A ground‐based neutron monitor (NM) is a standard tool to measure cosmic ray (CR) variability near Earth, and it is crucially important to know its yield function for primary CRs. Although there are ...several earlier theoretically calculated yield functions, none of them agrees with experimental data of latitude surveys of sea‐level NMs, thus suggesting for an inconsistency. A newly computed yield function of the standard sea‐level 6NM64 NM is presented here separately for primary CR protons and α‐particles, the latter representing also heavier species of CRs. The computations have been done using the GEANT‐4 PLANETOCOSMICS Monte‐Carlo tool and a realistic curved atmospheric model. For the first time, an effect of the geometrical correction of the NM effective area, related to the finite lateral expansion of the CR induced atmospheric cascade, is considered, which was neglected in the previous studies. This correction slightly enhances the relative impact of higher‐energy CRs (energy above 5–10 GeV/nucleon) in NM count rate. The new computation finally resolves the long‐standing problem of disagreement between the theoretically calculated spatial variability of CRs over the globe and experimental latitude surveys. The newly calculated yield function, corrected for this geometrical factor, appears fully consistent with the experimental latitude surveys of NMs performed during three consecutive solar minima in 1976–1977, 1986–1987, and 1996–1997. Thus, we provide a new yield function of the standard sea‐level NM 6NM64 that is validated against experimental data.
Key Points
A new calculation of the neutron monitor yield function is presented.
Effect of enhanced effective area of NM is considered for the first time.
The discrepancy between observed and modelled latitude surveys is resolved.
On the basis of neutron monitor data, we estimate the energy spectrum, anisotropy axis direction, and pitch-angle distribution of solar energetic particles during two major ground-level enhancements ...(GLE 59 on 14 July 2000 and GLE 70 on 13 December 2006). For the analysis we used a newly computed neutron monitor yield function. The method consists of several consecutive steps: definition of the asymptotic viewing cones of neutron monitor stations considered for the data analysis by computing the cosmic ray particle propagation in a model magnetosphere with the MAGNETOCOSMICS code, computing the neutron monitor model responses, and deriving the solar energetic particle characteristics on the basis of inverse problem solution. The pitch-angle distribution and rigidity spectrum of high-energy protons are obtained as a function of time in the course of ground-level enhancements. A comparison with previously reported results is performed and reasonable agreement is achieved. A discussion of the obtained results is included.
We have analyzed the data of the world neutron monitor network for the first ground level enhancement of solar cycle 24, the ground level enhancement (GLE) on 17 May 2012. A newly computed neutron ...monitor yield function and an inverse method are applied to estimate the energy spectrum, anisotropy axis direction, and pitch angle distribution of the high‐energy solar particles in interplanetary space. The method includes the determination of the asymptotic viewing cones of neutron monitor stations through computations of trajectories of cosmic rays in a model magnetosphere. The cosmic ray particle trajectories are determined with the GEANT‐based MAGNETOCOSMICS code using Tsyganenko 1989 and International Geomagnetic Reference Field models. Subsequent calculation of the neutron monitor responses with the model function is carried out, that represents an initial guess of the inverse problem. Derivation of the solar energetic particle characteristics is fulfilled by fitting the data of the global neutron monitor network using the Levenberg‐Marquardt method over the nine‐dimensional parameter space. The pitch angle distribution and rigidity spectrum of high‐energy protons are obtained as function of time in the course of the GLE. The angular distribution appears quite complicated. It comprises a focused beam along the interplanetary magnetic field line from the Sun and a loss‐cone feature around the opposite direction, possibly indicative of the particle transport in interplanetary magnetic field structures associated with previous coronal mass ejections.
Key Points
Spectra and pitch angle distributions of SEPs reconstructed for the GLE 71
Different populations of particles are found, and their origin is speculated
A sunward SEP population is explained by a preexisting magnetic mirror
The conventional definition of ground-level enhancement (GLE) events requires a detection of solar energetic particles (SEP) by at least two differently located neutron monitors. Some places are ...exceptionally well suitable for ground-based detection of SEP – high-elevation polar regions with negligible geomagnetic and reduced atmospheric energy/rigidity cutoffs. At present, there are two neutron-monitor stations in such locations on the Antarctic plateau: SOPO/SOPB (at Amundsen–Scott station, 2835 m elevation), and DOMC/DOMB (at Concordia station, 3233 m elevation). Since 2015, when the DOMC/DOMB station started continuous operation, a relatively weak SEP event that was not detected by sea-level neutron-monitor stations was registered by both SOPO/SOPB and DOMC/DOMB, and it was accordingly classified as a GLE. This would lead to a distortion of the homogeneity of the historic GLE list and the corresponding statistics. To address this issue, we propose to modify the GLE definition so that it maintains the homogeneity: A GLE event is registered when there are near-time coincident and statistically significant enhancements of the count rates of at least two differently located neutron monitors, including at least one neutron monitor near sea level and a corresponding enhancement in the proton flux measured by a space-borne instrument(s). Relatively weak SEP events registered only by high-altitude polar neutron monitors, but with no response from cosmic-ray stations at sea level, can be classified as sub-GLEs.
