The researchers referred to the 27-day solar rotational period, which is the average time it takes for the Sun to rotate on its axis. Since the Sun consists of plasma, the equator rotates quicker than its poles. When areas of high activity, such as sunspots, face Earth, there's an increase in ultraviolet rays and decrease in energetic particles showering the atmosphere.
Thunderstorm and cloud activities sometimes show a 27-day period, and this has long been studied to uncover a possible important link to solar rotation.
In this study, they examined in detail the intensity variations in the signal of the 27-day solar rotational period in thunder and lightning activity from the 18th to the 19th centuries based on 150-year-long records found in old diaries kept in Japan and discuss their relationship with the solar activity levels. Such long records enable them to examine the signals of solar rotation at both high and low solar activity levels.
The researchers examined the records for mentions of thunder and lightning events between May and September, when the influence from the cold Siberian air mass is weak in Japan and found peaks of lightning and thunder activity occurs every 24 to 31 days, the same time window it takes the sunspots to rotate completely.
They found that the signal of the solar rotational period in the thunder and lightning activity increases as the solar activity increases. They also discuss the possibility of the impact of the long-term climatological conditions on the signals of the 27-day period in thunder/lightning activities.
"Solar rotational cycle in lightning activity in Japan during the 18–19th centuries" -Hiroko Miyahara et al. - Annales Geophysicae (2018). DOI: 10.5194/angeo-36-633-2018 - OPEN ACCESS
Featured image: Lightning over Satellite Beach, Florida seen across the Indian River Lagoon from Melbourne, Florida. Credit: Michael Seeley