IceTop, the surface component of IceCube Neutrino Observatory at the South Pole, studies cosmic ray air showers with an array of ice Cherenkov detectors typically referred to as “IceTop Tanks.” In November 2009, collaborators from the University of Delaware, UW River Falls, and Uppsala University loaded an insulated shipping container containing an IceTop Tank on the icebreaker Oden which traversed the Atlantic Ocean from Helsingborg, Sweden to McMurdo, Antarctica, and return. Over an approximately 6-month interval, Oden carried the IceTop Tank through a wide range of geomagnetic cut-offs. The data obtained will allow the energy dependent effective area (yield function) to be determined using the Earth as a magnet spectrometer. The ultimate goal of the project is to calibrate the IceTop Tanks to study cosmic rays in the GeV primary energy range. We will report preliminary results for determining the yield functions.

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.

The energy spectrum of Galactic cosmic-ray (GCR) ions at Earth varies with solar activity as these ions cross the heliosphere. Thus, this "solar modulation" of GCRs provides remote sensing of heliospheric conditions throughout the similar to 11 yr sunspot cycle and similar to 22 yr solar magnetic cycle. A neutron monitor (NM) is a stable ground-based detector that measures cosmic-ray rate variations above a geomagnetic or atmospheric cutoff rigidity with high precision (similar to 0.1%) over such timescales. Furthermore, we developed electronics and analysis techniques to indicate variations in the cosmic-ray spectral index using neutron time-delay data from a single station. Here we study solar modulation using neutron time-delay histograms from two high-altitude NM stations: (1) the Princess Sirindhorn Neutron Monitor at Doi Inthanon, Thailand, with the world's highest vertical geomagnetic cutoff rigidity, 16.7 GV, from 2007 December to 2018 April; and (2) the South Pole NM, with an atmosphere-limited cutoff of similar to 1 GV, from 2013 December to 2018 April. From these histograms, we extract the leader fraction L, i.e., inverse neutron multiplicity, as a proxy of a GCR spectral index above the cutoff. After correction for pressure and precipitable water vapor variations, we find that L roughly correlates with the count rate but also exhibits hysteresis, implying a change in spectral shape after a solar magnetic polarity reversal. Spectral variations due to Forbush decreases, 27 day variations, and a ground-level enhancement are also indicated. These methods enhance the high-precision GCR spectral information from the worldwide NM network and extend it to higher rigidity.

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 neutron monitor, function much like standard neutron monitors but have different yield functions primarily because they are more sensitive to neutrons of lower energy. When operated together with standard monitors, the different yield functions allow estimates to be made of the energy spectrum of galactic or solar particles. In 2010 a new array of 12 bare neutron detectors was installed at the South Pole to operate together with the neutron monitor there. Prior to installation, two of the detectors were operated on a ship that traveled from Sweden to Antarctica and back from November 2009 to April 2010. The purpose of this latitude survey was to use Earth's magnetic field as a spectrometer, blocking cosmic rays below the local cutoff rigidity (momentum per unit charge), from which we determined the response function versus rigidity of these bare counters. By comparing that measured response function to direct measurements of the cosmic ray spectrum taken by the PAMELA spacecraft, we were able to make a direct determination of the yield function for these detectors.

The solar modulation of Galactic cosmic rays (GCRs) provides remote sensing of heliospheric conditions throughout the $\sim$11-yr sunspot cycle and $\sim$22-yr solar magnetic cycle. Neutron monitors (NMs) can measure cosmic ray rates above a cutoff rigidity with high precision ($\sim$0.1\%) over such time scales. To avoid systematic uncertainties in comparing NM count rates from different stations, here we study solar modulation of the GCR spectrum by independently using neutron time-delay histograms to determine the leader fraction $L$, i.e., the inverse neutron multiplicity, as a proxy of the GCR spectral index above the cutoff. Data were collected using specialized electronics at two high-altitude NM stations: 1) the Princess Sirindhorn Neutron Monitor (PSNM) at Doi Inthanon, Thailand, with the world's highest vertical geomagnetic cutoff rigidity, 16.8 GV, from 2007 December to 2018 September, and 2) the South Pole Neutron Monitor with an atmosphere-limited cutoff of $\sim$1 GV, from 2013 December to 2018 September. After correcting for pressure and (in Thailand) precipitable water vapor, we find that $L$ is roughly correlated with the count rate, but also exhibits hysteresis that indicates a change in spectral shape after the change in solar magnetic polarity. These methods enhance the high-precision GCR spectral information from the worldwide NM network and extend it to higher rigidity.

