Our understanding of the plasma motion below the solar surface and various phenomena occurring above the surface has advanced significantly in last couple decades. This has led to many new discoveries by expanding our knowledge of how the solar interior and atmosphere are interconnected. At the same time, they have raised many new challenges to our understanding of interior dynamics and magnetic field generation. For example, we do not have precise understanding of when and where active regions will emerge. Studies indicate puzzling and contradictory results regarding the cell structure of meridional circulation (Gizon et al., 2020). Furthermore, observations covering the last two deep and extended periods of low magnetic activity raise several additional questions about our understanding of the solar dynamo (Jain et al., 2022). Do we have precise knowledge that can be applied to forecast the strength and timing of future solar cycles with some confidence (Pesnell, 2020) for more than two solar cycles have played a crucial role in addressing these questions, however many unsolved problems remain that require better understanding of both observational and theoretical aspects.
Editorial23 May 2023
Editorial: Connecting solar flows and fields to understand surface magnetism
We measure differential rotation and meridional flow in the Sun`s surface shear layer by tracking the motions of the magnetic network seen in magnetograms from SOHO/MDI and SDO/HMI over solar cycles 23, 24, and the start of 25 (1996-2022). We examine the axisymmetric flows derived from 15-24 daily measurements averaged over individual 27-day Carrington rotations.
Variations in the differential rotation include the equatorial torsional oscillation - cyclonic flows centered on the active latitudes with slower flows on the poleward sides of the active latitudes and faster flows equatorward. The fast flow band starts at about 45 degrees latitude during the declining phase of the previous cycle and drifts equatorward, terminating at the equator at about the time of cycle minimum. Variations in the differential rotation also include a polar oscillation above 45 degrees with faster rotation at cycle maxima and slower rotation at cycle minima. The equatorial variations were stronger in cycle 24 than in cycle 23 but the polar variations were weaker.
Variations in the meridional flow include a slowing of the poleward flow in the active latitudes during cycle rise and maximum and a speeding up of the poleward flow during cycle decline and minimum. The slowing in the active latitudes was less pronounced in cycle 24 than in cycle 23.
Polar countercells (equatorward flow) extend from the poles down to 60 degrees latitude from time to time (1996-2000 and 2016-2022 in the south and 2001-2011 and 2017-2022 in the north).
While the axisymmetric flows are drivers of the Sun‘s global magnetic dynamo, the
variations in the flows are likely due to back reactions on the flows by the magnetic fields.