Observation of Alfven wave in ICME-HSS interaction region

TL;DR

This study investigates the generation and characteristics of Alfvén waves during the interaction between an interplanetary coronal mass ejection and a high-speed stream, revealing their directional properties and possible origins.

Contribution

It provides the first detailed analysis of Alfvén waves in an ICME-HSS interaction region, identifying their directional nature and potential formation mechanisms.

Findings

ICME exhibits Sunward Alfvén waves

HSS shows strong anti-Sunward Alfvén waves

Reconnection within ICME may generate Alfvén waves

Abstract

The Alfv\'en wave (AW) is the most common fluctuation present within the emitted solar wind from the Sun. Moreover, the interaction between interplanetary coronal mass ejection (ICME) and high-speed stream (HSS) has been observed on several occasions. However, can such interaction generate an AW? What will be the nature of AW in such a scenario remains an open question. To answer it, we have investigated an ICME-HSS interaction event observed on 21st October 1999 at 1 AU by Wind spacecraft. We have used the Wal\'en test to identify AW and estimated Elsasser variables to find the characteristics of the AWs. We explicitly find that ICME were dominant with Sunward AWs, whereas the trailing HSS has strong anti-Sunward AW. We suggest that the ICME-HSS interaction deforms the MC of the ICME, resulting in the AWs inside the MC. In addition, the existence of reconnection within the ICME early stage can also be the leading cause of the origin of AW within it.