On assumption in low-altitude investigation of dayside magnetospheric phenomena

Abstract

In the physics of large-scale phenomena in complicated media, such as space plasmas, the chain of reasoning from the fundamental physics to conceptual models is a long and winding road, requiring much physical insight and reliance on various assumptions and approximations. The low-altitude investigation of dayside phenomena provides numerous examples of problems arising from the necessity to make strong assumptions. In this paper we discuss some important assumptions that are either unavoidable or at least widely used. Two examples are the concepts of frozen-in field lines and convection velocity. Instead of asking what violates the frozen-in condition, it is quite legitimate to ask what freezes the plasma and the magnetic field in the first place. Another important complex of problems are the limitations introduced by a two-dimensional approach or linearization of equations. Although modern research is more and more moving toward three-dimensional and time-dependent models, limitations in computing power often make a two-dimensional approach tempting. In a similar way, linearization makes equations analytically tractable. Finally, a very central question is the mapping. In the first approximation, the entire dayside magnetopause maps down to the ionosphere through the dayside cusp region. From the mapping viewpoint, the cusp is one of the most difficult regions and assumptions needed to perform the mapping in practice must be considered with the greatest possible care. We can never avoid assumptions but we must always make them clear to ourselves and also to the readers of our papers.