Neutron Monitor Count Rate Research Articles

Modelling of radiation exposure at high altitudes during solar storms

A transport code analysis using Monte Carlo N-Particle eXtended code, MCNPX, has been used to propagate an extrapolated particle spectrum based on satellite measurements through the atmosphere to estimate radiation exposure during solar storms at high altitudes. Neutron monitor count rate data from stations around the world were used to benchmark the model calculations during a ground-level event (GLE). A comparison was made between the model predictions and actual flight measurements taken with various types of instruments used to measure the mixed radiation field during GLE 60. A computer code has been developed to implement the model for routine analysis.

Cosmogenic 3He and 21Ne measured in quartz targets after one year of exposure in the Swiss Alps

All currently used scaling models for Terrestrial Cosmogenic Nuclide (TCN) production rates are based on neutron monitor surveys. Therefore, an assumption underlying all TCN studies is that production rates are directly proportional to secondary cosmic ray intensities for all cosmogenic nuclides. To test this crucial assumption, we measured cosmogenic 3He and 21Ne in artificial quartz targets after one year of exposure at mountain altitudes in the Swiss Alps. The targets were inconel steel tubes containing 1 kg of artificial quartz sand (250–500 µm), degassed for one week at 700 °C in vacuum prior to exposure. From August 2006 until August 2007, ten of these targets were exposed at five locations in Switzerland and Italy: Zürich (556 m), Davos (1560 m), Säntis (2502 m), Jungfraujoch (3571 m), and Monte Rosa (4554 m). Additionally, a sixth set of two blank targets was kept in storage and effectively shielded from cosmic ray exposure. Cosmogenic noble gases were measured at room temperature and at 700 °C. Up to 9% of the cosmogenic 3He was measured in the cold step, indicating that 3He diffuses out of quartz at room temperature on short time scales. The remaining 3He and all 21Ne were released at 700 °C, as shown by a repeat measurement at 800 °C for the Monte Rosa target, which yielded no additional cosmogenic helium and neon. As expected, the Monte Rosa target contained the highest cosmogenic nuclide content, with 1.56 ± 0.07 × 10 6 atoms of excess 3He and 4.5 ± 1.2 × 10 5 atoms of excess 21Ne (all errors are 2 σ). The raw measurements were corrected for non-atmospheric blanks, shielding (roof + container wall), tritiogenic helium and solar modulation (normalised to the average neutron flux over the past five solar cycles). The 3He/ 21Ne production rate ratio of 6.8 ± 0.9 indicates that cosmogenic 3He production by the container walls is negligible. The main goal of the artificial target experiment was to determine the production rate attenuation length. Because all our targets had an identical design and were exposed under identical conditions, all systematic errors cancel out in the calculation of an attenuation length. Our best estimates for the 3He and 21Ne attenuation lengths are 134.8 ± 5.9 g/cm 2 and 135 ± 25 g/cm 2, respectively, agreeing very well with currently used scaling models. We conclude that TCN production rates are indeed proportional to neutron monitor count rates, and that 3He and 21Ne production rates follow the same altitudinal scaling relationships as the cosmogenic radionuclides. Finally, the measurements were scaled to sea level and high latitude using the empirical attenuation length, yielding weighted mean production rates of 107.6 ± 6.6 at/g/yr for 3He and 15.4 ± 2.1 at/g/yr for 21Ne. Despite the significant uncertainties associated with the corrections for shielding, solar modulation and especially the 3He/ 3H branching ratio, these estimates are in good agreement with production rates derived from long-term exposure experiments at natural calibration sites and physics-based simulations.

The effect of the thunderstorm activity on the tien shan neutron monitor data

Variations in the neutron monitor counting rate at the at the mountain level (3340 m above sea level) during the passage of electrically charged clouds above the installation have been investigated. It is established that the decrease in the counting rate with respect to fair-weather level is due to positive values of the atmospheric electric field (∼40–50 kV m−1). This effect is observed in the low-energy part of the neutron component intensity and is absent in the high-energy part (neutron emission multiplicity exceeds 6).

