Ilya G. Usoskin

Unusual activity of the Sun during recent decades compared to the previous 11,000 years

Direct observations of sunspot numbers are available for the past four centuries, but longer time series are required, for example, for the identification of a possible solar influence on climate and for testing models of the solar dynamo. Here we report a reconstruction of the sunspot number covering the past 11,400 years, based on dendrochronologically dated radiocarbon concentrations. We combine physics-based models for each of the processes connecting the radiocarbon concentration with sunspot number. According to our reconstruction, the level of solar activity during the past 70 years is exceptional, and the previous period of equally high activity occurred more than 8,000 years ago. We find that during the past 11,400 years the Sun spent only of the order of 10% of the time at a similarly high level of magnetic activity and almost all of the earlier high-activity periods were shorter than the present episode. Although the rarity of the current episode of high average sunspot numbers may indicate that the Sun has contributed to the unusual climate change during the twentieth century, we point out that solar variability is unlikely to have been the dominant cause of the strong warming during the past three decades.

Grand minima and maxima of solar activity: new observational constraints

Aims. Using a reconstruction of sunspot numbers stretching over multiple millennia, we analyze the statistics of the occurrence of grand minima and maxima and set new observational constraints on long-term solar and stellar dynamo models. Methods. We present an updated reconstruction of sunspot number over multiple millennia, from 14 C data by means of a physicsbased model, using an updated model of the evolution of the solar open magnetic flux. A list of grand minima and maxima of solar activity is presented for the Holocene (since 9500 BC) and the statistics of both the length of individual events as well as the waiting time between them are analyzed. Results. The occurrence of grand minima/maxima is driven not by long-term cyclic variability, but by a stochastic/chaotic process. The waiting time distribution of the occurrence of grand minima/maxima deviates from an exponential distribution, implying that these events tend to cluster together with long event-free periods between the clusters. Two different types of grand minima are observed: short (30–90 years) minima of Maunder type and long (>110 years) minima of Sporer type, implying that a deterministic behaviour of the dynamo during a grand minimum defines its length. The duration of grand maxima follows an exponential distribution, suggesting that the duration of a grand maximum is determined by a random process. Conclusions. These results set new observational constraints upon the long-term behaviour of the solar dynamo.

Millennium-scale sunspot number reconstruction: Evidence for an unusually active sun since the 1940s

The extension of the sunspot number series backward in time is of considerable interest for dynamo theory, solar, stellar, and climate research. We have used records of the (10)Be concentration in polar ice to reconstruct the average sunspot activity level for the period between the year 850 to the present. Our method uses physical models for processes connecting the (10)Be concentration with the sunspot number. The reconstruction shows reliably that the period of high solar activity during the last 60 years is unique throughout the past 1150 years. This nearly triples the time interval for which such a statement could be made previously.

Evolution of the solar irradiance during the Holocene

Context. Long-term records of solar radiative output are vital for understanding solar variability and past climate change. Measurements of solar irradiance are available for only the last three decades, which calls for reconstructions of this quantity over longer time scales using suitable models. Aims. We present a physically consistent reconstruction of the total solar irradiance for the Holocene. Methods. We extend the SATIRE (Spectral And Total Irradiance REconstruction) models to estimate the evolution of the total (and partly spectral) solar irradiance over the Holocene. The basic assumption is that the variations of the solar irradiance are due to the evolution of the dark and bright magnetic features on the solar surface. The evolution of the decadally averaged magnetic flux is computed from decadal values of cosmogenic isotope concentrations recorded in natural archives employing a series of physics-based models connecting the processes from the modulation of the cosmic ray flux in the heliosphere to their record in natural archives. We then compute the total solar irradiance (TSI) as a linear combination of the jth and jth + 1 decadal values of the open magnetic flux. In order to evaluate the uncertainties due to the evolution of the Earth’s magnetic dipole moment, we employ four reconstructions of the open flux which are based on conceptually different paleomagnetic models. Results. Reconstructions of the TSI over the Holocene, each valid for a different paleomagnetic time series, are presented. Our analysis suggests that major sources of uncertainty in the TSI in this model are the heritage of the uncertainty of the TSI since 1610 reconstructed from sunspot data and the uncertainty of the evolution of the Earth’s magnetic dipole moment. The analysis of the distribution functions of the reconstructed irradiance for the last 3000 years, which is the period that the reconstructions overlap, indicates that the estimates based on the virtual axial dipole moment are significantly lower at earlier times than the reconstructions based on the virtual dipole moment. We also present a combined reconstruction, which represents our best estimate of total solar irradiance for any given time during the Holocene. Conclusions. We present the first physics-based reconstruction of the total solar irradiance over the Holocene, which will be of interest for studies of climate change over the last 11 500 years. The reconstruction indicates that the decadally averaged total solar irradiance ranges over approximately 1.5 W/m 2 from grand maxima to grand minima.

