One of the most important astronomical discoveries of the mid-1980's was the discovery of ordered magnetic fields in spiral galaxies. These fields stretch tens of kiloparsecs and more.

Just as the Earth is surrounded by its own Magnetosphere, so our galaxy is surrounded by a magnetic field known as the galactic magnetosphere.

The magnetic field of the galaxy is very hard to determine. There is no way to measure it directly, because within the Solar System it is swamped by the much stronger fields produced by Earth and the Sun. Magnetic fields in galaxies are generally measured by observing the polarization of synchrotron light emitted by the galaxy. Since synchrotron light derives from fast, electric field accelerated electrons traveling along a magnetic field line, this method gives us information about both the electric field and magnetic field in a galaxy where plasma electrons are present. These electrons are confined to move in helical paths around the magnetic field of the galaxy.

Another method giving information about galactic magnetic fields was the discovery in the early 1900's that neutral hydrogen in the galaxy tended to be attached, or tethered, to regions of strong magnetic fields. "Neutral" hydrogen is actually a weakly ionized plasma that responds strongly to electromagnetic fields. The areas of neutral hydrogen, called HI regions, emit radiation at a wavelength of 21 centimeters (1.42 gigahertz) and are mapped by radio telescopes.

The pattern of polarization seen in the radio emission reveals the magnetic field of the galaxy. It demonstrates that the galactic magnetic field has a roughly circular pattern, similar to what has been deduced for the vicinity of the Sun. The center of the galaxy contains a bipolar magnetic field with a sheet-like structure wrapping around it. The very center of the galaxy has no field at all.

Bubbles, Tunnels and Sheets