The Interaction of the Solar Wind with the Interstellar Medium W
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THE INTERACTION OF THE SOLAR WIND WITH THE INTERSTELLAR MEDIUM W. I. Axford An invited review INTRODUCTION The interaction between the solar wind and the inter- stellar medium has been a topic of interest for many POSSIBLE years [Davis, 1955, 1962; Parker, 1961, 1963; Axford TERMINAL SHOCK et al., 19631 , but progress has been rather slow largely 100 AU I because of the paucity of relevant observations. Recently, however, the situation has changed as a result of the clear evidence for the penetration of interstellar gas into the inner solar system obtained from OGO 5 [momas and Krassa, 1971; Bertaux and Blamont, ,’. \ 19711. Furthermore, the prospect that there will be space probes traveling toward the outer regions of the solar system within a few years (see fig. 1) has created a need for a better understanding of the subject so that the most effective use can be made of the missions. This review is a survey of the work, both published and unpublished, that has been carried out up to the Figure 1 Spacecraft trajectories for the proposed time of writing. To the best of our knowledge, the “Grand Tour ’’ missions to Jupiter-Saturn-Pluto bibliography is complete up to mid-1971. First, the (1976-1 9 77), and Jupiter- Uranus-Neptune (19 79). me expected characteristics of the solar wind, extrapolated trajectories carry the spacecraft in the general direction from the vicinity of the earth are described, and several of the projection of the solar apex into the ecliptic simple models are examined for the interaction of the plane, the direction in which the distance to the solar solar wind with the interstellar plasma and magnetic wind termination is a minimum. mefinal portion of the field. The possibility that there is a substantial neutral trajectories (dashed) can be vaned, since it depends on component of the interstellar gas is ignored. In later sec- the last planetary encounter. tions, we consider various aspects of the penetration of neutral interstellar gas into the solar wind, and describe Allen, 19631. The sun is located on the inner edge of the the dynamic effects of the neutral gas on the solar wind local (Orion) spiral arm at a distance of -8 parsec north (to the extent that they are understood). Finally, we of the plane of the galaxy. The solar motion relative to discuss problems associated with the interaction of neighboring stars is 20 kmsec-’ in the direction cosmic rays with the solar wind. a=271”, 6 =+30°, or lrr=57”, b11=+22” (the “solar We briefly summarize here the present status of apex”). The nearest star is more than 1 parsec away, and knowledge concerning the properties of the interstellar the mean density of stars in the solar neighborhood is medium in the vicinity of the solar system [see, e.g., -0.057 solar masses per cubic parsec. Since the region containing the solar wind (the “heliosphere”) has dimen- sions that are at most of the order of 102-103 AU The author is with the Departments of Physics and Applied (<lg2parsec), and it seems reasonable to assume that Physics and Information Science, University of California, San stellar winds associated with nearby stars are not signifi- Diego, La Jolla, California. cantly more intense than the solar wind, we can be fairly 609 sure that the solar wind interacts directly with the inter- Staelin and Reifenstein, 1969; Goldstein and Meisel, stellar medium and that there is no overlapping of stellar 1969; Hewish, 19701, and of other polarized galactic winds in our vicinity. It should be noted that the sun sources [Morris and Berge, 1964;Milne, 19681 . Of these appears to be on the fringes of the Gum nebula; how- various types of observation, only the 21-cm Zeeman ever, according to Brandt etal. [1971] it lies some effect and the pulsar Faraday rotation measurements 60 parsecs outside and the local interstellar medium may provide a value for the magnetic field strength, and these not be directly affected by the presence of the nebula. refer only to some weighted mean of the line-of-sight The mean interstellar electron density ne in the disc component. Magnetic fields of the order of 10 pG have of the galaxy can be determined in two ways: (1) by been found in some absorbing clouds; however, it seems measurements of low frequency radio wave absorption likely that the field strength in such clouds is not typical [Ellis and Hamilton, 1966; Smith, 1965; Bridle, 1969; of the interstellar magnetic field as a whole, and is in Bridle and Venugopal, 1969; Alexander et al., 19691 ; fact amplified as a result of the contraction of the clouds and (2) by measurements of the frequency dispersion of [Verschuur, 1969~1.The pulsar observations provide the pulsar emissions [Rlkington et al., 1968; Lyne and