RADIO SCIENCE Journal of Research NBS/ USNC-URSI Vol. 69D, No.8, August 1965
Solar Wind and Its Interaction With the Magnetosphere1
Charles P. Sonett
Space Sciences Division NASA, Ames Research Center, Moffett Field, Calif.
(Received February 3, 1965)
A short su mmary of the properties of the solar wind, as they apply to the interaction process with the earth's magnetosphere, is given. Necessary conditions on the bow shock are discussed and are compared to experime ntal evidence from several space probes. The importance of the downstream or post·shock Aow is emphasized. Power s pectra and details of the turbulence in this region are reviewed. Limitations due to the very preliminary status of IMP data analys is are emphas ized.
1. Introduction motion, i.e., to propagate linear and angul ar momenta from particle to particle. Second, the fie ld appears This paper summarizes some of the properties of to have an energy density comparable to the th ermal the solar wind as they are known from space probe e nergy of the solar wind gas, but entirely ignorabl e data and attendant physical processes as they may be compared to the energy of bulk convective fl ow. Thus, regarded in li ght of our ge nerally rudimentary knowl the flow against the magnetosphere is ofte n regarded edge of collision-free plasmas. A pervasive aspect of as a high Mach number instance of aerodymanic fl ow the solar wind is its interaction with the magnetosphere with the electrodynamic conditions being subsidiary. of the earth and the subsequent generation of geo Though the interacti on is magnetohydrodynamic, the physical processes which are noted on the surface of fl ow properties are taken in this vi ew to be essentially our planet. aerodynamic. It is understood that the solar wind flows nearly continuously and that the outward flow carries along 2. Solar Wind the solar magneti c fi eld. The rotation of the sun results in the fi eld's being drawn out in a spiral pattern Mariner data are the most complete available in whose angle depends upon the velocity of the wind. regard to the properties of the solar wind. From Although the primary source of energy for the inter these data typical gas parameters indicate a volume action of the solar wind with the magnetosphere comes density of - 10 per cm 3, a temperature of - 105 OK, a from the convective energy of the wind, a small addi magne ti c field of 5 gamma, and a streaming velocity tional increment may be found from the rotation of of 400 km/sec as being typi cal of the wind, though the the earth through a drag interaction, such as that actual value of these parameters flu ctuates markedly proposed by Axford and Hines [1961]. from time to time. For example, in the experime nt of The simplest view of the total phenomenon is that Snyder and Neugebauer [1964] the velocity ranged the magnetos phere presents an obstacle to the passage from 300 to 800 km/sec for 90 percent of the data, the of the solar wind in the manner of a blunt body in temperature reached extremes of several hundred aerodynamic fl ow. The interruption of flow results thousand degrees Kelvin, while the magnetic field in a shock wave, followed by a shocked gas region attained values as high as 20 gamma. and boundary layer of subsonic flow around the mag From these measurements we calculate a mean-free netosphere, with the free s urface of the magnetosphere path ranging from 1 to 10 AU depending upon the itself determining the detailed fl ow properties near temperature and density values. The electrical con th e boundary. ductivity is some 6 X 1014 esu, 10- 3 that of copper, The validity of thi s aerodynamic point of vi ew is nevertheless, an extreme value over the cosmic dime n based primarily upon two factors. First, though sions characteristic of interplanetary space. The entirely colli sion-free over a scale determined by the absolute viscosity of the solar wind gas is some 10- 3 magnetosphere, the solar wind carries along a magnetic g/cm-sec. The kinematic viscosity is about 1020 fi eld which is thought to organize the gas into collective cm 2/sec, which is to be compared to air at standard temperature and pressure for which the value of kine
