Plasma convection in force-free magnetic fields as a mechanism for chemical separation in cosmical plasmas

Abstract

DURING the past two decades, progress in space research has forced us to abandon the earlier idea of a cosmical plasma as being a homogeneous medium. Filament formation and chemical separation are two important phenomena that are often observed in cosmical plasmas and illustrate the importance of inhomogeneity. An average composition from the Sun's atmosphere used to be taken as a standard when expressing cosmic abundancies in general. Numerous observations, however, of the chemical composition in the solar wind from in situ measurements and from lunar soil and meteorite samples, indicate a great variability in these ratios¹. The proton to α-particle flux ratio for solar flare energetic particle events varies widely between and within events². Recent observations show that this variability is connected with the flare process itself and the earlier history of the flare, rather than propagation effects between the flare site and the observer. Some local chemical differentiation mechanism in the preflare history of the specific active region is probably responsible for the composition of the emitted, energetic particles. The phenomena of filamentation and chemical separation may be coupled, as in the presence of a temperature gradient, the plasma convection associated with filamentary structures provides an effective means of selective transport. The general principles of this mechanism are described here. These principles may be important not only in solar flares but in many other cosmical plasmas where force-free fields and thermal gradients occur.