Observation of Alfvén Waves in an ICME-HSS Interaction Region

Abstract

The Alfvén wave (AW) is the most common fluctuation present within the solar wind emitted from the Sun. Whether or not AWs can originate after the collision of an Interplanetary Coronal Mass Ejection (ICME) and a High-Speed Stream (HSS) remains an open question. To find an answer to this question, we have investigated an ICME-HSS interaction event observed on $21^{st}$ October 1999 at 1 AU by the WIND spacecraft. We have used the Walén test to identify AWs and estimated the Elsässer variables to find its characteristics. We explicitly find dominant sunward AWs within the ICME, whereas the trailing HSS has strong anti-sunward AWs. We suggest that the ICME-HSS interaction deforms the Magnetic Cloud (MC) of the ICME, resulting in the generation of AWs inside the MC. Additionally, the existence of reconnection within the ICME’s early stage could also contribute to the origin of AWs within it.