A study led by University of Montreal researchers has confirmed that our Sun is indeed a solar-type star. Published in the journal Science, the findings help resolve an ongoing controversy about whether the Sun exhibits the same cyclic behavior as nearby solar-type stars.
NASA’s Solar Dynamics Observatory captured an X-class solar flare erupting on the Sun on December 19, 2014. This image blends two wavelengths of extreme UV light, 171 (gold) and 131 (purple). Image credit: NASA / SDO / Duberstein.
The Sun’s activity, including sun-spot activity, levels of radiation and ejection of material, varies on an 11-year cycle, driven by changes in its magnetic field.
Other nearby solar-type stars have their own cycles, but the Sun does not seem to match their behavior.
Understanding the Sun’s cycle is one of the biggest outstanding problems in solar physics.
In a series of simulations of stellar magnetic fields, University of Montreal researcher Antoine Strugarek and co-authors found that the magnetic cycle of the Sun depends on its rotation rate and luminosity.
“This relationship can be expressed in terms of the so-called Rossby number,” they said.
“What we showed is that the Sun’s magnetic cycle is inversely proportional to this number.”
The researchers then compared the results of their simulations with available observations of cyclic activity in a sample of nearby solar-type stars.
They found that the cycle periods of the Sun and other solar-type stars all follow the same relationship with the Rossby number.
“The magnetic field of a star draws its energy from the flows of matter which animate its interior,” the authors explained.
“Thanks to the simulations, we now know that the rotation of the star influences the efficiency of the transfer of energy between these turbulent flows and the magnetic field.”
“The same phenomenon also determines the cycle period, which has been shown to decrease with the Rossby number, a dimensionless number widely used in geophysical fluid dynamics that measures the effects of centrifugal forces.”
“The discovery of such a scaling law for the period of the star magnetic cycle from self-consistent turbulent 3D simulations is a world first.”
The results demonstrate that the Sun is indeed a solar-type star, and also advance scientists’ understanding of how stars generate their magnetic fields.
“These results provide a new theoretical interpretation of stellar magnetic cycles, and place the Sun as the cornerstone of our understanding of the dynamics of stars,” the scientists said.
“By characterizing the magnetism of solar-type stars, our simulations will make it possible in particular to prepare the scientific return of the next European missions Solar Orbiter and PLATO.”
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A. Strugarek et al. 2017. Reconciling solar and stellar magnetic cycles with nonlinear dynamo simulations. Science 357 (6347): 185-187; doi: 10.1126/science.aal3999
