50 research outputs found
New reconstruction of event-integrated spectra (spectral fluences) for major solar energetic particle events
Fluences of solar energetic particles (SEPs) are not easy to evaluate,
especially for high-energy events (i.e. ground-level enhancements, GLEs).
Earlier estimates of event-integrated SEP fluences for GLEs were based on
partly outdated assumptions and data, and they required revisions. Here, we
present the results of a full revision of the spectral fluences for most major
SEP events (GLEs) for the period from 1956 -- 2017 using updated low-energy
flux estimates along with greatly revisited high-energy flux data and applying
the newly invented reconstruction method including an improved neutron-monitor
yield function. Low- and high-energy parts of the SEP fluence were estimated
using a revised space-borne/ionospheric data and ground-based neutron monitors,
respectively. The measured data were fitted by the modified Band function
spectral shape. The best-fit parameters and their uncertainties were assessed
using a direct Monte Carlo method. As a result, a full reconstruction of the
event-integrated spectral fluences was performed in the energy range above 30
MeV, parametrised, and tabulated for easy use along with estimates of the 68%
confidence intervals. This forms a solid basis for more precise studies of the
physics of solar eruptive events and the transport of energetic particles in
the interplanetary medium, as well as the related applications.Comment: 19 pages, 3 figures, to be published in Astronomy and Astrophysic
A solar cycle lost in 1793--1800: Early sunspot observations resolve the old mystery
Because of the lack of reliable sunspot observation, the quality of sunspot
number series is poor in the late 18th century, leading to the abnormally long
solar cycle (1784--1799) before the Dalton minimum. Using the newly recovered
solar drawings by the 18--19th century observers Staudacher and Hamilton, we
construct the solar butterfly diagram, i.e. the latitudinal distribution of
sunspots in the 1790's. The sudden, systematic occurrence of sunspots at high
solar latitudes in 1793--1796 unambiguously shows that a new cycle started in
1793, which was lost in traditional Wolf's sunspot series. This finally
confirms the existence of the lost cycle that has been proposed earlier, thus
resolving an old mystery. This letter brings the attention of the scientific
community to the need of revising the sunspot series in the 18th century. The
presence of a new short, asymmetric cycle implies changes and constraints to
sunspot cycle statistics, solar activity predictions, solar dynamo theories as
well as for solar-terrestrial relations.Comment: Published by Astrophys. J. Let
Occurrence of extreme solar particle events: Assessment from historical proxy data
The probability of occurrence of extreme solar particle events (SPEs) with
the fluence of (>30 MeV) protons F30>10^{10} cm^{-2} is evaluated based on data
of cosmogenic isotopes 14C and 10Be in terrestrial archives
centennial-millennial time scales. Four potential candidates with
F30=(1-1.5)x10^{10} cm^{-2} and no events with F30>2x10^{10} cm^{-2} are
identified since 1400 AD in the annually resolved 10Be data. A strong SPE
related to the Carrington flare of 1859 AD is not supported by the data. For
the last 11400 years, 19 SPE candidates with F30=(1-3)x10^{10} cm^{-2} are
found and clearly no event with F30>5x10^{10} cm^{-2} (50-fold the SPE of
23-Feb-1956) occurring. This values serve as an observational upper limit for
the strength of SPE on the time scale of tens of millennia. Two events, ca. 780
and 1460 AD, appear in different data series making them strong candidates to
extreme SPEs. We built a distribution of the occurrence probability of extreme
SPEs, providing a new strict observational constraint. Practical limits can be
set as F30~1x, 2-3x, and 5x10^{10} cm^{-2} for the occurrence probability
~10^{-2}, 10^{-3} and 10^{-4} year^{-1}, respectively. Because of
uncertainties, our results should be interpreted as a conservative upper limit
of the SPE occurrence near Earth. The mean SEP flux is evaluated as ~40 (cm2
sec)^{-1} in agreement with estimates from the lunar rocks. On average, extreme
SPEs contribute about 10% to the total SEP fluence.Comment: accepted to Astrophys.
