 0°, the variation being almost uniform from ≥10 pfu (32°W) to ≥10,000 pfu (2°W). Finally, of the 10 events in cycles 19 to 24 with peak flux ≥10,000 pfu, all occurred between years 4 and 8 after solar minimum, all were the result of M- or X-class flares with associated halo CMEs and shock fronts, and the three most intense events (≥40,000 pfu) were all related to X-class flares which occurred in the longitude range 10–28°E. These results support and extend previous work by the cited authors, and have implications for solar-terrestrial relations and the effects of space weather within geospace.

Radiation Exposure in the Lower Atmosphere during Different Periods of Solar Activity

In recent years, there has been a huge increase in air travel, both for business and leisure. For this reason, entities such as the European Commission and the International Committee on Radiological Protection have provided several recommendations for the radiation protection of aviation crews and frequent flyers, as well as highlighted the need for accurate tools for radiation assessment in the atmosphere. With a focus on the most frequent commercial flying altitudes, this work has performed dosimetry calculations in the lower atmosphere of Earth for different values of cut-off rigidity, covering the recent solar cycles 23 and 24. Results are based on Monte Carlo simulations performed with the validated Geant4 software application Dynamic Atmospheric Shower Tracking Interactive Model Application (DYASTIMA) and its extension, DYASTIMA-R.

Disturbances during a geomagnetic storm: A case study on 7 October 2015

In this study, we present the detailed study of the geomagnetic storm that occurred 6-10 October 2015. In literature this storm was classified as a two-step storm due to the observation of two large decreases of the disturbance storm time (Dst) index during the main storm phase. In addition, the strong thermal emission velocity enhancement (STEVE) event occurred during the same storm phase was photographically documented by citizen scientists at Minnesota Nevis (USA). The storm of 7 October 2015, which exhibits high-intensity, long-duration, continuous AE activity (HILDCAA), is one of the strongest storms of the recent 24th solar cycle. We examined changes in Dst index, interplanetary magnetic field (IMF) Bz, flow velocity, proton density, solar wind pressure, epsilon parameters and presented the discussion of the physical mechanism happened during this geomagnetic storm. The results obtained for the storm on 7 October 2015 were also compared with geomagnetic storms occurred on 8 March 2008, 25 July 2016 and 8 September 2017.

A statistical analysis of X- class solar flares and their association with the super active regions during solar cycles 23- 24

The A statistical distribution function of X-class flares and their association with super-active regions (SARs) during two recent solar cycles 23-24 have been analyzed. The obtained results show that, during solar cycle 23, 57.60 % of the total number of Xclass flares coincides with a set of X1.0 - X1.9 flares, 15.20 % to X2.0 - X2.9, 12.80 % to X3.0 - X4.9, 9.60 % to X5.0 - X9.9, and 4.80 % to ≥ X10 flares. During solar cycle 24, 61.20 % of the total number of X-class flares coincides with a set of X1.0 - X1.9 flares, 22.40 % to X2.0 - X2.9, 8.16 % to both the X3.0 - X4.9 flares, and X5.0 - X9.9 flares. None of the ≥ X10 flares. All X-class flares are found to be in a scattered manner in solar cycles 23 and 24. Among the cycles, solar cycle 24 has been observed relatively very weak cycle and the total number of X-class flares diminished in comparison to previous solar cycles. During solar cycle 23, 29.16% of the X1.0 - X1.9 flares, 36.84 % of the X2.0 - X2.9 flares, 43.75 % of the X3.0 - X4.9 flares, 83.33 % of both the X5.0 - X9.9 flares, and 100.00 % of the ≥ X10 flares were produced by SARs. While during solar cycle 24, 20.00 % of the X1.0 - X1.9 flares, 36.36 % of the X2.0 - X2.9 flares, 25.00 % of the X3.0 - X4.9 flares, 75.00 % of the X5.0 - X9.9 flares, and 0.00 % of the ≥ X10 flares were produced by SARs.

