Examining Solar Wind Number Density Structures Observed in SECCHI HI 1

We present an analysis of small-scale periodic solar wind density enhancements observed in SECCHI HI 1. We discuss their possible relationship to periodic fluctuations of the proton density observed in-situ with the Wind SWE data. Viall et al. [2008] used 11 years of solar wind density measurements at 1 AU and demonstrated that in addition to turbulent fluctuations, non-turbulent, periodic density structures exist in the solar wind. In the slow wind, periodic density structures occurred most often with radial length-scales of approximately 73, 120, 136 and 180 Mm. In the fast wind, periodic density structures occurred most often with radial length-scales of approximately 187, 270 and 400 Mm. Event studies of the periodic density structures reveal instances in which the density structures have alpha/proton abundance ratio changes associated with the density structures. Specifically, the alpha density varies with the same periodicity as the protons, but in antiphase. For those events, this strongly suggests either time varying or spatially varying coronal source plasma that created the density structures. If such periodic density structures observed at 1 AU are generated in the corona, then they may be observable in SECCHI HI1 data. For instance, larger scale plasmoids have been observed in the corona [e.g. Sheeley et al., 2009] and it is plausible that smaller, periodic structures may exist as well. We identify periodic density structures as they convect with the solar wind into the field of view of SECCHI HI and follow the train of structures as a function of time. The periodic density structures we analyze are comparable in size to the larger structures identified in-situ at 1 AU.