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Observation of high energy and very high emission from Gamma Ray Bursts (GRBs) is crucial to study the gigantic explosion and the underline processes. With a large field-of-view and almost full duty cycle, the Water Cherenkov Detector Array (WCDA), a sub-array of the Large High Altitude Air Shower Observatory (LHAASO), is appropriate to monitor the very high energy emission from unpredictable transients such as GRBs. Nevertheless, the main issue for an extensive air shower array is the high energy threshold which limits the horizon of the detector. To address this issue a new trigger method is developed in this article to lower the energy threshold of WCDA for GRB observation. The proposed method significantly improves the detection efficiency of WCDA for gamma-rays around the GRB direction at 10-300 GeV. The sensitivity of the WCDA for GRB detection with the new trigger method is estimated. The achieved sensitivity of the quarter WCDA array above 10 GeV is comparable with that of Fermi-LAT. The data analysis process and corresponding fluence upper limit for GRB 190719C is presented as an example.

The extension of the cosmic-ray spectrum beyond 1 petaelectronvolt (PeV; 1015 electronvolts) indicates the existence of the so-called PeVatrons—cosmic-ray factories that accelerate particles to PeV energies. We need to locate and identify such objects to find the origin of Galactic cosmic rays1. The principal signature of both electron and proton PeVatrons is ultrahigh-energy (exceeding 100 TeV) γ radiation. Evidence of the presence of a proton PeVatron has been found in the Galactic Centre, according to the detection of a hard-spectrum radiation extending to 0.04 PeV (ref. 2). Although γ-rays with energies slightly higher than 0.1 PeV have been reported from a few objects in the Galactic plane3–6, unbiased identification and in-depth exploration of PeVatrons requires detection of γ-rays with energies well above 0.1 PeV. Here we report the detection of more than 530 photons at energies above 100 teraelectronvolts and up to 1.4 PeV from 12 ultrahigh-energy γ-ray sources with a statistical significance greater than seven standard deviations. Despite having several potential counterparts in their proximity, including pulsar wind nebulae, supernova remnants and star-forming regions, the PeVatrons responsible for the ultrahigh-energy γ-rays have not yet been firmly localized and identified (except for the Crab Nebula), leaving open the origin of these extreme accelerators. Observations of γ-rays with energies up to 1.4 PeV find that 12 sources in the Galaxy are PeVatrons, one of which is the Crab Nebula.

Neutron monitors were designed to measure atmospheric secondary neutrons from cosmic ray showers in order to track the cosmic ray flux vs. time. Furthermore, at the Princess Sirindhorn Neutron Monitor (PSNM), an 18-counter NM64 detector at 2560-m altitude at Doi Inthanon, Thailand, the leader fraction (inverse multiplicity) inferred from time delay distribution between successive neutron events in the same counter has been used to track spectral variations. More recent measurements of time delays between neutron events in different counters, as a function of counter separation, have confirmed that 1) the product neutrons from the interaction of a single atmospheric secondary neutron can spread among neighboring counters, with a cross-counter leader fraction that depends on whether the first counter is an end or middle counter, and 2) coincident counts between distant counters can be produced by multiple atmospheric secondary neutrons from the same primary cosmic ray, with a leader fraction that depends on whether the second counter is an end or middle counter. Here we report on measurements of neutron signals in amplifier outputs at PSNM using a 4-channel oscilloscope, in order to further investigate these phenomena. For a pair of neighboring counters located at the edge of the counter array, we find roughly equal event rates in either neighbor following a neutron trigger in one of them, implying that the difference in leader fraction relates to the base count rate of the first counter and is therefore lower if that is an end counter. In addition, an FPGA-based readout system was developed for more efficient collection of neutron events on two distant counters (Tubes 2 and 18) that were coincident within a 250-microsecond time window, while also monitoring Tube 10 in between. The time distributions and neutron multiplicities indicate that a small fraction of the cosmic ray events that triggered both Tubes 2 and 18 also led to neutron events on Tube 10 with an enhanced rate of high multiplicity, indicating a few air shower events that densely “carpeted” the neutron monitor, while the majority of such coincidences apparently involved a sparse distribution of isolated secondary particles near Tube 2 and near Tube 18 and not near the intermediate Tube 10, which is consistent with a cross-counter leader fraction dependence on the count rate of the second counter. This research was supported by the postdoctoral research sponsorship of Mahidol University, Thailand, by grant RTA6280002 from Thailand Science Research and Innovation, and by grant NSRF from the Program Management Unit for Human Resources & Institutional Development, Research and Innovation, NXPO, Thailand [grant number B05F640051].

