Why don't galaxies have a natural magnetic field like the earth does?
Category: Space Published: September 3, 2014
Each galaxy does have a natural magnetic field, but it is weak. The magnetic field of our galaxy is about 100 times weaker than the magnetic field of the earth. The magnetic field of a galaxy has two basic components: a large-scale ordered pattern that mimics the shape of the galaxy, and a small-scale random pattern. The magnetic field component that has a random pattern is about two times stronger than the large-scale magnetic field component. As a result, you can only detect the ordered pattern of a galaxy's magnetic field if you make many measurements at different locations and average away the random pattern. If you were to place a space probe at some point in interstellar space and have it take a single measurement of the magnetic field, it would mostly be measuring the small-scale random aspect of the field. For this reason, such a measurement could not be used to determine the space probe's orientation or location in the galaxy. In other words, a magnetic compass in interstellar space would not point to some kind of galactic north pole. On the other hand, if the space probe took many measurements at widely separated locations and averaged them, it could detect the large-scale pattern of the galactic magnetic field and use it for navigational purposes, assuming that the space probe already knew the large-scale pattern.
The large-scale galactic magnetic field component has the following pattern: the magnetic field lines lie parallel to the plane of the galaxy and trace out giant azimuthal spirals emanating from the center of the galaxy. These spiralling magnetic field lines coincide with the spiral arms of the galaxy. Interestingly, these field lines point in different directions on different spiral arms, some pointing along the spiral arms toward the galactic center and others pointing along the spiral arms away from the galactic center. The galaxy's magnetic field is strongest in and near the plane of the galaxy since that is where most of the matter exists. It gets weaker as you go away from the galactic plane.
The magnetic field of a galaxy is created in a similar way to how earth's magnetic field is created: through the dynamo effect. The charged interstellar gas throughout the galaxy is moving through space as part of the galaxy's overall rotation. Whenever charged particles move, they create a magnetic field. The moving interstellar gas therefore creates a magnetic field. But this is not the end of the story. The magnetic field then acts back on the charged gas particles, influencing their motion in such a way so as to amplify the magnetic field at the cost of the particles' kinetic energy. A complex, self-amplifying interplay of moving gas particles and magnetic fields results which tends to form a long-term, stable, large-scale galactic magnetic pattern.
In a review article, J.L. Han writes about the galactic magnetic field:
The increasing number of RMs [Rotation Measures], especially of newly discovered distant pulsars, enables us for the first time to explore the magnetic field in nearly one third of the Galactic disk. The fields are found to be coherent in directions... The magnetic fields reverse their directions from arm to arm. The coherent spiral structures and field direction reversals, including the newly determined counter-clockwise field near the Norma arm, are consistent with a bisymmetric spiral model for the disk field. At high latitudes, the antisymmetric RM sky is most probably produced by the toroidal field in the Galactic halo. Together with the dipole field in the Galactic center, it strongly suggests that an A0 dynamo is operating in the halo of our Galaxy.