Before coming to a more detailed discussion, let us see what ways exist to establish the presence of a magnetic field In the galaxy at all. (Table 1). We shall briefly review these methods one by one.Asociación Argentina de Astronomí

van de Hulst, H.C.

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-26-OBSERVING THE INTERSTELLAR MAGNETIC FIELD H.C. van de Hulst (Sterrewacht. Leiden, Netherlands)1. A LOOK AT HISTORYDiscoveries in present-day astronomy and radio astronomy come with such a rapid succession that results found five years ago are "old" and those of ten years ago "classical". Yet, sometimes, it is good to look hack in order to see the present efforts in a pro­per perspective.Exactly 15 years before today's dedication, on 25 March 1951»Even made the first positive observation of the 21-cm line in the attick of the Harvard physics laboratory. Two years later, when the first excitement of the discovery was over and the first spiral arms had been mapped, Lovell commented that relatively too much attention had been paid to this "predictable" branch of radio astronomer. The radio continuum iight prove far more exciting, Just because it was basically not understood. His remark was well taken, for the study of this continuous spectrum has now led us to such exciting studies as the quasars, new problems in cosmology and -closer to home- means to map the galactic magnetic fields. I shall deal in this pa­per only with the last subject.Astronomers did not wake up promptly to this opportunity. The theory of radiation of an electron moving in a magnetic field at relativistic speeds had been known for half a century. Suggestions that this "synchrotron mechanism" could be a source of continuum radio waves were in the literature since 1950» hut were not pursued. The breakthrough came with Sjiklovsky’s bold suggestion that the white optical spectrum of the Crab nebula, a notorious puzzling object, could arise from this mechanism. Soon this suggestion found a spectacular confirmation by several measurements culminating in Woltjer’s map.of the optical polarization on plates of the nebula taken by Baade (Woltjer, 1957).From that moment on, the determination of the polarization of the general galactic radio continuum with the aim of (a) establishing its character as synchrotron emission and (b) mapping the magnetic fields, has been high on the list of desiderata for radio astronomy. The first exploratory survey in the Netherlands was made in I960. Since that time many Improvements have been introduced. Surveys at- 27-four wavfelCrigttvs have been completed and Mr .Brouw at Leiden is preparing a thesis on these topics. Less extensive measurements Of the same kind have been performed at Cambridge (England) by Bingham. In Australia Matthewson has completed with a narrower beam a polarization survey of the southern sky at *4-08 Mc/s and has explored- interesting areas at three different frequencies.2. SEVERAL WAYS TO OBSERVE THE MAGNETIC FIELDBefore coming to a more detailed discussion, let us see what ways exist to establish the presence of a magnetic field In the galaxy at all. (Table 1). We shall briefly review these methods one by one.t a b l e 1Ways to observe the galactic magnetic fieldatsourceactionofmag- _neticfield enroutedyna­micalKind of observationoptical(synchrotron emission, only in special objects)interstellarpolarizationradioZeemarvfeffectKZeemanôffectOHSynchrotronemissionFaradayeffectstrangeshapescosmic ray confinementa. ZEEMAN EFFECTThis effect would provide the most direct way of measuring the field. The only eligible line is the 1*4-20 Mc/s line of hydrogen.A magnetic field in the line of sight can be observed as a frequency separation of 28 c/s per 10"^ gauss between the left-hand and right- hand circular polarization. Measuring this separation obviously requires line components with steep sides. These are present in the absorption profiles seen with the brighter discrete sources as background.- 28-Kore complete measurements have changed the initially positive results into (-2+5) x 10"^ and (-3+3) x 10”^ gauss for the best observed clouds (Verschuur and Davies, 1966). Hence so far the data are meagre: only a few HI clouds, for those only the line-of-sight component, and for that only an upper limit. Sharp emission compo­nents might be studied in the same way but again are found only in a few places.'An attempt to explain the line-profile and polarization of one OH line by means of Zeemanfeffect required several clouds with fields of the order of 1©“^ gauss (Davies et al., 1966). It does not seem that this explanation can be maintained if the other OH lines are also taken into account.b. INTERSTELLAR POLARIZATIONThis effect can be used as a tracer of the magnetic field, but not for a firm measurement of its magnitude. It provides a wealth of data, which have, since the discovery in 19*+9 formed the most important observational basis for all speculations regarding the topology of the magnetic field near us. The suggestion by Chandra­sekhar and Fermi (1953)» that spiral arms might be tubes of force, was inspired by these data.The traditional name does not say much, because any galactic field is interstellar and any method to observe it involves pola­rization. A more correct name would be: optical dichroism of the interstellar medium, in a medium two "modes" of electromagnetic wave propagation, each with a characteristic polarization, may propagate with a different velocity (birefringence) and/or unequal losses (dichroism). In Interstellar space the optical modes are linear, or very nearly so; Gehrels (1966) has observed a slight rotation of the plane of polarization, which would mean that the modes have an elliptical component. The losses (interstellar extinction) are generally ascribed to dust grains and the faot that the absorption may be some 5 -1 0 percent higher In one mode than in the other must be due to non-random orientation of these grains In a magnetic field.A great effort has been expanded by observers and theorists over the past 35 years In order to find the nature of these grains. Yet their physics and chemistry is by no means settled yet. Two recent symposia (Edihborough, l£f8 ; Troy, 1965) may serve for sufficient reference.Skipping these uncertainties and, consequently also the theories- 29-of the orientation Jprcech.ard.sm, we can still say that it is almost certain that the linear polarizations thus observed display the average transverse comments of the magnetic fields between the stars observed and us. The distances involved range from 100 pc to 2000 pc. Larger distances require measurements of fainter stars; shorter distances require measurement of weaker polarization. In both directions further progress is being made.C. FARADAY EFFECTFaraday effect, or circular birefringence is' another propagation effect. A linearly polarized wave passing through the medium suffers a rotation of the plane of polarization due to the presence of free electrons and a longitudinal component of the magnetic field. The rotation is byB cos0 = longitudinal component of magnetic field in microgauss d 0 = element of pathlength in parsecEither the general synchrotron continuum, or a discrete (usually extragalactic) source may provide the original, linearly polarized wave.The interpretation of these data is more secure than that of the optical polarization, because an accurate numerical value of the rotation measure is found. Uncertainties arise, first of all in estimating how much of the rotation occurs in the (extragalactic) source itself. The rotation in the terrestrial ionosphere can be eliminated quite nicely bjr comparing measurements made in the course of a night. An additional complication is that different values of the rotation measure are lumped in the finite beam and finite bandwidth. The effect is to reduce the amount of polariza- tion and to caus^ deviation from the A law. The initial fear was that these effects might delete all observable polarization. This may still be true in many directions and for most of the distant source regions. Hence, one interpretation of regions of strongA  X radianswhere A is the wavelength in meters and R is the, already traditio-2nal, rotation measure in radians/m :where Ne = number of free electrons per cm^-30-observed polarization! is that they simply are local "clearings" in the Faraday cover. One of the most pressing problems now is to find objective criteria for weighing this type of explanation against other explanations in which such reasons are structural features of a galactic arm.a .  DYNAMIC EFFECTSThe possible role of the magnetic field in the dynamics of the interstellar gas had been recognized by Spitzer, Alfv€n, Blermann and others, well before direct evidence of the field was ever obser­v e d .  We do not wish to review the theoretical arguments, which are many (see for instance, Woltjer 1961). Direct observational evidence for a magnetic field could show up by a striking pattern, as it does, e.g. in the '’polar plumes" of the sun's corona. The dark and bright nebulae in our galaxy displayed so beautifully in the Palomar sky atlas have indeed many striking features. But I should hesitate to quote any of these as evidence for magnetic fields.A second undeniable observation is the presence of cosmic rays in our galactic system. This has often been quoted as "proving" the existence of a confining magnetic field. Unfortunately, this argument breaks down at the very highest energies, for the gyration radius of a 10 eV proton in a 3 x 10 gauss field is 30000 parsec. The cri­terion at what energy it would start to be applicable would have to come from the position of a knee in the energy spectrum, which at the high-energy end is still somewhat vague (Kalhotra et al, 1966).3. WHAT CAN WE INFER FROM THE OBSERVED POLARIZATION?We now return to the synchrotron emission. The observational effort of measuring galactic polarization is wedged between two severe dificulties. At the low-frequency end there is too strong Faraday rotation; at the high frequencies the entire brightness, and hence also its polarized part, becomes extremely faint. Unfor­tunately, it is also a race against time, because radio astronomy h a s  not even been able to reserve through the International Tele­communications Union its claim for a free band every octave (which is already too widely spaced for Faraday measurements) and television stations are rapidly filling up their assigned frequency bands. Systematic errors, arising largely from ground radiation have been overcome and it not necessary anymore to conclude that "magnetic fields in the galaxy are parallel to the plnetrees in Dwlngeloo",- 3 1 -In trying to review the interpretation of these valuable data, and to assess the already heated arguments in the literature let us first look at the degree of polarization.The synchrotron emission process by itself gives 50 to 100 per cent polarization. More precisely, if the electron velocities are isotropic and have an energy spectrum proportional to E “̂  dE, the brightness is proportional to y“*where«f = ¿( Jf -1) and the degree of polarization is f  (see, for instance Ginzburg andSyrovatski, 