Variations of the Galactic Cosmic Rays in the Recent Solar Cycles

Abstract In this paper, we study the Galactic cosmic-ray (GCR) variations over the solar cycles 23 and 24, with measurements from NASA’s Advanced Composition Explorer/Cosmic Ray Isotope Spectrometer instrument and the ground-based neutron monitors (NMs). The results show that the maximum GCR intensities of heavy nuclei (5 ≤ Z ≤ 28, 50∼500 MeV nuc−1) at 1 au during the solar minimum in 2019–2020 break their previous records, exceeding those recorded in 1997 and 2009 by ∼25% and ∼6%, respectively, and are at the highest levels since the space age. However, the peak NM count rates are lower than those in late 2009. The difference between GCR intensities and NM count rates still remains to be explained. Furthermore, we find that the GCR modulation environment during the solar minimum P 24/25 are significantly different from previous solar minima in several aspects, including remarkably low sunspot numbers, extremely low inclination of the heliospheric current sheet, rare coronal mass ejections, weak interplanetary magnetic field and turbulence. These changes are conducive to reduce the level of solar modulation, providing a plausible explanation for the record-breaking GCR intensities in interplanetary space.

ASHLEY: A New Empirical Model for the High‐Latitude Electron Precipitation and Electric Field

AbstractIn this study, a new high‐latitude empirical model is introduced, named for Auroral energy Spectrum and High‐Latitude Electric field variabilitY (ASHLEY). This model improves specifications of soft electron precipitations and electric field variability that are not well represented in existing high‐latitude empirical models. ASHLEY consists of three components, ASHLEY‐A, ASHLEY‐E, and ASHLEY‐Evar, which are developed based on the electron precipitation and bulk ion drift measurements from the Defense Meteorological Satellite Program (DMSP) satellites during the most recent solar cycle. On the one hand, unlike most existing high‐latitude electron precipitation models, which have assumptions about the energy spectrum of incident electrons, the electron precipitation component of ASHLEY, ASHLEY‐A, provides the differential energy fluxes in the 19 DMSP energy channels under different geophysical conditions without making any assumptions about the energy spectrum. It has been found that the relaxation of spectral assumptions significantly improves soft electron precipitation specifications with respect to a Maxwellian spectrum (up to several orders of magnitude). On the other hand, ASHLEY provides consistent mean electric field and electric field variability under different geophysical conditions by ASHLEY‐E and ASHLEY‐Evar components, respectively. This is different from most existing electric field models which only focus on the large‐scale mean electric field and ignore the electric field variability. Furthermore, the consistency between the electric field and electron precipitation is better taken into account in ASHLEY.

Strong evidence of low levels of solar activity during the Maunder Minimum

ABSTRACT The Maunder Minimum (MM) was a period of prolonged solar activity minimum between 1645 and 1715. Several works have identified a significant number of problematic spotless days in the MM included in existing data bases. We have found a list of exact spotless (in the second half of 1709) and spot days (January and August 1709) provided by Johann Heinrich Müller. We computed the most probable value and upper/lower limits of the active day fraction (ADF) from Müller's data using the hypergeometrical probability distribution. Our sample is not strictly random because Müller recorded observations in consecutive days when he observed sunspots. Therefore, our result represents an upper threshold of solar activity for 1709. We compared this result with annual values of the ADF calculated for the Dalton Minimum and the most recent solar cycles. We concluded that, although 1709 is one of the most active years in the MM, it was less active than most years both in the Dalton Minimum and in the most recent solar cycles. Therefore, the solar activity level estimated in this work for 1709 represents robust evidence of low solar activity levels in the MM.

The Solar Corona during the Total Eclipse on 1806 June 16: Graphical Evidence of the Coronal Structure during the Dalton Minimum

Abstract Visible coronal structure, in particular the spatial evolution of coronal streamers, provides indirect information about solar magnetic activity and the underlying solar dynamo. Their apparent absence of structure observed during the total eclipses throughout the Maunder minimum has been interpreted as evidence of a significant change in the solar magnetic field from that during modern solar cycles. Eclipse observations available from the more recent Dalton minimum may be able to provide further information, with sunspot activity being between the levels seen during recent solar cycles and in the Maunder minimum. Here, we show and examine two graphical records of the total solar eclipse on 1806 June 16, during the Dalton minimum. These records show significant rays and streamers around an inner ring. The ring is estimated to be ≈0.44 R ⊙ in width and the streamers in excess of 11.88 R ⊙ in length. In combination with records of spicules or prominences, these eclipse records visually contrast the Dalton minimum with the Maunder minimum in terms of their coronal structure and support the existing discussions based on the sunspot observations. These eclipse records are broadly consistent with the solar cycle phase in the modeled open solar flux and the reconstructed slow solar wind at most latitudes.

