Tilt of sunspot bipoles in Solar Cycles 15 to 24

TL;DR

This study analyzes sunspot tilt angles over solar cycles 15 to 24 using historical data, revealing a non-monotonic latitudinal dependence, cycle-to-cycle variations, and a systematic offset related to solar cycle parity.

Contribution

It provides the first detailed analysis of tilt angle latitudinal dependence and cycle-related offsets using a century of digitized sunspot drawings.

Findings

Tilt angles peak at 25-30 degrees latitude.

Tilt dependence on latitude follows a sine function with specific parameters.

Cycle-to-cycle variations in tilt dependence are observed, with no correlation to cycle strength.

Abstract

We use recently digitized sunspot drawings from Mount Wilson Observatory to investigate the latitudinal dependence of tilt angles of active regions and its change with solar cycle. The drawings cover the period from 1917 to present and contain information about polarity and strength of magnetic field in sunspots. We identify clusters of sunspots of same polarity, and used these clusters to form ``bipole pairs''. The orientation of these bipole pairs was used to measure their tilts. We find that the latitudinal profile of tilts does not monotonically increase with latitude as most previous studies assumed, but instead, it shows a clear maximum at about 25--30 degree latitudes. Functional dependence of tilt ($\gamma$) on latitude ($\varphi$) was found to be $\gamma = (0.20\pm 0.08) \sin (2.80 \varphi) + (-0.00\pm 0.06)$. We also find that latitudinal dependence of tilts varies from one solar cycle to another, but larger tilts do not seem to result in stronger solar cycles. Finally, we find the presence of a systematic offset in tilt of active regions (non-zero tilts at the equator), with odd cycles exhibiting negative offset and even cycles showing the positive offset.