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.
The present work is a critical revision of the hypothesis of the planetary tidal influence on solar activity published by Abreu
et al.
(
Astron. Astrophys.
548
, A88,
2012
; called A12 here). A12 ...describes the hypothesis that planets can have an impact on the solar tachocline and therefore on solar activity. We checked the procedure and results of A12, namely the algorithm of planetary tidal torque calculation and the wavelet coherence between torque and heliospheric modulation potential. We found that the claimed peaks in long-period range of the torque spectrum are artefacts caused by the calculation algorithm (
viz.
aliasing effect). Also the statistical significance of the results of the wavelet coherence is found to be overestimated by an incorrect choice of the background assumption of red noise. Using a more conservative non-parametric random-phase method, we found that the long-period coherence between planetary torque and heliospheric modulation potential becomes insignificant. Thus we conclude that the considered hypothesis of planetary tidal influence on solar activity is not based on a solid ground.
The amplitude of the 11-year cycle measured in the cosmogenic isotope
10
Be during the Maunder Minimum is comparable to that during the recent epoch of high solar activity. Because of the virtual ...absence of the cyclic variability of sunspot activity during the Maunder Minimum this seemingly contradicts an intuitive expectation that lower activity would result in smaller solar-cycle variations in cosmogenic radio-isotope data, or in none, leading to confusing and misleading conclusions. It is shown here that large 11-year solar cycles in cosmogenic data observed during periods of suppressed sunspot activity do not necessarily imply strong heliospheric fields. Normal-amplitude cycles in the cosmogenic radio-isotopes observed during the Maunder Minimum are consistent with theoretical expectations because of the nonlinear relation between solar activity and isotope production. Thus, cosmogenic-isotope data provide a good tool to study solar-cycle variability even during grand minima of solar activity.
Here we present the first direct comparison of cosmogenic (10)Be and chemical species in the period of 38-45.5 kyr BP spanning the Laschamp geomagnetic excursion from the EPICA-Dome C ice core. A ...principal component analysis (PCA) allowed to group different components as a function of the main sources, transport and deposition processes affecting the atmospheric aerosol at Dome C. Moreover, a wavelet analysis highlighted the high coherence and in-phase relationship between (10)Be and nitrate at this time. The evident preferential association of (10)Be with nitrate rather than with other chemical species was ascribed to the presence of a distinct source, here labelled as "cosmogenic". Both the PCA and wavelet analyses ruled out a significant role of calcium in driving the (10)Be and nitrate relationship, which is particularly relevant for a plateau site such as Dome C, especially in the glacial period during which the Laschamp excursion took place. The evidence that the nitrate record from the EDC ice core is able to capture the Laschamp event hints toward the possibility of using this marker for studying galactic cosmic ray flux variations and thus also major geomagnetic field excursions at pluri-centennial-millennial time scales, thus opening up new perspectives in paleoclimatic studies.
A new model of cosmogenic tritium (3H) production in the atmosphere is presented. The model belongs to the CRAC (Cosmic Ray Atmospheric Cascade) family and is named as CRAC:3H. It is based on a full ...Monte Carlo simulation of the cosmic ray induced atmospheric cascade using the Geant4 toolkit. The CRAC:3H model is able, for the first time, to compute tritium production at any location and time, for any given energy spectrum of the primary incident cosmic ray particles, explicitly treating, also for the first time, particles heavier than protons. This model provides a useful tool for the use of 3H as a tracer of atmospheric and hydrological circulation. A numerical recipe for practical use of the model is appended.
Key Points
A new CRAC:3H model of cosmogenic tritium (3H) production in the atmosphere is presented
For the first time, it provides 3‐D production, also explicitly treating particles heavier than protons
This model provides a useful tool for the use of 3H as a tracer of atmospheric and hydrological circulation
This paper presents the results of a comparative wavelet coherence analysis of a multimillennial nitrate record with a number of climatic and solar activity proxies. Distinguishing between these ...factors is important in the view of a possibility of nitrate deposited in a polar region to represent galactic cosmic ray flux and, consequently, solar activity. We used the data from the TALDICE drilling project (Talos Dome, Antarctica), which covers the age range 12,000–700 BP (years before present, i.e. before 1950) and includes records of nitrate as well as climatic proxies, such as Na+, Ca2+, MSA (methanesulphonic acid), δ18O, SO42−. The solar activity series is represented by reconstructions of the heliospheric modulation parameter from the 14C and 10Be data. We found (1) a confirmation of multimillennial relation between nitrate and galactic cosmic ray flux; (2) no clear signature of long-term variations of nitrate transport from lower latitudes. We suggest that variations in the nitrate record in the time scale of hundreds–thousands years are most likely caused by local production, deposition and post-deposition processes.
