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

<p>Since the 1970s it has been empirically known that the area of solar coronal holes a ects the properties of high-speed solar wind<br />streams (HSSs) at Earth. We derive a simple analytical model for the propagation of HSSs from the Sun to Earth and thereby show<br />how the area of coronal holes and the size of their boundary regions a ect the HSS velocity, temperature, and density near Earth.<br />We assume that velocity, temperature, and density profiles form across the HSS cross section close to the Sun and that these spatial<br />profiles translate into corresponding temporal profiles in a given radial direction due to the solar rotation. These temporal distributions<br />drive the stream interface to the preceding slow solar wind plasma and disperse with distance from the Sun. The HSS properties at<br />1AU are then given by all HSS plasma parcels launched from the Sun that did not run into the stream interface at Earth distance.<br />We show that the velocity plateau region of HSSs as seen at 1AU, if apparent, originates from the center region of the HSS close<br />to the Sun, whereas the velocity tail at 1AU originates from the trailing boundary region. Small HSSs can be described to entirely<br />consist of boundary region plasma, which intrinsically results in smaller peak velocities. The peak velocity of HSSs at Earth further<br />depends on the longitudinal width of the HSS close to the Sun. The shorter the longitudinal width of an HSS close to the Sun, the<br />more of its &#8220;fastest&#8221; HSS plasma parcels from the HSS core and trailing boundary region have impinged upon the stream interface<br />with the preceding slow solar wind, and the smaller is the peak velocity of the HSS at Earth. As the longitudinal width is statistically<br />correlated to the area of coronal holes, this also explains the well-known empirical relationship between coronal hole areas and HSS<br />peak velocities. Further, the temperature and density of HSS plasma parcels at Earth depend on their radial expansion from the Sun<br />to Earth. The radial expansion is determined by the velocity gradient across the HSS boundary region close to the Sun and gives the<br />velocity-temperature and density-temperature relationships at Earth their specific shape. When considering a large number of HSSs,<br />the assumed correlation between the HSS velocities and temperatures close to the Sun degrades only slightly up to 1AU, but the<br />correlation between the velocities and densities is strongly disrupted up to 1AU due to the radial expansion. Finally, we show how<br />the number of particles of the piled-up slow solar wind in the stream interaction region depends on the velocities and densities of the<br />HSS and preceding slow solar wind plasma.</p>