Eleanna Asvestari

The Maunder minimum (1645–1715) was indeed a grand minimum: A reassessment of multiple datasets

Aims. Although the time of the Maunder minimum (1645–1715) is widely known as a period of extremely low solar activity, it is still being debated whether solar activity during that period might have been moderate or even higher than the current solar cycle #24. We have revisited all existing evidence and datasets, both direct and indirect, to assess the level of solar activity during the Maunder minimum. Methods. We discuss the East Asian naked-eye sunspot observations, the telescopic solar observations, the fraction of sunspot active days, the latitudinal extent of sunspot positions, auroral sightings at high latitudes, cosmogenic radionuclide data as well as solar eclipse observations for that period. We also consider peculiar features of the Sun (very strong hemispheric asymmetry of the sunspot location, unusual differential rotation and the lack of the K-corona) that imply a special mode of solar activity during the Maunder minimum. Results. The level of solar activity during the Maunder minimum is reassessed on the basis of all available datasets. Conclusions. We conclude that solar activity was indeed at an exceptionally low level during the Maunder minimum. Although the exact level is still unclear, it was definitely lower than during the Dalton minimum of around 1800 and significantly below that of the current solar cycle #24. Claims of a moderate-to-high level of solar activity during the Maunder minimum are rejected with a high confidence level.

Atmospheric impacts of the strongest known solar particle storm of 775 AD

Sporadic solar energetic particle (SEP) events affect the Earth’s atmosphere and environment, in particular leading to depletion of the protective ozone layer in the Earth’s atmosphere, and pose potential technological and even life hazards. The greatest SEP storm known for the last 11 millennia (the Holocene) occurred in 774–775 AD, serving as a likely worst-case scenario being 40–50 times stronger than any directly observed one. Here we present a systematic analysis of the impact such an extreme event can have on the Earth’s atmosphere. Using state-of-the-art cosmic ray cascade and chemistry-climate models, we successfully reproduce the observed variability of cosmogenic isotope 10Be, around 775 AD, in four ice cores from Greenland and Antarctica, thereby validating the models in the assessment of this event. We add to prior conclusions that any nitrate deposition signal from SEP events remains too weak to be detected in ice cores by showing that, even for such an extreme solar storm and sub-annual data resolution, the nitrate deposition signal is indistinguishable from the seasonal cycle. We show that such a severe event is able to perturb the polar stratosphere for at least one year, leading to regional changes in the surface temperature during northern hemisphere winters.

Heliospheric modulation of galactic cosmic rays: Effective energy of ground-based detectors

Variability of Galactic cosmic ray (GCR) is often expressed in terms of the modulation potential, which is typically assessed using energy-integrating ground-based detectors, such as neutron monitors (NMs) for the last decades or cosmogenic isotopes on the time scales of centuries and millennia. In order to estimate the energy dependence of the GCR variability we re-assess here the effective energy

Analysis of Ground-Level Enhancements: Strong events are hard

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Analysis of Ground Level Enhancements (GLE): Extreme solar energetic particle events have hard spectra

of each type of detector, which is defined so that the variability of the GCR particles at this energy is equal to that of the detectors count rate. We found that

Assessment of different sunspot number series using the cosmogenic isotope44Ti in meteorites

E_{\rm eff}

An empirical model of heliospheric cosmic ray modulation on long-term time scale

is 11--12 GeV/nuc for the standard polar sea-level neutron monitor, but it is essentially smaller for cosmogenic isotopes, being 6--7 GeV/nuc for

Neutron Monitors and Cosmogenic Isotopes as Cosmic Ray Energy-Integration Detectors: Effective Yield Functions, Effective Energy, and Its Dependence on the Local Interstellar Spectrum: COSMIC RAYS: EFFECTIVE YIELD FUNCTIONS

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Use of cosmogenic radionuclides 14C and 10Be to verify empirically reconstructed cosmic ray modulation since 1616

C and 5.5--6 GeV/nuc for

The Maunder minimum: A reassessment from multiple dataset

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