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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.

Since the 1970s it has been 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 assume 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 1 AU 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 1 AU, if apparent, originates from the center region of the HSS close to the Sun, whereas the velocity tail at 1 AU originates from the trailing boundary region. Small HSSs can be described to entirely consist of boundary region plasma, which intrinsically results in smaller peak velocities. The peak velocity of HSSs at Earth further 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 more of its “fastest” HSS plasma parcels from the HSS core and trailing boundary region have impinged upon the stream interface with 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 also explains 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 assumed correlation between the HSS velocities and temperatures close to the Sun degrades only slightly up to 1 AU, but the correlation between the velocities and densities is strongly disrupted up to 1 AU 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.

Energy and circulation in the Earth’s magnetosphere and ionosphere are largely determined by conditions in the solar wind and interplanetary magnetic field. When the driving from the solar wind is turned off (to a minimum), we expect the activity to die down but exactly how this happens is not known. Utilizing global MHD modelling, we have addressed the questions of what constitutes the quietest state for the magnetosphere and how it is approached following a northward turning in the IMF that minimizes the driving. We observed an exponential decay with a decay time of about 1 hr in several integrated parameters related to different aspects of magnetospheric activity, including the total field-aligned current into and out of the ionosphere. The time rate of change for the cessation of activity was also measured in total field aligned current estimates from the AMPERE project, adding observational support to this finding. Events of distinct northward turnings of the interplanetary magnetic field were identified, with prolonged periods of stable southward driving conditions followed by northward interplanetary magnetic field conditions. A well-defined exponential decay could be identified in the total hemispheric field-aligned current following the northward turning with a generic decay constant of 0.9, corresponding to an e-folding time of 1.1 hr. A possible physical explanation for the exponential decay follows from considering what needs to happen for the convection in the magnetosphere to slow down, or stop, namely the unwinding of the field-aligned current carrying flux tubes in the coupled magnetosphere-ionosphere system. A statistical analysis of the ensemble of events also reveals both a seasonal and a day/night variation in the decay parameter, with faster decay observed in the winter than in the summer hemisphere and on the nightside than on the dayside. These results can be understood in terms of stronger/weaker line tying of the ionospheric foot points of magnetospheric field lines for higher/lower conductivity. Additional global modeling results with varying conductance scenarios for the ionosphere confirm this interpretation.

We investigate a sample of persistent coronal holes which occurred over the time range 2007 – 2014 and were observed from multiple perspectives. From combined SOHO/EIT, STEREO and SDO EUV data, we extract coronal hole parameters such as area, location, magnetic field characteristics, and follow them in time. The resulting parameters are related to in–situ solar wind measurements at L1 from ACE/Wind. We show that coronal holes undergo several evolutionary steps that are characterized by specific value ranges in the extracted parameters. On a statistical basis, this supports the results from the case study performed by Heinemann et al. (2018), who found that the coronal hole evolution has three phases, growing, maximum and decaying phase. These phases are also mirrored in the in– situ proton bulk speed of the associated high–speed streams. In addition, we investigate if and how these evolutionary properties are reflected in the galactic cosmic ray measurements of the recurrent Forbush decreases.