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Physics LibreTexts

11.18 The Solar Cycle

Around 1830, an obscure German amateur astronomer named Heinrich Schwabe began observing sunspots as a hobby. For years he tabulated the number and position of sunspots. Schwabe noticed a regular pattern to the numbers and positions, and in 1851 he proposed the existence of a solar cycle. This discovery, which was followed a year later by the discovery that terrestrial magnetic compass deviations exactly follow the same cycle, was a key step in understanding the Sun and its effects on the Earth.

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Heinrich Schwabe. Click here for original source URL


The magnetic cycle’s duration averages 22 years and consists of two 11-year cycles of sunspot number. In a period of minimal sunspots, the few visible spots are grouped within about 10 degrees of the solar equator. When a new cycle begins after a year or so, groups of new spots appear at high latitudes, about 30 degrees from the solar equator. After a few years, the sunspot number reaches a maximum and the spots are at intermediate latitudes, about 20 degrees from the solar equator. After about 11 years, the spots appear mostly about 10 degrees from the equator, and a sunspot minimum occurs again.

The pattern of the sunspot cycle is intriguing, but even more remarkable is the fact that the magnetic field of the entire Sun reverses between each 11 year sunspot cycle! Imaginary observers on the Sun would find their compasses pointing north in one direction for 11 years (subject to disturbances by frequent magnetic storms) and in exactly the opposite direction for the next 11 years. This behavior is not entirely unknown: The Earth’s magnetic field reverses every few hundred thousand to every few million years. Both patterns of reversal may be caused by cyclic flow patterns in the deep fluid cores of the two bodies (liquid metal for the Earth, high temperature gas with over 150 times the density of water for the Sun). The sunspot cycle is important to us because during years of maximum sunspot activity, solar particles shooting off the Sun affect the magnetic field and upper atmosphere of Earth, disturbing radio communications and causing auroras — spectacular light displays in the upper atmosphere.

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SOHO extreme ultraviolet composite showing the number of sunspots through almost an entire solar cycle. Click here for original source URL.