1 Paper wa s present e d at th e ULt Symposium , Boulder. Colo. 17- 20 August 1964. matic viscosity is 1.5 X 10 - 1 cm2/sec. Since the kine- 1033 matic viscosity measures the ratio of viscous to inertial unobservable, lying outside of the passband of present ! forces, it is seen that the viscosity is extremely high day magnetometers and plasma probes. Mean col and that the solar wind is indeed a very sticky gas, li sion times are perhaps a day for protons. The greater even though the absolute viscosity is low. Another mobility of electrons means that the collision times are way to regard the kinematic viscosity is to understand more likely on the order of minutes to an hour. it in terms of the transport of momenta, i.e., the dif· The flow of the solar wind is often taken to be sheet fusion of momentum across a region, this having an like in the plane of the ecliptic, though there is no extremely large rate because of the long mean-free known spacecraft test of this property. Some evidence path. Thus, one is faced with a somewhat paradoxical for a dropoff in solar wind bulk flow with ecliptic lati· situation that the more rarefied the gas the higher the tude comes from comet tails observed out of the plane kinematic viscosity and the greater the " stickiness." of the ecliptic, indicating a dropoff in solar wind forces The aerodynamic Reynolds number which is given by [Beyer, 1953]. The magnetic field entrained in the the velocity times a characteristic length divided by solar wind is swept into a spiral pattern according to the kinematic viscosity also measures the effective arguments first given by Parker [1958] which involves viscosity of the gas. For the solar wind, the Reynolds the solar rotation. For a nonrotating sun, the field number, R", is of the order of 103 over a scale size of lines would be expected to be radially outward from the magnetosphere. For smaller scale sizes associated the sun. with detailed phenomena in the interaction region, The existence of magnetic fields in interplanetary the Reynolds number is reduced accordingly. For space was suspected long before spacecraft flight. magnetospheric scale size, Rn approaches the critical In particular, the ll-year modulation of the galactic number for the onset of ordinary turbulent flow. It component of cosmic rays, Forbush decreases, and is unfortunately not possible to extrapolate exactly 27-day and diurnal variations indicated the existence of from this because the magneti c fi eld contained in the a magnetic modulating mechanism sometime before the solar wind will inhibit the onset of turbulence, kine first spacecraft experiments on Pioneer V. Variations matic viscosity being reduced by the decrease trans in the intensity and direction of the interplanetary port across the field lines. It is possible that the inhi field are now known from the Mariner measurement bition of transport so decreases the viscosity that at [Davis, Smith, Coleman, and Sonett, 1964] to all times the flow of the solar wind against the magneto show a marked 27-day effect (fig. 1), which reflects a sphere is highly fluid. view of the radially outward component of the field; Transverse velocity spectra can only be inferred variations of 10 gamma over a 27-day period are seen from Mariner data. These are usually taken to be in this figure. The lower part of the figure displays the same as the longitudinal, isotropy of the velocity the standard deviation taken over different sampling distribution being assumed. (The Mariner probe was intervals up to 1 day. A small-scale variability is pointed at the sun and could not measure transverse seen in the field, indicating fluctuations in direction. velocity directly.) It is also indicated from Mariner In figure 2 is shown the interplanetary field as deter that some Q particles are present in the solar wind. mined by Mariner II in ecliptic spherical coordinates This conclusion is supported by Explorer 14 plasma with f3 being ecliptic latitude, A ecliptic longitude, data [Wolfe and Silva, 1964] taken during the flight and the magnitude of the field shown in the lower part of Mariner II on October 7,1962. Solar wind electrons of the fi gure. Here the magnitude displays a 27-day have never been detected in deep space, but would be variation, the longitude is plotted against the ideal expected to be transported with the same bulk velocity DAYS OF SOLAR ROTATION PERIOD as that of the ions, otherwise large currents which 8 3510152025 5 10152025 5 1015 are not observed would exist in interplanetary space. Actually, the velocity distribution for electrons in thermal equilibrium with the ions has a mean value 6 lying between 1.3 and 3.7 X 108 cm/sec, based upon the ion temperatures of the Mariner and a ratio of specific heats of 2. Therefore, the bulk and thermal velocities become comingled and their roles tend to be reversed from the ion case. Because of their higher O~--~~----+-~----~ mobility, electrons should be important in transport _2 L--L__ ~~~ ~~~~ __~ processes, such as heat conduction and kinematic viscosity. Properties of the solar wind vary strongly from the gommo base of the corona where it is created out to 1 astro '~,> :~ nomical unit. At the