Magnetic field reconnection

spaceweb@oulu.fi - last update: 18 May 1999, 1430 UT (RR)

The concept of merging or reconnection of magnetic field lines is widely used in magnetospheric and solar physics, although the physics behind the process are still under debate. The concept was developed to explain particle acceleration in solar flares (Giovanelli, 1946), and what is called the fast reconnection model was created by Petschek (1964). The basic idea behind reconnection is that (partly) antiparallel magnetic field lines can, when meeting, merge together and produce two topologically totally different field lines.

Reconnection is thought to be the main link in the solar wind - magnetosphere coupling process, occurring at the dayside magnetopause between southward directed IMF and the northward directed geomagnetic field. It is the main process that transports mass, momentum, and energy from the solar wind into the magnetosphere, and it drives the large scale magnetospheric convection. There are several reasons to believe the existence of such a process:

• there is a clear correlation between the IMF Bz direction and geomagnetic activity
• some measurements indicate the presence of unexplained processes at the magnetopause
• some theories can explain the formation of such processes (e.g., the tearing mode instability)

There are two basic types of reconnection models:

1. quasi-static reconnection (QSR)
2. flux transfer event (FTE)

In addition to the dayside reconnection, similar process should be occurring in the far tail. Finally, some substorm models are also based on reconnection. This is quite natural, since the magnetic field field lines above and below the cross-tail current sheet are oppositely directed, and the plasma sheet is typically thinning during the substorm growth phase. See, e.g., Ugai and Wang (1998).

References

• Giovanelli, R. G., A theory of chromospheric flares, Nature, 158, 81-, 1946.
• Petschek, H. E., Magnetic field annihilation, in AAS-NASA Symposium on the Physics of Solar Flares, edited by W. N. Hess, NASA Spec. Publ., SP-50, 425, 1964.
• Ugai, M., and W. B. Wang, Computer simulations on three-dimensional plasmoid dynamics by the spontaneous fast reconnection model, J. Geophys. Res., 103, 4573-4585, 1998.

See also:

• Space Physics Text Book of Oulu