Nonlinear energy transfer and current sheet development in localized Alfvén wavepacket collisions in the strong turbulence limit

Abstract

In space and astrophysical plasmas, turbulence is responsible for transferring energy from large scales driven by violent events or instabilities, to smaller scales where turbulent energy is ultimately converted into plasma heat by dissipative mechanisms. The nonlinear interaction between counterpropagating Alfvén waves, denoted Alfvén wave collisions, drives this turbulent energy cascade, as recognized by early work with incompressible magnetohydrodynamic (MHD) equations. Recent work employing analytical calculations and nonlinear gyrokinetic simulations of Alfvén wave collisions in an idealized periodic initial state have demonstrated the key properties that strong Alfvén wave collisions mediate effectively the transfer of energy to smaller perpendicular scales and self-consistently generate current sheets. For the more realistic case of the collision between two initially separated Alfvén wavepackets, we use a nonlinear gyrokinetic simulation to show here that these key properties persist: strong Alfvén wavepacket collisions indeed facilitate the perpendicular cascade of energy and give rise to current sheets. Furthermore, the evolution shows that nonlinear interactions occur only while the wavepackets overlap, followed by a clean separation of the wavepackets with straight uniform magnetic fields and the cessation of nonlinear evolution in between collisions, even in the gyrokinetic simulation presented here which resolves dispersive and kinetic effects beyond the reach of the MHD theory.

Role of reconnection in inertial kinetic-Alfvén turbulence

“…Large-scale low-frequency fluctuations in astrophysical systems such as the interstellar medium, the solar wind, and others, are associated with nearly incompressible magnetohydrodynamic (Alfvén) turbulence [e.g., [1][2][3]. The nonlinear Alfvén wave packets that compose such turbulence are three-dimensionally anisotropic: elongated along the strong background magnetic field and resembling current sheets in the perpendicular plane [1,[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. At scales smaller than the plasma microscales, such as the ion gyroscale or ion inertial scale, the shear-Alfvén modes transform into kinetic Alfvén modes.…”

The Alfvénic nature of energy transfer mediation in localized, strongly nonlinear Alfvén wavepacket collisions

“…Therefore, there is a very slight spreading of the wavepackets after nonlinear interactions have transferred energy into modes with k ⊥ ρ i 1. This behavior is noticeable in Figure 7 of our companion paper Verniero et al (2018) and is discussed in more detail in Section 3.4 of that paper.…”

“…But that study employed asymmetric initial Alfvén wavepackets (see Figure 1 of Verniero et al (2018)), where one of the wavepackets had a significant k = 0 component initially relative to the background magnetic field. Since it is the secondary mode with k = 0 that plays the key role in mediating the secular transfer of energy to smaller perpendicular scales in the periodic case, it is important to ensure that the non-zero k = 0 component of the wavepacket in Verniero et al (2018) does not affect the results in a fundamental way. To address this issue, we pursue here a detailed comparison of periodic Alfvén wave and localized Alfvén wavepacket collisions, where the initial wavepackets are symmetric and neither wavepacket has a significant k = 0 component.…”

Laboratory space physics: Investigating the physics of space plasmas in the laboratory

“…The success of this experimental investigation of Alfvén wave collisions has laid the foundation for subsequent advances in our theoretical understanding of how current sheets arise self-consistently in plasma turbulence, 70,124 the role played by resonant wave-particle interactions in the dissipation of these current sheets, 72 and how collisions between localized Alfvén wavepackets in the strongly nonlinear limit mediate the turbulent cascade of energy to small scales. 125,126 Observations of enhanced perpendicular ion temperatures in the solar corona 127 have lead to the important question of how ions are energized perpendicular to the magnetic field under low plasma beta conditions. Under similar low plasma beta and weakly collisional conditions, a broadband spectrum of anisotropic magnetic turbulence has been observed to arise during magnetic relaxation events in the Madison Symmetric Torus (MST ) experiment; 128 application of a Rutherford scattering diagnostic 129 has shown there to be anomalous ion heating coincident with this turbulence.…”