orbit of earth the Debye length, 8128 9/7 9/17 9/27 1017 10117 10/27 11/6 using the Mariner ion temperature of 105 OK and a 240 250260 270280 290 300 310 days volume density of 1O/cm3, is about 7 m. Although such a distance is large compared to the dimensions of present day spacecraft, this parameter must be FIGU RE 1. One-day average values o'f the radial component of the interplanetary magnetic field. referred to the streaming velocity of the wind. Thus, A 27·day periodicity is clearl y evident in this figure and is presumed to be due to asy m mctries in the field resulting fro m regions of hi gh field on the SUIl. In the lower parI of random fluctuations taking place within a Debye sphere the figure is shown the standard deviation of the radial component for different average occur in times of the order of microseconds and are times {D avis. Smith, Coleman, and Sonell, 19641. 1034 ------
DAYS OF SOLAR ROTATION PERIOD made reasonably exact calculations of the shape of the cavity formed by the pressure of this free molecule fl ow, and the results of their various techniques are ':::~;;;~~~I~i not grossly different. All results display neutral points corresponding to where the polar line of force is matched to the surface field of the boundary. Further, all solutions display the characteristic dou bling of the magnetic field due to the termination and -100· the consequent flow of surface currents_ Since the
- 200. "----'--''----1._ -'-----1------'_-'-- model used for these calculations has no fi eld carried in the solar wind, the matching of magnetic boundary conditions is simple except for the surface being a <,.gommo'lkJ1:6 free boundary. Clearly, the surface field must be wholl y contained in the boundary and the normal 0 8/28 917 9/17 9127 1017 10/ 17 10/27 component is zero_ Seasonal effects cause the sur 240 250 260 270 280 290 300 days face to change slightly as pointed out by Spreiter and Summers [1963] _ They calculated the boundary shape for the case of a wind flowing obliquely, cor FIGU RE 2. intensity and direction oj the interplanetary magnetic field shown in solar ecliptic coordinates; {3 is the ecliptic latitude, responding to seasonal changes_ Such c hanges, they II the longitude meosured from the earth-s un line, and B the foun d, cause s mall variations in the position of the intensity oj the field. polar neutral points and in the position of the s tag The 27-day variat ion is seen to be associated wi lh the magnitude of the field; the lungi tude is compared to the ideal s pi ral angle defin ed by the mean pl asma velocity, a nd , for these nati on point. However, they found the tail of the I ·day averages. di sp lays one gross c ha nge in polarit y that lasts a number of days; th e magnetosphere to lie in the plane of th e ecliptic regard latitude whkh mt:asures the out-or-th e-ecliptic component shows contin uous variati ons evell over th e average limes used here fD avis. Smith . Coleman. and Sonelt , 1964]. less of seasonal change. Such changes cause the positi on of th e polar ne ut ral points and of the s tag nation point to change sli ghtl y_ Calculations of the type di scussed he re are important angle of the magneti c fi eld as determined by the solar since a precise boundary appears often even though a wind bulk velocity, and the ecliptic latitude s hows mu ch more complicated overall view evolves from cur large variations in and out of the e cliptic component of rent experiments. Indeed even th e earl y results of the field. The general character of these data indi Pioneer I were not in agreement with the view of a cates that over large scales 'the fi eld takes on the simple smooth C hapman-Ferraro boundary. theoretical spiral angle direction and that over peri ods of a day or less gross variability in direction is to be seen_ Lastly, it is primarily the magnitude of the fi eld 4_ Bow Wave which displays the 27-day variation rather than direc In colli sion dominated fl ow, a shock wave will form tion as indicated in the lower part of fi gure 2. Fluctu wh en the local s peed of flow exceeds the so und veloc ations of the type displayed in the Mariner data indicate ity. Such a wave is a manifestation of the role of the that the magneti c field is in approximate energy equi fluid motion in transmitting changes in the fluid rather librium with the thermal energy of the solar wind gas, than by means of sound waves. In this primitive so that in a frame of reference moving with the solar view there is no essential difference between sho ck wind, the field and gas di splay equipartition, the ther waves in free motion and those that occur before an mal motions of the gas being shared with a di sorder of obstacle