The flux of low-energy (GeV-range) Galactic cosmic rays at Earth is modulated by the long term magnetic variations of the Sun (11-year sunspot cycle and 22-year magnetic solar cycle). This process known as Solar modulation is most pronounced at 1 GeV and below. However, it also operates at much higher energy, still exhibiting solar magnetic polarity dependence. For the last decades, ground-based neutron monitors provided valuable observations of the solar modulation up to a rigidity cutoff of about 17 GV. To extend the energy range of the neutron monitor observations, we recently upgraded the electronics of the Princess Sirindhorn Neutron Monitor in Thailand (PSNM, the operating neutron monitor at the highest geomagnetic rigidity cutoff) to record complex combinations of hits in multiple proportional counters. We present here the detection of multiple-hit events recorded at the PSNM. We discuss these observations with the help of a detailed Monte-Carlo simulation of energetic neutrons interacting in the detector. Finally, we estimate the nucleonic spectrum of the atmospheric secondary particles at the altitude of the detector.

Modeling of time profiles of solar energetic particle (SEP) observations typically considers transport along a large-scale magnetic field with a fixed path length from the source to the observer. Chhiber et al. (2021) pointed out that the path length along a turbulent magnetic field line is longer than that along the large scale field, and that the path along the particle gyro-orbit can be substantially longer again; they also considered the global variation in these quantities. Here we point out that variability in the turbulent field line path length can affect the fits to SEP data and the inferred mean free path and injection profile. To explore such variability, we perform Monte Carlo simulations in representations of homogeneous 2D MHD + slab turbulence in spherical geometry and trace trajectories of field lines, particle guiding centers, and full particle orbits, considering ion injection from a narrow or wide angular region near the Sun, corresponding to an impulsive or gradual solar event, respectively. We analyze our simulation results in terms of path length statistics within and among square-degree pixels in heliolatitude and heliolongitude at 0.35 and 1 AU from the Sun. For a given representation of turbulence, there are systematic effects on the path lengths vs. heliolatitude and heliolongitude. Field line path lengths relate to the fluctuation amplitudes experienced by the field lines, which in turn partly relate to the local topology of 2D turbulence. Particles from an impulsive event that arrive at a distant angular separation (up to ~25 degrees from the mean field connection) generally have longer path lengths, not because of the angular distance per se but because of strong magnetic fluctuations experienced to drive the guiding field lines to such angular distances and because of the associated scattering of the particles. We describe the effects of such path length variations on observed time profiles of solar energetic particles, both in terms of path length variability at specific locations and motion of the observer with respect to turbulence topology during the course of the observations. This research was partially supported by Thailand Science Research and Innovation grant RTA6280002 and the Parker Solar Probe mission under the ISOIS project (contract NNN06AA01C) and a subcontract to University of Delaware from Princeton University (SUB0000165). Additional support is acknowledged from the NASA LWS program (NNX17AB79G) and HSR program (80NSSC18K1210 & 80NSSC18K1648).

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.

The passage of an interplanetary shock and/or interplanetary coronal mass ejection often causes a rapid decrease in the Galactic cosmic-ray (GCR) flux, known as a Forbush decrease, followed by a recovery of the flux over some days. These local effects are of short duration and strongly rigidity dependent, with higher-rigidity particles exhibiting much weaker effects. In contrast, we present data for two events in which the cosmic-ray flux gradually decreased for about 1 week after shock passage, then recovering over the following week, with the highest anisotropy levels observed throughout Solar Cycle 24. These extended decreases have a weak rigidity dependence and are much more prominent in observations at higher cutoff rigidity, where the initial Forbush decrease is not clearly detected and other variations are generally weak, as we demonstrate using data from the Princess Sirindhorn Neutron Monitor at Doi Inthanon, Thailand with a cutoff rigidity of about 17 GV. We propose that these extended decrease events were initiated upon the passage of an interplanetary shock that inhibited the inflow of GCRs along the interplanetary magnetic field, possibly due to magnetic mirroring at the shock. We also discuss the general behavior of GCR anisotropy as observed at this high cutoff rigidity.

