Solar Wind Periodic Density Structures: Size Scales and Geo-effectiveness

Mesoscale transient structures affecting the solar wind-magnetosphere coupling can be either generated in the near-Earth environment or already present in the pristine solar wind. Among the solar wind mesoscale structures, in recent years, there has been a growing attention to Periodic Density Structures (PDSs), that are quasi-periodic enhancements of solar wind density ranging from a few minutes to a few hours. These structures have been extensively observed in remote sensing observations of the solar corona, and in in situ observations up to 1 AU where they manifest radial length scales (L_x) greater than or equal to the size of the Earth’s dayside magnetosphere, i.e., from tens to hundreds Earth’s radii (R_E). The PDSs have significant impact on the dynamics of the Earth’s magnetosphere and space weather for example driving Ultra-Low-Frequency (ULF) waves and affecting the dynamics and precipitation of electrons in the outer radiation belt. One key aspect to understand the PDSs role in the dynamics of space weather is to characterize their 3D size scales. Current interplanetary multi-spacecraft observations mostly occur at spatial separations unable to measure the 3D size scale of PDSs. Here, we focused on high density slow solar wind intervals observed by the Wind and ARTEMIS-P1 spacecraft.  We classified solar wind parcels based on the occurrence or not of quasi periodic density enhancements. When both spacecraft observe PDSs, we further classify each interval based on the level of coherence of the relative periodicities. Combining our results with a simulation of PDSs transit, we provide, for the first time, an estimate of the PDSs azimuthal size scale, that is their extent in the direction perpendicular to the Sun-Earth direction. For two PDSs groups with radial length scales of L_x1≈86 R_E and L_x2≈35 R_E, we obtained azimuthal scales of L_y1≈340 R_E and L_y2≈187 R_E. After discussing the consequence of these findings in the context of solar wind-magnetosphere coupling, we remark that the magnetosphere response to PDS become even more relevant when these structures are compressed in Stream Interaction Regions (SIRs) creating larger fluctuations in solar wind density/dynamic-pressure. Finally, we select time intervals of PDSs in SIRs and, using GOES and RBSP constellations, we investigate the PDSs geo-effectiveness in term of ULF waves and outer belt/GEO electron response.