A wave model interpretation of the evolution of rotational discontinuities

Abstract

The evolution of rotational discontinuities (RDs) is followed using a hybrid numerical code. An extensive parameter variation is carried out, with particular emphasis on β, Ti/Te, θB (the angle between the normal and total magnetic field), and the helicity of the RD. The RD structure is shown to have features in common with the evolution of both strongly modulated, nonlinear wave packets and linear dispersive wave propagation in oblique magnetic fields. For small θB, the RD disperses linearly giving fast and Alfvén waves upstream and downstream, respectively, and the familiar S‐shaped hodograms. At larger θB (≈ 30°), nonlinearity becomes important and strong coupling to a compressional (sonic) component can occur in the main current layer. When the ions are cold, there is a critical value of βe(=β*) when the intermediate wave train moves from the downstream side of the RD to the upstream and is replaced on the downstream side by slow modes. This is reflected in the hodograms by a change of the wave polarization on both sides, and represents an important modification to the original Goodrich and Cargill (1991) wave model of RDs. As Ti/Te increases, the spreading rate of the current layer increases for moderate θB. For large θB(≈ 60°), RDs with electron (ion) sense of rotation show increased (decreased) spreading with increasing Ti/Te. These results are applied to RDs observed in the solar wind and at the magnetopause.

Cluster observations of fast magnetosonic waves in the heliosphere current sheet

“…10.100210. /2014GL059223 1991Vasquez and Cargill, 1993]. Higher-dimensional hybrid simulation [Vasquez and Hollweg, 1998] have traced the steepening of large-amplitude Alfvén wave train Vasquez and Hollweg [1998], which more resemble the observation in this event.…”

“…The phase‐steepened edge of Alfvén waves is usually considered to be a rotational discontinuity [ Tsurutani et al , ; Tsurutani and Ho , ]. Numerous theoretical studies have predicted the generation of fast modes at the rotational discontinuity (RD) [ Goodrich and Cargill , ; Vasquez and Hollweg , ; Vasquez and Cargill , ]. Hybrid simulations that traced the evolution of a kink in the magnetic field predicted the generation of fast (Alfvén) mode wave at the RD and its propagation upstream (downstream) from the RD layer [ Goodrich and Cargill , ; Vasquez and Cargill , ].…”

“…Numerous theoretical studies have predicted the generation of fast modes at the rotational discontinuity (RD) [ Goodrich and Cargill , ; Vasquez and Hollweg , ; Vasquez and Cargill , ]. Hybrid simulations that traced the evolution of a kink in the magnetic field predicted the generation of fast (Alfvén) mode wave at the RD and its propagation upstream (downstream) from the RD layer [ Goodrich and Cargill , ; Vasquez and Cargill , ]. Higher‐dimensional hybrid simulation [ Vasquez and Hollweg , ] have traced the steepening of large‐amplitude Alfvén wave train Vasquez and Hollweg [], which more resemble the observation in this event.…”

One‐dimensional hybrid simulations of current sheets in the quiet magnetotail

“…In general, such waves are dispersive and elliptically polarized, so that in a magnetotail-like field any wave structures would be expected to have a By, as seen in the simulations. Such contrasting wave properties also play a significant role in determining the structure of tangential and rotational discontinuities [Cargill, 1990;Vasquez and Cargill, 1993] A useful way to analyse the results is by Fourier transforming By into k-space. Figure 3 shows [ Rich et al, 1972;Holland and Chen, 1993].…”

A possible generation mechanism of interplanetary rotational discontinuities

“…Several different generation mechanisms of RDs were proposed in the past and two of them are well known. One is from the nonlinear evolution of Alfvén waves in the solar wind [ Cohen and Kulsrud , 1974; Vasquez and Cargill , 1993; Tsurutani et al , 1994; Vasquez and Hollweg , 1996] and the other is through the process of magnetic reconnection near the sun [ Lee et al , 1996]. For magnetic reconnection, the rate is related to V 1n / V A1 (= B n / B ) at a value <0.2 [e.g., Vasyliunas , 1975; Phan et al , 1996, 2006, 2007; Davis et al , 2006], and thus the generated RDs should have a small B n / B < 0.2.…”