which is placed within the fluid, except that the magnetic fi eld. downstream of the bow wave a region of subso nic flow That the interplanetary magnetic field is closely of great complexity is developed which may expand associated with events which take place in the atmos back to the free-stream superso nic conditions far phere of the sun has been suggested by Greenstadt from the obstacle_ This cannot take place, for [1963] who has shown a close correlation between example, in the one-dimensional shock tube_ both the coronal green line integrated index and plage The bow wave is greatly complicated by several index and the intensity of a model field based upon factors which do not exist in the ordinary collision Pioneer V. An unexplained aspect of these data is dominated gas dynamic shock wave_ The two issues a time dependence indicating a 5- to 7-day delay complicating the physics of this problem are the between an increase in solar indices and the cor magnetohydrodynamics and the collisionless property responding fi eld in crease at the s pacecraft. of the gas_ Introducing these problems also suggests that shock stability and the modes of propagation of 3. Magnetospheric Boundary the fluid need to be considered. The energetics of the shock wave are determined primarily by the fl ow Prior to the introduction of the shock concept of energy of the wind since the thermal and magne ti c s topping the solar wind by the magnetosphere, most energies of the gas are so much lower. The bulk flow investigations of the termination consisted in solving energy is some 10- 9 to 10- 8 ergs/cm 3 which is us ually the problem of free-molecule flow of a solar wind, one to two orders greater than that of either the thermal without a magneti c fi eld, against a plasma-free mag or magne ti c energies. Because of this, it is plausible netospheric cavity_ A number of investigators have to expect some similarity to aerodynamic flow in the 1035
------gross sense. Spreiter and Jones [1963] have calcu assume the ideal spiral angle of 45° with respect to lated a model shock based on an aerodynamic theory the earth-sun line for the direction of the interplane for a blunt body as developed earlier. Using values tary field, then the fast mode velocity at the face of for the ratio of specific heats of 5/3 and 2 and restricting the shock will vary in a complicated manner. Depend their considerations to regions near to the stagnation ing upon the geometry of the surface of the shock, point, they arrive at shock surfaces which are similar conditions can be anticipated where the normal com to those in the true aerodynamic case. ponent of the flow may be along the direction of the Modifying the Spreiter-Jones calculation to the MHD field while on the other side of the magnetosphere the case requires that the interplanetary field be included. normal component of the flow may turn out to be nor Walters [1964] studied the combination of a weak mal to the local field direction. Thus, the sonic speed interplanetary field together with the rapid changes in may vary over wide ranges on the face of the shock and, direction. He developed the downstream parameters indeed, where the flow is along the field, one must con of the shock from assumptions regarding the upstream sider that the details of the shock and the specification or solar wind properties. As expected he found that of the local Mach number become difficult to define, the downstream properties varied with the free-stream in view of the collisionless aspect of the flow. The parameters. In his calculation he used the de-Hoffman fact that it is not difficult to find a fast mode velocity Teller [1950] equations together with Spreiter and for typical solar wind parameters means that from time Jones' gross geometry for the aerodynamic shock. to time the flow may actually become subsonic for this The potential weakness of his calculation lies in the mode. If the diffusion of hot electrons produced in lack of self-consistency since, in the computation, the the shocked flow takes place in the upstream direc shock surface position is fixed. In reality the conse tion, this alone could contribute to the modified sonic quences of changes in upstream parameters may be velocity's becoming greater than the flow speed of the reflected in variations in the position of the, shock as solar wind. well as in the downstream direction of flow of the gas In attempting to obtain an understanding of collision and the direction of the magnetic field. free shocks we have studied an especially favorable When the bow wave is considered, it is essential shock event which took place in interplanetary space also to take into account the