Expanding CGL: The CGL double-adiabatic approximation in the Expanding Solar Wind
arxiv(2024)
摘要
Different in situ satellite observations within 0.3 to 1 AU from the Sun
reveal deviations in the thermodynamics of solar wind expansion. Specifically,
these deviations challenge the applicability of the double adiabatic or CGL
theory, indicating potential influences such as perpendicular heating and/or
parallel cooling of ions. The study aims to investigate the plasma expansion
phenomena using the Expanding Box Model (EBM) coupled with an ideal MHD
description of the plasma. The primary objective is to understand the observed
deviations from the CGL predictions, and how the expansion can affect the
conservation of the adiabatic invariants, particularly focusing on the impact
of transverse expansion on the CGL equations. To address the plasma expansion,
we employed the Expanding Box Model (EBM) coupled with the ideal-MHD formalism
used for CGL theory. This model provides a unique system of reference co-moving
with the solar wind, allowing for the incorporation of transverse expansion
into the double adiabatic equations. Solving the equations for different
magnetic field profiles, we compute the evolution of anisotropy and plasma
beta, which deviates from CGL predictions and empirical observations. This
deviation is attributed to the plasma cooling effect induced by the Expanding
Box Model (EBM). Results suggest that heating mechanisms play a crucial role in
counteracting plasma cooling during expansion.
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