Evolution of switchbacks in the inner Heliosphere

TL;DR

This study investigates how magnetic switchbacks evolve with distance from the Sun using data from multiple spacecraft, revealing their amplitude decay, scale-dependent occurrence, and potential generation mechanisms in the solar wind.

Contribution

It provides a comprehensive statistical analysis of switchback evolution across different heliocentric distances, highlighting their decay, scale dependence, and possible in-situ generation processes.

Findings

Switchback amplitudes decrease faster than turbulent fluctuations.

Longer duration switchbacks become more common with distance, shorter ones less so.

Switchbacks can be generated by solar wind expansion, but other sources are possible.

Abstract

We analyze magnetic field data from the first six encounters of PSP, three Helios fast streams and two Ulysses south polar passes covering heliocentric distances $0.1\lesssim R\lesssim 3$ au. We use this data set to statistically determine the evolution of switchbacks of different periods and amplitudes with distance from the Sun. We compare the radial evolution of magnetic field variances with that of the mean square amplitudes of switchbacks, and quantify the radial evolution of the cumulative counts of switchbacks per km. We find that the amplitudes of switchbacks decrease faster than the overall turbulent fluctuations, in a way consistent with the radial decrease of the mean magnetic field. This could be the result of a saturation of amplitudes and may be a signature of decay processes of large amplitude Alfv\'enic fluctuations in the solar wind. We find that the evolution of switchback occurrence in the solar wind is scale-dependent: the fraction of longer duration switchbacks increases with radial distance whereas it decreases for shorter switchbacks. This implies that switchback dynamics is a complex process involving both decay and in-situ generation in the inner heliosphere. We confirm that switchbacks can be generated by the expansion although other type of switchbacks generated closer to the sun cannot be ruled out.