Abstract
Electromagnetic expulsion acts on a body suspended in a conducting fluid or plasma, which is subject to the influence of electric and magnetic fields. Physically, the effect is a magnetohydrodynamic analogue of the buoyancy (Archimedean) force, which is caused by the nonequal electric conductivities inside and outside the body. It is suggested that electromagnetic expulsion can drive the observed plasma counter-streaming flows in solar filaments. Exact analytical solutions and scaling arguments for a characteristic plasma flow speed are reviewed, and their applicability in the limit of large magnetic Reynolds numbers, relevant in the solar corona, is discussed.
Highlights
The boundary conditions are controlled by the photospheric plasma flows and magnetic flux emergence
The gas pressure gradient and the gravity force are among the local forces acting on a filament in the corona, whose effects can be described by perturbing a large-scale force-free model [23]
Whereas the equilibrium in filaments is primarily determined by a balance between the Lorentz and gravity forces, the observed counter-streaming remains an unsolved problem [24]
Summary
More recent theoretical models employed a force-free approximation, (∇ × B) × B ≈ 0, to successfully describe the overall filament structure [4,5]. Realistic theoretical models should satisfy a number of observational conditions for the formation and maintenance of filaments [6]. The modeling of plasma flows in filaments appears to be of particular interest in relation to their structure and evolution. Observations clearly demonstrated that even quiescent filaments are not static formations but rather are systems of jets streaming along the filaments with speeds up to 30 km s−1 [7]. One of the puzzles of the small-scale dynamics in filaments is the physical mechanism of counterstreaming—the observed simultaneous flows with speeds of 5–20 km s−1 in opposite directions in filament barbs (feet) [10,11]. The purpose of this paper is to advocate the electromagnetic expulsion force, whose effects are well-known in engineering and industrial applications, as a mechanism of counter-streaming in solar filaments
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