Radial evolution of switchbacks in the inner heliosphere: observations from PSP to Ulysses *

Measurements from Parker Solar Probe have shown the ubiquitous presence of the so-called switchbacks. These 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. While there is not yet a general consensus on the origins of switchbacks and their connection to coronal activity, a first necessary step is to understand how they evolve and how long they can propagate undisturbed in the solar wind. Characterizing the dynamical evolution of switchbacks in the solar wind can help us determine whether they are generated in-situ or not, and whether they contribute to the turbulent cascade by evolving nonlinearly. In this work, we have analyzed magnetic field data from the first six encounters of Parker Solar Probe, three fast streams observed by Helios 1 and 2, and two Ulysses south polar passes, covering the range of heliocentric distances 0.1 < R < 3 au. We have compared the radial evolution of the magnetic energy density of switchbacks with that of the overall turbulent fluctuations, and we have characterized the radial evolution of the occurrence rate of switchbacks as a function of their duration. Our results show that switchbacks both decay and reform in-situ in the inner heliosphere, in-situ generation being more efficient at the larger scales. Our results confirm that switchbacks can be generated in the inner heliosphere by the expansion, although other types of switchbacks, generated closer to the sun, cannot be ruled out.