arXiv subjects

I. G. Usoskin

Publications and source records attributed to I. G. Usoskin.

40 records · Page 3Linked to original sources

arXiv2007

Grand minima and maxima of solar activity: New observational constraints

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. We present an updated reconstruction of sunspot number over multiple millennia, from $^{14}$C data by means of a physics-based 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. 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 Spörer 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. These results set new observational constraints upon the long-term behaviour of the solar dynamo.

astro-ph↗

arXiv2005

Preferred sunspot longitudes: Non-axisymmetry and differential rotation

As recently found, the distribution of sunspots is non-axisymmetric and spot group formation implies the existence of two persistent active longitudes separated by 180 degrees. Here we quantitatively study the non-axisymmetry of sunspot occurrence. In a dynamic reference frame inferred from the differential rotation law, the raw sunspot data show a clear clustering around the persistent active longitudes. The differential rotation describing the dynamic frame is quantified in terms of the equatorial angular velocity and the differential rotation rate, which appear to be significantly different from those for individual sunspots. This implies that the active longitudes are not linked to the depth of sunspot anchoring. In order to quantify the observed effect, we introduce a measure of the non-axisymmetry of the sunspot distribution. The non-axisymmetric component is found to be highly significant, and the ratio of its strength to that of the axisymmetric one is roughly 1:10. This provides additional constraints for solar dynamo models.

astro-ph↗

arXiv2004

Latitudinal dependence of low cloud amount on cosmic ray induced ionization

A significant correlation between the annual cosmic ray flux and the amount of low clouds has recently been found for the past 20 years. However, of the physical explanations suggested, none has been quantitatively verified in the atmosphere by a combination of modelling and experiment. Here we study the relation between the global distributions of the observed low cloud amount and the calculated tropospheric ionization induced by cosmic rays. We find that the time evolution of the low cloud amount can be decomposed into a long-term trend and inter-annual variations, the latter depicting a clear 11-year cycle. We also find that the relative inter-annual variability in low cloud amount increases polewards and exhibits a highly significant one-to-one relation with inter-annual variations in the ionization over the latitude range 20--55$^\circ$S and 10--70$^\circ$N. This latitudinal dependence gives strong support for the hypothesis that the cosmic ray induced ionization modulates cloud properties.

physics.ao-ph↗

arXiv2003

Reconstruction of solar activity for the last millennium using $^{10}$Be 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 Earth's atmosphere, cosmic ray transport in the heliosphere, and evolution of the Sun's 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.

astro-ph↗