Two-field transport models for magnetized plasmas

Two-field fluid models for low-frequency density and electrostatic potentia l fluctuations in a purely toroidal magnetized low-beta plasma are discusse d. In particular, we present some simple models suitable for investigating plasma transport close to marginal stability. For gradient-driven fluctuati ons, linear normal mode analysis of the interchange and resistive drift wav e instabilities are reviewed, with attention to the relative phase and ampl itude of density and electrostatic potential fluctuations. This reveals man y characteristic features of drift-wave fluctuations that are also observed in laboratory experiments and during ionospheric irregularities. Finally, these results are used to investigate the nonlinearly conserved energy func tionals, discussing the effect of magnetic field curvature and particle col lisions on energy, enstrophy, and cross-correlation between density and vor ticity fluctuations. Implications for turbulent fluctuations and nonlinear transport in laboratory and ionospheric plasmas are discussed.