Wednesday, July 22, 2020

Newer Solar Cycle 25 Forecast Runs Counter to Consensus

From the July 9, 2020, ARRL Newsletter

Scientists associated with the National Center for Atmospheric Research, the University of Maryland, NASA Goddard Space Flight Center, and other institutions are offering a "bold prediction" on how Solar Cycle 25 will play out. In a paper, "Overlapping Magnetic Activity Cycles and the Sunspot Number: Forecasting Sunspot Cycle 25 Amplitude," they assert that the next sunspot cycle will be of major proportions. The forecast stands in stark contrast to the consensus of forecasters who predict that the magnitude of the nascent Cycle 25 may not be much different from the current unremarkable solar cycle, which appears to have reach its low point.

"From the dawn of modern observational astronomy, sunspots have presented a challenge to understanding -- their quasi-periodic variation in number, first noted 160 years ago, stimulates community-wide interest to this day," the abstract points out. "A large number of techniques are able to explain the temporal landmarks, (geometric) shape, and amplitude of sunspot 'cycles,' however, forecasting these features accurately in advance remains elusive."

Monthly sunspot numbers since 1749. The data values are represented by dots, and the 12-month running average values are illustrated as a red shaded area. Vertical blue dashed lines signify the magnetic activity cycle termination times that trigger the rapid growth of sunspot activity.

The paper notes that recent studies have illustrated a relationship between the sun's 22-year Hale magnetic cycle and the production of sunspot cycle landmarks and patterns, but not the amplitude of the cycle.

"Using discrete Hilbert transforms on 270 years of monthly sunspot numbers to robustly identify the so-called 'termination' events -- landmarks marking the start and end of sunspot and magnetic activity cycles -- we extract a relationship between the temporal spacing of terminators and the magnitude of sunspot cycles," the abstract explains. "Given this relationship and our prediction of a terminator event in 2020, we deduce that Sunspot Cycle 25 will have a magnitude that rivals the top few since records began. This outcome would be in stark contrast to the community consensus estimate of Sunspot Cycle 25 magnitude."

NASA's Solar Dynamic Observatory captured this image of a sunspot, with a core larger than Earth.

According to the paper, low-amplitude solar cycles appear to correspond with widely separated terminators, while larger-amplitude cycles correspond to more narrowly separated terminators.

"[O]ur best estimate for the [sunspot number] amplitude of Solar Cycle 25 is 233 spots, with a 68% confidence that the amplitude will fall between 204 and 254 spots," the paper posits. "We predict with 95% confidence that the Cycle 25 amplitude will fall between 153 and 305 spots."

The researchers concede that their forecast is outside of the scientific consensus, based on different paradigms. "Over the coming months, as [Solar Cycle] 25 matures, it will become evident which of these paradigms is most relevant," the paper says. "Very early indications of the spot pattern are appearing at higher-than-average latitudes (∼40°). Historically, high-latitude spot emergence has been associated with the development of large amplitude sunspot cycles -- only time will tell."