The Origins of a Near-Ecliptic Merged Interaction Region as a Magnetic-Cloud like Structure Embedded in a Co-rotating Interaction Region

Abstract. Using remote-sensing and in-situ observations across multiple spacecraft with complimentary methods of analysis, we investigate a Magnetic Cloud Like-structure (MCL) observed in-situ on 3–4 July 2007 near the ecliptic at OMNI, STEREO-A and -B (all within 15° longitude of Earth). The MCL is entrained in a Corotating Interaction Region (CIR) originating in the Northern heliospheric sector, to create a Merged Interaction Region (MIR). This event allows the comparison of MIR observations at different longitudes showing differences in size, formation of sheath, presence of forward and reverse waves and small-scale structuring, demonstrating the progression of the interaction between the CIR and MCL from West to East. In order to explore its origins further, we compare the MIR with the (Interplanetary) Coronal Mass Ejection (ICME/CME) studied in Maunder et al. (2022) in the mid-latitudes at Ulysses containing a Magnetic Cloud (MC) and present a comprehensive discussion of the challenges posed by observing and relating transients not in alignment, across different latitudes and longitudes, and in different solar wind environments. As the CME propagates almost directly towards Ulysses, we find through fitting and modelling that its flanks could also potentially skim the near-ecliptic spacecraft. Length-scale analysis appears to be consistent with this configuration. However, local expansion velocities of the MCL/MC indicate compression near the ecliptic and expansion at Ulysses and the magnetic flux rope orientations and helicities at the different latitudes oppose each other. The CIR likely causes more compression and re-aligns the transient axis orientation near the ecliptic while a High Speed Stream (HSS) from the Southern sector propagates directly into the back of the ICME/MC near the mid-latitude. Opposing signs of helicity could provide indications of flux added in the first stages of CME evolution or magnetic reconnection with the Heliospheric Current Sheet (HCS). These observations and analyses demonstrate the continued challenge of modelling and fitting the propagation of transients embedded in complex solar wind environments. We note some of the caveats and limitations in the methods and highlight the use of multi-spacecraft analysis to disentangle the origin and formation of ICME substructures from the solar wind and other transients.