Methods for Tracking Cosmic-Ray Spectral Changes Using Neutron Monitors at High Cutoff Rigidity

<p>A Forbush decrease (FD) is the decrease in Galactic cosmic ray (GCR) flux, e.g., as observed by a neutron monitor count rate, in association with a coronal mass ejection (CME) and/or its shock. The FD amplitude is known to decrease at higher cutoff rigidity. During Solar Cycle 24, the Mawson neutron monitor in Antarctica, with a low (atmosphere-limited) cutoff rigidity of ~1 GV, observed numerous FDs, while the Princess Sirindhorn Neutron Monitor (PSNM) located near the Earth's equator at Doi Inthanon, Thailand, with the world’s highest geomagnetic cutoff rigidity (17 GV), observed only a fraction of these as FDs.  Instead, we find that the shock arrival is often followed by repeated dips in the PSNM count rate at only certain times of day, while the GCR flux from other directions remains near the pre-shock level.  We refer to decreases of this type as diurnal dips.  In this work, we have surveyed FDs and diurnal dip events observed by PSNM and the dependence of their maximum amplitude on the solar wind speed, ICME speed, and interplanetary magnetic field.  We acknowledge logistical support from Australia's Antarctic Program for operating the Mawson NM and support from the National Astronomical Research Institute of Thailand and grant RTA6280002 from Thailand Science Research and Innovation.</p>

Neutron monitors are the premier instruments for precise measurements of time variations (e.g., of solar origin) in the galactic cosmic ray (GCR) flux in the range of ∼1–100 GeV. However, it has proven challenging to accurately determine the yield function (effective area) versus rigidity in order to relate a neutron monitor's count rate with those of other monitors worldwide and the underlying GCR spectrum. Monte Carlo simulations of the yield function have been developed, but there have been few opportunities to validate these observationally, especially regarding the particular environment surrounding each monitor. Here we have precisely measured the count rate of a calibration neutron monitor near the Princess Sirindhorn Neutron Monitor (PSNM) at Doi Inthanon, Thailand (18.59∘N, 98.49∘E, 2560 m altitude), which provides a basis for comparison with count rates of other neutron monitors worldwide that are similarly calibrated. We directly measured the effect of surrounding structure by operating the calibrator outside and inside the building. Using Monte Carlo simulations, we clarify differences in response of the calibrator and PSNM, as well as the calibrator outside and inside the building. The dependence of the calibrator count rate on surrounding structure can be attributed to its sensitivity to neutrons of 0.5–10 MeV and a shift of sensitivity to nucleons of higher energy when placed inside the building. Simulated calibrator to PSNM count rate ratios inside and outside agree with observations within a few percent, providing useful validation and improving confidence in our ability to model the yield function for a neutron monitor station.

Neutron monitor counting rates and solar wind velocity, interplanetary magnetic field, sunspot number and total solar irradiance measurements from 2013 to 2018 corresponding to the end of solar maximum and the decreasing phase of the Solar Cycle 24 have been used. The main objective is to check whether the periodicities observed in the cosmic rays are affected by the magnetic rigidity or the height at which the neutron monitors are placed. A Global Neutron Monitor (GNM) has been defined as representative of the neutron monitor global network. This GNM is constructed by averaging the counting rates of a set of selected neutron monitors. The selection process is based on the combination of three new data quality criteria, which are applied to neutron monitors in the Neutron Monitor Data Base giving a final pool of 22 selected neutron monitors. Morlet wavelet analysis is applied to the GNM and the selected solar activity parameters to find the common periodicities. Short-term periodicities of 13.5, 27, 48, 92, 132 and 298 days have been observed in cosmic ray intensity. A clear inverse relationship between rigidity and spectral power has been obtained for the 13.5-, 48-, 92-, 132-day periods. A not so clear but still observed direct relationship between the height of the neutron monitors and the spectral power for the 48-, 92-, 132-day periods has been also found. The periodicity of 92 days is the one which shows the highest dependence with rigidity cutoff and height. As far as we know, this is the first time that these dependencies are reported. We think that these observations could be explained by assuming some cosmic ray intensity energy dependence in such periodicities and a competitive effect between rigidity and height.

