Marta Florido-Llinas

Coronal mass ejections (CMEs) are one of the main drivers of adverse space weather. They are large eruptions of magnetized plasma with the ability to severely impact telecommunications and space systems, due to the sudden injection of magnetic energy into the magnetosphere. CMEs are typically modeled as magnetic flux ropes (MFRs), which are collections of magnetic field lines wrapped around an internal main axis in a twisting way.

The twist of the field lines is defined as the angle they cover around the axis per unit length. Its distribution along the cross-sectional radius of a MFR is an important property, since it has a fundamental connection with initiation processes at the Sun, and it is closely related to the tendency to develop particular plasma instabilities.

One of these instabilities is called the kink instability. It occurs when the twist of the MFR exceeds a critical threshold, making the axis become a helix itself. Previous studies show that it plays an important role during the eruption of MFRs at the Sun, as well as in laboratory plasmas, where it needs to be avoided in order to allow the fusion reactions to take place.

The aim of this work is to present a theoretical study of the conditions under which interplanetary MFRs like CMEs, with different twist profiles and physical properties, become kink unstable. A code has been developed in Python that is able to study the stability of any cylindrical MFR model. Its theoretical foundations are based on the article by Linton et al. (1996), where it is assumed that the flux rope boundary is free to move, and that there is no external magnetic field. This code finds the critical thresholds and the growth rates of the instability, as well as its minimum and maximum wavelengths, and the shape of the perturbation.

The magnetic field has been modeled according to the circular-cylindrical (CC) analytical flux rope model in Nieves-Chinchilla et al. (2016), as well as the Lundquist and Gold-Hoyle models. The results are displayed in the center of this poster, where the graph depicts the boundary values of the parameters between the stable and unstable regimes of the CC model.

These stability ranges give us an indicator of when CMEs could start to rotate in the interplanetary medium. This is important in space weather research, because we need to know when a CME is going to change its orientation in order to better predict its evolution and dynamics.

We have also found that different expansion regimes in MFRs described by the CC model could have a kink stabilizing or destabilizing effect, so for example if the axis expands more slowly than the cross-sectional radius, the MFR will likely become unstable at some point of its interplanetary journey. Moreover, the analysis of the angle between the current density and the magnetic field suggests that the presence of magnetic forces in opposite directions within the MFR, or of inward forces around the boundary, among others, could destabilize the system.

Future research should explore, for instance, other boundary conditions for the stability analysis and more MFR models, the full nonlinear evolution of these rotations, and check with observational data if the conclusions regarding magnetic forces and expansion regimes hold. Therefore, this work provides a theoretical background and a method to gain a better insight into the internal magnetic structure of interplanetary MFRs, and the data obtained by new missions like Parker Solar Probe or Solar Orbiter will allow us to explore these results and ideas by observing MFRs closer than ever to the Sun.