[Paper Review] Latitudinal structure and dynamic of the photospheric magnetic field

E. Gavryuseva|arXiv (Cornell University)|Feb 7, 2018

Solar and Space Plasma Dynamics17 references3 citations

TL;DR

This study analyzes 31 years of Wilcox Solar Observatory data (1976–2007) to reveal a stable four-zone latitudinal structure in the Sun's photospheric magnetic field, with polarity waves propagating from the equator to the poles every 2–3 years. The key finding is a synchronized, quasi-22-year periodicity in North-South magnetic field asymmetry, confirming long-term organization of solar magnetic dynamics across three solar cycles.

ABSTRACT

Analysis of the structure and dynamics of the magnetic field of the Sun is fundamental for understanding of the origin of solar activity and variability as well as for the study of solar-terrestrial relations. Observations of the large scale magnetic field in the photosphere taken at the Wilcox Solar Observatory from 1976 up to 2007 have been analysed to deduce its latitudinal and longitudinal structures, its differential rotation, and their variability in time. This paper is dedicated to the analysis and dynamics of the latitudinal structure of the solar magnetic field over three solar cycles 21, 22, 23. The main results discussed in this paper are the following: the large scale latitudinal structure is antisymmetric and composed of four zones with boundaries located at the equator, -25 and + 25 degrees, stable over 10-11 years with a time delay of about 5-6 years in near-equatorial zones. The variability and North-South asymmetry of polarity waves running from the equator to the poles with 2-3 - year period was studied in detail.

Motivation & Objective

• To understand the long-term latitudinal structure and dynamics of the photospheric magnetic field (SMF) over three solar cycles.
• To investigate the origin and periodicity of polarity waves propagating from the equator to the poles.
• To quantify the North-South asymmetry and temporal correlation of SMF variability across hemispheres.
• To determine whether large-scale magnetic field structures are stable over multiple solar cycles and how they relate to solar activity.

Proposed method

• Analysis of daily line-of-sight magnetograms from the Wilcox Solar Observatory (1976–2007), covering solar cycles 21, 22, and 23.
• Averaging of SMF intensity and polarity over Carrington Rotations (CR) and years to identify stable latitudinal patterns.
• Use of filtered magnetic field (FMF) to isolate short-term variability and study synchronization between hemispheres.
• Computation of cross-correlation coefficients between northern and southern hemisphere magnetic field variability at symmetric latitudes, with time delays up to 22 years.
• Application of phase reconstruction and Fourier analysis to detect periodicities in magnetic field dynamics.
• Visualization of magnetic field intensity and polarity across latitude and time, including butterfly diagram-like plots and correlation maps.

Experimental results

Research questions

• RQ1 What is the long-term latitudinal structure of the photospheric magnetic field, and is it stable over multiple solar cycles?
• RQ2 Do polarity waves propagate from the equator to the poles, and what is their period and spatial evolution?
• RQ3 How is the North-South asymmetry of the solar magnetic field related in time, and what periodicities emerge in their correlation?
• RQ4 Is there a consistent phase relationship between magnetic field variability in the northern and southern hemispheres at symmetric latitudes?
• RQ5 What is the role of intermediate-intensity magnetic fields (5–2000 μT) in generating the observed wave-like structures?

Key findings

• The photospheric magnetic field exhibits a stable four-zone latitudinal structure with boundaries at ±25° and the equator, persisting over 10–11-year intervals with a 5–6-year time delay in near-equatorial zones.
• Polarity waves propagate from the equator to the poles with a quasi-2-year period, visible across all three solar cycles.
• A strong 22-year periodicity is observed in the North-South correlation of magnetic field variability, especially near the zonal boundaries of the four-zone structure.
• High correlation (r > 0.9) in short-term FMF variability between symmetric latitudes (θ and −θ) occurs at 1.8, 9.4, 11.7, 20, and 22 years, indicating synchronized dynamics.
• The filtered magnetic field (FMF) reveals that the 2-year periodicity in North-South synchronization is significant and robust over 29 years of data.
• The four-zone structure and wave dynamics are primarily driven by magnetic fields of intermediate intensity (5–2000 μT), as confirmed by recent analysis (Gavryuseva, 2008a).