Fast magnetic reconnection and particle energization

7 problems

• Why the collisionless reconnection rate is close to 0.1

  Partially resolved

  Computer simulations and spacecraft find that reconnection without particle collisions always proceeds at about one tenth of a natural speed limit. Nobody has a derivation that explains why this number is so universal.

• Collisionless onset of reconnection in a thinning magnetotail current sheet

  Open

  In Earth's magnetic tail the solar wind slowly stretches a sheet of electric current for about an hour before reconnection suddenly starts and releases a substorm (a burst of auroral activity). A small magnetic field threading the sheet is predicted to stabilize it, and why and when the sheet nevertheless tears is not known.

• Onset of fast reconnection in forming coronal current sheets

  Open

  Before a solar flare, a sheet of electric current in the Sun's atmosphere slowly thins while it stores magnetic energy. Theory suggests the sheet breaks up once it is thin enough compared with its length, but the threshold and timing for a sheet that is still forming are not established.

• Partition of released energy between ions and electrons

  Partially resolved

  When reconnection releases magnetic energy, some goes to heating ions, some to heating electrons and some to fast flows. Spacecraft see ions heated much more than electrons, and there is no first-principles theory of the split.

• Power-law index and cutoff of particles accelerated by reconnection

  Contested

  Reconnection in strongly magnetized astrophysical plasmas makes particles with a power-law energy distribution, few particles at high energy but many more than a thermal gas has. How steep that power law is and how high it extends is not settled.

• Reconnection rate at very high Lundquist number in three dimensions

  Partially resolved

  In very large, highly conducting plasmas the reconnecting layer breaks into a chain of magnetic bubbles called plasmoids, which speeds reconnection up. In two-dimensional simulations the resulting rate is known, but real systems are three-dimensional, where the bubbles become twisted tubes and turbulence develops.

• Why sawtooth crashes in tokamaks leave core reconnection incomplete

  Open

  The tokamak core heats up and then periodically collapses in a sawtooth crash, which the classic model of the Soviet theorist Boris Kadomtsev explains as complete reconnection of the central field lines. Measurements show the core field is only partly reconnected, and the crash is faster than the model allows.