The role of aerodynamic drag in propagation of interplanetary coronal mass ejections

Context. The propagation of interplanetary coronal mass ejections (ICMEs) and the forecast of their arrival on Earth is one of the central issues of space weather studies. Aims. We investigate to which degree various ICME parameters (mass, size, take-off speed) and the ambient solar-wind parameters (density and velocity) affect the ICME Sun-Earth transit time. Methods. We study solutions of a drag-based equation of motion by systematically varying the input parameters. The analysis is focused on ICME transit times and 1 AU velocities. Results. The model results reveal that wide ICMEs of low masses adjust to the solar-wind speed already close to the sun, so the transit time is determined primarily by the solar-wind speed. The shortest transit times and accordingly the highest 1 AU velocities are related to narrow and massive ICMEs (i.e. high-density eruptions) propagating in high-speed solar wind streams. We apply the model to the Sun-Earth event associated with the CME of 25 July 2004 and compare the results with the outcome of the numerical MHD modeling

The Dependence of the Peak Velocity of High-Speed Solar Wind Streams as Measured in the Ecliptic by ACE and the STEREO satellites on the Area and Co-Latitude of their Solar Source Coronal Holes

We study the properties of 115 coronal holes in the time‐range from 2010/08 to 2017/03, the peak velocities of the corresponding high‐speed streams as measured in the ecliptic at 1AU, and the corresponding changes of the Kp index as marker of their geo‐effectiveness. We find that the peak velocities of high‐speed streams depend strongly on both the areas and the co‐latitudes of their solar source coronal holes with regard to the heliospheric latitude of the satellites. Therefore, the co‐latitude of their source coronal hole is an important parameter for the prediction of the high‐speed stream properties near the Earth. We derive the largest solar wind peak velocities normalized to the coronal hole areas for coronal holes located near the solar equator, and that they linearly decrease with increasing latitudes of the coronal holes. For coronal holes located at latitudes >∼ 60°, they turn statistically to zero, indicating that the associated high‐speed streams have a high chance to miss the Earth. Similar, the Kp index per coronal hole area is highest for the coronal holes located near the solar equator and strongly decreases with increasing latitudes of the coronal holes. We interpret these results as an effect of the three‐dimensional propagation of high‐speed streams in the heliosphere, i.e., high‐speed streams arising from coronal holes near the solar equator propagate in direction towards and directly hit the Earth, whereas solar wind streams arising from coronal holes at higher solar latitudes only graze or even miss the Earth