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1.3. Aims of this Review

"Cherchez le Champ Magnétique" is slowly becoming a well known saying in astronomy. Magnetic fields exist almost everywhere in Space, filling Space to a very large extent. The review below includes the small scale magnetic fields, on the scale from ~ 10-17 pc up to 1 pc. The search for a magnetic field ouside the Earth starts with magnetometers inside spacecrafts (planetary and interplanetary magnetic fields) and continues further with polarimeters inside telescopes on Earth (stellar and interstellar magnetic fields). The large majority of recent observations in interplanetary space have been made with detectors on board spacecrafts, notably Galileo, Ulysses, Voyager I, and Vega I and II.

The study of magnetic fields in circumstellar, protostellar and interstellar objects do impose some extra complexities for theorists in their equations (more terms are added, often non-linear), and do require extra time for observers in their measurements (to see the weak polarized signals above a detection threshold).

The large majority of recent observations of magnetic fields in protostellar disks and interstellar cloudlets have been made in the field of radio astronomy called the Extreme Infrared (submillimeter wavelengths), Current theories for the formation of protostellar disks and interstellar cloudlets prefer a strong magnetic field. Yet strong magnetic fields could (i) prevent gravitational free-fall collapse of the gas, decreasing the star formation rate there; (ii) limit the random motions of individual disks and cloudlets, dragging along low density gas; (iii) transfer momentum between cloudlets, removing angular momentum during condensation; (iv) be a source of extra pressure, creating non-linear magneto-hydro-dynamical (MHD) waves (e.g., Elmegreen 1993). Can we match theories with observations ?

Section 2 deals with magnetism in small bodies and planets, as sampled by artificial satellites launched from Earth. Section 3 deals with stars and the large interplanetary medium. Section 4 deals with circumstellar magnetic fields. Section 5 deals with the methodology and instrumentation involved in sampling interstellar space with radio telescopes. Section 6 deals with the predictions while Section 7 deals with the observations of magnetic field B for protostellar disks and small interstellar cloudlets, up to 1 parsec in size. Section 8 deals with large stripes and cloudlets seen at near IR, optical, and UV wavelengths. In what follows, one Astronomical Unit equals 150 million km (1 AU = Earth-Sun distance).

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