COMPARISON OF TWO-FLUID AND GYROKINETIC MODELS FOR KINETIC ALFVEN WAVES IN SOLAR AND SPACE PLASMAS

An analytical comparative study of a two-fluid and a gyrokinetic model of kinetic Alfven waves (KAWs) is presented for various solar and space plasma environments. Based on the linear KAW dispersion relation for gyrokinetics (Howes et al. 2006), the wave group velocity and electromagnetic polarizations are obtained analytically. Then the gyrokinetic wave properties are compared with those of the two-fluid model. The results show that both models agree well with each other not only in the long wavelength regime (>> the ion gyroradius rho(i)) for all cases considered, but also in wavelengths similar to rho(i) and <<rho(i) (still much larger than the electron gyroscale) for a moderate or low (less than or similar to 1) and a high (>> 1) ion/electron temperature ratio T-0i/T-0e, respectively. However, the fluid model calculations deviate strongly from the gyrokinetic model at scales < rho(i) for a relatively low T-0i/T-0e due to the electron gyroradius effect. Meanwhile, the plasma beta(i) can make the gyrokinetic dispersion relation of KAWs become complex and sometimes have an oscillation-like structure. With the inherent simplicity of the fluid theory, these results may improve our understanding of the applicability of the two-fluid model, and may have important implications for computer simulation studies of KAWs in the solar and space plasma surroundings.