Plasma convection in force-free magnetic fields as a mechanism for chemical separation in cosmical plasmas
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.
Ion-ion collisions and element separation effects in the boundary region of a plasma surrounded by neutral gas
In the boundary region between a plasma and neutral gas, the temperature is low (some thousand degrees). Then the Coulomb cross-section is large compared with other collision cross-sections. The influence of ion-ion collisions in the boundary region is discussed. In particular a simple formula is derived for the separation between different kinds of constituents. This separation depends on different collision cross-sections for diffusion and seems to be especially large when resonant charge exchange collisions are involved. These diffusive processes of importance in impermeable plasmas are also compared with separation processes due to a difference in ionization potential, which is known to occur in permeable plasmas. Different isotopes have the same ionization potential and the same charge exchange cross-sections. The separation processes described above are, therefore, likely to give element, but not isotope, separation.
Observation of solar wind heavy ions
Plasma experiment on board of the ESRO satellite HEOS-1 observed heavy ions with energy per unit charge 2.5 and 4 times higher than protons. Observations refer to the driving piston of the interplanetary shock occurred on 31 March 1970.