Waves Generated by Electron Beam in a Crater-Shaped Flux Rope

Kyunghwan Dokgo,Kyoung-Joo Hwang,James L Burch,Peter H Yoon

doi:10.3389/fphy.2021.734437

Kyunghwan Dokgo, Kyoung-Joo Hwang + Show 2 more

Open Access

https://doi.org/10.3389/fphy.2021.734437

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Abstract

Understanding the nature and characteristics of high-frequency waves inside a flux rope may be important as the wave-particle interaction is important for charged-particle energization and the ensuing dissipation process. We analyze waves generated by an electron beam in a crater-shaped magnetic flux rope observed by MMS spacecraft on the dawnside tailward magnetopause. In this MMS observation, a depression of magnetic field, or a crater, of ∼100 km is located at the center of the magnetic flux rope of ∼650 km. There exist parallel and perpendicular electrostatic wave modes inside the depression of the magnetic field at the center of the flux rope, and they are distinguished by their locations and frequencies. The parallel mode exists at the center of the magnetic depression and its power spectrum peaks below Fce (electron cyclotron frequency). In contrast, the perpendicular mode exists in the outer region associated with the magnetic depression, and its power spectrum peaks near Fce. The linear analysis of kinetic instability using a generalized dispersion solver shows that the parallel mode can be generated by the electron beam of 5,000 km/s. They can thermalize electrons ≲100 eV effectively. However, the generation mechanism of the perpendicular mode is not clear yet, which requires further study.

Highlights

Magnetic flux ropes are 3-D helical structures with coherently twisted magnetic field lines winding about a common axis
They showed that the averaged quantities from the magnetosheath side and the magnetosphere side satisfy the threshold condition for the Kelvin-Helmholtz waves (KHW), [k · (Vsh − Vsp)]2 > μ−01(ρ−sp1 + ρ−sh1)[(Bsp · k)2 + (Bsh · k)2], where subscripts sp and sh stand for magnetopause and magnetosphere, respectively; V, B, and ρ, are flow velocity, magnetic field vector, and plasma density, respectively; k is the wave vector associated with KHW; and μ0 denotes the vacuum permeability [18]
In the present manuscript the issue of high-frequency waves measured inside the center of the magnetic field depression in a crater-shaped flux rope is discussed

Summary

Introduction

Magnetic flux ropes are 3-D helical structures with coherently twisted magnetic field lines winding about a common axis. We investigate waves inside the magnetic field depression (crater) at the center of a crater-shaped flux rope on the May 5.

Results

Conclusion