Interstellar Magnetic Field

one of the components of the interstellar medium. The strength and structure of the interstellar magnetic field can be estimated from various kinds of astronomical observations. One of these involves the investigation of radio emissions in the Milky Way Galaxy, which occur as a result of the motion of relativistic electrons (that is, electrons having velocities close to the velocity of light) in the interstellar magnetic field. In order to obtain reliable results, it is necessary to know the number of such electrons; however, this is not known very accurately.

Another method of estimating the interstellar magnetic field strength is based on measurement of the polarization of starlight in the interstellar medium. Polarization is caused by the fact that elongated interstellar dust particles are spatially oriented in a definite manner by the interstellar magnetic field and absorb light with different polarizations differently. Inasmuch as the characteristics of the dust particles are not well known, such investigations lead to only approximate results, but they do permit the determination of the direction of the lines of force as projected on the celestial sphere.

A third method of estimating the field strength is based on the Faraday effect, which causes the plane of polarization of polarized radio waves passing through a plasma in a magnetic field to rotate by an angle proportional to the path length, the electron concentration, and the mean component of the magnetic field strength along the line of sight. Since many radio sources emit polarized radio waves, this method allows the radial component of the field to be estimated for many directions in the Milky Way Galaxy.

A fourth, very direct, method of measuring the strength of the interstellar magnetic field is applicable only to relatively dense and massive gas clouds, which are in front of powerful sources of radio emission. Such clouds produce a 21-cm absorption line in the source’s spectrum, for which it is possible to measure the Zeeman effect and thus estimate the longitudinal component of the field strength in the cloud. In some cases, the field strength can be estimated by its dynamical action upon the gas, which produces the elongated shape of certain gaseous nebulae and facilitates the formation of the thin filaments observed in reflection nebulae. Finally, the interstellar magnetic field significantly affects the thickness of the Milky Way Galaxy’s gaseous disk.

Comparison of all the foregoing methods has led to the following conception of the Milky Way Galaxy’s interstellar magnetic field. The magnitude of the field is several microgauss and varies somewhat in different regions of the galaxy. It is of the order of 1 microgauss between galactic arms, approximately twice as large in the arms, and still larger in clouds, especially dense ones. The lines of force on the average are nearly circular in the galactic disk. However, in certain regions measuring several hundred parsecs, the structure of the field can be quite complex.

The origin of the galactic magnetic field is not yet very clear. It may have already existed in the medium out of which our galaxy was formed. However, it is more probable that it was formed as a result of magnetohydrodynamic processes in turbulent motions of a conducting medium. On the other hand, the field may have been formed during the formation of the first stars. Subsequent explosions may have ejected the magnetic field into interstellar space, where the field was strengthened by turbulent motions and the differential rotation of the galaxy. The interstellar magnetic field plays an essential role in star formation.