The AD775 cosmic event revisited: the Sun is to blame

Aims. Miyake et al. (2012, Nature, 486, 240, henceforth M12) recently reported, based on 14 C data, an extreme cosmic event in about AD775. Using a simple model, M12 claimed that the event was too strong to be caused by a solar flare within the standard theory. This implied a new paradigm of either an impossibly strong solar flare or a very strong cosmic ray event of unknown origin that occurred around AD775. However, as we show, the strength of the event was significantly overestimated by M12. Several subsequent works have attempted to find a possible exotic source for such an event, including a giant cometary impact upon the Sun or a gamma-ray burst, but they are all based on incorrect estimates by M12. We revisit this event with analysis of new datasets and consistent theoretical modelling. Methods. We verified the experimental result for the AD775 cosmic ray event using independent datasets including 10 Be series and newly measured 14 C annual data. We surveyed available historical chronicles for astronomical observations for the period around the AD770s to identify potential sightings of aurorae borealis and supernovae. We interpreted the 14 C measurements using an appropriate carbon cycle model. Results. We show that: (1) The reality of the AD775 event is confirmed by new measurements of 14 C in German oak; (2) by using an inappropriate carbon cycle model, M12 strongly overestimated the event’s strength; (3) the revised magnitude of the event (the global 14 C production Q = (1.1−1.5) × 10 8 atoms/cm 2 ) is consistent with different independent datasets ( 14 C, 10 Be, 36 Cl) and can be associated with a strong, but not inexplicably strong, solar energetic particle event (or a sequence of events), and provides the first definite evidence for an event of this magnitude (the fluence >30 MeV was about 4.5 × 10 10 cm −2 ) in multiple datasets; (4) this interpretation is in agreement with increased auroral activity identified in historical chronicles. Conclusions. The results point to the likely solar origin of the event, which is now identified as the greatest solar event on a multimillennial time scale, placing a strong observational constraint on the theory of explosive energy releases on the Sun and cool stars.

Reconstruction of solar activity for the last millenium using 10Be data

In a recent paper (Usoskin et al. 2002a), we have reconstructed the concentration of the cosmogenic 10 Be isotope in ice cores from the measured sunspot numbers by using physical models for 10 Be production in the Earths atmosphere, cosmic ray transport in the heliosphere, and evolution of the Suns open magnetic flux. Here we take the opposite route: starting from the 10 Be concentration measured in ice cores from Antarctica and Greenland, we invert the models in order to reconstruct the 11-year averaged sunspot numbers since 850 AD. The inversion method is validated by comparing the reconstructed sunspot numbers with the directly observed sunspot record since 1610. The reconstructed sunspot record exhibits a prominent period of about 600 years, in agreement with earlier observations based on cosmogenic isotopes. Also, there is evidence for the century scale Gleissberg cycle and a number of shorter quasi-periodicities whose periods seem to fluctuate in the millennium time scale. This invalidates the earlier extrapolation of multi-harmonic representation of sunspot activity over extended time intervals.

Heliospheric modulation of cosmic rays and solar activity during the Maunder minimum

Modern models and direct cosmic ray experiments deal with heliospheric modulation of cosmic rays only during the recent times of rather high overall solar activity level. On the other hand, the question of cosmic ray modulation during the exceptional conditions of very quiet heliosphere is important. In the present paper we compare the variations of cosmic ray intensity with solar and auroral activity during the Maunder minimum (1645-1715) when the Sun was extremely quiet. We use the newly presented group sunspot number series as a measure of early solar activity, the auroral observations in central Europe as an indicator of transient phenomena in the inner heliosphere, and the radiocarbon data as a proxy of cosmic ray intensity. We find that both cosmic ray intensity and auroral activity closely follow the dominant 22-year cyclicity with sunspot activity during the Maunder minimum. Moreover, the strict antiphase between the 22-year variation of cosmic ray intensity and sunspot activity suggests that the 22-year variation in cosmic ray intensity can be explained by the diffusion-dominated terms of cosmic ray modulation without significant drift effects. We also discuss the possible origin of the behavior of the 10 Be data which is different from all other parameters during the Maunder minimum.

OCCURRENCE OF EXTREME SOLAR PARTICLE EVENTS: ASSESSMENT FROM HISTORICAL PROXY DATA

The probability of occurrence of extreme solar particle events (SPEs) with proton fluence (>30 MeV) F 30 ≥ 1010 cm–2 is evaluated based on data on the cosmogenic isotopes 14C and 10Be in terrestrial archives covering centennial-millennial timescales. Four potential candidates with F 30 = (1-1.5) × 1010 cm–2 and no events with F 30 > 2 × 1010 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 11,400 years, 19 SPE candidates with F 30 = (1-3) × 1010 cm–2 are found and clearly no event with F 30 > 5 × 1010 cm–2 (50 times the SPE of 1956 February 23) has occurred. These values serve as observational upper limits on the strength of SPEs on the timescale of tens of millennia. Two events, ca. 780 and 1460 AD, appear in different data series making them strong candidates for extreme SPEs. We build a distribution of the occurrence probability of extreme SPEs, providing a new strict observational constraint. Practical limits can be set as F 30 ≈ 1, 2-3, and 5×1010 cm–2 for occurrence probabilities ≈10–2, 10–3, and 10–4 yr–1, respectively. Because of the uncertainties, our results should be interpreted as a conservative upper limit on the SPE occurrence near Earth. The mean solar energetic particle (SEP) flux is evaluated as ≈40 (cm2 s)–1, in agreement with estimates from lunar rocks. On average, extreme SPEs contribute about 10% to the total SEP fluence.

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.

Revisited Sunspot Data: A New Scenario for the Onset of the Maunder Minimum

The Maunder minimum forms an archetype for the Grand minima, and detailed knowledge of its temporal development has important consequences for the solar dynamo theory dealing with long-term solar activity evolution. Here, we reconsider the current paradigm of the Grand minimum general scenario by using newly recovered sunspot observations by G. Marcgraf and revising some earlier uncertain data for the period 1636-1642, i.e., one solar cycle before the beginning of the Maunder minimum. The new and revised data dramatically change the magnitude of the sunspot cycle just before the Maunder minimum, from 60-70 down to about 20, implying a possibly gradual onset of the minimum with reduced activity started two cycles before it. This revised scenario of the Maunder minimum changes, through the paradigm for Grand solar/stellar activity minima, the observational constraint on the solar/stellar dynamo theories focused on long-term studies and occurrence of Grand minima.