Observed Properties of the Interplanetary Plasma

Abstract

termed by him the solar wind. It is also known that a very weak magne tic field of solar origin thre ads through this pla sma continuously while generally maintaining direct connection to the Sun. Representative values measured at 1 a. u. are : the interplanetary mag­ netic-field magnitude ranges between 2 to 40 'Y (1 'Y = 10 ,ugauss) with an average magnitude of 6 'Y; positive ion flux values range from 108 to 1010 ions/cm2 /sec with an average 3 X 108 ions/cm 2/sec; densities range between 0.4 to 80 ions/cm3 with an average of approximately 5. The average energy of an individual ion ranges between 500 eV to 4 keV, corresponding to velocities of 300 to 900 km/sec with an average of 400-500 km/sec. Variations of these parameters over several years have now been observed and an increase of solar activity has gener ally led to an enha nced interplanetary magnetic-field strength and plasma flux. The chemical composition is not yet well estab­ lished but it appears to be principally ionized hydrogen with the fraction of helium nuclei ranging between 0 to 15 per cent with an average value of 4 per cent. Additional first-order results suggest that the sola r-wind sou rce is located at 10° to 25° north heliographic latitude and that the pla sma flow, as measured at 1 a.u., comes on average from 1.5 ° east of the Sun, which is a direct result of physical corotation of the outer solar corona. Synoptic studies of the phy sical properties of the interpla netary medium and in partic ula r the interpl anetary magnetic field have indicated a sectoring of the direction and correlated variations of plasma velocity, density, terres­ trial magnet ic activity, and long-lived streams of solar cosmic rays. Second­ order results recently obta ined reveal a fine-scale fila mentary structure of the interplanetary medium which appears to be directly connected to the Sun. Thermal anisotropies of the solar plasma are found to be field aligned as are solar cosmic -ray anisotropies. From detailed satellite studies