What is the butterfly diagram?
Every roughly 11 years, the Sun swings from quiet to stormy and back. Early in each cycle, active regions, sunspots and their bright chromospheric counterparts called plages, emerge at mid latitudes, around 30 to 35 degrees north and south of the solar equator. As the cycle matures, new regions appear closer and closer to the equator. When you plot the latitude of every active region against time, each cycle traces two wedges converging on the equator: a pair of butterfly wings.
This pattern, first charted by Edward and Annie Maunder in 1904, is direct evidence of the solar dynamo: a deep flow of plasma and magnetic field that winds up, erupts through the surface, migrates equatorward, flips polarity, and starts again.
Interactive simulator: watch the wings form
Press play to simulate active regions emerging over several solar cycles. Each dot is an active region plotted at its latitude and time. Watch how each cycle starts high and drifts toward the equator, overlapping with the start of the next cycle.
If the chart does not render, your browser may not support canvas. In short: active regions begin each cycle near 30 degrees latitude in both hemispheres and migrate to below 10 degrees over about a decade, producing the butterfly shape.
A century of Sun watching at Kodaikanal
The Kodaikanal Solar Observatory in southern India has photographed the Sun almost daily since the early 1900s, building one of the longest continuous solar archives in the world. Its calcium K-line images reveal bright plage regions, the magnetic footprints of solar activity.
Recently, AI models have been trained to scan this 100-year archive, automatically detecting and tracking bright regions across tens of thousands of photographic plates. Machines can do in days what would take humans decades, extending the measured butterfly diagram far beyond the modern satellite era and giving scientists a much longer view of how solar activity changes over time.
Why it matters
Space weather
Knowing where activity emerges in a cycle helps forecast solar flares and coronal mass ejections that can disrupt satellites, GPS, and power grids on Earth.
Testing dynamo theory
The equatorward drift constrains models of the plasma flows inside the Sun. A longer data record means stronger tests of how the solar dynamo really works.
Climate context
Century-scale records reveal grand trends in solar output, helping separate the Sun's small influence on climate from human-driven change.
AI plus archives
Digitizing and mining historical plates with machine learning turns fragile glass photographs into precise, searchable science, a model for rescuing old data everywhere.
Read the pattern yourself
Three things to look for in any butterfly diagram: the tilt of each wing (activity drifting equatorward), the overlap where a new cycle begins at high latitudes while the old one finishes near the equator, and the asymmetry, since the northern and southern hemispheres rarely behave identically. Try the simulator above and see if you can spot all three.