-A magnetohydrodynamic model f o r the inceraction o f t h e s o l a r wind and t h e geomagnetic f i e l d i s described, t h e degree t o which t h e I n t h e hydromagnetic model, the magnetosphere boundary and d i s t a n t t a i l a r e represented by t a n g e n t i a l and contact d i s c o n t i n u i t i e s , and the bow wave by a fast hydromagnetic shock wave. The connectivity of i n t e r p l a ne t a r y and geomagnetic f i e l d s , and t h e asymptotic d i r e c t i o n s of t h e wake and shock waves at g r e a t d i s t a r c e s f r o n t h e e a r t h a r e discussed i n terms of p r o p e r t i e s of these d i s c o n t i n u i t i e s . Detailed numerical r e s u l t s f o r t h e location of the bow wave, and t h e density, velocity, and temperature of t h e flow i n t h e region between t h e bow wave and t h e magnetosphere are presented f o r Mach numbers 5, 8, and 12 f o r 7 = 5/3 and 2. The calcul a t e d position of t h e bow wave i s shown t o be i n good accordance with t h a t observed i n shadowgraph photographs of supersonic flow p a s t a model magnetosphere i n the Ames Supersonic Free-Flight Wind Tunnel. Results Ia r e a l s o presented that i l l u s t r a t e t h e d i s t o r t i o n of t h e i n t e r p l a n e t a r y magnetic f i e l d i n t h e region between t h e bow and t h e magnetosphere f o r cases i n which t h e magnetic f i e l d i n t h e incident stream i s i n c l ' ed at 45' and 90' t o t h e free-stream direction.

A review is provided of the salient features of the. flow of solar plasma past the magnetosphere, as they are revealed by observations in space and by theory. Discussed first are the properties of the solar wind, including the velocity, density, temperature and temperature anisotropies, and magnetic field, and their variations in space and time as observed beyond the disturbing influence of the earth and its magnetic field. Theoretical aspects of the steady-state interaction of the solar wind and the geomagnetic field are taken up next, using the continuum description provided by the equations of magnetohydrodynamics of a perfect dissipationless gas as a basis. An outline is given of the series of approximations, and their observational and theoretical justifications, used to reduce the complex free-boundary magnetohydrodynamic flow problem to a series of tractable problems; and a set of results is provided for representative conditions in the solar wind. The results are shown to be in good accord, even to numerous details, with observations made in space during quiet times. The results are also shown to be useful in interpreting many features of observations during disturbed times. Attention is drawn to other features of the flow of solar plasma past the magnetosphere for which quantitative theoretical descriptions must still be devised. These include the irregular or fluctuating character of the flow downstream of the bow wave, the nature of the magnetosphere tail %nd surrounding flow at great distances from the earth, numerous transient aspects of the flow, and the mechanisms underlying the many statistical correlations being discovered between variations of the surface geomagnetic field and various properties of the solar wind. x Invited review paper presented at the International Symposium on the Physics of the Magnetosphere11 12 SP.REITER AND ALKSNE SOLAR WIND M>>I MA>> I p,,,B2/8w -FLUCTUATING FLOW >>p MAGNETOSPHERE TAIL ALLEN BELT FREE STREAMLINE IC NEUTRAL SHEET SHEET • BOW WAVI ERE BOUNDARYstretches out almost indefinitely to oeorm an extended tail within which the magnetic field is directed away from the earth in the southern half and toward the earth in the northern half. Within the tail and separating the regions ooe opposite field polarity is oeound the relatively thin electrical current sheet associated with the magnetic neutral sheet and a substantially thicker enclosing plasma sheet. Although these features are ooe considerable importance to the subject ooe this symposium, as are also the energetic particles ooe the Van Allen belts, they are to be found within the magnetosphere, and their properties must therefore be left for others to summarize. PROPERTIES OF THE INCIDENT SOLAR WINDFundamental to all understanding of the present subject is a knowledge of the properties of the solar wind as it approaches the bow wave marking the limit PLASMA FLOW AROUND MAGNETOSPHERE 13 of the earth's perturbing influence. Although an appreciation tha• ionized gas may be flowing •rom the sun to the earth and beyo...

On February 13, 1967, a class 3B solar flare occurred at 20°N, 10°W. The resultant disturbance in the solar wind was observed by the Los Alamos plasma probe on Vela 3A and the Ames Research Center magnetometer on Explorer 33. The initial discontinuity in the solar wind was identified as a shock. The normal to the shock made an angle of 60° with the plane of the ecliptic. This extreme angle of tipping indicates that the shock from this flare did not propagate spherically from the sun as described by simple theory. Nine hours after the shock passed, plasma containing 22% helium was observed. Since the solar wind normally contains 4% helium, this observation adds to the increasing body of evidence that flares occur in regions relatively rich in helium. The velocity of the solar wind continued to increase after the helium plasma passed, i.e., at 1 AU the helium‐enriched material was still being propelled from behind. This observation is evidence that the plasma continued to be accelerated at the sun for an extended period of time after the flash phase of the flare.

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