most satisfactory results for our purposes since they Rickett, 1968;Habingand Pottasch, 1968;Davies, 1969; refer to a region relatively close to the sun. If it is pos- Prentice and Ter Haar, 1969a,b; Davidson and Terzian, sible to obtain reasonably good values for the rotation 1969; Mills, 1969; Gould, 1971; Grewingand Walmsley, measure (RM=0.85 JneB dZ, with B in pG and 1 in 19711 . The first method yields the quantity (n:Te-3'2)l, parsecs) and the dispersion measure (DM = Jnedl) from where 1 is the distance in the galaxy along the line of the ratio RM/DM, one can obtain a value of the mean sight and Te is the electron temperature; this tends to line-of-sight component of B, weighted with respect to overemphasize the contributions from denser regions the electron density. The measured values of BIIrange [Gould, 19711 . The second method yields be)l and thus from 0.7 pG to 3 pG, and it seems reasonable therefore permit, us to estimate 02,) directly if we know the dis- to assume that the largest value is representative of the tance to the pulsar by independent means (as in the case interstellar magnetic field strength. This value is consis- of the Vela and Crab pulsars), or if an estimate can be tent with that found using the mean value of ne made of the neutral hydrogen content in the line of sight obtained from pulsar measurements together with the (e.g., from 21-cm absorption of the pulsar emission, as in rotation measures of extragalactic sources [Davies, the case of CP0329) so that a comparison can be made 19693. Furthermore, a magnetic field strength of this with 21-cm emission in the same direction. In fact, the order is required to account for the nonthermal galactic interpretation of the observations is not easy because of radio emission [Felten, 1966; Webber, 1968; Goldstein the confusion introduced by dense WI regions, and the et al., 1970al , although there is some uncertainty con- structure associated with the spiral arms of the galaxy. cerning the unmodulated spectrum of cosmic ray elec- According to Davidson and Terzian [1969] the mean trons. These and other arguments concerning the electron density in the disc of the galaxy is interstellar magnetic field have been reviewed by 0.03-0.10 ~m-~.However, the result is model dependent, Burbidge [ 19691 . and according to Gould [ 19711 ,who notes that the path Interstellar atomic hydrogen can be detected by to the Crab pulsar crosses the interarm region where the observations of (1) the 21-cm line in emission and density could be low, the local mean electron density in absorption [Kerr and Westerhout, 1965; Kerr, 1968, the galactic plane is -0.12 cm-3 . 19691 , including absorption of pulsar emissions [deJager Evidence for the presence of a large-scale interstellar etal., 1968; Guelin etal., 1969; Gordon etal., 1969; magnetic field arises from observations of (1) the polari- Gordon and Gordon, 1970;Hewish, 19701 ;(2) Lyman a zation of starlight [Davis and Greenstein, 1951; absorption in the ultraviolet spectra of nearby early type Mathewson, 1968) ; (2) the polarization of galactic non- stars [Morton, 1967; Jenkins and Morton, 1967; thermal radio emission [Mathewson and Milne, 19651 ; Gzrruthers, 1968, 1969, 197Oa; Morton etal., 1969; (3) the Zeeman effect at 21-cm wavelength [Verschuur, Jenkins et al., 1969; Smith, 1969; Wilson and 1968, 1969a,b,c, 1970; Davies etal., 19681 ; (4) the Boksenberg, 19691 ; (3) the absorption of soft X rays Faraday rotation of polarized extragalactic radio sources from discrete sources [Gorenstein et al., 1967; Fritz [Gardner and Whiteoak, 1966; Gardner and Davies, et al., 1968; Rappaport et al., 1969; Grader et al., 1966; Gardner et al., 1967; van de Hulst, 1967; Berge 19701 ; and (4) the backscattering of solar Lyman a and Seielstad, 1967; Mathewson and Nicholls, 19681 ; [Chambers etal., 1970; Barth, 197Oa; Thomas and and (5) the Faraday rotation of pulsar emissions [Smith, Krassa, 1971; Bertaux and Blamont, 19711 . Observa- 1968a,b; Radhakrishnan et al., 1969; Ekers etal., 1969; tions of 21-cm radio emission suggest that the mean 610 density of neutral hydrogen in the vicinity of the sun is observations (all of which refer to path lengths of the h~)x 0.7 ~m'~.The 21-cm absorption measurements order of 100 parsecs or more), and indeed is quite con- yield an estimate for h~)l(where I is the distance to the sistent with most of the ultraviolet absorption source), provided one assumes a value for the spin measurements. temperature T,;it is generally assumed that Ts - 100" K. It has been conjectured that the interstellar gas might However, discrete sources such as pulsars do not show contain a significant fraction of molecular hydrogen the absorption one would expect on the basis of [Gould and Salpeter, 1963; Gould et al., 19631, and in measurements made using extended sources.