Poynting-Robertson-like Drag at the Sun's Surface

Area of Science:

• * Solar physics
• * Astrophysics
• * Helioseismology

Background:

• * The Sun's internal rotation is a complex function of radius and colatitude.
• * Surface rotation is slower than interior rotation, a phenomenon not fully understood.
• * This discrepancy is vital for stellar rotation theories and solar interior models.

Purpose of the Study:

• * To investigate the cause of the Sun's surface slow-down.
• * To analyze the role of the photosphere in solar angular momentum loss.
• * To explore the implications of photospheric angular momentum loss for stellar rotation.

Main Methods:

• * Analysis of 5-minute solar p-mode limb oscillations as rotation tracers.
• * Measurement of velocity gradients in the upper photosphere.
• * Calculation of photospheric torque and angular momentum loss.

Main Results:

• * A significant velocity gradient was identified in a thin layer at the top of the photosphere.
• * This shear layer is where the solar atmosphere radiates energy and angular momentum.
• * Photospheric angular momentum loss, analogous to photon braking, was quantified.

Conclusions:

• * Photospheric angular momentum loss is a key factor in the Sun's near-surface angular momentum deficit.
• * The calculated torque over the Sun's lifetime matches the observed deficit.
• * This finding provides a plausible mechanism for stellar surface spin-down.