Solar 11-Year Cycle Signal in Stratospheric Nitrogen Dioxide—Similarities and Discrepancies Between Model and NDACC Observations

NOx (NO2 and NO) plays an important role in controlling stratospheric ozone. Understanding the change in stratospheric NOx and its global pattern is important for predicting future changes in ozone and the corresponding implications on the climate. Stratospheric NOx is mainly produced by the reaction of N2O with the photochemically produced O(1D) and, therefore, it is expected to vary with changes in solar UV irradiance during the solar cycle. Previous studies on this topic, often limited by the relatively short continuous data, show puzzling results. The effect of the 1991 Pinatubo eruption might have caused interference in the data analysis. In this study, we examine the NO2 vertical column density (VCD) data from the Network for the Detection of Atmospheric Composition Change (NDACC). Data collected at 16 stations with continuous long-term observations covering the most recent Solar Cycles 23 and 24 were analyzed. We found positive correlations between changes in NO2 VCD and solar Lyman- $\alpha $ over nine stations (mostly in the Northern Hemisphere) and negative correlations over three stations (mostly in the Southern Hemisphere). The other four stations do not show significant NO2 solar-cycle signal. The varying NO2 responses from one location to another are likely due to different geo-locations (latitude and altitude). In particular, two high-altitude stations show the strongest positive NO2 solar-cycle signals. Our 1D chemical-transport model calculations help explain the altitude dependence of NO2 response to the solar cycle. NO2 solar-cycle variability is suggested to play an important role controlling O3 at an altitude range from $\approx20~\mbox{km}$ to near 60 km, while OH solar-cycle variability controls O3 at 40 – 90 km. While observations show both positive and negative NO2 responses to solar forcing, the 1D model predicts negative NO2 responses to solar UV changes throughout the middle atmosphere. 3D global model results suggest complex roles of dynamics in addition to photochemistry. The energetic particle-induced NO2 variabilities could also contribute significantly to the NO2 variability during solar cycles.

Characteristics of probability distribution functions of low- and high-latitude current systems during Solar Cycle 24

Recent solar cycles (SCs) 21–24 have experienced a gradual decrease in their activity with considerable weakening during current SC 24. This is a unique opportunity to examine the long-term response of Earth’s low-latitude ring-current and high latitude auroral electrojet current systems during such systematically decreasing solar activity. With the advancement in technology, continuous recordings of ground/space magnetic field are available for the last few decades that allow us to explore the behaviour of probability distribution functions (PDFs) linked with the ring-current and auroral electrojet current systems for past five SCs (20–24). Also, PDFs linked with solar wind parameters that drive these current systems like magnetic field and velocity at Earth’s bow shock are examined. We noticed the significant narrowing of PDF of ring-current and auroral electrojet during SC 24. The number of one-hour intervals with Dst<-150 nT are less than 600 for SCs 20–23, which constitutes less than 0.7% of respective PDF, and number of one-hour intervals with Dst<-250 nT are less than 100 for SCs 20–23, which corresponds to less than 0.1% of respective PDF. But for SC 24 the Dst<-150 nT encountered only for 58 h, which corresponds to 0.06% of PDF and there are no intervals when Dst was <−250 nT. For auroral electrojet, the number of one-hour intervals with AE>750 nT and AE>1500 nT are less than 3060 and 70, respectively for SCs 20–23, which corresponds to <4% and <0.06% of respective PDFs. But for SC 24 the AE>750 nT encountered only for 1398 h, which corresponds to 1.7% of PDF and there are only 9 intervals when AE increased above 1500 nT, which is 0.01% of PDF. It implies that the probability of intense ring-current and auroral electrojet current during SC 24 was unusually low. Such narrowing is seen in PDFs of the interplanetary magnetic field and solar wind velocity as well. This fair quiet space weather experienced during SC 24 is attributed to the weakening of solar activity, which has subsequently influenced the strength of the interplanetary magnetic field and solar wind velocity at Earth’s bow shock.

