| Popis: |
This study covers a thorough statistical investigation of the evolution of interplanetary coronal mass ejections (ICMEs) with and without sheaths, through a broad heliocentric distance and temporal range. The analysis treats the sheath and magnetic obstacle (MO) separately to gain more insight about their physical properties. In detail, we aim to unravel different characteristics of these structures occurring over the inner and outer heliosphere. The method is based on a large statistical sample of ICMEs probed over different distances in the heliosphere. For this, information about detection times for sheath and MO from 13 individual ICME catalogs were collected and cross-checked. The time information was then combined into a main catalog used as basis for the statistical investigation. The data analysis based on that covers a wealth of spacecraft missions enabling in-situ solar wind measurements from 1975--2022. This allows to study differences between solar cycles. All the structures under study (sheath, MO with and without sheath) show the biggest increase in size together with the largest decrease in density at a distance 0.75 AU. At 1 AU we find different sizes for MOs with and without sheath, with the former being larger. Up to 1 AU, the upstream solar wind shows the strongest pile-up close to the interface with the sheath. For larger distances the pile-up region seems to shift and recedes from that interface further into the upstream solar wind. This might refer to a change in the sheath formation mechanism (driven versus non-driven) with heliocentric distance, suggesting the relevance of the CME propagation and expansion behavior in the outer heliosphere. Comparison to previous studies shows inconsistencies over the solar cycle, which makes more detailed studies necessary to fully understand the evolution of ICME structures. |
