From CME - CH proximity on the Sun to ICME - CIR interaction at Earth: a case study

The interaction between interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs) leads to a variety of changes both in the trajectory and the morphology of the former. This means that the solar wind (SW) conditions at Earth's orbit due to interacting heliospheric structures may deviate significantly from the case of sole ICME or CIR passage. Studying the interaction of these large-scale structures in the heliosphere thus provides a basis for a more accurate space weather prediction of the associated near-Earth effects and enhances the accuracy of CME propagation models. The eruption of a flare related CME southwest to the center of the solar disk was observed by SDO/AIA on February 4, 2014. A coronal hole (CH) east of the disk center is also present at that time. The CME is listed by the DONKI database and associated to the in-situ magnetic field and plasma signatures detected at L1 3 days later. After the arrival of a fast forward shock on February 7, 2014 typical sheath properties are observed, followed by a region of low fluctuations, plasma beta and temperature - typical ICME signatures. A SW flow angle reversal is observed at the beginning of this interval, indicating east-west flow deflection normally observed around stream interfaces. The region of typical ICME signatures is interrupted by a phase of decreased field magnitude simultaneously occurring with the disturbance of the flow speed. Finally, at the trailing part of the in-situ event we observe the passage of a high-speed stream. The proximity of the coronal source regions of ICME and CIR clearly results in their interaction and thus morphological changes visible in plasma and magnetic field data. This view is also supported by WSA-ENLIL simulations showing the coincident arrival and apparent merging of the CIR and ICME at Earth. The separation of two intervals of weak magnetic field fluctuations by a region of strong fluctuations indicates a fundamental rearrangement of the magnetic field associated with the ICME. This is supported by the non-bidirectional electron pitch angle data. The complexity of this event demonstrates the need to study the interaction of the coronal source regions and their respective SW structures in a holistic way.