Collisionless turbulence, heating and plasma dynamos

6 problems

• Fluctuation dynamo in a weakly collisional plasma

  Partially resolved

  In hot, thin gas such as that between galaxies, turbulence should stretch and amplify weak magnetic fields. Because particles rarely collide there, the usual fluid theory fails, and how strong the field gets is uncertain.

• Which mechanisms dissipate turbulence in the solar wind

  Open

  Several processes can turn turbulent wave energy into particle heat without collisions, such as particles moving in step with waves or being pushed around by strong fluctuations. Their relative importance in the solar wind is unknown.

• Ion-to-electron heating ratio in collisionless turbulence

  Partially resolved

  Turbulence in a collision-free plasma heats ions and electrons by different amounts. The ratio sets what telescopes see from hot astrophysical gas, and it is not yet pinned down.

• Does the helicity barrier control ion heating in imbalanced solar wind

  Partially resolved

  Near the Sun most solar-wind waves travel outward, an imbalance that theory says blocks the energy cascade at the scale of ion orbits. The blocked energy would then accumulate and heat the ions through resonant waves, and whether this happens in the real wind is under test.

• Laboratory plasma dynamo at low magnetic Prandtl number

  Open

  Laser experiments have shown turbulent plasma amplifying a magnetic field when the gas's viscosity and electrical resistance are comparable. In stars and planets resistance is much larger than viscosity, and no laboratory plasma has shown a dynamo in that regime.

• Nature and spectral slope of sub-ion-scale turbulence

  Partially resolved

  Below the size of ion orbits, the solar-wind magnetic fluctuation spectrum steepens. Whether these fluctuations are a kind of wave or a collection of thin current sheets, and why the slope has its value, are open.