The flux of galactic cosmic rays (GCR) at Earth is modulated in the heliosphere by the heliospheric magnetic field and solar wind, that vary in the course of solar cycle. The modulation is caused by diffusion, convection, adiabatic cooling and drifts, where very important is scattering of particles on magnetic inhomogeneities in the turbulent solar wind and heliospheric magnetic field (HMF). Turbulent variations are often characterized by a power law in the power spectrum of the cosmic-ray variations. Previous works revealed a power law behaviour in the power spectral density (PSD) of GCR fluxes in the frequency range between $5.56\cdot 10^{-6}$ and $2.14\cdot 10^{-6}$ Hz (50 – 130 hours) that varies with the solar cycle phases. Here we further develop these results by employing a broader and higher-quality dataset obtained from the world data repository (the WDCCR). Using data from 31 ground-based neutron monitors (NMs) spanning 65 years (nearly six solar cycles, 1953—2018) of measurements, we have studied the spectral slopes of the PSD of NM count rates. Using 2-year overlapping PSD intervals, we found that the spectral slope varies in time between approximately -1 and -2.6, with a mean value of $1.84\pm 0.01$. Separating the results to specific solar cycle phases, we found that steep slopes close to -2.0 (typical for a random walk) appear during solar maximum times, and flatter slopes close to -1.7 (Kolmogorov-type spectrum) appear during solar cycle minimum times, indicating that different physical processes determine GCR transport in the Heliosphere at different phases of the solar cycle. We note that these results are in close agreement with the earlier results that used a different dataset.

In April 2017, a mini neutron monitor (NM) was installed at King Abdulaziz City for Science and Technology (KACST) central Saudi Arabia (Riyadh; cut-off rigidity, Rc = 14.4 Gv) for continuous observation of the cosmic ray (CR) neutrons. The detector was built as a major aspect of the international scientific joint effort between the Centre of Space Research (North-West University, Potchefstroom, South Africa) and KACST. The recorded data correspond to low energy neutrons that primarily have energies lower than 20 GeV. In this paper, a brief description about the mini NM detector will be given. The influence of atmospheric pressure on the recorded CR neutrons was studied and the barometric coefficient was calculated and used to eliminate the pressure effects from the measured data. The obtained coefficient was consistent with those previously obtained by several investigators. The daily variation of the CR neutron was studied and characterized. Short-term CR periodicities, such as the 27-day period, and its two harmonics, were identified. The obtained periodicities are in agreement with those reported by different researchers. The obtained results from this detector have been compared to the existing 1 m2 scintillator detector showing comparable results. Long-term data from this detector will be of incredible significance to the research community to investigate several types of CR variations resulting from solar activity at such high cut off rigidity site.

The Baksan neutron monitor (NM) is installed at the Baksan neutrino observatory, the Northern Caucasus, which is located at the bottom of the Baksan gorge, at height of 1700 m above sea level. The cosmic rays flux recorded at the ground level depends on the amount of the substance (air), through which the particles are passing, from the uppermost layers of the atmosphere to a cosmic ray detector. So, besides the cosmic rays flux data, the station records pressure; the pressure meter recordings interval is 1 minute, like that of the cosmic rays. The perennial barometric data have been analysed to show that at the Baksan station one can often observe a daily pressure variation that is related to the topography features. In general, the local conditions (wind, local orographic effect) result in pressure variations, which do not occur in the cosmic rays, because they do not change the thickness of the atmosphere but result from the effect of the ground level. The barometric variation discovered is not so big (about 1 mb), but is also synchronously observed in the cosmic rays. It means that the pressure variation is not a local phenomenon. In this case, the NM detects the amount of the substance in the atmosphere, showing that pressure changes are not due to dynamic reasons (the Bernoulli effect) and it is the atmosphere strata that changes in reality. Hence, the orographic effect covers a significant part of the troposphere, resulting in the change of the atmosphere strata located over the NM. No similar pressure variation is observed in other NMs.

Abstract. The diurnal variation (DV) in galactic cosmic ray (GCR) flux is a widely observed phenomenon in neutron monitor data. The background variation considered primarily in this study is due to the balance between the convection of energetic particles away from the Sun and the inward diffusion of energetic particles along magnetic field lines. However, there are also times of enhanced DV following geomagnetic disturbances caused by coronal mass ejections or corotating interaction regions. In this study we investigate changes in the DV over four solar cycles using ground-based neutron monitors at different magnetic latitudes and longitudes at Earth. We divide all of the hourly neutron monitor data into magnetic polarity cycles to investigate cycle-to-cycle variations in the phase and amplitude of the DV. The results show, in general, a similarity between each of the A < 0 cycles and A > 0 cycles, but with a phase change between the two. To investigate this further, we split the neutron monitor data by solar magnetic polarity between times when the dominant polarity was either directed outward (positive) or inward (negative) at the northern solar pole. We find that the maxima and minima of the DV changes by, typically, 1–2 h between the two polarity states for all non-polar neutron monitors. This difference between cycles becomes even larger in amplitude and phase with the removal of periods with enhanced DV caused by solar wind transients. The time difference between polarity cycles is found to vary in a 22-year cycle for both the maximum and minimum times of the DV. The times of the maximum and minimum in the DV do not always vary in the same manner between A > 0 and A < 0 polarity cycles, suggesting a slight change in the anisotropy vector of GCRs arriving at Earth between polarity cycles. Polar neutron monitors show differences in phase between polarity cycles which have asymptotic directions at mid-to-high latitudes. All neutron monitors show changes in the amplitude of the DV with solar polarity, with the amplitude of the DV being a factor of 2 greater in A < 0 cycles than A > 0 cycles. In most cases the change in timing of the maximum /minimum is greatest with the stations' geomagnetic cut-off rigidity shows little variation in the DV phase with latitude. We conclude that the change in the DV with the dominant solar polar polarity is not as simple as a phase change, but rather an asymmetric variation which is sensitive to the neutron monitor's asymptotic viewing direction.

