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* News 04/04/20 * The archive is using a new backend database. This has thrown up a few SQL errors in the last few days. If you have any issues please email adavey@nso.edu with either the number of eprint you are trying to edit or a link to your preprint.

Spectral characteristic of mid-term quasi-periodicities in sunspots data  

Valery Pipin   Submitted: 2019-11-21 00:26

Numerous analyses suggest the existence of various quasi-periodicities in solar activity. The power spectrum of solar activity recorded in sunspot data is dominated by the ∼11-year quasi-periodicity, known as the Schwabe cycle. In the mid-term range (1 month - 11 years) a pronounced variability known as a quasi-biennial oscillation (QBO) is widely discussed. In the shorter time scale a pronounced peak, corresponding to the synodic solar rotation period (∼ 27 days) is observed. Here we revisited the mid-term solar variability in terms of statistical dynamic of fully turbulent systems, where solid arguments are required to accept an isolated dominant frequency in a continuous (smooth) spectrum. For that, we first undertook an unbiased analysis of the standard solar data, sunspot numbers and the F10.7 solar radioflux index, by applying a wavelet tool, which allows one to perform a frequency-time analysis of the signal. Considering the spectral dynamics of solar activity cycle by cycle, we showed that no single periodicity can be separated, in a statistically significant manner, in the specified range of periods. We examine whether a model of solar dynamo can reproduce the mid-term oscillation pattern observed in solar data. We found that a realistically observed spectrum can be explained if small spatial (but not temporal) scales are effectively smoothed. This result is important because solar activity is a it global feature, although monitored via small-scale tracers like sunspots.

Authors: P. Frick, D. Sokoloff, R. Stepanov, V. Pipin, I. Usoskin
Projects: Other

Publication Status: accepted in MNRAS
Last Modified: 2019-11-21 14:53
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On the Origin of Solar Torsional Oscillations and Extended Solar Cycle  

Valery Pipin   Submitted: 2019-11-20 18:43

We present a nonlinear mean-field model of the solar interior dynamics and dynamo, which reproduces the observed cyclic variations of the global magnetic field of the Sun, as well as the differential rotation and meridional circulation. Using this model, we explain, for the first time, the extended 22-year pattern of the solar torsional oscillations, observed as propagation of zonal variations of the angular velocity from high latitudes to the equator during the time equal to the full dynamo cycle. In the literature, this effect is usually attributed to the so-called "extended solar cycle". In agreement with the commonly accepted idea our model shows that the torsional oscillations can be driven by a combinations of magnetic field effects acting on turbulent angular momentum transport, and the large-scale Lorentz force. We find that the 22-year pattern of the torsional oscillations can result from a combined effect of an overlap of subsequent magnetic cycles and magnetic quenching of the convective heat transport. The latter effect results in cyclic variations of the meridional circulation in the sunspot formation zone, in agreement with helioseismology results. The variations of the meridional circulation together with other drivers of the torsional oscillations maintain their migration to the equator during the 22-year magnetic cycle, resulting in the observed extended pattern of the torsional oscillations.

Authors: V.V. Pipin and A.G. Kosovichev
Projects: Other

Publication Status: accepted in ApJ
Last Modified: 2019-11-20 22:52
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Evolution of Magnetic Helicity in Solar Cycle 24  

Valery Pipin   Submitted: 2019-06-04 19:46

We propose a novel approach to reconstruct the surface magnetic helicity density on the Sun or Sun-like stars. The magnetic vector potential is determined via decomposition of vector magnetic-field measurements into toroidal and poloidal components. The method is verified using data from a non-axisymmetric dynamo model. We apply the method to vector field synoptic maps from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory to study the evolution of the magnetic helicity density during solar cycle 24. It is found that the mean helicity density of the non-axisymmetric magnetic field of the Sun evolves in a way similar to that reported for the current helicity density of the solar active regions. It predominantly has a negative sign in the northern hemisphere, while it is mainly positive in the southern hemisphere. Also, the hemispheric helicity rule for the non-axisymmetric magnetic field showed the sign inversion at the end of cycle 24. The evolution of the magnetic helicity density of a large-scale axisymmetric magnetic field is different from what is predicted by dynamo theory. On one hand, the mean large- and small-scale components of magnetic helicity density display the hemispheric helicity rule of opposite signs at the beginning of cycle 24. However, later in the cycle, the two helicities exhibit the same sign, in contrast with theoretical expectations.

