Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 2071–2080 Multimoment convecting flux tube model of the polar wind system with return current and microprocesses Supriya Banerjee à , Valeriy V. Gavrishchaka Science Applications International Corporation, T1-9-1, 1710 SAIC Drive, McLean, VA 22102, USA Accepted 31 July 2007 Available online 31 August 2007 Abstract Multimoment fluid simulation frameworks, which effectively account for anomalous transport due to microprocesses, combine best features of small-scale kinetic and global-scale MHD models. The most practical models of this type, 1D flux tube models, have been successfully used for realistic simulations of space plasmas including polar wind and magnetosphere–iono- sphere coupling processes characterized by a wide range of temporal and spatial scales. Our earlier flux tube models with field- aligned current and microprocesses have been formulated for spatially stationary flux tubes. However, horizontal convection due to electric fields is an important aspect of the high-latitude ionosphere–polar wind system and typical time scales of the polar wind upflow are comparable to the transit time across the polar cap. To take into account this important feature we have added flux tube convection to our earlier model. Using typical convecting flux tube that starts outside auroral oval, then enters and leaves downward current region, it has been shown that anomalous transport effects due to current-driven microinstabilities significantly alter dynamics of several plasma moments and should be taken into account for an accurate interpretation and prediction of the observed data. Future applications of our new model have also been discussed. r 2007 Elsevier Ltd. All rights reserved. Keywords: Polar wind; Current-driven instabilities; Multi-moment simulation 1. Introduction Ionospheric and magnetospheric plasma dy- namics is characterized by a wide range of temporal and spatial scales. In many cases the processes of different scales are coupled (e.g., magnetospher- e–ionosphere coupling phenomena). Small-scale phenomena usually require a kinetic description. However, existing computer power limits the kinetic model applications to rather small regions in space and time. Therefore, global-scale models are usually based on the MHD equations which assume near- Maxwellian distribution functions and neglect temperature anisotropies, heat flows, and numerous kinetic effects. Although these simplifications allow numerical analysis of such models in large space and time regions, they can significantly limit the quantitative and even qualitative accuracy of the obtained results. To fill the gap between small-scale kinetic and global-scale MHD descriptions multimoment fluid models have been introduced (Barakat and Schunk, 1982; Mitchell and Palmadesso, 1983; Ganguli ARTICLE IN PRESS www.elsevier.com/locate/jastp 1364-6826/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2007.08.004 à Corresponding author. Tel.: +1 703 6768645. E-mail addresses: SupriyaBanerjee@cox.net, Supriya.Banerjee@saic.com (S. Banerjee), gavrish@verizon.net (V.V. Gavrishchaka).