The dependence of periodic density structures’ amplitude and length scale on solar wind density within stream interaction regions

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Abstract
Context. Periodic density structures (PDSs) are a type of solar wind mesoscale structure characterised by quasi-periodic variations in the density of the solar wind ranging from a few minutes to a few hours. They are trains of advected density structures with radial length scales of LR ≈ 100 − 10 000 Mm. Analysis of case studies shows that PDSs can be compressed when embedded in a stream interaction region (SIR), leading to larger density variations and an increased impact on the magnetospheric and radiation belt dynamics. Aims. We perform an extensive statistical study to identify PDSs embedded in SIRs as well as their corresponding frequency and radial length scale distributions. Methods. We used an extensive list of 186 SIRs and 1217 embedded PDS events from the entire Wind dataset (1995−2022), spanning more than two solar cycles, to investigate the frequency and radial length scales of PDSs. With the use of wavelet methods, we classified these PDSs as coherent or incoherent, based on the shared periodic behaviour between proton density and the alpha-to-proton ratio, and we derived the corresponding occurrence distributions. Results. We found that 130 out of 186 SIR events have embedded coherent PDSs, which exhibit an increasing probability of occurrence with increasing frequency (up to ≈3 mHz). Furthermore, the investigation of radial length scales of coherent PDSs in SIRs reveals significant compression compared to PDSs in the ambient solar wind, as the most probable LR values are 120−130 Mm and 160−190 Mm for the slow and fast compressed solar wind, respectively. The coherent PDS LR decreases with a rate of −0.74, while the corresponding amplitude increases with a rate of 0.74 with increasing solar wind proton density, both following a power law function. Conclusions. Our results indicate that coherent PDSs occur more often than not in SIRs. This is consistent with a picture in which PDSs are formed at the Sun, advected by the solar wind, and enhanced by their interaction with SIRs, while both their radial length scale and amplitude are controlled by the level of compression in the interaction region.

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