New method for estimating the absolute flux and energy spectrum of solar cosmic rays based on neutron-monitor data

A new method is described for estimating the absolute flux of solar cosmic rays based on the data from a single neutron monitor. The method is capable of yielding the energy spectrum at the isotropic phase of a solar flare using the available data from the currently operable worldwide network of cosmic-ray stations. The method is based on the determination of the effective momentum or energy for which the particle flux derived from the neutron-monitor count rate is weakly sensitive to small variations in the exponent of the power-law spectrum. A comparison of the calculations with direct space-borne measurements and calculations by other authors based on the data from the neutron-monitor network shows their satisfactory agreement for the last ground-level enhancement of solar cosmic rays observed on December 13, 2006.

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

In this paper we will report the results of the computation of cutoff rigidities of vertical and non-vertical incident cosmic ray particles. Non-vertical effective cutoff rigidities have been computed by tracing particle trajectories through the “real” geomagnetic magnetic field comprising the International Geomagnetic Reference Field model (IGRF95, IAGA Division 5 Working Group 8, 1996: Sabaka, T.J., Langel, R.A., Baldwin, R.T., Conrad, J.A. The geomagnetic field, 1900–1995, including the large scale fields from magnetospheric sources and NASA candidate models for the 1995 IGRF revision. J. Geomag. Geoelect. 49, 157–206, 1997.) and the Tsyganenko [Tsyganenko, N.A. A magnetospheric magnetic field model with a warped tail current sheet. Planet. Space Sci. 37, 5–20, 1989.] magnetosphere model. The computation have been done for the backward route (from Antarctica to Italy) of the Italian Antarctic ship survey 1996–1997, for geographic points corresponding to the daily average coordinates of the ship; for zenith angles 15°, 30°, 45° and 60°, and azimuth angles from 0° to 360° in steps of 45°. By means of the obtained non-vertical cutoffs the apparent cutoff rigidities have been calculated. The information on integral multiplicities of secondary neutrons detected by the neutron monitor in dependence of the zenith angle of incoming primary cosmic ray particles have also been used. This information is based on the theoretical calculations of meson-nuclear cascades of primary protons with different rigidities arriving to the Earth’s atmosphere at the zenith angles of 0°, 15°, 30°, 45°, 60° and 75°. The difference between the computed apparent and vertical cutoff rigidities reaches ∼1 GV at rigidities >7–8 GV. At rigidities of 10–16 GV, the difference between the apparent and vertical cutoff rigidities is larger than that obtained earlier by Clem et al. [Clem, J.M., Bieber, J.W., Duldig, M., Evenson, P., Hall, D., Humble, J.E. Contribution of obliquely incident particles to neutron monitor counting rate. J. Geophys. Res. 102, 26919–26926, 1997.] and Dorman et al. [Dorman, L.I., Villoresi, G., Iucci, N., Parisi, M., Tyasto, M.I., Danilova, O.A., Ptitsyna, N.G. Cosmic ray survey to Antarctica and coupling functions for neutron component near solar minimum (1996–1997), 3. Geomagnetic effects and coupling functions. J. Geophys. Res. 105, 21047–21056, 2000.].

