The solar dynamo begins near the surface.

Area of Science:

• Solar physics
• Plasma astrophysics
• Heliophysics

Background:

• The Sun's 11-year magnetic dynamo cycle involves sunspot migration and torsional oscillations.
• Previous theories proposed deep solar origins for these phenomena.
• Helioseismology localizes torsional oscillations to the Sun's near-surface shear layer.

Purpose of the Study:

• To investigate whether the solar dynamo is a near-surface instability.
• To challenge traditional models focusing on the solar tachocline.

Main Methods:

• Analysis of helioseismology data to pinpoint torsional oscillations.
• Development of simple analytic estimates for near-surface magneto-rotational instability.
• Conducting state-of-the-art numerical simulations.

Main Results:

• The near-surface magneto-rotational instability model better explains torsional oscillation scales and subsurface magnetic field amplitudes.
• Numerical simulations corroborate analytic estimates.
• Simulations reproduce hemispherical magnetic current helicity laws.

Conclusions:

• The solar dynamo is likely a near-surface instability.
• This finding contrasts with traditional models.
• Understanding this near-surface phenomenon can enhance predictions of solar magnetic cycles and space weather.