Using neutron time-delay data from neutron monitors (NMs), we can extract the leader fraction, L, of neutron counts that do not follow a previous neutron count in the same counter tube due to the cosmic ray shower. L is the inverse of the neutron multiplicity and serves as a proxy of the cosmic ray spectral index over the rigidity range of the NM response function. We have outfitted several Antarctic NMs with special electronics to collect neutron time delay distributions. We present a comparative analysis of L during two time periods: 1) during December 2015 to January 2017, for NMs at South Pole (SP), McMurdo (MC), and Jang Bogo (JB), and 2) during February 2020 to February 2021, for NMs at SP, JB, and Mawson (MA). To first order L varies in concert with the count rate C, reflecting unrolling of the Galactic cosmic ray (GCR) spectrum as part of solar modulation during the declining phase of solar cycle 24 and during solar minimum. However, during 27-day variations inCdue to high-speed solar wind streams (HSSs) and corotating interaction regions (CIRs), L usually had a very weak variation. Our results indicate weak GeVrange GCR spectral variation due to HSSs and CIRs, relative to the flux variation, in contrast with the strong observed spectral variation due to solar modulation.

Using a setup for testing a prototype for a satellite-borne cosmic-ray ion detector, we have operateda stack of scintillator and silicon detectors on top of the Princess Sirindhorn Neutron Monitor(PSNM), an 18-counter NM64 detector at 2560-m altitude at Doi Inthanon, Thailand. Monte Carlosimulations have indicated that about 15% of the neutron counts by PSNM are due to interactions(mostly in the lead producer) of GeV-range protons among the atmospheric secondary particlesfrom cosmic ray showers, which can be detected by the scintillator and silicon detectors. Detectionof incoming charged particles associated with neutron counts in the NM64 allows a measurementof the travel time distribution of such neutrons as they scatter and propagate through the NM64,processes that are nearly the same whether the interaction was initiated by an energetic proton (for15% of the count rate) or neutron (for 80% of the count rate). This travel time distribution underliesthe time delay distribution between successive neutron counts, from which we can determine theleader fraction (inverse multiplicity), which has been used to monitor Galactic cosmic ray spectralvariations over∼1-40 GV. In the present experiment we have measured both the coincidence rateof incident charged shower particles with neutron counts in the NM64 and the neutron travel timedistribution. We utilize these measurements to validate Monte Carlo simulations of atmosphericsecondary particle detection by the NM64 and the resulting yield functions used to interpret thecount rate and the leader fraction.

Magnetic activity on the sun influences the flux of galactic cosmic rays at Earth in the process known as solar modulation. While most pronounced at 1 GeV and below, it also operates at much higher energy, still exhibiting solar magnetic polarity dependence. Historically an observational gap exists between approximately 17 GeV (the highest geomagnetic cutoff) neutron monitor data and muon observations of primary cosmic rays that are mostly above 50 GeV. Detecting multiple neutrons from the same primary particle has recently been used to monitor the primary energy spectrum using data from a single neutron monitor. Over the past few years we have used details of the timing distribution from individual neutron detectors and pairwise correlations among adjacent detectors to improve the energy resolution of this technique. We present a further extension of our observations to pattern recognition of events comprising hits in multiple detectors in order to identify and study cases where more than one secondary particle from the same primary interacts in the neutron monitor. Our particular focus at present is to distinguish interactions of energetic hadrons from those generated by cores of small air showers.

We report the discovery of a new unidentified extended $\gamma$-ray source in the Galactic plane named LHAASO J0341+5258 with a pre-trial significance of 8.2 standard deviations above 25 TeV. The best fit position is R.A.$=55.34^{\circ}\pm0.11^{\circ}$ and Dec$=52.97^{\circ}\pm0.07^{\circ}$. The angular size of LHAASO J0341+5258 is $0.29^\circ \pm 0.06^\circ_{stat} \pm0.02^\circ_{sys}$. The flux above 25 TeV is about $20\%$ of the flux of Crab Nebula. Although a power-law fit of the spectrum from 10 TeV to 200 TeV with the photon index $\alpha=2.98 \pm 0.19_{stat} \pm 0.02_{sys}$ is not excluded, the LHAASO data together with the flux upper limit at 10 GeV set by the Fermi LAT observation, indicate a noticeable steepening of an initially hard power-law spectrum %($\alpha \leq 1.75$) spectrum with a cutoff at $\approx 50$ TeV. We briefly discuss the origin of UHE gamma-rays. The lack of an energetic pulsar and a young SNR inside or in the vicinity of LHAASO J0341+5258 challenge, but do not exclude both the leptonic and hadronic scenarios of gamma-ray production., Comment: Accepted by APJL