Possible influence of cosmic rays on climate through thunderstorm clouds

With decreasing of cosmic ray (CR) intensity caused by increasing of solar activity (SA) or in some short periods of Forbush-decreases, the intensity of secondary CR relativistic electrons decreases and the probability of formation of thunderstorm clouds and discharges between clouds or between clouds and ground is also expected to decrease. This will influence on weather and climate. In this case is very important to have more detail information on the atmospheric electric field distribution in the atmosphere, additional to information what gave now electric field sensors (EFS) only in about one point near the ground. We show that CR not only influenced on atmospheric electric field phenomenon, but can give practically continuous information on the atmospheric electric field distribution in the atmosphere. We extend our theory of CR atmospheric electric field effect on electron–photon, muon and neutron component including different multiplicities. We take into account that about 0.07 of neutron monitor counting rate caused by negative soft muons captured by lead nucleons and formed mesoatoms with generation of several MeV energy neutrons from lead. In this case the neutron monitor or neutron super-monitor works as analyzer that detects muons of only one, negative sign. It is very important because the atmospheric electric field effect have opposite signs for positive and negative muons that main part of this effect in the muon telescope or in ionization chamber is compensated and we can observe only small part of total effect of one sign muons. On the basis of our general theory of CR atmospheric electric field effects with taking into account of negative soft muon acceleration and deceleration in the Earth atmosphere (in dependence of direction and intensity of electric field) we discuss the possibility of existing this effect in CR neutron monitor counting rate and in different multiplicities and calculate the expected effects in dependence of atmospheric electric field distribution in the atmosphere. We show that the comparison of observed effects with theoretically expected will give important information on the value of atmospheric electric field and its distribution in the atmosphere. We consider also the possible influence of secondary relativistic electrons of CR and relativistic electrons precipitated from the Earth’s radiation belts on thunderstorms and lightnings, and through this – on climate change.

The relative transit time of high-rigidity and low-rigidity cosmic rays through the solar system.

A study of neutron monitor count rates with vertical cut-off rigidities from 0.6 to 13 GV indicates that high-energy and low-energy galactic cosmic rays arrive at earth orbit with small time differences compared to a month. This is in contradiction to the theory of the deceleration potential which states that cosmic-ray particles with rigidities greater than 36 V arrive at earth orbit about three months before cosmic-ray particles with lesses rigidines.

Heliospheric modulation strength: effective neutron monitor energy

The widely used concept of the neutron monitor energy range is not well defined. Also, the median energy of a neutron monitor varies in the course of the solar cycle. Here we present a new concept of the effective energy of cosmic rays as measured by neutron monitors. Using a spherically-symmetric model of the heliospheric transport of cosmic rays and the specific yield function of a neutron monitor, we show that there is such an effective energy that the count rate of a given neutron monitor is directly proportional to the flux of cosmic rays with energy above this effective energy, irrespectively of the phase of the solar cycle. The new concept of the effective energy allows to regard the neutron monitor count rate as a direct measurement of the galactic cosmic ray flux with energy above this value. The effective energy varies from about 6 GeV for polar up to about 50 GeV for equatorial stations (e.g., it is about 6.5 GeV for high-latitude Oulu, 8 GeV for mid-latitude Climax and 40 GeV for equatorial Huancayo NM).

The atmospheric radiation response to solar-particle-events

High-energy solar particles, produced in association with solar flares and coronal mass ejections, occasionally bombard the earth's atmosphere,. resulting in radiation intensities additional to the background cosmic radiation. Access of these particles to the earth's vicinity during times of geomagnetic disturbances are not adequately described by using static geomagnetic field models. These solar fluxes are also often distributed non uniformly in space, so that fluxes measured by satellites obtained at great distances from the earth and which sample large volumes of space around the earth cannot be used to predict fluxes locally at the earth's surface. We present here a method which uses the ground-level neutron monitor counting rates as adjoint sources of the flux in the atmosphere immediately above them to obtain solar-particle effective dose rates as a function of position over the earth's surface. We have applied this approach to the large September 29–30, 1989 ground-level event (designated GLE 42) to obtain the magnitude and distribution of the solar-particle effective dose rate from an atypically large event. The results of these calculations clearly show the effect of the softer particle spectra associated with solar particle events, as compared with galactic cosmic rays, results in a greater sensitivity to the geomagnetic field, and, unlike cosmic rays, the near-absence of a “knee” near 60° geomagnetic latitude.