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. Recently, a new capability has been developed to record and analyze the neutron time delay distribution (related to neutron multiplicity) to infer variations in the cosmic ray spectrum as well. In particular, from time delay histograms 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 atmospheric secondary particle. Using data taken during 2000–2007 by a shipborne neutron monitor latitude survey, we observe a strong dependence of the count rate and L on the geomagnetic cutoff. We have modeled this dependence using Monte Carlo simulations of cosmic ray interactions in the atmosphere and in the neutron monitor. We present new yield functions for the count rate of a neutron monitor at sea level. The simulation results show a variation of L with geomagnetic cutoff as observed by the latitude survey, confirming that these changes in L can be attributed to changes in the cosmic ray spectrum arriving at Earth's atmosphere. We also observe a variation in L with time at a fixed cutoff, which reflects the evolution of the cosmic ray spectrum with the sunspot cycle, known as solar modulation.

The Galactic cosmic ray spectrum exhibits subtle variations over the 22 yr solar magnetic cycle in addition to the more dramatic variations over the 11 yr sunspot cycle. Neutron monitors are large ground-based detectors that provide accurate measurements of variations in the cosmic ray flux at the top of the atmosphere above the detector. At any given location the magnetic field of the Earth excludes particles below a well-defined rigidity (momentum per unit charge) known as the cutoff rigidity, which can be accurately calculated using detailed models of the geomagnetic field. By carrying a neutron monitor to different locations, e.g., on a ship, the Earth itself serves as a magnet spectrometer. By repeating such latitude surveys with identical equipment, a sensitive measurement of changes in the spectrum can be made. In this work, we analyze data from the 1994 through 2007 series of latitude surveys conducted by the Bartol Research Institute, the University of Tasmania, and the Australian Antarctic Division. We confirm the curious "crossover" in spectra measured near solar minima during epochs of opposite solar magnetic polarity, and show that it is directly related to a sudden change in the spectral behavior of solar modulation at the time of the polarity reversal, as revealed from contemporaneous variations in the survey data and a fixed station. We suggest that the spectral change and crossover result from the interaction of effects due to gradient/curvature drifts with a systematic change in the interplanetary diffusion coefficient caused by turbulent magnetic helicity.

A ground level enhancement (GLE) is a solar event that accelerates ions (mostly protons) to GeV range energies in such great numbers that ground-based detectors, such as neutron monitors, observe their showers in Earth's atmosphere above the Galactic cosmic ray background. GLEs are of practical interest because an enhanced relativistic ion flux poses a hazard to astronauts, air crews, and aircraft electronics, and provides the earliest direct indication of an impending space radiation storm. The giant GLE of 2005 January 20 was the second largest on record (and largest since 1956), with up to 4200% count rate enhancement at sea level. We analyzed data from the Spaceship Earth network, supplemented to comprise 13 polar neutron monitor stations with distinct asymptotic viewing directions and Polar Bare neutron counters at South Pole, to determine the time evolution of the relativistic proton density, energy spectrum, and three-dimensional directional distribution. We identify two energy-dispersive peaks, indicating two solar injections. The relativistic solar protons were initially strongly beamed, with a peak maximum-to-minimum anisotropy ratio over 1000:1. The directional distribution is characterized by an axis of symmetry, determined independently for each minute of data, whose angle from the magnetic field slowly varied from about 60° to low values and then rose to about 90°. The extremely high relativistic proton flux from certain directions allowed 10 s tracking of count rates, revealing fluctuations of period ≳ 2 minutes with up to 50% fractional changes, which we attribute to fluctuations in the axis of symmetry.