Using a setup for testing a prototype for a satellite-borne cosmic-ray ion detector, we have operated a stack of scintillator and silicon detectors on top of the Princess Sirindhorn Neutron Monitor (PSNM), an NM64 detector at 2560-m altitude at Doi Inthanon, Thailand (18.59 N, 98.49 E). Monte Carlo simulations have indicated that about 15% of the neutron counts by PSNM are due to interactions (mostly in the lead producer) of GeV-range protons among the atmospheric secondary particles from cosmic ray showers, which can be detected by the scintillator and silicon detectors. Those detectors can provide a timing trigger for measurement of the propagation time distribution of such neutrons as they scatter and propagate through the NM64, processes that are similar whether the interaction was initiated by an energetic proton (for 15% of the count rate) or neutron (for 80% of the count rate). This propagation time distribution underlies the time delay distribution between successive neutron counts, from which we can determine the leader fraction (inverse multiplicity), which has been used to monitor Galactic cosmic ray spectral variations over $\sim$1-40 GV. Here we have measured and characterized the propagation time distribution from both the experimental setup and Monte Carlo simulations of atmospheric secondary particle detection. We confirm a known propagation time distribution with a peak (at $\approx$70 microseconds) and tail over a few ms, dominated by neutron counts. We fit this distribution using an analytic model of neutron diffusion and absorption, for both experimental and Monte Carlo results. In addition we identify a group of prompt neutron monitor pulses that arrive within 20 microseconds of the charged-particle trigger, of which a substantial fraction can be attributed to charged-particle ionization in a proportional counter, according to both experimental and Monte Carlo ..., Comment: 14 pages, 8 figures. Published in Astroparticle Physics

Ground-based neutron counters are a standard tool for detecting atmospheric showers from GeV range primary cosmic rays of either solar or galactic origin. Bare neutron counters, a type of lead-free ...

The Water Cherenkov Detector Array (WCDA) is a major component of the Large High Altitude Air Shower Array Observatory (LHAASO), a new generation cosmic-ray experiment with unprecedented sensitivity, currently under construction. The WCDA is aimed at the study of TeV $\gamma$-rays. In order to evaluate the prospects of searching for TeV $\gamma$-ray sources with the WCDA, we present in this paper a projection for the one-year sensitivity of the WCDA to TeV $\gamma$-ray sources from TeVCat (footnote: http://tevcat.uchicago.edu) using an all-sky approach. Out of 128 TeVCat sources observable to the WCDA up to a zenith angle of $45^\circ$, we estimate that 42 would be detectable for one year of observations at a median energy of 1 TeV. Most of them are Galactic sources, and the extragalactic sources are Active Galactic Nuclei (AGN)., Comment: 10 pages,10 figures, it is to be published in Chinese Physics C

The main scientific goal of the LHAASO WFCTA experiment is to measure the cosmic ray energy spectra and composition from 10 TeV to 1 EeV. Cherenkov photons in the extensive air shower measured by the SiPM camera of Cherenkov telescopes can be used to reconstruct the cosmic ray energy. The absolute calibration of the camera is a crucial step to achieve the accurate measurement of the cosmic ray energy spectrum. A multi-wavelength cylindrical illuminator based on LEDs is developed and mounted inside the telescope to calibrate and monitor the camera, and the illuminator’s stability is better than 0.5% under the temperature variation from -26 to 26 ° C. A portable probe with a single photoelectron resolution of 21.6% is developed. After calibration by National Institute of Metrology, China (NIM), the probe is taken to the LHAASO site to measure the absolute photon density of the cylindrical illuminator inside the telescope. Based on the illuminator with known photon density, the photon conversion factor of the camera can be calibrated, and the overall calibration uncertainty is less than 2.6%.

