Magnetic field kinks and folds in the solar wind

Abstract: Parker Solar Probe (PSP) observations during its first encounter at 35.7 $R_\odot$ have shown the presence of magnetic field lines which are strongly perturbed to the point that they produce local inversions of the radial magnetic field, known as switchbacks. Their counterparts in the solar wind velocity field are local enhancements in the radial speed, or jets, displaying (in all components) the velocity-magnetic field correlation typical of large amplitude Alfv\'en waves propagating away from the Sun. Switchbacks and radial jets have previously been observed over a wide range of heliocentric distances by Helios, WIND and Ulysses, although they were prevalent in significantly faster streams than seen at PSP. Here we study via numerical MHD simulations the evolution of such large amplitude Alfv\'enic fluctuations by including, in agreement with observations, both a radial magnetic field inversion and an initially constant total magnetic pressure. Despite the extremely large excursion of magnetic and velocity fields, switchbacks are seen to persist for up to hundreds of Alfv\'en crossing times before eventually decaying due to the parametric decay instability. Our results suggest that such switchback/jet configurations might indeed originate in the lower corona and survive out to PSP distances, provided the background solar wind is sufficiently calm, in the sense of not being pervaded by strong density fluctuations or other gradients, such as stream or magnetic field shears, that might destabilize or destroy them over shorter timescales.

• The paper identifies the persistence of magnetic switchbacks in the solar wind using close-proximity PSP observations.
• The paper employs MHD simulations to model kinked Alfvén wave packets, showing switchbacks endure for hundreds of Alfvén crossing times.
• The paper demonstrates that homogeneous plasma conditions are crucial for sustaining switchbacks, with implications for solar wind acceleration models.

Analysis of Magnetic Field Kinks and Folds in the Solar Wind

The study "Magnetic Field Kinks and Folds in the Solar Wind" by Anna Tenerani et al. provides a comprehensive examination of the phenomenon known as magnetic field switchbacks in the solar wind, particularly using data from the Parker Solar Probe (PSP). The primary goal of this research is to understand the nature and origin of these switchbacks, characterized by local inversions of the radial magnetic field and enhancements in the radial solar wind velocity, hypothesized to be large amplitude Alfvén waves.

Overview of Observations

PSP observations at a distance of 35.7 solar radii revealed the presence of switchbacks in the solar wind. These switchbacks were not only prevalent but exhibited significant angular deflections of the magnetic field, akin to large amplitude kinks. The velocity-magnetic field correlations observed suggested the presence of Alfvénic turbulence. What makes the PSP findings compelling is the persistence of these phenomena even at a closer proximity to the Sun compared to previous observations by the Helios, WIND, and Ulysses spacecraft.

Numerical Simulations and Results

The authors conducted magnetohydrodynamic (MHD) simulations to model the evolution of these large amplitude Alfvénic fluctuations. Key features of the simulation include:

• Kinked Alfvén Wave Packet: The study models these packets as propagating in a homogeneous plasma, with constraints ensuring a constant total magnetic pressure.
• Persistence of Switchbacks: Despite large amplitudes, switchbacks persist for significant durations—hundreds of Alfvén crossing times—before being dissipated by parametric decay instability.
• Role of Plasma Conditions: The robustness of the switchbacks is contingent upon the background solar wind conditions being relatively homogeneous and devoid of strong gradients that might accelerate their decay.

Implications of the Research

The simulations support the possibility that switchbacks could originate in the lower solar corona, traversing significant distances out to the PSP without losing integrity due to dissipation. This has profound implications for understanding solar wind origins and the physics of magnetic field dynamics close to the Sun.

The research also suggests that a refined understanding of switchbacks could inform models of solar wind acceleration and magnetic field configuration in the solar corona. The persistence of these phenomena, if correlated with specific coronal features or processes like reconnection, could offer insights into solar magnetic topology and energy transfer processes.

Future Directions

Further exploration into the longevity and stability of switchbacks, particularly under varying solar wind conditions, might involve:

• Inclusion of Variable Plasma Environments: To better replicate realistic solar conditions, future studies could introduce varying degrees of turbulence and non-periodicity into the models.
• Incorporation of Solar Wind Expansion Effects: Understanding how switchbacks evolve under solar expansion regimes could elucidate stability mechanisms in prevailing solar wind models.
• Extended Observations Beyond PSP: As PSP continues its mission, it should provide additional data that can help refine models and confirm theoretical predictions on the nature of these magnetic folds.

In summary, the study by Tenerani et al. elucidates critical aspects of solar wind dynamics and offers a framework for future investigation into the persistence and evolution of magnetic field switchbacks. The interplay between theoretical modeling and empirical observations in this domain remains a fertile ground for advancing our understanding of heliophysical processes.