‘Is the source of the solar wind steady or intermittent?’
Various in-situ studies have suggested that the inner heliosphere is filled with a network of entangled magnetic flux tubes and that the flux tubes are fossil structures that originate at the solar surface (e.g., Zaqarshvili et al. 2014; Borovsky et al. 2008). The tube walls are associated with large changes in the ion entropy density and the alpha-to-proton ratio. The median size of the flux tubes at 1 AU is 4.4 °ø 105 km (Borovsky 2006; Borovsky et al. 2008). The magnetic flux in the tubes at 1 AU corresponds to the magnetic flux in field concentrations in the photospheric magnetic carpet. Using 11 years (1995–2005) of solar wind observations from the Wind spacecraft, Viall et al. (2009) showed that periodic proton density structures occurred at particular radial length scales more often than others. An analysis of the alpha to proton solar wind abundance ratio variations strongly suggests that these periodic solar wind density structures originate in the solar corona. Some recent models of abundance variations predict that they are set in the chromosphere (Laming 2009). Because the observed emission is related to the number of electrons along the LOS, intensity variations provide a direct measure of solar wind density variations, which can be compared to Earth-based interplanetary scintillation or SPP in-situ measurements. Viall et al. (2010) have identified specific periodicities by following individual blobs of <1200Mm size through the SECCHI/HI FOV (Fig. 5). The minimum size that could be measured is determined by the cadence and exposure times of the instrument (40 min and 30 min, respectively for HI-1). Our analysis of density data from the SECCHI/HI suggests that we can obtain measures of the fine-scale solar wind variability directly from the WISPR images down to length scales of ∼11 Mm at closest perihelion. This estimate is scaled from the results in Viall et al. (2010) using the expected cadence for WISPR (4 s).