Identification of Periodic Density Structures in Solar Orbiter Data: Radial Evolution

Abstract The quasi-periodic density structures (PDSs) are quasiperiodic variations of solar wind density ranging from a few minutes to a few hours. They are trains of advected density structures with radial length scales $L_{R}\approx $ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>L</mml:mi> <mml:mi>R</mml:mi> </mml:msub> <mml:mo>≈</mml:mo> </mml:math> 100 – 10,000 Mm, thus belonging to the class of solar wind “mesoscale structures”. Even though PDS at L1 have been extensively studied both through statistical and event analysis, their investigation at distances closer to the Sun is limited. This study performs a statistical investigation of PDS at various distances from the Sun between 0.3 and 1 AU by exploiting Solar Orbiter data. We compiled and made publicly available an extensive list of PDSs following a well-established methodology that combines the Multitaper method as well as wavelet analysis to reveal the distribution of PDS radial length scales and how they vary with respect to the radial distance. Our results indicate that PDS advected with the ambient slow solar wind are expanded, while PDS detected during fast solar wind segments show compression indicative of their interaction with stream interaction regions.