Neutron monitors of standard design (IGY or NM64) are employed worldwide to study variations in the flux of galactic cosmic rays and solar energetic particles in the GeV range. The design minimizes detector response to neutrons below ∼10 MeV produced by cosmic ray interactions in the ambient medium. Increasingly, however, such neutrons are of interest as a means of obtaining spectral information on cosmic rays, for studies of soil moisture, and for nuclear threat detection. Bare neutron counters, a type of lead-free neutron monitor, can detect such neutrons, but comparatively little work has been done to characterize the dependence of their count rate on cutoff rigidity. We analyze data from three bare neutron counters operated on a ship together with a three-tube NM64 monitor from November 1995 to March 1996 over a wide range of magnetic latitude, that is, a latitude survey. The bare counter design used foamed-in-place polyurethane insulation to keep the temperature uniform and to some extent moderate high-energy neutrons. When the ship was near land, the bare/NM64 count rate ratio was dramatically higher. Considering only data from open sea, the bare and NM64 pressure coefficients are not significantly different. We determine the response function of these bare counters, which is weighted to Galactic cosmic rays of lower energy than the NM64. This measurement of the response function may improve determination of the spectral index of solar energetic particles and Galactic cosmic rays from a comparison of bare and NM64 count rates.

Ionization in Earth's troposphere is mainly due to Galactic cosmic rays. Occasionally, solar storms produce intense relativistic ion beams that significantly increase such ionization. One of the largest recorded solar radiation storms, on 20 January 2005, resulted in up to 55-fold increases in the count rates of ground-based particle detectors in polar regions. We use McMurdo and Inuvik neutron monitor data to estimate accurate time profiles of ion energy spectra above the atmosphere at each location. Using data-driven atmospheric models, we perform Monte Carlo simulations of particle-air interactions and calculate atmospheric ionization and potential biological dosage versus altitude and time for each location. We found that if airplane passengers had traversed the south polar region, they could have been exposed to the typical annual cosmic radiation dosage at sea level within 1 h. These techniques can help evaluate possible influences of solar activity on atmospheric properties.

We present an update on the analysis of cross-counter time-delay histograms with 5 μs bins as recorded since late 2015 in the Princess Sirindhorn Neutron Monitor, an 18NM64 (with 18 counter tubes in a continuous row) at 2560 m altitude in Doi Inthanon, Thailand. A cross-counter relative leader fraction $L_{ij}$ can be defined as the fraction of pulses measured in counter number $i$ that are uncorrelated in time with the most recent pulse in counter $j$. For low tube separation, $L_{ij}$ rapidly increases with increasing tube separation $|i-j|$, as expected from the spatial spreading of neutrons produced by the same atmospheric secondary particle inside the neutron monitor. At high tube separation, $L_{ij}$ increases slowly with increasing tube separation and remains different from unity, which we interpret as due to multiple atmospheric secondaries arriving in rapid succession after originating from the same primary cosmic ray. We study temporal variations of the cross-counter relative leader fraction at large tube separation ($|i-j|>6$, or roughly more than 3 m) and analyze the possible contribution of atmospheric effects to the data.

The flux of Galactic cosmic rays (GCRs) can undergo a Forbush decrease (FD) during the passage of a shock, sheath region, or magnetic flux rope associated with a coronal mass ejection (CME). Cosmic-ray observations during FDs can provide information complementary to in situ observations of the local plasma and magnetic field, because cosmic-ray distributions allow remote sensing of distant conditions. Here we develop techniques to determine the GCR anisotropy before and during an FD using data from neutron monitors worldwide. For example, for the FD starting on 2013 April 13, we find that at times with strong magnetic fluctuations and strong cosmic-ray scattering, there were spikes of high perpendicular anisotropy and weak parallel anisotropy. These results, along with the near constancy of parallel anisotropy across magnetic field reversals, are consistent with diffusive barriers causing the decrease in GCR flux before the arrival of the flux rope. In contrast, within the CME flux rope there was a strong parallel anisotropy in the direction predicted from a theory of drift motions into one leg of the magnetic flux rope and out the other, confirming that the anisotropy can remotely sense a large-scale flow of GCRs through a magnetic flux rope.

A neutron monitor (NM) is a ground- (or sea-) based detector of the flux of cosmic ray particles in space. The high-energy cosmic rays in the GeV primary range interact in the upper atmosphere, producing a cascade of subatomic particles, some of which reach Earth’s surface. A neutron monitor is mostly sensitive to the neutron component of the atmospheric cascade. The standard-design neutron monitor (NM64) contains lead, the nuclei of which fragment when struck by a high-energy particle. Some of the fragments are neutrons which are moderated and trapped by polyethylene acting as a reflector and moderator. These neutrons can then be detected by induced nuclear fission of 10B in a 10BF3 gas proportional counter. The Changvan neutron monitor is a portable neutron monitor assembled in Thailand and housed in a standard insulated shipping container to conduct long-term research in polar regions. There are three proportional counters in the Changvan, but the central counter lacks the lead producer. Since the detector has a non-standard semi-leaded design, we examine the detection efficiency of the Changvan for neutrons and other atmospheric secondary particles. We are also developing an electronic board and a highly sensitive control module to reduce dead time to a minimum, for monitoring neutrons in the GeV energy range. Simulation results accounting for the dead time will also be reported.

