Author:

Keywords:

Science & Technology, Physical Sciences, Astronomy & Astrophysics, solar-terrestrial relations, solar wind, Sun, corona, VELOCITY, GAS, C14/19/089#55221731, G0D0719N#54970748, 0201 Astronomical and Space Sciences, 5101 Astronomical sciences, 5107 Particle and high energy physics, 5109 Space sciences

Abstract:

Since the 1970s, it is empirically known that the area of solar coronal holes affects the properties of high-speed solar wind streams (HSSs) at Earth. We derive a simple analytical model for the propagation of HSSs from the Sun to Earth, and thereby show how the area of coronal holes and the size of their boundary regions affect the HSS velocity, temperature, and density near Earth. We presume that velocity, temperature, and density profiles form across the HSS cross-section close to the Sun, and that these spatial profiles translate into corresponding temporal profiles in a given radial direction due to the solar rotation. These temporal distributions drive the stream interface to the preceding slow solar wind plasma, and disperse with distance from the Sun. The HSS properties at 1AU are then given by all HSS plasma parcels launched from the Sun that did not impinge 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 close to the Sun, whereas the velocity tail at 1AU originates from the trailing boundary region. The peak velocity of HSSs at Earth depends on the longitudinal width of the HSS close to the Sun. The shorter the longitudinal width of a HSS close to the Sun, the more of its ’fastest’ HSS plasma parcels from the HSS core and trailing boundary region have impinged into the stream interface to the preceding slow solar wind, and the smaller is the peak velocity of the HSS at Earth. As the longitudinal width is statistically correlated to the area of coronal holes, this explains also the well-known empirical relationship between coronal hole areas and HSS peak velocities. Further, the temperature and density of HSS plasma parcels at Earth depend on their radial expansion from the Sun to Earth. The radial expansion is determined by the velocity gradient across the HSS boundary region close to the Sun, and gives the velocity-temperature and density-temperature relationships at Earth their specific shape. When considering a large number of HSSs, the presumed correlation between the HSS velocities and temperatures close to the Sun degrades only slightly up to 1AU, but the correlation between the velocities and densities is strongly disrupted up to 1AU due to the radial expansion. Finally, we show how the number of particles of the piled-up slow solar wind in the stream interaction region depends on the velocities and densities of the HSS and preceding slow solar wind plasma.