Using data obtained with neutron monitors and space-borne instruments, we analyzed the second ground-level enhancement (GLE) of Solar Cycle 24, namely the event of 10 September 2017 (GLE 72), and ...derived the spectral and angular characteristics of associated GLE particles. We employed a new neutron-monitor yield function and a recently proposed model based on an optimization procedure. The method consists of simulating particle propagation in a model magnetosphere in order to derive the cutoff rigidity and neutron-monitor asymptotic directions. Subsequently, the rigidity spectrum and anisotropy of GLE particles are obtained in their dynamical evolution during the event on the basis of an inverse-problem solution. The derived angular distribution and spectra are discussed briefly.
As a result of notable solar activity observed in April 2001, one of the strongest ground level enhancements (GLE) of solar cycle 23 occurred, namely GLE # 60 on 15 April 2001. In this paper, we ...derived the spectral and angular characteristics, and apparent source position of the solar protons during the GLE # 60, using a verified by direct measurements model and employing the calibrated neutron monitor yield function. Subsequently, employing the updated and verified by balloon measurements dosimetric model: Oulu CRAC:DOMO (Cosmic Ray Atmospheric Cascade: Dosimetric Model) we computed the dose rates throughout the event at several altitudes using the obtained spectra as an input. A global map of the ambient dose at an altitude of 35 kft is computed. A comparison with direct dosimetric measurements obtained by a Liulin device during an intercontinental flight is performed and good agreement is achieved.
Plain Language Summary
During, and after, solar eruptions solar energetic particles (SEPs) can be produced. SEPs with enough energy, if directed at Earth, penetrate the Earth's magnetic environment and enter the atmosphere, upon which they create complex atmospheric cascades of secondary particles that are detected at the ground. These events are called ground level enhancements (GLEs). GLEs bring with them space weather risks caused by the increase in SEPs arriving at Earth, which can damage spacecraft and increase the radiation dose at aviation altitudes. Analyzing GLE events to determine their characteristics and space weather impacts is crucial for developing important space weather nowcasting capabilities that can help mitigate damage caused by future GLE events. This work presents the analysis of one such GLE event observed on 15 April 2001, this event is called GLE 60, also known as the Easter event. The spectral and angular characteristics of the SEPs during GLE 60 were derived and used to compute the radiation dose at aviation altitudes during the event using a new radiation model. The computed doses were then compared to direct dosimetric measurements obtained during an intercontinental flight that occurred during GLE 60 and a good agreement was obtained.
Key Points
Rigidity spectra and angular distribution of solar energetic protons during GLE 60 were derived
Time profiles of the effective dose rates at several altitudes during GLE 60 and the global exposure map at 35 kft were computed
Good agreement between the radiation model and measurements with Liulin dosimetric device from an intercontinental flight was achieved
Aims.
The first relativistic solar proton event of solar cycle 25 was detected on 28 October 2021 by neutron monitors (NMs) on the ground and particle detectors on board spacecraft in near-Earth ...space. This is the first ground-level enhancement (GLE) of the current cycle. A detailed reconstruction of the NM response together with the identification of the solar eruption that generated these particles is investigated based on in situ and remote-sensing measurements.
Methods.
In situ proton observations from a few MeV to ∼500 MeV were combined with the detection of a solar flare in soft X-rays, a coronal mass ejection, radio bursts, and extreme ultraviolet (EUV) observations to identify the solar origin of the GLE. Timing analysis was performed, and a relation to the solar sources was outlined.
Results.
GLE73 reached a maximum particle rigidity of ∼2.4 GV and is associated with type III, type II, and type IV radio bursts and an EUV wave. A diversity of time profiles recorded by NMs was observed. This points to the event having an anisotropic nature. The peak flux at
E
> 10 MeV was only ∼30 pfu and remained at this level for several days. The release time of ≥1 GV particles was found to be ∼15:40 UT. GLE73 had a moderately hard rigidity spectrum at very high energies (
γ
∼ 5.5). Comparison of GLE73 to previous GLEs with similar solar drivers is performed.
