Deriving the interaction point between a Coronal Mass Ejection and High Speed Stream: A case study

We analyze the interaction between an Interplanetary Coronal Mass Ejection (ICME) detected in situ at the L1 Lagrange point on 2016 October 12 with a trailing High-Speed Stream (HSS). We aim to estimate the region in the interplanetary (IP) space where the interaction happened/started using a combined observational-modeling approach. We use Minimum Variance Analysis and the Walen test to analyze possible reconnection exhaust at the interface of ICME and HSS. We perform a Graduated Cylindrical Shell reconstruction of the CME to estimate the geometry and source location of the CME. Finally, we use a two-step Drag Based Model (DBM) model to estimate the region in IP space where the interaction took place. The magnetic obstacle (MO) observed in situ shows a fairly symmetric and undisturbed structure and shows the magnetic flux, helicity, and expansion profile/speed of a typical ICME. The MVA together with the Walen test, however, confirms reconnection exhaust at the ICME HSS boundary. Thus, in situ signatures are in favor of a scenario where the interaction is fairly recent. The trailing HSS shows a distinct velocity profile which first reaches a semi-saturated plateau with an average velocity of 500 km/s and then saturates at a maximum speed of 710 km/s . We find that the HSS interaction with the ICME is influenced only by this initial plateau. The results of the two-step DBM suggest that the ICME has started interacting with the HSS close to Earth (approx 0.81 au), which compares well with the deductions from in situ signatures.
Comment: 14 pages, 8 Figures, Accepted for publication in the Astrophysical Journal