Context. Flows are a common feature of many processes occurring in the solar atmosphere, such as the formation of prominences where evaporated plasma from the chromosphere condensates along thin prominence threads that are seen to twist and oscillate. Aim. We aim to investigate the twisting of these threads by plasma condensation during their formation. Methods. We introduce a simple model with fixed critical points where the flow speed matches the Alfvén speed. This allows us to study the problem separately in the sub-Alfvénic and super-Alfvénic regimes. The temporal and spatial evolution of small amplitude initial twists along a thread is investigated analytically and numerically. Results. Analytical solutions are constructed in terms of the generalised hypergeometric functions. The solutions grow in time, despite the absence of any influxes of energy or magnetic fields. These results are confirmed numerically: We find oscillations with an amplifying amplitude and increasing period in the sub-Alfvénic regime. In the super-Alfvénic regime, we find twist amplification without any accompanying oscillations. An interesting result is the convergence of the twists at the critical points that leads to the formation of steep gradients and small scales. Energy is transferred from the flow to the amplifying twists. Conclusions. Magnetic field lines may be twisted by a stagnation point flow without the influx of any azimuthal field or energy. This twisting could assist in the formation of topology that is able to support the growth of prominences. The formation of steep gradients and small scales at the critical point is a new phenomenon which requires further investigation in the non-linear regime with the inclusion of magnetic diffusion.

Axisymmetric magnetohydrodynamic (MHD) models are useful in studies of magnetised winds and nonlinear Alfvén waves in solar and stellar atmospheres. We demonstrate that a condition often used in these models for the determination of a nearly vertical magnetic field is applicable to a radial field instead. A general divergence-free condition in curvilinear coordinates is self-consistently derived and used to obtain the correct condition for the variation of a nearly vertical magnetic field. The obtained general divergence-free condition along with the transfield equation complete the set of MHD equations in curvilinear coordinates for axisymmetric motions and could be useful in studies of magnetised stellar winds and nonlinear Alfvén waves.

Context. Plasma inflows accompany a variety of processes in the solar atmosphere such as heating of coronal loops and formation of prominences.Aims. We model a stratified solar atmosphere, within which a simulated prominence thread experiences density accumulation via a plasma inflow designed to mimic the formation process. We aim to investigate the interaction of such a system with torsional perturbations, and the possible consequences. Methods. The linearised equations of motion and induction are integrated to analyse the spatial and temporal evolution of torsional perturbations that are randomly driven at the photospheric footpoints. Results. Our results demonstrate that magnetic threads will experience twist amplification. Different sources and sinks of energy and the corresponding amplification mechanisms are identified. Threads reaching chromospheric heights are most susceptible to magnetic twisting with the maximum twist occurring near their footpoints. The amplifying twists are associated with a standing wave behaviour along the simulated threads. Conclusions. Our work suggests that torsional perturbations may be amplified within prominence threads, with strong magnetic twists forming at the footpoints. The amplification process is facilitated by small length scales in the background magnetic field. On the other hand, a small length scale in the background density inhibits growth. Possible consequences of the amplified twists, including their role in supporting the dense plasma within a prominence structure are discussed.