HELIOSPHERIC MODULATION STRENGTH DURING THE NEUTRON MONITOR ERA

Using a stochastic simulation of a one-dimensional heliosphere we calculate galactic cosmic ray spectra at the Earth's orbit for different values of the heliospheric modulation strength Φ. Convoluting these spectra with the specific yield function of a neutron monitor, we obtain the expected neutron monitor count rates for different values of Φ. Finally, inverting this relation, we calculate the modulation strength using the actually recorded neutron monitor count rates. We present the reconstructed annual heliospheric modulation strengths for the neutron monitor era (1953–2000) using several neutron monitors from different latitudes, covering a large range of geomagnetic rigidity cutoffs from polar to equatorial regions. The estimated modulation strengths are shown to be in good agreement with the corresponding estimates reported earlier for some years.

Coronal electron acceleration and relativistic proton production during the 14 July 2000 flare and CME

The large solar flare of 14 July 2000 10 UT occurred in an active region near the central meridian. It was accompanied by the eruption of a lament and a rapid halo-type coronal mass ejection (CME). Large particle fluxes were detected up to relativistic energies at 1 AU. In this paper accelerated particles and plasma structures in the corona are traced using radio, X-ray, EUV and visible light observations, together with neutron monitor measurements of relativistic protons at 1 AU. Both the bulk of the radio emission at decimetric and longer waves and the escape of suprathermal electrons and relativistic protons from the Sun were delayed by 10{20 min with respect to the hard X-ray emission. Despite the delay and the association with a flare near the central meridian the neutron monitor time prole was impulsive. We show that the escape of the relativistic protons occurred in time coincident both with a coronal shock wave, which may be the bow shock of the CME, and with radio sources which trace electron acceleration and magnetic eld reconguration in the western hemisphere. Three observations support the idea that the relativistic protons were accelerated during this reconguration, at heights between 0.1 and 1 R above the photosphere, and not in the flaring active region or at the bow shock of the CME: (i) the rise of the neutron monitor count rates is simultaneous with the brightening of a new continuum radio source; (ii) the duration of the continuum emission is similar to the rise time of the neutron monitor count rates; (iii) the radio source is close to the Earth-connected interplanetary magnetic eld line.

Sensitivity of a neutron monitor to galactic cosmic rays

Abstract We studied the sensitivity of a neutron monitor to Galactic cosmic rays depending on energy of cosmic ray particles in the interstellar medium. This is important for studying of the interstellar spectrum of cosmic rays and details of solar modulation by means of the world neutron monitor network. We calculated the expected response of a standard sea-level neutron monitor vs. energy of cosmic ray particles. First, we studied modulation of a monoenergetic flux of cosmic rays. Then the specific atmospheric yield function of a neutron monitor was applied to the calculated flux of cosmic rays at the Earth's orbit. The obtained response function, being convoluted with the model interstellar spectrum of cosmic rays, gives a maximum of neutron monitor sensitivity at around several GV of rigidity of cosmic ray particles. We performed calculations for weak (φ = 350 MV) and medium (φ = 750 MV) modulation strength. A normalisation of neutron monitor count rate is suggested which can give an experimental measure of the overall solar modulation of cosmic rays.

An investigation of short-term solar wind modulation of atmospheric electricity at Mauna Loa Observatory

A search has been made for short-term signatures in the global electric circuit of solar wind modulated inputs into the atmosphere, using atmospheric electricity observations made at Mauna Loa, Hawaii, in 1976–1984. The short-term events were Forbush decreases of the galactic cosmic ray flux (characterized by decreases in neutron monitor count rates); geomagnetic disturbances (characterized by the Ap index); and ring current changes (characterized by the Dst index). Each set of events was designated by a set of key days, which were used in superposed epoch analyses of the time-series of daily averages of the atmospheric electricity parameters. The observations consisted of measurements of air-earth current density, potential gradient, positive conductivity, and negative conductivity. The Mauna Loa observatory is at an altitude of 3.4 km in a relatively clean and dust free oceanic environment. The meteorologically-induced electrical noise there is less than at most other atmospheric electricity observatories, with the exception of high altitude sites on the polar ice caps. Nevertheless the noise level was still a problem even after making superposed epoch analyses using several tens of events. We find responses similar to those expected from solar wind inputs but of marginal statistical significance. Diurnal and annual variations were found to be well defined, and consistent with those found by others.