Authors: V.V. Pipin, A.A. Pevtsov, Yang Liu, A.G. Kosovichev
Projects: None

Publication Status: Published online in ApJL
Last Modified: 2019-06-05 14:16
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Nonkinematic solar dynamo models with double-cell meridional circulation  

Valery Pipin   Submitted: 2018-08-06 19:51

Employing the standard solar interior model as input we construct a dynamically-consistent nonlinear dynamo model that takes into account the detailed description of the - effect, turbulent pumping, magnetic helicity balance, and magnetic feedback on the differential rotation and meridional circulation. The background mean-field hydrodynamic model of the solar convection zone accounts the solar-like angular velocity profile and the double-cell meridional circulation. We investigate an impact of the nonlinear magnetic field generation effects on the long-term variability and properties of the magnetic cycle. The nonlinear dynamo solutions are studied in the wide interval of the α effect parameter from a slightly subcritical to supercritical values. It is found that the magnetic cycle period decreases with the increasing cycle's magnitude. The periodic long-term variations of the magnetic cycle are excited in case of the overcritical α effect. These variations result from the hemispheric magnetic helicity exchange. It depends on the magnetic diffusivity parameter and the magnetic helicity production rate. The large-scale magnetic activity modifies the distribution of the differential rotation and meridional circulation inside convection zone. It is found that the magnetic feedback on the global flow affects the properties of the long-term magnetic cycles. We confront our findings with solar and stellar magnetic activity observations.

Authors: V.V. Pipin
Projects: None

Publication Status: JASTP (online)
Last Modified: 2018-08-08 11:37
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On the Origin of the Double-cell Meridional Circulation in the Solar Convection Zone  

Valery Pipin   Submitted: 2018-02-13 19:49

Recent advances in helioseismology, numerical simulations and mean-field theory of solar differential rotation have shown that the meridional circulation pattern may consist of two or more cells in each hemisphere of the convection zone. According to the mean-field theory the double-cell circulation pattern can result from the sign inversion of a nondiffusive part of the radial angular momentum transport (the so-called Λ-effect) in the lower part of the solar convection zone. Here, we show that this phenomenon can result from the radial inhomogeneity of the Coriolis number, which depends on the convective turnover time. We demonstrate that if this effect is taken into account then the solar-like differential rotation and the double-cell meridional circulation are both reproduced by the mean-field model. The model is consistent with the distribution of turbulent velocity correlations determined from observations by tracing motions of sunspots and large-scale magnetic fields, indicating that these tracers are rooted just below the shear layer.

Authors: V.V. Pipin and A.G. Kosovichev
Projects: None

Publication Status: ApJ (online)
Last Modified: 2018-02-14 11:41
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A new dynamo pattern revealed by the tilt angle of bipolar sunspot groups  

Valery Pipin   Submitted: 2013-04-16 10:48

We obtain the latitude-time distribution of the averaged tilt angle of solar bipoles. For large bipoles, which are mainly bipolar sunspot groups, the spatially averaged tilt angle is positive in the Northern solar hemisphere and negative in the Southern, with modest variations during course of the solar cycle. We consider the averaged tilt angle to be a tracer for a crucial element of the solar dynamo, i.e. the regeneration rate of poloidal large-scale magnetic field from toroidal. The value of the tilt obtained crudely corresponds to a regeneration factor corresponding to about 10% of r.m.s. velocity of solar convection. These results develop findings of Kosovichev and Stenflo (2012) concerning Joy's law, and agree with the usual expectations of solar dynamo theory. Quite surprisingly, we find a pronounced deviation from these properties for smaller bipoles, which are mainly solar ephemeral regions. They possess tilt angles of approximately the same absolute value, but of opposite sign compared to that of the large bipoles. Of course, the tilt data for small bipoles are less well determined than those for large bipoles; however they remain robust under various modifications of the data processing.

Authors: A.Tlatov, E.Illarionov, D.Sokoloff, V.Pipin
Projects: None

Publication Status: accepted in MNRAS
Last Modified: 2013-04-17 12:21
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Abstracts by Author
Spectral characteristic of mid-term quasi-periodicities in sunspots data
On the Origin of Solar Torsional Oscillations and Extended Solar Cycle
Evolution of Magnetic Helicity in Solar Cycle 24
Nonkinematic solar dynamo models with double-cell meridional circulation
On the Origin of the Double-cell Meridional Circulation in the Solar Convection Zone
A new dynamo pattern revealed by the tilt angle of bipolar sunspot groups

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