Hemispheric asymmetry of long-term sunspot activity: sunspot relative numbers for 1939–2019

ABSTRACT. Hemispheric asymmetry of solar magnetic structures is an important feature of solar cycles, and it involves dynamical processes in the interior of the Sun. For the first time, the data of the monthly sunspot relative numbers derived from the National Astronomical Observatory of Japan (NAOJ) is used to perform a long-term study (1939 March–2019 November) of the north–south asymmetry. The results show the following: (1) the sunspot relative numbers derived from NAOJ/Mitaka observatory are highly correlated with the international sunspot numbers obtained from the World Data Center Sunspot Index and Long-term Solar Observations, which justifies the use of Mitaka sunspot time series for hemispheric variation; (2) the change in the signs of the slopes of the regression lines fitted to the absolute asymmetry index suggest a kind of periodic variation in the hemispheric asymmetry, but the time interval of the current data base is not long enough to derive 8 or 12 cycles periodicity; and (3) for mid-term periods, there are enhanced powers for the period ranges around 3.7yr, around 9yr, between 30 and 50yr. Our analysis results implicate a possible mechanism responsible for the generation and variation of the hemispheric coupling in the Sun.

摘要。太阳磁结构的半球不对称是太阳活动周的一个重要特征，它涉及太阳内部的动力学过程。首次使用日本国立天文台（NAOJ）得出的月太阳黑子相对数数据对南北不对称进行了长期研究（1939年3月 - 2019年11月）。结果如下：（1）日本国立天文台/三鹰天文台得出的太阳黑子相对数与从世界数据中心太阳黑子指数和长期太阳观测获得的国际太阳黑子数高度相关，这证明了使用三鹰太阳黑子时间序列研究半球变化是合理的；（2）拟合绝对不对称指数的回归线斜率符号的变化表明半球不对称存在一种周期性变化，但当前数据库的时间间隔不够长，无法得出8或12个周期的周期性；（3）就中期而言，在3.7年左右、9年左右以及30到50年之间的周期范围内功率增强。我们的分析结果暗示了一种可能的机制，该机制负责太阳中半球耦合的产生和变化。