Dynamics of Phase Synchronization between Solar Polar Magnetic Fields Assessed with Van Der Pol and Kuramoto Models

We establish the similarity in two model-based reconstructions of the coupling between the polar magnetic fields of the Sun represented by the solar faculae time series. The reconstructions are inferred from the pair of the coupled oscillators modelled with the Van der Pol and Kuramoto equations. They are associated with the substantial simplification of solar dynamo models and, respectively, a simple ad hoc model reproducing the phenomenon of synchronization. While the polar fields are synchronized, both of the reconstruction procedures restore couplings, which attain moderate values and follow each other rather accurately as the functions of time. We also estimate the evolution of the phase difference between the polar fields and claim that they tend to move apart more quickly than approach each other.

Can the long-term hemispheric asymmetry of solar activity result from fluctuations in dynamo parameters?

Context. The hemispheric asymmetry of sunspot activity observed possesses a regular component varying on a timescale of several solar cycles whose origin and properties are currently debated. Aims This paper addresses the question of whether the long-term hemispheric asymmetry can result from random variations of solar dynamo parameters in time and latitude. Methods. Scatter in the observed tilt angles of sunspot groups was estimated to infer constraints on fluctuations in the dynamo mechanism for poloidal field regeneration. A dynamo model with fluctuations in the Babcock-Leighton type α-effect was designed in accordance with these constraints and then used to compute a large number of magnetic cycles for statistical analyses of their hemispheric asymmetry. Results Hemispheric asymmetry in the simulated dynamo results from the presence of an equator-symmetric part in the oscillating magnetic field. The sub-dominant quadrupolar oscillations are stochastically forced by dominant dipolar oscillations via the equator-symmetric part of the fluctuating α-effect. The amplitude and sense of the asymmetry of individual cycles varies on a timescale of the order of four dynamo-cycle periods. The variations are irregular and not periodic. The model suggests that asymmetry in the polar magnetic fields in the solar minima can be used as a precursor for asymmetry of sunspot activity in the following solar cycle.

Geomagnetic polar minima do not arise from steady meridional circulation

Observations of the Earth’s magnetic field have revealed locally pronounced field minima near each pole at the core–mantle boundary (CMB). The existence of the polar magnetic minima has long been attributed to the supposed large-scale overturning circulation of molten metal in the outer core: Fluid upwells within the inner core tangent cylinder toward the poles and then diverges toward lower latitudes when it reaches the CMB, where Coriolis effects sweep the fluid into anticyclonic vortical flows. The diverging near-surface meridional circulation is believed to advectively draw magnetic flux away from the poles, resulting in the low intensity or even reversed polar magnetic fields. However, the interconnections between polar magnetic minima and meridional circulations have not to date been ascertained quantitatively. Here, we quantify the magnetic effects of steady, axisymmetric meridional circulation via numerically solving the axisymmetric magnetohydrodynamic equations for Earth’s outer core under the magnetostrophic approximation. Extrapolated to core conditions, our results show that the change in polar magnetic field resulting from steady, large-scale meridional circulations in Earth’s outer core is less than 3% of the background field, significantly smaller than the ∼ 100% polar magnetic minima observed at the CMB. This suggests that the geomagnetic polar minima cannot be produced solely by axisymmetric, steady meridional circulations and must depend upon additional tangent cylinder dynamics, likely including nonaxisymmetric, time-varying processes.

Association of calcium network brightness with polar magnetic fields

Recent dedicated HINODE polar region campaign revealed the presence of concentrated kilogauss patches of magnetic field in the polar regions of Sun which are also shown to be correlated with facular bright points at the photospheric level. In this work, we demonstrate that this spatial intermittency of the magnetic field persists even up to the chromospheric heights. Polar network bright points are the ones which are present in the polar regions of the Sun (above 70° latitudes). We use special HINODE campaigns devoted to observe polar regions of the Sun to study the polar network bright points during the phase of last extended solar minimum. We are able to find a considerable association between the polar network bright points and magnetic field concentrations which led us to conclude that these bright points can serve as a good proxy for polar magnetic fields where the direct and regular measurements of polar magnetic fields are not available (before 1970).