1965). Inserting, the values ¿f = 2.3, ci = 0.65 we get p = 71 per cent.Typical observed figures at ^08 Mc/s are 2 OK polarization temperature on 30 flK total brightness temperature (Seeger et al,1965)» which makes 6 per cent polarization. Why is this so much lower than the theoretical value? There is no physical depolariza­tion effect. Only incoherent superposition of radiation with diffe­rent directions of polarization can produce this effect. We may think of superposition offa. unpolarized radiation from a different mechanism (e.g. thermal);,b. different intrinsic polarization along the line of sight;c. different Faraday rotation because sources are spread slong the line of sightd. different Faraday rotation or intrinsic polarization side by side in the beam;e. different Faraday rotation side by side in the bandwidth.We can exclude â because thermal radiation is weak and no other mechanism is known. Likewise, A cannot be the major cause because of the fairly regular pattern shown by the maps and £  because it would be felt only at rotation measures larger than 100. Hence and £, both of which are line-of-sight effects, remain. In integra­ting over the line of sight no bias in favour of the nearer of farther portions exists, which would again make the regularity of the maps hard to understand. However, effect d clearly is stronger at larger distance because the beam covers a larger domain of space. So the conclusion seems warranted that we see mainly the polarization produced in the regions of space near us and that ninety percent of the observed intensity arises at larger distances where a combination of effects b, £, and g causes virtually complete depolarization.This conclusion is consistent with the values of the rotation measure. Only 11 out of U-9 discrete (extragalactic) sources listed by Gardner and Davies (1966  ̂have R < 5  and many come as high as- 32-IfO or 50» Yet the majority of the galactic fields with measurable polarization at two frequencies have R < 2  (Mathewson et al,(1966) Brouw, priv. commjThe next task is to look for structural details. The main "local" structure in optical astronomy and in hydrogen line studies is formed by the spiral arms. The main features in radio continuum studies ih meter waves are three arcs, each part of a circle: the north galactic spur, the Cetus arc, and "loop 3W (Quigley and Haslam, 1965)* Rougoor (1966) wishes to string them all together. The most striking feature on the polarization maps so far is Mathewson's ring, a band 600 wide perpendicular to the galactic plane and cutting it at Z 11 = 3500 and 1500. (Mathewson and Milne, 1965). This band contains all areas of locally high polarization. One of these near /  ** = lM-0a,/** = 3fl» has been discussed in detail by Berkhuysen et.al. (1965).How are these features,, observed by different techniques, inte­rrelated? In my opinion it is still too early to decide on a preci­se model. Muc)i will depend on what the surveys at lM-00 Mc/s, now in progress, will show us.The most convincing suggestion, so far, is that the ring of rela­tively high-polarization features is perpendicular to the local direction of the spiral arm (Mathewson and Milne, 1965} Mathewson, Broten and Cole, 1966). The magnetic field in the arm then runs from /**= 2500 t o /  1 = 700. To interpret the map of rotation measures presented by Gardner and Davies (1966) in a similar manner seems more questionable, because the values are so large and because Maltby (1966) finds absence of correlation between the degree of polariza­tion and the galactic latitude. Probably the Faraday rotation In the sources themselves can assume higher values than Gardner and Davies ¿dmtt.- 33-REFERENCESBerkhuysen, E.M.; Brouw, W.N., Muller, C.A., Tinbergen, J. (1965)» Bull. Astron. Inst. Neth. 17. *4-65.Brouw, W.N,, Muller, C.A* and Tinbergen, J. (1962). Bull Astron.faaUJtett» 16.« 2i3.Coyne, G.V. and Gehrels, T. (1966). Astron.J. 71. to be published Davies, R.D., Jager de, C., Verschuur, G.L. (1966). Preprint "The detection of circular and linear polarization in OH emission sources near W3 and VJV9%Edinborough, (196M-). Publ.Roval Observatory Edlnborough. *+; Pratt, N.M. and Reddish, V.C. editors.Gardner, F.F., Davies, R.D. (1966), Austral.J.Phys. 12. 129. Ginzburg, V.L., Syrovatskii, S.I. (1965). Annual Rev. Astron. and Astrophvs. 297.Mathewson, D.S.j Broten, N.W., Cole, D.J. (1966). Austral.J.Phys.12« 93«Mathewson, D.S, and Milne-, D.I. (1965). Austral.J.Phys. 18. 635» Maltby, P. (1966). Astrophys.J. lMt. 219.Malhotra, P.K. and nine more authors,(1966). Nature. 209. 567. Quigley, M.J.S. and Haslam, C.G.T. (1965). Nature. 208. 7^1.Rougoor, W. (1966). preprint.Seeger, Ch.fr.; Westerhout, G.; Conway, R.G.; and Hoekema, T. (1965). Bull. Astron. Inst. Neth. 18. ll.Troy, (1965). Symposium on interstellar grains, organized by I.A.TJ.Comm, 3*M Greenberg, J.M. editor; to be published. Voider, Li (1957). Bull. Astro. Inst. Neth. H .  301.Volt^er, L. (editor) (1962). Interstellar matter in galaxies (Ben­jamin Publ. CV), New York.- 3 V -Fig. 1 Part of Continuum survey at ^00 Mc/s (Seeger et. al. 1965) showing galactie center «id base of north galactic spur.Fig. 2 Part of polarization survey at 820 Mc/s (Brown, unpubli­shed) showing approximately the same area of the sky as Figure 1. Depolarization appears to become less effective toward higher latitudes 1-n the spur.- 35 -¿o° o° 320

Observing the interstellar magnetic field

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Observing the interstellar magnetic field

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