Princeton University Astroplasmas Seminar

Large amplitude, turbulent Alfvénic fluctuations have been commonly observed in the solar wind since the first in-situ measurements, and they are thought to provide a possible mechanism to heat the solar corona and accelerate the solar wind. Despite Alfvénic fluctuations have been studied for many decades now, a complete understanding of their origin and nonlinear evolution still remains elusive. Observations show that such fluctuations are characterized by a nearly constant magnetic field amplitude, a condition which remains largely to be understood and that points to an intrinsic degree of coherence of s such fluctuations. The Parker Solar Probe mission, by probing regions of space never explored before closer to the sun, has provided a wealth of data showing the ubiquitous and persistent occurrence of the so-called switchbacks. Switchbacks are magnetic field lines which are strongly perturbed to the point that they lead to local inversions of the radial magnetic field. The corresponding signature in the velocity field is that of a local radial speed jet displaying the well-known velocity/magnetic field correlation that characterizes Alfvén waves propagating away from the Sun. Switchbacks are thus an extreme case of those Alfvénic fluctuations that permeate the solar wind further away. While there is not yet a general consensus on what is the origin of switchbacks and their connection to coronal activity, a first necessary step is to understand how they evolve and how long they can persist in the solar wind. In this talk, we will discuss the evolution of Alfvénic fluctuations and switchbacks in the solar wind. We will focus on how their evolution is affected by parametric instabilities and the possible role of expansion and kinetic effects by comparing theory with observations in the inner heliosphere. We will finally discuss what are the implications of our results for models of switchback generation and related open questions.