Long term trends in solar photospheric fields and solar wind turbulence levels: Implications to the near-Earth space

We re-examined solar polar magnetic fields, using ground based synoptic photospheric magnetograms, during solar cycle 24. IThe signed polar magnetic fields showed an unusual hemispheric asymmetry in the polar field reversal process with a single unambigous reversal in the Southern hemisphere around late 2013 while the polar reversal in the Northern hemisphere started earlier around June 2012, but was completed only by the end of 2014. The examination of the unsigned polar magnetic fields in cycle 24 showed a continuing decline of fields in the Northern hemisphere whereas in the Southern hemisphere, it had partially recovered. However, the overall declining trend in solar polar fields, which began in the mid-1990’s, is still in progress. The continued decline seen in solar photospheric fields raises thequestion of whether we are heading towards a Grand or Maunder like solar minimum.

On the Characteristics of Geomagnetic Storms Observed During the Past 415 Years

In this paper we will present our investigations on the characteristics of geomagnetic storms deduced from direct and proxy observations for the years 1601–2016 AD. We show that we could infer epoch of reversal of solar polar magnetic fields from geomagnetic data. Such an inference is done back to the 18th century using geomagnetic and Aurora observations. We could also infer secular changes in the intensity of geomagnetic storms for the past 415 years.

Corona during the total solar eclipse on March 20, 2015, and 24 cycle development

We analyzed the structure of coronal features, using data on the March 20, 2015 total solar eclipse. The Ludendorff index characterizing the flattening of the corona is 0.09. The solar corona structure in the Northern and Southern hemispheres corresponds to the maximum and post-maximum phases of solar activity, respectively. The asynchronous development of magnetic activity in the Sun’s Northern and Southern hemispheres caused a substantial asymmetry of coronal features observed at the reversal of polar magnetic fields in the current cycle. The polar ray structures in the Southern Hemisphere are associated with the polar coronal hole, while in the Northern Hemisphere a polar hole has not been formed yet. We examine the relation between large-scale magnetic fields and location of high coronal structures.

Corona during the total solar eclipse on March 20, 2015, and 24 cycle development

We analyzed the structure of coronal features, using data on the March 20, 2015 total solar eclipse. The Ludendorff index characterizing the flattening of the corona is 0.09. The solar corona structure in the Northern and Southern hemispheres corresponds to the maximum and post-maximum phases of solar activity, respectively. The asynchronous development of mag-netic activity in the Sun’s Northern and Southern hemi-spheres caused a substantial asymmetry of coronal features observed at the reversal of polar magnetic fields in the current cycle. The polar ray structures in the Southern Hemisphere are associated with the polar cor-onal hole, while in the Northern Hemisphere a polar hole has not been formed yet. We examine the relation between large-scale magnetic fields and location of high coronal structures.

The reversal of the Sun’s magnetic field in cycle 24

Analysis of synoptic data from the Vector Spectromagnetograph (VSM) of the Synoptic Optical Long-term Investigations of the Sun (SOLIS) and the NASA/NSO Spectromagnetograph (SPM) at the NSO/Kitt Peak Vacuum Telescope facility shows that the reversals of solar polar magnetic fields exhibit elements of a stochastic process, which may include the development of specific patterns of emerging magnetic flux, and the asymmetry in activity between Northern and Southern hemispheres. The presence of such irregularities makes the modeling and prediction of polar field reversals extremely hard if possible. In a classical model of solar activity cycle, the unipolar magnetic regions (UMRs) of predominantly following polarity fields are transported polewards due to meridional flows and diffusion. The UMRs gradually cancel out the polar magnetic field of the previous cycle, and rebuild the polar field of opposite polarity setting the stage for the next cycle. We show, however, that this deterministic picture can be easily altered by the developing of a strong center of activity, or by the emergence of an extremely large active region, or by a ‘strategically placed’ coronal hole. We demonstrate that the activity occurring during the current cycle 24 may be the result of this randomness in the evolution of the solar surface magnetic field.