Galactic cosmic rays (CRs) entering the heliosphere are affected by interplanetary magnetic fields and solar wind disturbances resulting in the modulation of the CR total flux observed in the inner heliosphere. The so‐called force field model is often used to compute the galactic CR spectrum modulated by the solar activity due to the fact that it characterizes this process by only one parameter (the modulation potential, ϕ). In this work, we present two types of an empirical simplification (ES) method used to reconstruct the time variation of the modulation potential (Δϕ). Our ES offers a simple and fast alternative to compute the Δϕ at any desired time. The first ES type is based on the empirical fact that the dependence between Δϕ and neutron monitor (NM) count rates can be parameterized by a second‐degree polynomial. The second ES type is based on the assumption that there is a inverse relation between Δϕ and NM count rates. We report the parameters found for the two types, which may be used to compute Δϕ for some NMs in a very fast and efficient way. In order to test the validity of the proposed ES, we compare our results with Δϕ obtained from literature. Finally, we apply our method to obtain the proton and helium spectra of primary CRs near the Earth at four randomly selected times.

Cosmic ray variations due to changes in the magnetosphere are evaluated for severe magnetic storm on 20 November 2003 using data from the worldwide neutron monitor network and the global survey method. From these results the changes in the planetary distribution of magnetic cutoff rigidities during this disturbed period are obtained in dependence of latitude. A correlation between Dst index and cutoff rigidity variations was defined for each cosmic ray station. The maximum changes in cutoff rigidities occurred while Dst index was around −472 nT. Geomagnetic effect in cosmic ray intensity reached at some stations 6–8%, and it seems to be the greatest one over the history of neutron monitor observations. The latitudinal distribution shows a maximum changes at geomagnetic cutoff rigidities around 7–8 GV. This corresponds to unusually low latitudes for maximal effect. Cutoff rigidity variations were also calculated utilizing the last model of Tsyganenko for a disturbed magnetosphere (T01S). A comparison between experimental and modeling results revealed a big discrepancy at cutoff rigidities less than 6 GV. The results on the geomagnetic effect in cosmic rays can be used for validating magnetospheric field models during very severe storms.

Effective non-vertical and apparent cutoff rigidities for a cosmic ray latitude survey from Antarctica to Italy in minimum of solar activity

The pressure corrected hourly data from the global network of cosmic ray detectors, measurements of the interplanetary magnetic field (IMF) intensity (B) at Earth’s orbit and its components B x , B y , B z (in the geocentric solar ecliptic coordinates) are used to conduct a comprehensive study of the galactic cosmic ray (GCR) intensity fluctuations caused by the halo coronal mass ejection of 13 May 2005. Distinct differences exist in GCR timelines recorded by neutron monitors (NMs) and multidirectional muon telescopes (MTs), the latter respond to the high rigidity portion of the GCR differential rigidity spectrum. The Forbush decrease (FD) onset in MTs is delayed (∼5 h) with respect to the onset of a geomagnetic storm sudden commencement (SSC) and a large pre-increase is present in MT data before, during, and after the SSC onset, of unknown origin. The rigidity spectrum, for a range of GCR rigidities (≤200 GV), is a power law in rigidity (R) with a negative exponent (γ=−1.05) at GCR minimum intensity, leading us to infer that the quasi-linear theory of modulation is inconsistent with observations at high rigidities (>1 GV); the results support the force field theory of modulation. At present, we do not have a comprehensive model for the FD explaining quantitatively all the observational features but we present a preliminary model listing physical processes that may contribute to a FD timeline. We explored the connections between different phases of the FD and the power spectra of IMF components but did not find a sustained relationship.

