Scientists have used centuries-old reports of auroras to reconstruct eight solar cycles between 1560 and 1640, with more than 90% reliability for identifying solar minima within two years of actual occurrences. Kristof Petrovay, Laura G. Magyar, and Hisashi Hayakawa developed a statistical method using Monte Carlo simulations that can accurately trace solar behavior during an era when systematic telescopic observations were not yet common. The study, published in Scientific Reports, validates the historical aurora sightings as reliable indicators of past solar activity patterns, aligning well with reconstructions from cosmogenic radionuclides and available sunspot data.
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Centuries-old reports of strange lights in the night sky have helped scientists reconstruct a missing chapter in the Sun’s history.
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scientists → help → history
A study published in Scientific Reports by Kristof Petrovay, Laura G. Magyar and Hisashi Hayakawa has reconstructed eight solar activity cycles between 1560 and 1640 using historical records of aurorae, offering a way to trace solar behaviour from an era when systematic telescopic observations were only beginning.
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observations → publish → era
When the sky became a record of the Sun
Before telescopes began producing regular observations of sunspots, scientists had few direct ways to determine how active the Sun was.
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Sun → become → ways
One of the clues available to researchers today comes from historical accounts of aurorae, the glowing displays that occur when energetic particles associated with solar activity interact with Earth’s atmosphere.
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particles → come → atmosphere
Low-latitude aurorae are relatively rare and tend to capture some of the strongest space-weather events rather than providing a continuous measure of solar activity.
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aurorae → tend → activity
Their occurrence also does not follow the sunspot cycle closely enough to simply use every auroral sighting as an indicator of a solar maximum or minimum.
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occurrence → follow → maximum
Petrovay, Magyar and Hayakawa therefore developed a statistical approach to extract information about the underlying solar cycles from these sparse observations.
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Petrovay → develop → observations
More than 90% reliability for solar minima
The researchers used Monte Carlo simulations, repeatedly modelling large numbers of possible solar activity cycles, to determine how historical auroral observations could best be interpreted.
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observations → use → cycles
Their analysis found that the method can identify the phase of a solar cycle with more than 90% reliability when the long-term average rate of major space-weather events producing low-latitude aurorae is around three events per year or higher.
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rate → find → year
Under those conditions, more than 90% of the reconstructed solar minima fall within two years of the actual minimum.
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% → reconstruct → minimum
This is particularly useful because precisely identifying individual cycles in the period before systematic sunspot observations has been difficult.
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identifying → identify → observations
Eight cycles reconstructed between 1560 and 1640
Using the method, the team reconstructed eight solar activity cycles, designated T−5 through T2, spanning roughly 1560 to 1640.
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team → reconstruct → 1640
The researchers introduce the term “telescopic era” for this sequence, with T0 referring to the solar cycle that was underway in 1610, around the beginning of the telescopic era of solar observation.
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that → introduce → observation
They also found that cycles extending back to about 1540 could be reconstructed, although with lower reliability.
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cycles → find → reliability
The reconstructed sequence largely agrees with other independent evidence of past solar activity, including reconstructions based on cosmogenic radionuclides and available historical sunspot observations.
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sequence → reconstruct → radionuclides
That comparison is important because radionuclides such as carbon-14 and beryllium-10 provide another way of studying solar activity before the modern observational record.
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radionuclides → provide → record
Their production in Earth’s environment is influenced by cosmic radiation, which in turn is affected by solar magnetic activity.
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which → influence → activity
A window into the years before the Maunder Minimum
The period examined by the researchers falls within what they describe as the Early Modern Active Period, a century-long interval of broadly normal solar activity between the Spörer and Maunder Minima.
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they → examine → Spörer
The Maunder Minimum, beginning in the mid-17th century, is associated with an extended period of exceptionally low sunspot activity.
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Minimum → begin → activity
The new reconstruction is therefore particularly useful for understanding what happened as the Sun approached that prolonged minimum.
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Sun → understand → minimum
While the reconstructed cycles show good overall agreement with radionuclide-based reconstructions and historical sunspot observations, the researchers found a notable discrepancy for the final cycle immediately preceding the Maunder Minimum.
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researchers → reconstruct → Minimum
The study does not turn medieval and early-modern aurora sightings into a perfect historical sunspot record.
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study → turn → record
Instead, it shows that even an incomplete collection of observations, recorded long before modern solar monitoring existed, can contain enough information to reconstruct the rhythm of the Sun with surprising precision.
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monitoring → show → precision
And in doing so, centuries-old descriptions of lights in the sky become more than historical curiosities: they become data points in the effort to understand how our star has changed over time.
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star → do → time
