The magnetic field of the sun is created by a magnetohydrodynamic dynamo under conditions bearing some qualitative similarities to the apparent generation of the galactic field in the gaseous disk of the galaxy. There is a similarity, too, in the extension of bipolar lobes of the solar field above the surface of the sun and the extension of bipolar lobes of the galactic field outward from both sides of the disk. Hence one can learn a lot about the expected origin and activity of the galactic field by studying the behavior of the magnetic field of the sun. In particular, the mysteries associated with the “simple” circumstances of the origin of the solar magnetic field far below the surface are no less than the mysteries in the theoretical origin of the galactic field, where there is so little direct observation of the small scale motions and magnetic fields. There is reason to think that the activity of the magnetic field of the sun, producing prominences, flares and X-ray corona, a solar wind, and coronal mass ejection may all have counter parts in the activity of the galactic field above the surface of the gaseous disk.

The solar corona is a hot (106K) highly ionized plasma structured by its magnetic field into open regions where the solar wind escapes and closed regions where relatively dense plasma is trapped in near static equilibrium. Observed in Thomson-scattered light at times of solar eclipse or by artificial occultation using a coronagraph, these closed regions show up conspicuously as helmet-shaped bright structures. The large scale corona evolves in time in response to the solar dynamo that continually injects new magnetic flux into the corona with the eventual reversal of the global magnetic polarity at the end of each half cycle of eleven years. It was discovered in the 1970s using spaceborne coronagraphs that in addition to its long-term evolution, the corona also undergoes dynamical reconfiguration with ejection of mass of the order of 1015g into interplanetary space (MacQueen 1980). These time dependent phenomena take place once every few days at solar activity minimum and as often three times a day at solar activity maximum. Since the 1970s, coronal mass ejections have been studied at the High Altitude Observatory by the use of the coronagraph on the NASA Solar Maximum Mission Satellite and groundbased instruments at Hawaii. This brief review presents three points of interest in the coronal mass ejection as a hydromagnetic process, emphasizing the unique opportunity offered by the corona to study hydromagnetic phenomena by direct observation (Hundhausen 1987, Kahler 1987, Low 1986).

In addition to the more or less steady solar wind, the Sun also ejects mass in highly time dependent events taking place in the corona once every few days at solar activity minimum and as often as three times a day at solar activity maximum (Hundhausen 1988, Low 1986). These events involve large scale reconfiguration of the corona with an expulsion of some 1015g of ionized material into interplanetary space. The High Altitude Observatory (HAO) operates a groundbased internally occulted coronagraph at Mauna Loa, Hawaii, with a field of view of the corona from 1.2 to 2.2R⊙ in heliocentric distance, registering polarization brightness. A second instrument at the same site in Hawaii observes the solar limb in Hα emission to detect chromospheric material from the limb out to 1.5R⊙. HAO also operates an externally occulted coronagraph/polarimeter onboard the NASA Solar Maximum Mission Satellite (SMM) launched in 1980, capitalizing on the advantage of space with a field of view from 1.5 to 6R⊙ to cover the fainter outer corona. Coronal mass ejections involve magnetic field reconfiguration from high in the corona down to the base lying below 1.1R⊙. Important physical insights can be had when simultaneous observations by HAO's three instruments are put together with a common scale and orientation to reveal a mass ejection in the full extent of the solar atmosphere from the limb outward. Combined observations of two mass ejections are presented, one associated with an eruptive prominence and the the other associated with a flare. The significance of these two events is that in both cases, the mass ejection was in fully developed motion and had traveled high into the corona well before the onset of the associated prominence or flare eruption, pointing to an instability in the large scale coronal magnetic field as the underlying cause of the global reconfiguration.

A few authors (Barnes and Sturrock, 1972; Ma, 1977; Svestka, 1977) have calculated the quantitative relationship between the static force-free field connecting the magnetic field and the twisting processes. They pointed out that the potential magnetic field without the current may be twisted into the force-free field with the enhanced current produced by the plasma rotation. Li et al. (1982) and Li and Hu (1984) have stated that the processes should be unsteady, and especially that they should not be static. The magnetic Reynold number is usually much larger than 100 in stellar atmosphere (Li et al., 1982). We adopt the following MHD equations: where the force - free factor α (t, r) depends on both, t and r. According to t h e kinematical momentum conservation, the following constraint is easily obtained: where V = (u, v, w) is the velocity field in the cylindrical coordinates. When studying the evolution of the kinematical force - free field, the in fluence of a reasonable flow on the variations of the magnetic field should be taken into account. After some reasonable simplification we deduce the specific expression of the variation law of the toroidal magnetic energy where J1 is the Bessel function of the first order. In the active region, magnetic energy including the term of a twisted effect f(t) is larger than that of the static force - free field.

The Coronal Magnetic Structures Observing Campaign (CoMStOC) was designed to measure the magnetic field strength and determine its structure in the solar corona. Simultaneous soft X-ray and microwave data separate the contributions of the two dominant microwave emission mechanisms - gyroresonance and thermal bremsstrahlung. Where gyroresonance dominates, the magnetic field can be determined.

Observations of circular polarization of molecular masers associated with late type giant and supergiant stars can be used to estimate the magnetic field strength in the masing region. Magnetic field strengths of ~ 5 mG are deduced for OH masers in circumstellar envelopes at distances of ~ 1016 cm from the star, and magnetic field strengths of ~ 50 G are deduced for SiO masers that reside above the photosphere. Extrapolation to the stellar photosphere suggests that average surface magnetic fields are on the order of 103 G.