modes of propagation in on October 7, 1962. The preliminary results of this the solar wind. If an interplanetary field of 10 gamma calculation have been reported elsewhere [Sonett, and a particle density of 1O/cm3 are assumed, the Colburn, Davis, Smith, and Coleman, 1964] and here Alfven velocity is about 10 7 cm/sec. For a ratio of we provide only a summary of the findings. The specific heats of 5/3 and a temperature of 10 5 OK the event noted was a sudden commencement storm with sound velocity becomes 6 X 106·cm/sec. Thus for an a degenerate main phase and which was seen at the ideal field spiral angle of 45°, the slow mode velocity Mariner spacecraft 4.7 hr earlier than the geomag in the direction of flow of the solar wind becomes netic effects. The spacecraft events allow the appli 1.8 X 106 cm/sec and the fast mode, 1.2 X 10 7 cm/sec. cation of the Rankine-Hugoniot equations and the For a solar wind velocity of 400 km/sec this means discontinuity in plasma velocity and magnetic field is that even for the fast mode the velocity is supersonic, treated as a shock wave in a wall-less environment. though cases may arise where the sound velocity grows The change in the field is shown in figure 3 which to where the wind is not supersonic to the magneto marks the interplanetary onset. The shock plane sphere. Little is known of the detailed properties of appears to be slightly tipped with respect to the earth the shocks corresponding to these modes, especially sun line, denoting an oblique shock. This together regarding their stability. The slow shock is expected with the solar wind aberration required that the most to display a decrease in field through the discontinuity, general form of the shock equations be applied. Tem while the Alfven shock is incompressible and, in effect, perature effects were important in correcting the den- a large amplitude linear wave. It seems proper until the stability problems are further clarified to consider that the bow wave is indeed the fast mode shock. PLAN VIEW IN ECLIPTIC PLANE Support for this view comes from the calculation of the Mariner October 1962 interplanetary event dis cussed elsewhere in this paper where the Mach num bers for the flow fit only a fast magnetosonic mode. An especially important peculiarity of the blunt-body ELEVATION VIEW IN ECLIPTIC PLANE TO SUN shock is the variation of Mach number over the face >~~~m of the shock due to the curved geometry. Thus the
\ g6~=I08~=90~~72~-5~4~3...J6~-L18~dO~~18~~3=6=-5·4~72 \ 0.0 I L-~-'---'----L...l-'-'...LJ.J'--'-'----'-L..L-"--L-L.lJ EA ST ANGLE OF INCIDENCE, deg WEST 0.01 0.10 1.0 FREQUENCY, Hz FIGURE 9. Angle of arrival of peak plasma /lux versus number of observations. FIGURE 8. Longitudinal wave power density in the transition region The peak a l about 35° is approximately toward th e subsolar point dis tin ctl y showing the magnetic field for three typical swaths of data. expected blunt body fl ow pattern [Wolfe and Silva, 19641. The straight line is for a theoreticaJ Kolmogoroff spectrum. Magnetometer response was about 2 Hz between half-power points [Sonell . 1963).
000,000
,-, , \ 1.2 ~~~~~~~~~~~ 0 ,000 \ I ~ \ ~ \ 1.0 10.50e 15. loe \ """''' no " ...," .,,, "\/ \ 0.8 \ I \ ~ \ \ \ MILL/GAUSS \ 0 .6 \ , COUNTS E , 0 .4 SEC U"'l Pioneer TIl (Van Allen, 1959) 1.6 1.6 I I I 1.4 1.4 I I ~ 12 12 .6oe 1.2 ~ en 14. 8ae en 1.0 '"~ 1.0 ~ 08 ~ 08 3 I :J ~ 06 1M ' ~ 06 ~ ll'. li'.j 11 \ I~1 I Ii 1'\ 0.4 0.4 \ 1\ ,\'\I f\I ~\. 1'1 Iy\ ~ I~ 1/'1 1. 11 VI I V: 02 1\ /I . . 0.2 '. /) .": ... .' '.. ... o "J ~ o '-'".... '" ~ 00 00 00 ~ ~ ~ 00 ~ .0 ~ ~ ro 00 00 ~ ~ ~ 00 = o ro TIME, SECONDS TIME, SECO NDS Pioneer I (Soneff, Smifh and Sims, 1960) F I GURE 10. Summary of early observations of the transition region oj the m.agnetosphere showing Pioneer / , Pioneer IV, and Pioneer V data [Spreiter and J ones, 1963). 1039 r- in the range of tens of kilovolts could exist in the transi underwent significant changes, confirming the view tion zone. This result together with the early obser that the solar wind is a relatively cool gas compared to vations of the interaction region between the solar the gas of the transition region. Some of the pre wind and the magnetosphere is shown in this figure. liminary results are summarized in figure 11 which Reference to the discussion concerning the electron shows a display of the boundary crossings according observations of Van Allen are given by Sonett [1963]. to a prescribed set of rules given by Wolfe et aI., In regard to the transmission properties of the transi together with a comparison to the theory of Spreiter tion region insofar as propagating disturbances from and Jones [1963]. The time the plasma