Interplanetary Gas. XIII. Gross Plasma Velocities from the Orientations of Ionic Comet Tails

Results are presented concerning the velocity of the solar-wind plasma as deduced from the observations of Type I (ion) tails contained in the comet catalogue of Belton and Brandt The analysis is in the spirit of the early work by Biermann, where the orientations are interpreted on the basis of dynamical aberration The major conclusions are as follows: (i) The solar wind has a fairly sharp velocity minimum at 150 + 50 km/sec; this agrees with the prediction of Axford, Dessler, and Cottlieb. (ii) The solar-wind velocity is correlated with geomagnetic activity in a manner which confirms the relationship found from the Mariner II data by Snyder, Neugebauer, and Rao and from the travel times for geomagnetic storms by Hirshberg. The comet tail, Mariner II, and storm data are consistent with the expressions w = 330 + 9 0 K (km/sec) or w = 316 Ap0 i0 (km/sec), where w is the solar-wind velocity, Kp is the sum of Kp for the day in question, and A is the surface magnetic disturbance index. These statistical relations hold over nearly the entire range of the magnetic indices (iii) These relations predict a mean plasma velocity of 500 km/sec with only a 50 km/sec spread in the means between years of solar maximum and solar minimum (iv) Evidence is found for a mean tangential component of the solar-wind velocity near Earth of 10 km/sec directed in the sense of the solar rotation; the value of the tangential velocity is apparently lower for quiet conditions and higher for disturbed conditions in the interplanetary medium (v) The sample of Type I comet tails (both direct and retrograde) is not uniformly distributed in space and time, and care must be exercised in drawing conclusions from the sample presently available; such bias may (at least in part) account for the curious dependence of the solar-wind velocity on heliographic latitude found by Pflug, where the minimum velocity is found in the sunspot zone. It appears that the latitude dependence of the plasma velocity cannot be determined with the information presently available.