Role of entropy in magnetotail dynamics

The role of entropy conservation and loss in magnetotail dynamics, particularly in relation to substorm phases, is discussed on the basis of MHD theory and simulations, using comparisons with particle‐in‐cell (PIC) simulations for validation. Entropy conservation appears to be a crucial element leading to the formation of thin embedded current sheets in the late substorm growth phase and the potential loss of equilibrium. Entropy conservation also governs the accessibility of final states of evolution and the amount of energy that may be released. Entropy loss (in the form of plasmoids) is essential in the earthward transport of flux tubes (bubbles, bursty bulk flows). Entropy loss also changes the tail stability properties and may render ballooning modes unstable and thus contribute to cross‐tail variability. We illustrate these effects through results from theory and simulations.