The name “Changvan” is given to a transportable neutron monitor, modernized by researchers in Thailand and housed inside a standard-size shipping container. It contains three neutron-sensitive proportional counters, which contain enriched10BF3gas, to detect cosmic ray showers. Primary cosmic rays are high-energy particles arriving from outer space. By colliding with atoms in the atmosphere, they produce a shower of sub-atomic particles, called secondary particles. Some particles can reach the Earth’s surface, including neutrons. The Changvan is designed to measure atmospheric neutrons during a latitude survey. The side counter tubes are ringed with lead producer to produce evaporation neutrons when secondary particles collide with it. The center counter lacks the lead producer but can receive neutrons from nearby counters. The reflector and moderator made from polyethylene thermalize the evaporation neutrons and protect against disturbance by external low-energy neutrons. In this work, we pursue methods to determine spectral changes in Galactic cosmic rays using data from the Changvan, avoiding the systematic inconsistencies of cross-station comparisons. We examine neutron time delay histograms from the three counter tubes, as recorded by specially designed readout electronics, where the time delay refers to the time interval between consecutive neutron detections in one counter. We perform a preliminary analysis of such histograms to measure the leader fraction (L) of neutrons that do not follow a previous neutron from the same primary cosmic ray. The electronic dead time will be considered in theLcalculation.

Neutron monitors (NMs) are ground-based detectors that measure variations of the Galactic cosmic ray flux at GV range rigidities. Differences in configuration, electronics, surroundings, and location induce systematic effects on the calculation of the yield functions of NMs worldwide. Different estimates of NM yield functions can differ by a factor of 2 or more. In this work, we present new Monte Carlo simulations to calculate NM yield functions and perform an absolute (not relative) comparison with the count rate of the Princess Sirindhorn Neutron Monitor (PSNM) at Doi Inthanon, Thailand, both for the entire monitor and for individual counter tubes. We model the atmosphere using profiles from the Global Data Assimilation System database and the Naval Research Laboratory Mass Spectrometer, Incoherent Scatter Radar Extended model. Using FLUKA software and the detailed geometry of PSNM, we calculated the PSNM yield functions for protons and alpha particles. An agreement better than 9% was achieved between the PSNM observations and the simulated count rate during the solar minimum of December 2009. The systematic effect from the electronic dead time was studied as a function of primary cosmic ray rigidity at the top of the atmosphere up to 1 TV. We show that the effect is not negligible and can reach 35% at high rigidity for a dead time >1 ms. We analyzed the response function of each counter tube at PSNM using its actual dead time, and we provide normalization coefficients between count rates for various tube configurations in the standard NM64 design that are valid to within ∼1% for such stations worldwide.

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.

On 2017 January 18 scaler rates in the IceTop detectors at the South Pole revealed an impulsive increase in the galactic cosmic ray flux lasting a few hours. In addition to the neutron monitor at Pole the event was detected clearly by the Mawson neutron monitor and faintly at Jang Bogo. No other neutron monitors appear to have seen the increase. The event was in many ways reminiscent of the 2015 June 22 event observed by the GRAPES muon detectors. Both events occurred during the declining phase of a Forbush decrease, at a time of increasing geomagnetic activity, and were observed by a limited number of neutron monitors with similar asymptotic directions. The magnitude of the impulse was in both cases such that the flux returned briefly to approximately the pre-decrease level. Distinctly unlike the 2015 June 22 event, a changing geomagnetic cutoff cannot explain the 2017 January 18 event because the cutoff at South Pole is nearly zero and the detector response is atmosphere limited. We therefore interpret the 2017 January 18 event in terms of the structure of the Forbush decrease and (possibly changing) asymptotic directions. With our interpretation of the January event in mind we also comment on possible alternative interpretations of the GRAPES event.