In this work, a method of neutron monitor data correction for the snow effect caused by the snow accumulation on and around the detector housing is described. This is particularly important for some high‐latitude and mountain cosmic ray stations. The results of manual correction are compared with the results of automatic correction on the basis of the algorithm developed here. The proposed method has been applied to a number of cosmic ray stations where the snow accumulation is large and variable during the winter, for example, Magadan, Emilio Segre Observatory in Israel, Moscow Cosmic Ray Laboratory, Jungfraujoch, and Nain.

We describe a practical system for forecasting peak intensity and fluence of solar energetic protons in the tens to hundreds of MeV energy range. The system could be useful for forecasting radiation hazard, because peak intensity and fluence are closely related to the medical physics quantities peak dose rate and total dose. The method uses a pair of ground‐based detectors located at the South Pole to make a measurement of the solar particle energy spectrum at relativistic (GeV) energies, and it then extrapolates this spectrum downward in energy to make a prediction of the peak intensity and fluence at lower energies. A validation study based upon 12 large solar particle events compared the prediction with measurements made aboard GOES spacecraft. This study shows that useful predictions (logarithmic correlation greater than 50%) can be made down to energies of 40–80 MeV (GOES channel P5) in the case of peak intensity, with the prediction leading the observation by 166 min on average. For higher energy GOES channels, the lead times are shorter, but the correlation coefficients are larger.

We seek to determine whether South Pole neutron monitor observations can be used to predict radiation storm intensity as measured from GOES spacecraft. Using the “Polar Bare” method which compares a standard neutron monitor (NM64) and a monitor lacking the usual lead shielding (Bare), we estimate the particle spectrum from the Bare and NM64 count rates. Selected ground level enhancements (GLEs) are divided into two groups. One group consists of 12 GLEs where spectra derived from the South Pole observations are compared to 4 proton energy channels (P4-P7) from GOES. The other group consists of 7 GLES where spectra are additionally compared to higher energy proton channels (P8-P11). The second group of 7 GLEs is a subset of the first group of 12 GLEs. We conclude that South Pole GLE observations can be used to predict radiation intensity of the higher energy proton channels from GOES.

IceTop is an air shower array now under construction at the South Pole. It is the surface component of IceCube, an observatory primarily focused on cosmic neutrinos. When completed, IceTop will have approximately 500 square meters of collecting area in the form of 160 separate ice Cherenkov detectors. These detectors are sensitive to electrons, photons, muons and neutrons. With the high altitude and low geomagnetic cutoff at the South Pole, IceTop promises to have unprecedented statistical preci- sion, coupled with spectral sensitivity that can be used to observe solar energetic particles and transient phenomena in the flux of galactic cosmic rays. We discuss the potential of IceCube to contribute to he- liospheric physics in general, and present a preliminary analysis of a complex interplanetary disturbance that occurred in August of 2006.

Recent studies have stressed the importance of solar energetic particle (SEP) transport under disturbed interplanetary conditions, including the case of detection inside a closed interplanetary magnetic loop ejected by a preceding solar event. In this case, particles might be observed to arrive from the far leg of the loop, thus arriving at the detector while traveling sunward. We perform numerical simulations of the focused transport of SEPs along Archimedean spiral and magnetic loop configurations. For loop configurations, we consider injection along either the near leg or the far leg of the loop, either with or without compression at the leading edge. We show that there are specific anisotropy signatures of transport in a closed magnetic loop configuration. SEPs traveling sunward cannot have a high, sustained anisotropy due to the effect of inverse focusing. As an example, the relativistic SEP event of 2003 October 28 exhibited unusual directional distributions, with an early peak of particle flow approximate to 120 degrees and a main peak approximate to 80 degrees from the radial direction. However, quantitative fitting of data from the Spaceship Earth network of polar neutron monitors indicates that injection along the far leg of an interplanetary loop is not a good description; our analysis strongly favors transport from the Sun to the Earth over a short path length of similar to 1 AU.