Most theoretical works, which aim to interpret the radio radiation observed from pulsars, are based on the idea that coherence of the pulsar emission process could result from the development of plasma instabilities in the magnetosphere of the star. In particular the Ruderman-Sutherland (1975) magnetic pole model reproduces in a satisfying manner several features of the emission. It is based on the development of the two-stream instability. This instability results from the interaction of a beam and a plasma, both in relativistic motion along the open and curved magnetic field lines issued from the magnetic poles of the star. The possibility to obtain radio radiation through this instability process is limited to part of the open cone of magnetic field lines which form an hollow-cone. We describe the linear and nonlinear evolution of the two-stream instability, in the presence of an extremely strong magnetic field. The linear description, including oblique wave propagation (and eventually curvature of the magnetic lines), shows the existence of a natural coupling between the electrostatic and electromagnetic features of the unstable plasma waves which provides the possibility for a direct electromagnetic emission mechanism. The nonlinear evolution of these waves into Langmuir solitons, with also both electrostatic and electromagnetic features, shows the possibility for a new type of radio radiation mechanism, induced by the presence of transverse perturbations of the solitons, which is of interest for the interpretation of pulsar radio observations.

Difficulties of the dynamo and alternative theories of the magnetic field generation are briefly discussed. The correlation between η = lg μ/μo and ζ = lg J/Jo for rotating celestial bodies is considered. μ is the magnetic and J the angular momentum of the body. Existing theories do not explain such a correlation, and it may be an evidence for some new fundamental interaction.

Since its discovery 40 years ago as a confiner of cosmic rays and an aligner of interstellar dust grains, the Galactic magnetic field has been studied through emission and polarization of synchrotron radiation, Faraday rotation, Zeeman splitting, and effects on gas flows and morphology. The local field has a coherent, few microgauss, component roughly along the local spiral arm and a comparable chaotic component. Field direction in the plane is reversed one or more times both inside and outside the solar circle. Loops and spurs extend outside the plane and may merge with a general dipole, which is most conspicuous near the Galactic center. There may be a separate thick disc magneto-ionic component. Comparison with other galaxies, especially grand design vs. flocculent spirals should eventually prove fruitful. Parameters are not currently well enough known to rule out dipole or primordial origin.

More than 300 measurements of magnetic field strengths in H I regions now exist. Interpretation of about 100 shows that magnetic pressure either is comparable with or dominates other pressures near at least some dark clouds and in at least some H I shells. There appears to be direct evidence for Alfvén waves, but this needs to be confirmed by additional examples. In many regions the field is highly uniform, but in other regions it varies considerably over 30 arcminutes or less. The field directions derived from Zeeman splitting and from Faraday rotation do not correlate well.

The high linear polarization of pulsar radiation allows a reasonably easy determination of the Faraday rotation measure. Unlike other radio sources, the impulsive nature of pulsar emission also permits a determination of the number of electrons responsible for the rotation, through the dispersion measure, DM.

We have modeled the local Galactic magnetic field using pulsar rotation measures (RMs), of which there are now about 200 available. The North Polar Spur has a significant effect on pulsar RMs. Using RMs of 116 pulsars nearer than 3 kpc, we find that the local field has a strength B0 = 1.6 ± 0.2 μG toward longitude lB =96° ± 4°, with a reversal of the field at a distance Dr = 600 ± 80 pc toward the inner Galaxy. Relaxing the 3 kpc distance restriction, we find that a concentric ring model with reversals is superior to a bisymmetric spiral model as a fit to the data.

We have measured the degree of Faraday rotation towards 73 extra-galactic sources at low Galactic latitudes, predominantly within the first quadrant of longitude. The Faraday rotation towards extragalactic sources follows the trends of that towards pulsars, a fact which vindicates an initial assumption that the rotation arises mostly from within the Galaxy. When combined with the pulsar data, our data imply the existence of magnetic field reversals along several lines of sight. Extragalactic sources with l > 60° have rotation measures that are systematically larger than those for pulsars in the same region, which suggests that the structure responsible for the excess rotation must lie more distant than ~ 7 kpc.

Parameters of the interstellar magnetic field - the strength ratio of the regular and the isotropic component and the orientation of the regular component - have been derived in the Local Galactic arm over a scale ≤ 1000 pc in the direction of the Galactic anticentre. The results have been obtained by an independent method involving analysis of the Galactic longitude dependences of the volume density of the low frequency Galactic background. An estimate of the ratio of the arm and interarm magnetic field strengths has been made.

The present paper is devoted to a study of the magnetic field configuration of our Galaxy based on Faraday rotation measures (RM) of 185 pulsars and 802 extragalactic radio sources. RM data of pulsars located near the plane of the Galaxy are used for the study of magnetic fields in neighbouring spiral arms. For the distribution of spiral arms the well-known model of Georgelin and Georgelin (1976) is used. The calculations show (for details see Andreassian and Makarov, 1987, 1989) that in the Perseus spiral arm and the local Orion arm the magnetic fields have approximately the same directions (lo;bo) ≈ (80°;0°), while in the Sagittarius-Carina arm the magnetic field has an opposite direction.

We discuss small scale structure in the Galactic magnetic field as inferred from Faraday rotation measurements of extragalactic radio sources. The rotation measure data suggest a continuum of length scales extending from parsec scales down to at least 0.01 pc and perhaps to as small as 109 cm. Such turbulence in the magnetic field comprises a reservoir of energy that is comparable to the energy in the large scale field.