First analysis of in-situ observation of surface Alfv\'en waves in ICME flux rope

TL;DR

This paper reports the first in-situ observation of surface Alfvén waves in an interplanetary coronal mass ejection flux rope, confirming simulation predictions and exploring their potential excitation mechanisms.

Contribution

It provides the first observational evidence of surface Alfvén waves in a flux rope, validating previous simulation-based hypotheses.

Findings

First in-situ detection of surface Alfvén waves in ICME flux rope

Application of Walén test confirms wave identification

Surface Alfvén waves may be excited by flux rope foot point movement or boundary instabilities

Abstract

Alfv\'en waves (AWs) are inevitable in space and astrophysical plasma. Their crucial role in various physical processes, occurring in plasma, has triggered intense research in solar-terrestrial physics. Simulation studies have proposed the generation of AWs along the surface of a cylindrical flux rope, referred to as Surface AWs (SAWs); however the observational verification of this distinct wave has been elusive to date. We report the first \textit{in-situ} observation of SAWs in an interplanetary coronal mass ejection flux rope. We apply the Wal\'en test to identify them. The Elsa\"sser variables are used to estimate the characterization of these SAWs. They may be excited by the movement of the flux rope's foot points or by instabilities along the plasma magnetic cloud's boundaries. Here, the change in plasma density or field strength in the surface-aligned magnetic field may trigger SAWs.