Meridional Circulations of the Solar Magnetic Fields of Different Strength

Abstract

The meridional circulation of the solar magnetic fields in Solar Cycles 21 – 24 was considered. Data from both ground-based and space observatories were used. Three types of time–latitude distributions of photospheric magnetic fields and their meridional circulations were identified depending on the magnetic-field intensity. (i) Low-strength magnetic fields. Positive- and negative-polarity magnetic fields were distributed evenly across latitude and they weakly depended on the magnetic fields of active regions and their cycle variation. (ii) Medium-strength magnetic fields. For these positive- and negative-polarity magnetic fields a sinusoidal wave-like, pole-to-pole, antiphase meridional circulation with a period of ≈22 yr was revealed. The velocities of meridional flows were slower at the minima of solar activity, when they were at high latitudes in the opposite hemispheres, and maximal at the solar maxima, when the centers of positive- and negative-polarity flows crossed the equator. The time–latitude dynamics of these fields coincides with that of coronal holes and reflects the solar global magnetic-field dynamics including the solar polar-field reversals. (iii) High-strength (local, active-region) magnetic fields. They were distributed symmetrically in the northern and southern hemispheres. The magnetic fields of active regions were formed only during the periods when the medium-strength positive- and negative-polarity magnetic fields approached at low latitudes. Magnetic fields of both leading and following sunspot polarity migrated from high to low latitudes. The meridional-flow velocities of high-strength magnetic fields were higher in the rising and maximum than in the declining phases. Some of the high-latitude active-region magnetic fields were captured by the second type of meridional circulation flows and transported along with them to the appropriate pole. However, the magnetic fields of active regions are not the main ones in the solar polar-field reversals. The results indicate that high-strength magnetic fields were not the main source of weak and medium-strength ones. The butterfly diagram is the result of a superposition of these three types of magnetic-field time–latitude distributions and their cycle evolution. The results suggest that different strength magnetic fields have different sources of their generation and cycle evolution.