•Wavelet coherence analysis of nitrate with climatic and solar activity proxies.•Confirmation of multimillennial relation between nitrate and galactic cosmic ray flux.•No clear signature of long-term variations of nitrate transport from lower latitudes.•Variations of nitrate in the scale of hundreds–thousands years are caused by local processes.
New consistent and precise computations of the production of five cosmogenic radioisotopes, 7Be, 10Be, 14C, 22Na, and 36Cl, in the Earth's atmosphere by cosmic rays are presented in the form of ...tabulated yield functions. For the first time, a detailed set of the altitude profiles of the production functions is provided which makes it possible to apply the results directly as input for atmospheric transport models. Good agreement with most of the earlier published works for columnar and global isotopic production rates is shown. Altitude profiles of the production are important, in particular for such tasks as studies of strong solar particle events in the past, precise reconstructions of solar activity on long‐term scale, tracing air mass dynamics using cosmogenic radioisotopes, etc. As an example, computations of the 10Be deposition flux in the polar region are shown for the last decades and also for a period around 780 A.D. and confronted with the actual measurements in Greenland and Antarctic ice cores.
Key Points
A new consistent set of yield functions for cosmogenic isotopes 7Be, 10Be, 14C, 22Na, and 36Cl by cosmic rays in the atmosphere is presented
For the first time, a detailed altitudinal profile of the production is given
The results can be straightforwardly used in the atmospheric chemistry and dynamics models
Aims.
Solar energetic particles (SEPs) have been measured directly in space over the past decades. Rare extreme SEP events are studied based on terrestrial cosmogenic proxy data for the past ten ...millennia. Lunar rocks record the average SEP fluxes on the megayear timescale. The question of whether the SEP fluxes averaged over different timescales are mutually consistent is still open. Here we analyze these different datasets for mutual consistency.
Methods.
Using the data from directly measured SEPs over the past decades and reconstructions of extreme SEP events in the past, we built a distribution function of the occurrence of annual SEP fluences for SEPs with energies above 30, 60, 100, and 200 MeV. The distribution function was fit with the Weibull and other types of distributions, and the long-term average SEP flux was computed and compared with the megayear SEP flux estimated from lunar data.
Results.
In contrast to the current paradigm, the direct space-era data are not representative of the long-term averaged SEP flux because they are only 20–55% of it, while the major fraction was formed by rare extreme SEP events in the past. The combined statistics of direct and proxy data are fully consistent with megayear lunar data, implying that our knowledge of the whole range of the SEP fluxes, from frequent weak to rare extreme events, is now consistent.
Aims.
Lunar soil and rocks are not protected by a magnetic field or an atmosphere and are continuously irradiated by energetic particles that can produce cosmogenic radioisotopes directly inside ...rocks at different depths depending on the particle’s energy. This allows the mean fluxes of solar and galactic cosmic rays to be assessed on the very long timescales of millions of years.
Methods.
Here we show that lunar rocks can serve as a very good particle integral spectrometer in the energy range 20–80 MeV. We have developed a new method based on precise modeling, that is applied to measurements of
26
Al (half-life ≈0.7 megayears) in lunar samples from the
Apollo
mission, and present the first direct reconstruction (i.e., without any a priori assumptions) of the mean energy spectrum of solar and galactic energetic particles over a million of years.
Results.
We show that the reconstructed spectrum of solar energetic particles is totally consistent with that over the last decades, despite the very different levels of solar modulation of galactic cosmic rays (
ϕ
= 496 ± 40 MV over a million years versus (
ϕ
= 660 ± 20 MV for the modern epoch). We also estimated the occurrence probability of extreme solar events and argue that no events with the
F
(>30 MeV) fluence exceeding 5×10
10
and 10
11
cm
−2
are expected on timescales of a thousand and million years, respectively.
Conclusions.
We conclude that the mean flux of solar energetic particles hardly depends on the level of solar activity, in contrast to the solar modulation of galactic cosmic rays. This puts new observational constraints on solar physics and becomes important for assessing radiation hazards for the planned space missions.
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