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Generic anticorrelation between density and generalized temperatures

Let us first consider the classical case where tex2html_wrap_inline1955 is a Maxwellian of temperature tex2html_wrap_inline1979 . One sees from (4) that the distribution remains Maxwellian for tex2html_wrap_inline2005 , with the same temperature, and, from (6), that all the moments tex2html_wrap_inline1965 vary with z as tex2html_wrap_inline2011 . This would justify the widely-used assumption of constant temperatures along magnetic field lines, if the particle velocity distributions were actually Maxwellian; of course, in this case, the density and temperature are not anticorrelated.

If the distribution tex2html_wrap_inline1955 is now a linear combination of Maxwellians, so that there is no more thermal equilibrium, the temperatures tex2html_wrap_inline1969 are no longer equal to each other, nor independent of z (although the temperature of each Maxwellian is independent of z). The tex2html_wrap_inline1969 generally increase with q, since higher-order moments favor components of higher temperatures. In particular, the effective temperature given by Eq.(1) is then tex2html_wrap_inline2025 . In the Appendix, we show analytically that with such a distribution and a monotonic potential which attracts particles to z = 0, all the generalized temperatures tex2html_wrap_inline1969 increase with z. Hence, since the density decreases with z, all the temperatures tex2html_wrap_inline1969 vary in anticorrelation with tex2html_wrap_inline1647 . An important consequence is that if a polytrope law tex2html_wrap_inline2039 does exist, its index is necessarily smaller than one (or just equal to one in the limiting case of a Maxwellian distribution.)

This generalizes to the temperatures tex2html_wrap_inline1969 the anticorrelation between density and temperature first shown by Scudder (1992a) in a general context, for the traditional temperature, using graphical arguments; this is a generic property of non-thermal distributions.



Michel Moncuquet
Mon Feb 2 16:12:15 MET 1998