[P-EM08_2PM1] Space Weather and Space Climate

[PEM08-21] 太陽ダイナモの南北半球対称性の破れに関する研究

*宿谷 大志¹、草野 完也¹ (1.名古屋大学太陽地球環境研究所)

キーワード：極域磁場反転, 太陽ダイナモ, 数値シミュレーション, 南北非対称性

Solar dynamo is a mechanism whereby the kinetic energy of the plasma in the sun is converted to the magnetic energy. This mechanism works to generate and maintain all solar magnetic activities. Because the Earth's climate can be influenced by solar activities, variability of the solar dynamo is an important issue to understand long-term evolution of the Earth's climate. Comparisons of the solar activities in each solar hemisphere show hemispheric asymmetry. Sunspots were found preferentially in one hemisphere and not the other in often long periods of time (Spoerer, 1889; Maunder, 1890, 1904). This asymmetry was extended to other measures of activity including faculae, prominences and flares (Waldmeier, 1971; Roy, 1977). The asymmetry happens in the solar polar magnetic field reversals. The polarity of the solar magnetic fields on the north and south poles periodically reverses at every sunspot maxima. However, the reversals at both poles actually don't occur at the same time. In other words, the reversal at one pole is followed by that on the other pole. This time difference of magnetic field reversals between the poles was first noted by Babcock (1959) from the very first observation of polar field. Recently, it was confirmed by detailed observations with the HINODE satellite (Shiota et al. 2012). As above, we have ever obtained many observation facts. However, the mechanisms of hemispheric asymmetry of the solar dynamo haven't been revealed theoretically yet. In this paper, we study the asymmetric feature of the solar dynamo based on the flux transport dynamo model (Chatterjee et al. 2004) to explain the time difference of magnetic polarity reversal between the north and south poles. In order to calculate long-term variations of solar activities, we use the mean field kinematic dynamo model, which is derived from magnetohydrodynamics (MHD) equation through the mean field and other approximations. We carried out the mean field dynamo simulations using the updated SURYA code which was developed originally by Choudhuri and his collaborators (2004). We decomposed the symmetric and asymmetric components of magnetic field, which correspond respectively to the quadrupole and dipole-like components (Nishikawa and Kusano 2008), and analyzed the phase relation between them. As a result, we found that the two components are mixed even if the dipole-like component is predominant and that the two components spontaneously form 90 or -90 degree out of phase oscillation. The solutions with 90 and -90 degree out of phase oscillation form the different attracters of dynamo solutions. We found that the time difference of the polar field reversals between the different hemispheres can be explained by the out of phase relation between the different components of magnetic field.