probe crossed the solar wind to the surface of the earth, it is well a zone was indicated by the appearance or disap known that sudden commencements are seen on the pearance of the characteristic spectrum in both surface ostensibly caused by rapid compression of the velocity and angular distribution. The crossing of magnetospheric boundary in response to a sudden the inner boundary, i.e., the termination of the regular change in the momentum density flux of the solar wind. magnetosphere, was indicated by the cessation of Thus it appears that whatever the details of the flow sensible readings on the plasma probe for inbound pattern it seems safe to regard the transition region orbits and an onset for outbound cases. The dashed as capable of transmitting pressure changes from the lines are those cases where the definition was difficult free-stream wind to the magnetosphere with the sub to satisfy. In particular, there were many times when sequent generation of magnetohydrodynamic waves an outer or shock boundary was difficult or impossible which we regard as transmitted to the surface of the to find and at other times the time of crossing was earth to produce the sudden commencement or impulse. different from that indicated by the magnetometer. At other times the agreement was quite good and the sup position for the shock appears to rest upon reasonable 6. Recent Experiments data. This may suggest a high degree of variability The Explorer 18 or IMP satellite is the latest in a in the gas parameters or perhaps tends to underline series of spacecraft designed for investigating the present ignorance of the true meaning of these experi ma~netosph~re and. adjacent interplanetary space. ments. Results of some of the other experiments of BasIcally, thIs satellIte had an orbit of extreme eccen this spacecraft tend to confirm the view that the transi tricity with apogee at about 32 Re. The extreme orbit tion region contains a remarkably hot gas. Little can meant that the period was 3 to 4 days and that the be said regarding the manner of leakage of this gas into spacecraft spent about two-thirds of its time beyond the regular magnetosphere and whether it supplies 20 Re. The data system is quite complex, utilizing a some or all of the flux of electrons of the Van Allen data burst technique where a high data rate is inter radiation. spersed with blanks, for anyone instrument. Only Calculations being carried out by Olbert (private the S-min averages of the magnetometer data have communication) seem to confirm that collision-free been reported to date, and these tend to confirm earlier shocks require a scale far in excess of the gyroradius spacecraft findings regarding the magnetic properties geometric mean to damp into the post-shock down of the transition region and the rather turbulent stream state irreversibly. These calculations, carried character of the interplanetary gas. The data have out for the bow shock and based upon the plasma al~o yielded substantial information regarding the experiment of Bridge et aI., [1964] and Ness et aI., eXIstence of a sudden discontinuity in the magnetic [1964] also suggest the applicability of the MHD field which is interpreted as a shock wave [Ness, Rankine-Hugoniot equations. In the instance of Searce, and Seek, 1964] though the plasma experi Olbert's calculation it is found that the downstream ment~ do not correspond to this view rigorously all the tIme. Two plasma experiments were carried on this space craft; these were of radically different design, one being SHOCK WAVE a Faraday cage and the other an electrostatic analyzer. Y'2 The limitations of telemetry allowed space to be Y'5/3 divided into three sectors for the electrostatic analyzer, h~nceforth referred to as the Ames experiment [Wolfe, SIlva, and Myers, 1964]. Threshold for this instru ment was about 10- 14 A, corresponding to 3 X 105 t SUN , THEORY H+/cm2 sec. The energy resolution was 6.5 to 7.5 (SPREITER - JONES, JGR (1963)) percent and the electrometer was of the peak readina V· 600 km/sec n· 2.5 PROTONs/cm3 variety, reading the peak current in each of the anaula~ B' 5y,M A·8.71 sectors. Results of this experiment are com pIe; and 20 o 10 20 the reader is referred to the original paper for details. r/Re The results show variations in the direction of arrival of flux changing from a highly directed beam when in FIGURE 11. Locus of outward and inward bound IMP orbits as the satellite crossed the transition region. the solar wind to a pseudoisotropic flux when in the The definition of crossing for the purposes of this diagram is based upon the Wolfe experi transition region, though this behavior is by no means ment and will vary from the findings of other experimente rs bec ause of the differing instru· ment response and definition of crossing. The reference idealized shocks for the two common to all the orbits. When this type of behavior different ratios of specific heat are from the calculations of Spreil er and Jones {l963]. was noted, the character of the velocity spectrum also The y = 2 case is the outermost of the two shocks. 