Neutron Monitors and Cosmogenic Isotopes as Cosmic Ray Energy-Integration Detectors : Effective Yield Functions, Effective Energy, and Its Dependence on the Local Interstellar Spectrum
The method of assessment of galactic cosmic rays (GCR) variability over different timescales, using energy-integrating ground-based detectors such as a neutron monitor and cosmogenic isotopes 10Be and 14C stored in natural archives is revisited here. The effective yield functions for cosmogenic 14C (globally mixed in the atmosphere) and 10Be (realistically deposited in the polar region) are calculated and provided, in a tabulated form, in the supporting information. The effective energy of a detector is redefined so that the variability of the flux of GCR particles at this energy is equal to that of the detector's count rate. The effective energy is found as 11–12 GeV/nucleon for the standard polar neutron monitor, and 6–7 GeV/nucleon and 5.5–6 GeV/nucleon for 14C and 10Be, respectively. New “calibration” relations between the force-field modulation potentials, based on different models of local interstellar spectra (LIS) are provided. While such relations are typically based on refitting the modeled cosmic ray spectra with a prescribed LIS model, the method introduced here straightforwardly accounts for the exact type of the detector used to assess the spectrum. The relations are given separately for ground-based neutron monitors and cosmogenic isotopes. This work allows for harmonization of different works related to variability of galactic cosmic ray flux in the vicinity of Earth, on long-term scale.Peer reviewe
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Assessment of different sunspot number series using the cosmogenic isotope 44 Ti in meteorites
Many sunspot number series exist suggesting different levels of solar activity during the past centuries. Their reliability can be assessed only by comparing them with alternative indirect proxies. We test different sunspot number series against the updated record of cosmogenic radionuclide 44Ti measured in meteorites. Two bounding scenarios of solar activity changes have been considered: the HH-scenario (based on the series by Svalgaard and Schatten), in particular, predicting moderate activity during the Maunder minimum, and the LL-scenario (based on the RG series by Lockwood et al.) predicting moderate activity for the 18th–19th centuries and the very low activity level for the Maunder minimum. For each scenario, the magnetic open solar flux, the heliospheric modulation potential and the expected production of 44Ti were computed. The calculated production rates were compared with the corresponding measurements of 44Ti activity in stony meteorites fallen since 1766. The analysis reveals that the LL-scenario is fully consistent with the measured 44Ti data, in particular, recovering the observed secular trend between the 17th century and the Modern grand maximum. On the contrary, the HH-scenario appears significantly inconsistent with the data, mostly due to the moderate level of activity during the Maunder minimum. It is concluded that the HH-scenario sunspot number reconstruction significantly overestimates solar activity prior to the mid-18th century, especially during the Maunder minimum. The exact level of solar activity after 1750 cannot be distinguished with this method, since both H- and L- scenarios appear statistically consistent with the data
New reconstruction of event-integrated spectra (spectral fluences) for major solar energetic particle events
Aims. Fluences of solar energetic particles (SEPs) are not easy to evaluate, especially for high-energy events (i.e. ground-level enhancements, GLEs). Earlier estimates of event-integrated SEP fluences for GLEs were based on partly outdated assumptions and data, and they required revisions. Here, we present the results of a full revision of the spectral fluences for most major SEP events (GLEs) for the period from 1956 to 2017 using updated low-energy flux estimates along with greatly revisited high-energy flux data and applying the newly invented reconstruction method including an improved neutron-monitor yield function.Methods. Low- and high-energy parts of the SEP fluence were estimated using a revised space-borne/ionospheric data and ground-based neutron monitors, respectively. The measured data were fitted by the modified Band function spectral shape. The best-fit parameters and their uncertainties were assessed using a direct Monte Carlo method.Results. A full reconstruction of the event-integrated spectral fluences was performed in the energy range above 30 MeV, parametrised and tabulated for easy use along with estimates of the 68% confidence intervals.Conclusions. This forms a solid basis for more precise studies of the physics of solar eruptive events and the transport of energetic particles in the interplanetary medium, as well as the related applications.</p
The impact of geomagnetic spikes on the production rates of cosmogenic 14C and 10Be in the Earth's atmosphere
We seek corroborative evidence of the geomagnetic spikes detected in the Near East ca. 980 BC and 890 BC in the records of the past production rates of the cosmogenic nuclides 14C and 10Be. Our forward modeling strategy rests on global, time-dependent, geomagnetic spike field models feeding state-of-the-art models of cosmogenic nuclide production. We find that spike models with an energy budget in line with presently inferred large-scale flow at Earth's core surface fail to produce a visible imprint in the nuclide record. Spike models able to reproduce the intensity changes reported in the Near East require an unaccountably high-magnitude core flow, yet their computed impact on cosmogenic isotope production rates remains ambiguous. No simple and unequivocal agreement is obtained between the observed and modeled nuclide records at the epochs of interest. This indicates that cosmogenic nuclides cannot immediately be used to confirm the occurrence of these two geomagnetic spikes