SuperDARN Observations During Geomagnetic Storms, Geomagnetically Active Times, and Enhanced Solar Wind Driving

AbstractThe Super Dual Auroral Radar Network (SuperDARN) was built to study ionospheric convection at Earth and has in recent years been expanded to lower latitudes to observe ionospheric flows over a larger latitude range. This enables us to study extreme space weather events, such as geomagnetic storms, which are a global phenomenon, on a large scale (from the pole to magnetic latitudes of 40°). We study the backscatter observations from the SuperDARN radars during all geomagnetic storm phases from the most recent solar cycle and compare them to other active times to understand radar backscatter and ionospheric convection characteristics during extreme conditions and to discern differences specific to geomagnetic storms and other geomagnetically active times. We show that there are clear differences in the number of measurements the radars make, the maximum flow speeds observed, and the locations where they are observed during the initial, main, and recovery phase. We show that these differences are linked to different levels of solar wind driving. We also show that when studying ionospheric convection during geomagnetically active times, it is crucial to consider data at midlatitudes, as we find that during 19% of storm time the equatorward boundary of the convection is located below 50° of magnetic latitude.

The Periodic and Temporal Behaviors of Solar X-Ray Flares in Solar Cycles 23 and 24

Abstract The recent solar cycles 23 and 24 are thought to be unusually quiet with more prolonged periods. Both sunspot groups and solar soft X-ray (SXR) flares show a decline in numbers comparable to cycles 21 and 22. In order to understand the periodic variations and distributions of solar flares with the sunspot group numbers in these cycles, this study investigates the periodicities and distributions of the solar SXR flares, 36354 in total (B (13351) 36.7%, C (20699) 56.9%, M (2141) 5.9%, and X (163) 0.45%) between 1996 July and 2016 December. The continuous wavelet transforms and cross-correlation methods were used to carry out the study. First, we found that the B- class flare number is asynchronous and negatively correlated with the sunspot group numbers. The flare class exhibits deviation when sunspot group numbers rise or fall around 100. The other class flares are in phase, synchronous with the sunspot group numbers. The C-class flares show the highest level of correlation of 0.868 with sunspot group numbers. A different number of short and intermediate periodicities was also noticed among the flare classes. Notable 256-day periodicity is found for B flares, 7, 64, and 300 days for C, 32, 64, and 256 days for M flares, and 40, 32, and 25 days for X flares. The difference in periodic variations of the flare classes could be attributed to the magnetic flux system of sunspot groups producing them.

The Behaviour of Galactic Cosmic-Ray Intensity During Solar Activity Cycle 24

We have studied long-term variations of galactic cosmic-ray (GCR) intensity in relation to the sunspot number (SSN) during the most recent solar cycles. This study analyses the time lag between the GCR intensity and SSN, and hysteresis plots of the GCR count rate against SSN for Solar Cycles 20 – 23, to validate a methodology against previous results in the literature, before applying the method to provide a timely update on the behaviour of Cycle 24. Plots of SSN versus GCR show a clear difference between the odd- and even-numbered cycles. Linear and elliptical models have been fit to the data, with the linear fit and elliptical model proving the more suitable model for even- and odd-numbered solar-activity cycles, respectively, in agreement with previous literature. Through the application of these methods for Solar Cycle 24, it has been shown that Cycle 24 experienced a lag of two to four months between the GCR intensity and SSN, and this follows the trend of the preceding activity cycles, albeit with a slightly longer lag than previous even-numbered cycles. It has been shown through the hysteresis analysis that the linear fit is a better representative model for Cycle 24, as the ellipse model does not show a significant improvement, which is also in agreement with previous even-numbered cycles.