Identification of periodic density structures in Solar Orbiter data: Helical structures and flux ropes

Context. Quasi-periodic density structures (PDSs) are quasiperiodic variations in the solar wind density that range from a few minutes to a few hours. They are trains of advected density structures with radial length scales LR ≈ 100 − 10 000 Mm, and they thus belong to the class of so-called mesoscale structures in the solar wind. Although the precise mechanisms that form PDSs are still debated, cumulative evidence from multiple studies using in situ and remote data supports the view that most PDSs have a solar origin and do not form through dynamics during their propagation in interplanetary space. Low-frequency (< 1 mHz) PDSs have been associated with small-scale flux ropes, which further indicates a solar origin. Aims. We further investigated the origin and properties of PDSs by searching for coherent small-scale helical structures and flux ropes within PDS intervals. Methods. We used an extensive list of PDSs, compiled from Solar Orbiter measurements, and we applied a wavelet analysis technique to obtain the reduced magnetic helicity, cross helicity, and residual energy. Results. Our results indicate that small flux ropes are a constituent of PDS events, while the occurrence probability of helical Alfvénic structures is similar within and outside the PDSs. Conclusions. These results are consistent with the scenario in which PDSs are formed by processes involving magnetic reconnection.