As a result of intense solar activity during the first 10 days of September, a ground level enhancement occurred on 10 September 2017. Here we computed the effective dose rates in the polar region at ...several altitudes during the event using the derived rigidity spectra of the energetic solar protons. The contribution of different populations of energetic particles, namely, galactic cosmic rays and solar protons, to the exposure is explicitly considered and compared. We also assessed the exposure of a crew members/passengers to radiation at different locations and at several cruise flight altitudes and calculated the received doses for two typical intercontinental flights. The estimated received dose during a high latitude, 40 kft, ∼10‐hr flight is ∼100 μSv.
Plain Language Summary
As a result of intense solar activity during the first 10 days of September, a ground level enhancement occurred on 10 September 2017. We computed the exposure, namely, the effective dose rates in the polar region at several altitudes during the event using the derived spectra of the solar protons. The contribution of different populations of energetic particles, namely, galactic cosmic rays and solar protons, to the exposure is explicitly considered and compared. We also assessed the exposure of a crew members/passengers to radiation at different locations and at several cruise flight altitudes and calculated the received doses for two typical intercontinental flights.
Key Points
Rigidity spectra of solar energetic protons during the GLE 72 were reconstructed using data from the global neutron monitor network
Time profiles of the effective dose rates at several altitudes during the GLE 72 were estimated
Received doses for crew/passenger of a typical intercontinental high‐latitude ∼10‐hr flight were estimated as ∼100 μSv
We retrieve ionization rates in the atmosphere caused by energetic electron precipitation from balloon observations in the polar atmosphere and compare them against ionization rates recommended for ...the Phase 6 of the Coupled Model Intercomparison Project. In our retrieval procedure, we consider the precipitating electrons with energies from about tens of keV to 5 MeV. Our simulations with 1‐D radiative‐convective model with interactive neutral and ion chemistry show that the difference of the Phase 6 of the Coupled Model Intercomparison Project and balloon‐based ionization rate can lead to underestimation of the NOx enhancement by more than 100% and ozone loss up to 25% in the mesosphere. The atmospheric response is different below 50 km due to considering highly energetic electrons, but it is not important because the absolute values of atmospheric impact is tiny. Ionization rates obtained from the balloon observations reveal a high variability.
Plain Language Summary
The main idea of our manuscript is to demonstrate that the atmospheric ionization rates (IR) can be successfully retrieved from the long‐term balloon observations of the energetic electron precipitation (EEP) events and used to evaluate the uncertainties of the other IR data sets, for example, IR recommended for the Phase 6 of the Coupled Model Intercomparison Project (CMIP6). IR obtained from the balloon observations reveal a high variability. This means that the time resolution used in CMIP6 probably is not enough to consider high frequency variability of the precipitating electron fluxes. Using 1‐D radiative‐convective model with neutral and ion chemistry, we compared the atmospheric response to the one particular EEP observed by balloons and presented in CMIP6 data. We show that the difference of the CMIP6 and balloon‐based IRs can lead to underestimation of the NOx enhancement by more than 100% and ozone loss by up to 25% in the mesosphere. Our results are new and needed for the understanding of the potential uncertainties in CMIP6 EEP forcing. Our paper will give inspiration for the continuation of the balloon measurements of EEP‐related processes using improved instruments.
Key Points
Ionization rates (IR) from energetic electron precipitation (EEP) are calculated using balloon observations and compared to the CMIP6 data set
The difference in the atmospheric response calculated with these two data sets can exceed 100% for NOx and reach 25% for O3
IR obtained from balloon measurements reveal a high temporal variability, which is absent in CMIP6 data
Polar regions are the most exposed to secondary particles and radiation produced by primary cosmic rays in the atmosphere, because naturally they are with marginal geomagnetic shielding. In addition, ...the secondary particle flux contributing to the complex radiation field is enhanced at high-mountain altitudes compared to sea level because of the reduced atmospheric attenuation. At present, there are very few systematic experimental measurements of environmental dose at high southern latitudes, specifically at high-altitude region. Here, we report a campaign of measurements with different devices, that is passive and Liulin-type dosimeters, of the radiation background at high-mountain Antarctic station Vostok (3488 m above sea level, 78° 27′ S; 106° 50′ E). We compare the measurements with a Monte Carlo-based model for the propagation of the cosmic rays through the atmosphere and assessment of the radiation field in the atmosphere. We employed the model to estimate the radiation dose at Vostok station during the ground-level enhancement at 28 October 2021. As in previous studies by other teams, we show that the annual dose equivalent at high-altitude Antarctic facilities can significantly exceed the limit of 1 mSv established for the general population by the ICRP.
Display omitted
•Measurement of radiation background at high-altitude Antarctic station Vostok•Comparison with model for propagation of the cosmic rays through the atmosphere•Assessment of the contribution of cosmic rays to radiation during solar proton event•The dose at high-altitude Antarctic stations significantly exceeds the limit of 1 mSv.
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