On 2006 December 13 the IceTop air shower array at the South Pole detected a major solar particle event. By numerically simulating the response of the IceTop tanks, which are thick Cerenkov detectors with multiple thresholds deployed at high altitude with no geomagnetic cutoff, we determined the particle energy spectrum in the energy range 0.6-7.6 GeV. This is the first such spectral measurement using a single instrument with a well-defined viewing direction. We compare the IceTop spectrum and its time evolution with previously published results and outline plans for improved resolution of future solar particle spectra.

The count rate recorded by a neutron monitor at South Pole, Antarctica, displays a long‐term decline over the 32‐year span from 1965 to 1997. The neutron rate follows an 11‐year cycle with maxima at times of low solar activity, but the 1997 peak rate was approximately 8% lower than the 1965 peak rate based on 27‐d averages. This change is much larger than that recorded by any other neutron monitor. We suggest that the South Pole monitor, owing to its unique position at both high latitude and high altitude (2820 m), has enhanced sensitivity at ∼1–3 GV relative to a sea level monitor and may be responding to a change in the intensity of primary cosmic rays in this rigidity region. Measurements of cosmic rays made aboard stratospheric balloons and on the IMP‐8 spacecraft support the possibility of a long‐term change in cosmic ray intensity.

Large solar energetic particle (SEP) events occur in association with fast coronal mass ejections (CMEs) and flares. We have studied in detail the rise phase of the SEP event of 1998 May 2 observed with the particle telescope ERNE aboard the Solar and Heliospheric Observatory (SOHO) spacecraft and ground-based neutron monitors. Using the ERNE data and numerical modeling of the SEP transport, we present improved evaluations of the solar release profile of deka-MeV protons. The SOHO EIT images are used to study the CME liftoff processes and possible sources of deka-MeV and hecto-MeV proton streams. In a first stage of the deka-MeV proton production, which starts not later than 4 minutes after the radio flash and the Moreton wave start, particles get accelerated from a few MeV through 20 MeV in ≈15 minutes. Both ERNE and neutron monitor data are used to study the release of solar protons in the hecto-MeV range. The proton acceleration to above 400 MeV was completed not later than 15-20 minutes after the onset of the eruption. However, injection profiles of deka-MeV protons and hecto-MeV protons were different. Differences in the release scenarios, energy spectra, and composition of deka-MeV protons versus hecto-MeV protons suggest two different acceleration regions involved, perhaps situated on initially open lines and initially closed lines of the coronal magnetic field. The first SEP productions were followed by a prolonged period of proton reacceleration, which continued in the ~10-100 MeV range for many hours and during which a common energy spectrum was formed.

We have developed a system that watches for count rate increases recorded in real time by eight neutron monitors, which triggers an alarm if a ground level enhancement (GLE) is detected. In this work, we determine optimal strategies for detecting the GLE event at a very early stage, while still keeping the false alarm rate at a very low level. We study past events to optimize appropriate intensity threshold values and a baseline to determine the intensity increase. The highest‐level alarm, which we term an “alert,” is generated when a 4% increase is recorded at three stations in 3 min averaged data. At this level, the false alarm rate obtained by backtesting over the past 4.4 years is zero. Ten GLEs occurred in this period, and our system produced GLE alarms for nine events. Alarm times for these nine events are compared with satellite proton data. The GLE alert precedes the earliest alert from GOES (100 MeV or 10 MeV protons) by ∼10–30 min. Real‐time GLE data may be viewed at http://neutronm.bartol.udel.edu/spaceweather. An automated e‐mail alert system is under development.