The Princess Sirindhorn Neutron Monitor (PSNM) is an 18NM64 (with 18 counter tubes in a continuous row) at 2560~m altitude in Doi Inthanon, Thailand. In late 2015, the electronics in the PSNM were upgraded so as to return an absolute time, referenced to the GPS time, for each neutron count recorded. The timing accuracy is approximately $\pm3$ $\mu$s. This permits extending the well-known concept of multiplicity (multiple correlated counts in one detector) to what we refer to as `cross multiplicity' or correlation of counts in one detector with those in another. At present, the data are recorded as cross-counter time-delay histograms with 5-$\mu$s bins. A cross-counter relative leader fraction $L_{ij}$ can be defined as the fraction of pulses measured in counter number $i$ that are uncorrelated in time with the next pulse in counter $j$. For simplicity, we derive average cross-counter leader fractions $L_\Delta$ with $\Delta=|i-j|$ as a function of counter tube separation, from a tube separation $\Delta=0$ (i.e., using the same tube) to $\Delta=17$. For low tube separation, $L_\Delta$ rapidly increases with increasing $\Delta$, as expected from the spatial spreading of neutrons produced by the same atmospheric secondary particle inside the neutron monitor. At high tube separation, $L_\Delta$ increases slowly with increasing $\Delta$ and remains different from unity, which we interpret as due to multiple atmospheric secondaries arriving in rapid succession after originating from the same primary cosmic ray.

Only three Ground Level Enhancements (GLEs) produced by GeV-range solar energetic particles have been confirmed in the present solar cycle, and those have been quite small by historical standards. At the same time direct observations of high energy solar particles from spacecraft have become available. The combination of instruments at the Amundsen - Scott Station at the geographic South Pole offers an opportunity to span the two disparate sets of measurements. Operating at high altitude and low geomagnetic cutoff the neutron monitor at Pole has nevertheless traditionally been accepted as making a "ground level" observation. An enhanced array of bare neutron detectors and IceTop (surface ice Cherenkov detectors in the IceCube Neutrino Observatory) in principle allow spectral information to be extracted from events that previously were too small to use geomagnetic techniques to obtain spectra. We report our spectral measurements for the three confirmed GLE and compare them to other available data on these events. We also present preliminary results of an ongoing study of 34 particle events selected to have significant enhancements in the > 100 MeV channel in GOES data in terms of possible detections and sensitivity limits.

Solar modulation refers to Galactic cosmic ray (GCR) variations with the $\sim$11-year sunspot cycle and $\sim$22-year solar magnetic cycle, and is relevant to the space radiation environment and effects on Earth's atmosphere. Its complicated dependence on solar and heliospheric conditions is only roughly understood but has been empirically modeled in terms of a single modulation parameter. Most analyses of solar modulation used neutron monitor (NM) data from locations with relatively low geomagnetic cutoff rigidity, i.e., the threshold for cosmic rays to penetrate Earth's magnetic field. The Princess Sirindhorn Neutron Monitor (PSNM) at Doi Inthanon, Thailand has the highest cutoff rigidity ($\approx$17 GV) where observations span a complete solar modulation cycle (since late 2007). The pattern of solar modulation at Doi Inthanon during 2011-2014 was qualitatively very different from that at low geomagnetic cutoff, and is not well described by the same modulation parameter. At other times, NM count rates from Doi Inthanon and McMurdo, Antarctica (cutoff $

Neutron monitors are the premier instruments for precisely tracking time variations in the Galactic cosmic ray flux at GeV-range energies above the geomagnetic cutoff at the location of measurement. In addition to the count rate, recording and analyzing the time delays between successive counts allows us to infer variations in the cosmic ray spectrum as well. In particular, we can determine the leader fraction L, defined as the fraction of neutrons that did not follow a previous neutron detection in the same tube from the same nuclear interaction, from time delay histograms. By analyzing data taken during 2001–2007 by a ship-borne neutron monitor latitude survey we confirm a strong dependence of L on the geomagnetic cutoff up to 16 GV. The data of the Princess Sirindhorn Neutron Monitor located in Thailand at a higher vertical cutoff of 16.8 GV have also been analyzed for the period 2007–2014. We have developed Monte Carlo simulations of cosmic ray interactions in the atmosphere and in both neutron monitors. Both data and simulations show that the absolute value of L depends significantly of the configuration of the detector and that experimental conditions such as the electronic dead time must be well monitored. The simulation results show a change in L with the geomagnetic cutoff as observed by the latitude survey, confirming that this change in L can reasonably be attributed to changes in the cosmic ray spectrum.