Alfvena

Alfvena plays the Sun like a harp. Every magnetic field line is a taut string, and when she plucks one, the plasma sings.

The Physics

In a plasma, magnetic field lines behave like stretched strings. Shake one sideways and a transverse ripple runs along it, carrying the plasma with it. This is the shear Alfvén wave of ideal magnetohydrodynamics, and its speed is set by the field strength and the plasma's density.

v_A = B / √(μ₀ρ) and ω = k∥ v_A

• v_A: Alfvén speed (m/s)
• B: magnetic field (T)
• μ₀: vacuum permeability (H/m)
• ρ: plasma mass density (kg/m³)
• ω: angular frequency (rad/s)
• k∥: wavenumber along B (1/m)

Hannes Alfvén predicted these waves in Nature in 1942 and shared the 1970 Nobel Prize in Physics for his work in magnetohydrodynamics.

Honest caveat: The Sun's corona is about 1–2 million K while its surface is about 5800 K. Alfvén waves are a leading candidate for heating it, but the question is still open.

On the shirt

Front: Alfvena, the pixel-art character, with the title. Back: the field-line strings, both Alfvén equations, the coronal-heating note and Alfvén's 1942 paper. Printed on a unisex tee.