Alfvena: Alfvén Waves
Electromagnetism & Plasma arc, page 83 of 193. Back to The Pixelated Physics Universe
Who they are
Alfvena plays the Sun like a harp. Every magnetic field line is a taut string, and when she plucks one, the plasma sings.
Alfvena is a character of the Electromagnetism & Plasma arc: drawn in pixels, ruled by one piece of real physics, and never allowed to break it.
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. Hannes Alfvén predicted these waves in Nature in 1942 and shared the 1970 Nobel Prize in Physics for his work in magnetohydrodynamics.
$$ \begin{gathered} v_A = \frac{B}{\sqrt{\mu_0 \rho}} \\ \omega = k_{\parallel}\, v_A \qquad (\text{shear Alfvén wave, ideal MHD}) \end{gathered} $$
| Symbol | Meaning | SI unit |
|---|---|---|
| $v_A$ | Alfvén speed | m/s |
| $B$ | magnetic field | T |
| $\mu_0$ | vacuum permeability | H/m |
| $\rho$ | plasma mass density | kg/m³ |
| $\omega$ | angular frequency | rad/s |
| $k_{\parallel}$ | wavenumber along the magnetic field | 1/m |
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.
Sources
- Alfvén (1942) Existence of Electromagnetic-Hydrodynamic Waves, Nature 150, 405
- Nobel Prize in Physics 1970, Hannes Alfvén, NobelPrize.org
- De Pontieu et al. (2007) Chromospheric Alfvénic Waves Strong Enough to Power the Solar Wind, Science 318, 1574
Crossovers
Alfvena meets Coronix (Forbidden Fe XIV Coronal Line). Alfvena's waves run along magnetic field lines in the solar corona. Coronix's green line shows the corona is above a million kelvin. Wave heating is one candidate for keeping it that hot, and the question is unsolved.
Alfvena meets Plasma Drift (Ionized Gas / Plasma). Plasma Drift's charges circle magnetic field lines and drift across them in crossed fields. Alfvena treats the same field lines as strings under tension; in a plasma, the charges and the field move together.
Wear the lore
Concept keywords: Alfvén Waves, Alfvena, Electromagnetism & Plasma, physics lore, Pixelated Physics