Connections between neutron monitor count rate and solar modulation strength

We suggest a new approach to the normalisation of neutron monitor response to galactic cosmic rays. The reference normalisation count rate is the neutron monitor response to the model unmodulated flux of galactic cosmic rays. A comparison of the actually recorded neutron monitor count rate with the calculated normalisation count rate can provide one with an observationally obtained true-of-date integral measure of the current level of solar modulation of galactic cosmic rays.

Correlative study of solar activity and cosmic ray intensity

We perform a correlative study of solar activity (sunspot numbers) and cosmic ray intensity (neutron monitor count rates) for the last four solar cycles. Analysis of the running cross correlation between the two series shows that the behavior of cosmic ray modulation is similar, in general, for particles with different energy. However, a strong rigidity dependence as well as an unusual behavior of the cross correlation function is found for the descending phase of cycle 20. We study the evolution of cosmic ray and solar activity cycles in a three‐dimensional phase space by means of the delayed component method. While all solar activity cycles and most cosmic ray cycles are planar, cosmic ray cycle 20 is significantly three‐dimensional. A concept of the momentary phase of a cycle is introduced, and the phase evolution of cosmic ray and solar activity cycles is studied. We also discuss the heliospheric conditions responsible for the unusual behavior of cosmic ray modulation in the descending phase of cycle 20.

Contribution of obliquely incident particles to neutron monitor counting rate

We describe a new method for calculating geomagnetic cutoffs that incorporates obliquely incident primaries, and we use it to interpret a sea level neutron monitor latitude survey. Effects due to obliquely incident primaries are significant and may be responsible for anomalies observed in this and other latitude surveys. We define an “apparent” cutoff that takes these obliquely incident particles into account. Use of our apparent cutoff in a Dorman function fit to the 1994–1995 Bartol Research Institute‐University of Tasmania latitude survey data results in a significant improvement over use of the more conventional effective vertical cutoff.

Cosmic-ray atmospheric electric field effects

Experimental data on the atmospheric electric field effect in the cosmic-ray muon component are discussed on the basis of the general theory of cosmic-ray meteorological effects. In this framework, we develop the theory of atmospheric electric field effects in the hard- and soft-muons of secondary cosmic rays and in the neutron-monitor counting rates as well. We show that the experimental results can be understood on the basis of this theory. We also show that a sufficient atmospheric electric field effect in the cosmic-ray neutron component is to be expected because the neutron monitors work as analyzers of soft muons and really detect only negative muons as well as neutrons.

The 1991 March 22 flare: Possible anisotropy of high-energy neutral emission

We made a parameter fit to the Haleakala neutron monitor counting rate during the 1991 March 22 solar flare (Pyle and Simpson, 1991) using the time profiles of γ-rays at 0.42–80 MeV obtained with the GRANAT satellite (Vilmeret al., 1994) and the microwave data from Owens Valley Radio Observatory. We use a two-component neutron injection function to find that either an impulsive injection or the ‘impulsive-plus-prolonged’ neutron injection is possible. In both cases, the number of > 300 MeV neutrons emitted towards the Earth is estimated as ≈ 2 × 1027 sr−1, which is less than that of the 1990 May 24 flare by an order of magnitude. We tested if such a big difference in neutron number detected on the Earth can be accounted for solely by their different positions on the solar disk. For the estimation of the degree of anisotropy of high-energy secondary emission, we made use of macroscopic parameters of the flare active region, in particular, the vector magnetogram data from the Big Bear Solar Observatory. In our result, the anisotropy factor for the neutral emissions of the 1991 March 22 flare is only ≈ 1 – 10, which is rather small compared with previous theoretical predictions for a disk flare. Such a moderate anisotropy is due to the relatively large inclination angles of the magnetic fields at the footpoints of the flaring loop where accelerated particles are trapped. We thus concluded that the smaller number of neutrons of the 1991 March 22 flare would be not only due to its location on the disk, but also due to fewer protons accelerated during this event as compared with the 1990 May 24 limb event. For a more precise determination of the anisotropy factor in a flare, we need a detailed spectrum of electron bremsstrahlung in 0.1 – 10 MeV and the fluence of γ-ray emission from the π0-decay.

