Sunspots have been observed for over four centuries and the magnetic nature of sunspot cycles has been known for about a century. However, some of its underlying physics still remain elusive. In new research, astronomers at IISER Kolkata discovered a new relationship between the rise rate of the sunspot cycle and the Sun’s magnetic field. Their results indicate that the maximum intensity of solar cycle 25, the ongoing sunspot cycle, is imminent and likely to occur within a year.
Flaring, active regions of the Sun are highlighted in this image combining observations from several telescopes. High-energy X-rays from NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) are shown in blue; low-energy X-rays from Japan’s Hinode spacecraft are green; and extreme ultraviolet light from NASA’s Solar Dynamics Observatory (SDO) is yellow and red. All three telescopes captured their solar images around the same time on April 29, 2015. Image credit: NASA / JPL-Caltech / GSFC / JAXA.
Our host star, the Sun, is a dynamic star whose magnetic activity varies across a wide range of time-scales spanning from minutes to millennia and beyond.
The most prominent signature of this variability is captured by the waxing and waning of sunspots — dark, magnetized patches on the Sun’s surface — that repeats almost every 11 years, known as the sunspot cycle.
Sunspot cycles exhibit significant fluctuations in both amplitude and duration that occasionally result in extreme activity phases like solar grand minima and grand maxima.
The Sun’s dynamic activity output influences the entirety of the heliosphere including our home planet by shaping its space environmental conditions and determining the habitability.
Therefore, developing accurate predictive capabilities pertaining to the long-term solar activity is crucial in planning future space missions and safeguarding space-reliant technologies.
“The solar cycle is produced by a dynamo mechanism driven by energy from plasma flows inside the Sun,” said IISER researcher Dibyendu Nandy and colleagues.
“This dynamo mechanism is understood to involve two primary components of the Sun’s magnetic field, one which manifests in the cycle of sunspots and another which manifests in a recycling of the large-scale dipole field of the Sun; the latter is much like the Earth’s magnetic field — stretching from one pole of the Sun to another.”
“With the cycle of sunspots, the Sun’s dipole field is also observed to wax and wane in strength, the north and south magnetic poles swap places, also every 11 years.”
“In 1935, the Swiss astronomer Max Waldmeier discovered that the faster the rate of rise of a sunspot cycle the stronger its strength, so stronger cycles take less time to rise to their peak intensity.”
“This relationship has often been utilized to forecast the strength of a sunspot cycle based on observations of its early rising phase.”
In the new research, the authors discovered a new relationship between the rise rate of the sunspot cycle and the decay rate of the solar dipole moment.
This discovery complements the Waldmeier effect, connecting the two primary magnetic field components of the Sun and supporting the theory that the evolution of sunspots are integral to the functioning of the solar dynamo process rather than being a mere symptom of it.
The scientists also demonstrated how observations of the rate of decrease of the Sun’s dipole magnetic field can be usefully combined with sunspot observations to predict when the ongoing cycle would peak.
“Our analysis indicates solar cycle 25 is expected to be a weak-moderate cycle, peaking in 2024,” they said.
Their paper was published in the Monthly Notices of the Royal Astronomical Society: Letters.
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Priyansh Jaswal et al. 2024. Discovery of a relation between the decay rate of the Sun’s magnetic dipole and the growth rate of the following sunspot cycle: a new precursor for solar cycle prediction. MNRAS 528 (1): 27-32; doi: 10.1093/mnrasl/slad122
