The effect of the morphology of coronal holes on the propagational evolution of high-speed solar wind streams in the inner heliosphere

Abstract

Since the 1970s it has been empirically known that the area of solar coronal holes a ects the properties of high-speed solar windstreams (HSSs) at Earth. We derive a simple analytical model for the propagation of HSSs from the Sun to Earth and thereby showhow the area of coronal holes and the size of their boundary regions a ect the HSS velocity, temperature, and density near Earth.We assume that velocity, temperature, and density profiles form across the HSS cross section close to the Sun and that these spatialprofiles translate into corresponding temporal profiles in a given radial direction due to the solar rotation. These temporal distributionsdrive the stream interface to the preceding slow solar wind plasma and disperse with distance from the Sun. The HSS properties at1AU are then given by all HSS plasma parcels launched from the Sun that did not run into the stream interface at Earth distance.We show that the velocity plateau region of HSSs as seen at 1AU, if apparent, originates from the center region of the HSS closeto the Sun, whereas the velocity tail at 1AU originates from the trailing boundary region. Small HSSs can be described to entirelyconsist of boundary region plasma, which intrinsically results in smaller peak velocities. The peak velocity of HSSs at Earth furtherdepends on the longitudinal width of the HSS close to the Sun. The shorter the longitudinal width of an HSS close to the Sun, themore of its “fastest” HSS plasma parcels from the HSS core and trailing boundary region have impinged upon the stream interfacewith the preceding slow solar wind, and the smaller is the peak velocity of the HSS at Earth. As the longitudinal width is statisticallycorrelated to the area of coronal holes, this also explains the well-known empirical relationship between coronal hole areas and HSSpeak velocities. Further, the temperature and density of HSS plasma parcels at Earth depend on their radial expansion from the Sunto Earth. The radial expansion is determined by the velocity gradient across the HSS boundary region close to the Sun and gives thevelocity-temperature and density-temperature relationships at Earth their specific shape. When considering a large number of HSSs,the assumed correlation between the HSS velocities and temperatures close to the Sun degrades only slightly up to 1AU, but thecorrelation between the velocities and densities is strongly disrupted up to 1AU due to the radial expansion. Finally, we show howthe number of particles of the piled-up slow solar wind in the stream interaction region depends on the velocities and densities of theHSS and preceding slow solar wind plasma.