1040 ------gas has a very low ratio of speci fi c heats, this co n Chandrasekh ar, S. (Dec. 1955), Hydromagneti c turbule nce H. An clusion bein g arri ved at from the very large density ele me nt ary theory, Proc. Roy. Soc. London, A233, No . 1194, 330-50. jump across the shock. Insofar as the generation of Dav is, L. , Jr. , E. J . Smith, P. 1. Coleman, Jr .. and C. P. Sonett e ntropy is regarded as the true test for a shock even in (1964), Inte rplane tary magnetic measurements, presented at Solar the colli sion-free domain, it appears that the properties Wind Confere nce, JPL, Pasadena, Calif., April 1- 4 1964. of the downstream flow directly behind the onset of the de- Hoffmann, F., and E. Te ll er (1950), Magneto-hydrodynamic shocks, Phys. Re v. 80, No.4, 692- 703. s hock will playa leading role in the next few years in Freeman, J . W. , 1. A. Van Allen, and L. 1. Cahill (1963), Explorer 12 establishing a foundation for collision-free shock observations of the magnetosphere boundary and the associated theory. solar plasma on September 13, 1961, J. Geophys. Res. 68, No.8, The very preliminary status of reporting of the data 2121-2130. Greenstadt, E. W. (April 1963), Effect of solar activity region s on from the majority of the IMP experiments together the interplanetary magneti c field, Astrophys. J. 137, No.3, with the necessary brevity of this report preclude 999-1002. the possibility of reviewing the majority of the results. Ness, N. F., C. S. Scearce, and 1. B. Seek (1964), Initial result s of It is probable that forthcoming results from the IMP the IMP- l magnetic fi eld experime nt, J. Geophys. Res. 69, No. 17, 3531-3569. seri es will provide much of the information required Parker, E. N. (Nov. 1958), Dynamics of the interplanetary ~as and in order to obtain a meaningful physical basis for much magnetic fi eld s, Astrophys. 1. 128, No.3, 664-676. of what today must be regarded as only partially under Sears, W. R. (Oct. 1960), Some remarks about fl ow past bodies, Rev . stood physical phenomena. Mod. Phys. 32, No. 4, 701-705. Sone tt, C. P. (1963), The distant geomagnetic fie ld 4: Microstructu re of a disordered hydromagneti c medium in th e co lli sionl ess li mit, 1. Ceoph ys. Res. 68, No.5, 1265-1294. Sone tt, C. P. , D. S. Colburn, L. Dav is, Jr. , E . ./. S mith, a nd P. J. The author acknowledges several stimulating di s Cole ma n, Jr. (1964), Ev idence for a cul li sion·free magnetohy· drodyna mi c shock in int erpl a netary s pace, Ph ys. Re v. Lett e rs cussions with .T . R. Spreiter and, in particular, hi s 13, No.5, 153-156. calling attention to the work of Sears regarding shock Sfl re it e r, 1. R. , a nd W. P. Jones (1963), On the effect of a wea k modes. interpl aneta ry magnetic fi eld on the in teracti on between the solar wind a nd th e geomagneti c fi e ld, J . Geophys . Res. 68, No . 12, 3555-3564. (Paper 69D8-537) Spre it e r, J . R., and A. L. Summe rs (1963), Effect of uniform e xt e rnal fi e ld a nd oblique in cid e nce of the solar wind on th e te rminal shape of th e geo magne ti c fi e ld , NASA TR R- IS!. Snyder, C. W. , and M. Ne ugebauer (l964), Int erplanetary solar wind measureme nts by Ma rineI'll , Space Res. IV . Van All e n, 1. A., a nd L. A. Frank (1959), Radiati on measure ments to 7. References 658,300 km with Pioneer IV , Natu re 184, No. 4682- 1, 219-24. Walters, G. K. (1964), Effect of ob liq ue inte rplanetary magneti c Axford, W. I., and C. O. Hines (1961), A unifyin g theory of hi gh. fi eld on shape a nd be hav ior of the magnetosphere, J. Geophys. latitude geophys ical ph enomena and geo ma~n e t ic storms, Can. 1. Res. 69, No.9, 1769-83. Phys. 39, No. 10, 1433- 1464. Wolfe, J. H., and R. Sil va (1965), Explorer 14 plasma probe observa· Beyer, M. (1953), Brightness of co mets and solar activity, Soc. Roy. tions during the Octobe r 7, 1962, geomagnetic d isturbance, 1. Sci. Li ege 13, No. 1- 2,276-86. Geophys. Res., 1 Augu st 1965 (in press). Brig:ht, H. , A. Egidi , A. La zarus, E. Lyons, and L. Jacobson (May Wolfe, 1. H. , R. S il va, and M. Myers. Observati ons of th e solar win d 1964), Prelim in ary result s of Il lasma measure ments on IMP- A c1uring the flight of IMP- l (i n preparati on-to be s ubmitted to COSPAR, Preprin!, Fl orence, It aly. 1. Geophys . Res.). 1041 J