Neutron and electromagnetic emissions during the 1990 May 24 solar flare

In this paper, we are primarily concerened with the solar neutron emission during the 1990 May 24 flare, utilizing the counting rate of the Climax neutron monitor and the time profiles of hard X-rays and gamma-rays obtained with the GRANAT satellite (Pelaez et al., 1992; Talon et al., 1993; Terekhov et al., 1993). We compare the derived neutron injection function with macroscopic parameters of the flare region as obtained from the H-alpha and microwave observations made at the Big Bear Solar Observatory (BBSO) and the Owens Valley Radio Observatory (OVRO) respectively. Our results are summarized as folows: (1) to explain the neutron monitor counting rate and 57.5-110 MeV and 2.2 MeV gamma-ray time profiles, we consider a two-component neutron injection function, Q(E, t). (2) From the H-alpha observations, we find a relatively small loop of length approximattely equal to 2 x 10(exp 4) km, which may be regarded as the source for the fast-decaying component of gamma-rays (57.5-110 MeV) and for the first component of neutron emission. From microwave visibility and the microwave total power spectrum we postulate the presence of a rather big loop (approximately equal to 2 x 10(exp 5) km), which we regard as being responsible for the slow-decaying component of the high-energy emission. We show how the neutron and gamma-ray emission data can be explained in terms of the macroscopic parameters derived from the H-alpha and microwave observations. (3) The H-alpha observations also reveal the presence of a fast mode MHD shock (the Moreton wave) which precedes the microwave peak by 20-30 s and the peak of gamma-ray intensity by 40-50 s. From this relative timing and the single-pulsed time profiles of both radiations, we can attribute the whole event as due to a prompt acceleration of both electrons and protons by the shock and subsequent deceleration of the trapped particles while they propagate inside the magnetic loops.

Long-term modulation of galactic cosmic radiation and its model for space exploration

As the human exploration of space has received new attention in the United States, studies find that exposure to space radiation could adversely impact the mission design. Galactic Cosmic Radiation (GCR), with its very wide range of charges and energies, is particularly important for a mission to Mars, because it imposes a stiff mass penalty for spacecraft shielding. Dose equivalent versus shielding thickness calculations, show a rapid initial drop in exposure with thickness, but an asymptotic behavior at a higher shielding thickness. Uncertainties in the radiobiology are largely unknown. For a fixed radiation risk, this leads to large uncertainties in shielding thickness for small uncertainties in estimated dose. In this paper we investigate the application of steady-state, spherically-symmetric diffusion-convection theory of solar modulation to individual measurements of differential energy spectra from 1954 to 1989 in order to estimate the diffusion coefficient, κ(r,t), as a function of time. We have correlated the diffusion coefficient to the Climax neutron monitor rates and show that, if the diffusion coefficient can be separated into independent functions of space and time: κ(r,t) = K(t)κ 0βPκ 1(r), where β is the particle velocity and P the rigidity, then (i) The time dependent quantity l/K(t), which is proportional to the deceleration potential, φ(r,t), is linearly related to the Climax neutron monitor counting rate. (ii) The coefficients obtained from hydrogen or helium intensity measurements are the same. (iii) There are different correlation functions for odd and even solar cycles. (iv) The correlation function for the Climax neutron monitor counting rate for given time, t, can be used to estimate mean deceleration parameter φ(t) to within ±15% with 90% confidence. We have shown that κ(r,t) determined from hydrogen and/or helium data, can be used to fit the oxygen and iron differential energy spectra with a root mean square error of about ±10%, and essentially independent of the particle charge or energy. We have also examined the ion chamber and 14C measurements which allow the analysis to be extended from the year 1906 to 1990. Using this model we have defined reference GCR spectra at solar minimum and solar maximum. These can be used for space exploration studies and provide a quantitative estimate of the error in dose due to changes in GCR intensities.