Physics of auroral phenomena : proceedings of the 36th Annual seminar, Apatity, 26 February – 01 March, 2013 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2013. - 215 с. : ил., табл.

ROLE OF TILT ORIENTATION OF BMRs AND MERIDIONAL CIRCULATION IN THE POLAR MAGNETIC FIELD REVERSAL ON THE SUN N .V . Zolotova, D.I. Ponyavin (St-Petersburg State University, St-Petersburg, Russia) A b stra c t. Using Greenwich catalogue of sunspots observations it was shown that impulses o f sunspot activity during a course of solar cycle are responsible for residual magnetic field transported by meridional circulation toward the poles. This, in turn, is related to the polarity reversal of the axisymmetric large-scale magnetic fields. We perform parameter study of tilt angle of the Bipolar Magnetic Regions (BMRs) with imposed meridional flow profiles to model poleward magnetic field surges. Simulation was done in terms o f point density distributions of leading and following sunspots according to Hale and Joy polarity rules. A faster meridional velocity reduces polar field. However results depend on size of the mesh grid. Latitudinal profile of meridional flow influences the polar field strength as well. “Physics o f Auroral Phenomena'’, Proc. XXXVI Annual Seminar, Apatity, pp. 130 - 132, 2013 © Kola Science Centre, Russian Academy of Science, 2013 Polar Geophysical Institute In tro d u c tio n The time-latitude diagram of the radial photospheric magnetic field of the Sun or the magnetic butterfly diagram ( Hathaway , 2010) represents several main features: the 11-year periodicity of solar activity, Hale’s law, SpOrer’s law, Joy’s law, the reversal of the polar magnetic fields. Spots (or Bipolar Magnetic regions - BMRs) on the Sun emerge at latitudes of the so-called royal-zone. On time-latitude plane sunspots are distributed as impulses (Antalova and Gnevyshev, 1983). It was found that width of these impulses is some tens of degrees in latitude and duration from one 0.5 to 2 years. The times at which the impulses appear in both hemispheres commonly do not coincide. Also impulses may violate the Sporer’s law, typically in long activity cycles (Zolotova and Ponyavin, 2012a). The other “thin feature” of the magnetic butterfly diagram is intermittent polarity pattern of poleward streams from sunspot latitudes. These streams have been called magnetic surges by Wang et al. (1989), and explained as a result of meridional flow action on a diffusion background. Wang et al. (2002b) suggested that giant surges denote periods of increased flow velocity and amplified rate of flux emergence. Currently to reproduce behavior of the polar magnetic fields the temporal changes of meridional flow velocity are popular to apply (Schrijver and Liu, 2008; Wang et al., 2009; Sheeley, 2010; Nandy et al., 2011). In the flux-transport model even a ~15% increase of flow velocity is enough to reproduce the 35-40% weaker polar field ( Wang et al., 2009). In this paper we reconstruct magnetic surges from sunspot impulses. To consider impact of only sunspot population to the surge strength we perform modeling without explicit diffusivity assignment. We consider influence of tilt angle of BMRs and latitudinal profile of the meridional flow on the strength and geometry of poleward surges. M e th o d Fig. 1 schematically demonstrates procedure. Fig.la displays modeled number o f BMRs per day. The first cycle has symmetrical ascending and descending phases. The second one is long cycle; it reproduces the Waldmeier effect ( Waldmeier, 1935). The third cycle is short double-peak cycle with the so-called Gnevyshev gap ( Gnevyshev , 1963, 1967). Fig. 2b illustrates the butterfly diagram for these three cycles constructed as bivariate Gaussians. Parameters are described by Zolotova and Ponyavin (2012a). Each individual BMRs is shown according to the Joy’s law. In asymmetrical case we define the latitudinal separation between leading and trailing spots of BMR as Al = /W-tan(0.5-/), A d is longitudinal separation, / is latitude. Fig. lc illustrates probability density function (PDF) o f BMRs: . - ----------- 1 --------- fCY./i.S ) = - s ( A ^ r 4 v - p ) J \ № n ) d where X = (I, t ) is 2-dimensional vector of sunspot positions on time-latitude plane with mean ц - [u,, //,] and covariance matrix £ . |£| is determinant of the matrix E (Bishop, 2006). For convenience a flux in term of PDF is normalized according to its magnitude. Thus simulation is performed in dimensionless units. Now one can see that the first cycle was composed from one impulse in each hemisphere; the second — from two impulses; the third — from three impulses. In Fig. lb overlapping of impulses hides internal structure of these cycles. We set the magnetic flux from each sunspot impulse equal to its magnitude. Separate fluxes of leading and trailing spots: FluxuaiVmg(l,t) = Flux\t.iim%(l+Al,t)- 1.01, where 1.01 is flux imbalance. In absence of diffusivity we specify a slight imbalance between leading and trailing fluxes of BMR - less than 10% for benefit o f total flux of trailing spots in each hemisphere. Then we calculate flux surplus: S(l,t) = F lu x ^ img (/,/) - FluxBilling (ft) (Fig. Id). Finally, we use autoregression to reconstruct surges (Fig. le): SResult(/n+, ,/n+1) = 130