Cosmic Ray Neutron Sensing (CRNS) is a technique to measure water content, for example soil moisture, on the hectare scale through the measurementof epithermal neutrons. The neutrons are results of particle showers in the earth’s atmosphere caused by cosmic rays impinging on it. The abundance andglobal distribution of neutrons is changed in time through different factors. On the largest scale, the heliosphere and therefore the solar cycle greatly affect the amount of galactic cosmic rays that are able to reach earth. Large solar events, such as Forbush decreases, also cause rapid changes in the cosmic ray flux. The aim of any correction method is ultimately to account for these heliospheric changes. The next smaller scale is the earth’s magnetic field. The biggest consequence here is an uneven distribution of neutrons across latitudes, as with increasing cut-off rigidity less cosmic rays are able to reach the atmosphere. This presents a challenge to correction methods based on neutron monitors, since any neutron measurement is always localised to specific geomagnetic and atmospheric conditions. Lastly, the abundance of neutrons at different energy bands is strongly tied to the local conditions. In particular hydrogen abundance is of vital importance. The CRNS technique is based upon the sensitivity of neutrons to hydrogen, which is dependent on the energy of any given neutron. As these neutrons are still dependent on heliospheric and geomagnetic conditions, their measurement needs to be corrected for the amount and variation in incoming radiation. Neutron monitor based correction methods are tackling two challenges at the same time. Firstly, a neutron signal, free of local interference, needs to be extracted from the neutron monitor data. This signal is representative of the heliospheric conditions localised through the geomagnetic conditions to the neutron monitor site. Secondly, the signal needs to be localised to the point of the CRNS measurement, which rarely coincides with the position of a neutron monitor. Multiple correction methods have been evaluated and compared, with consideration towards both challenges.

We study 27-day variations of the galactic cosmic ray (GCR) intensity for 2005–2008 period of the solar cycle #23. We use neutron monitors (NMs) data corrected and uncorrected for geomagnetic disturbances. Besides the limited time intervals when the 27-day variations are clearly established, always exist some feeble 27-day variations in the GCR intensity related to the constantly present weak heliolongitudinal asymmetry in the heliosphere. We calculate the amplitudes of the 27-day variation of the GCR intensity based on the NMs data corrected and uncorrected for geomagnetic disturbances. We show that these amplitudes do not differ for NMs with cut-off rigidities smaller than 4-5 GV comparing with NMs of higher cut-off rigidities. Rigidity spectrum of the 27-day variation of the GCR intensity found in the uncorrected data is soft while it is hard in the case of the corrected data. For both cases exists definite tendency of softening the temporal changes of the 27-day variation's rigidity spectrum in period of 2005 to 2008 approaching the minimum of solar activity. We believe that a study of the 27-day variation of the GCR intensity based on the data uncorrected for geomagnetic disturbances should be carried out by NMs with cut-off rigidities smaller than 4-5 GV.

Diurnal anisotropy of polar neutron monitors: Dome C looks poleward

The original design by J. A. Simpson of the neutron monitor enabled continuous monitoring of the primary cosmic-ray flux by ground-based recordings of the nucleonic component with only a rather simple correction for atmospheric effects. Simpson (1957) extended the original pile to the 12 counter IGY neutron monitor which was deployed in a world wide network during the International Geophysical Year 1957/8. The desirability for monitors with higher counting rates became evident soon afterwards. Subsequently the NM64 super neutron monitor was designed by H. Carmichael for deployment in time for the International Quiet Sun Year 1964. Using unusually large 10BF3 proportional counters made at Chalk River, Hatton and Carmichael (1964) studied comprehensively the experimental design of the NM64. Consequently the efficiency of neutron counters to record evaporation neutrons produced in the lead of a monitor increased from 1.9% for the IGY to 5.7% for the NM64, an increase of 3.3 times the counting rate per unit area of lead producer. During the years much attention was given to the neutron multiplicity spectrum in neutron monitors. This spectrum is related to the energy spectrum of the nucleonic component incident on the neutron monitor, but is only weakly dependent on the spectrum of galactic cosmic rays at the top of the atmosphere. Contrary to galactic cosmic rays, solar flare protons and neutrons are observed predominantly as single counts per interaction, in multiplicity 1, because of the softness of solar flare particle energy spectra. Neutron monitors have also been specially designed to record solar neutrons with increased sensitivity. Newly developed 3He counters with a largely reduced thermal neutron absorption mean free path should lead to improved efficiency in recording primary cosmic radiation. Design criteria are discussed.