Turbulent heating and cross‐field transport near the magnetopause from THEMIS

Abstract

[1] We demonstrate that the low frequency broadband magnetic fluctuations observed during THEMIS spacecraft traversals near the Earth's magnetopause may be described as a turbulent spectrum of Doppler shifted kinetic Alfvén waves (KAWs). These waves are most intense along reconnected flux-tubes in the magnetosheath just outside the magnetopause. We identify distinct power-law scalings of wave spectral energy density in wavenumber and show that Landau (LD) and transit time damping (TD) on ions and electrons is largest at the wavenumber where the power-law index changes. The threshold amplitude for stochastic ion scattering/acceleration is also exceeded by these waves. These acceleration processes are manifest in observations of field-aligned and transverse heating of electrons and ions respectively. From integration over the range of observed wavenumbers we show that, if the wave-normal angles are sufficiently large, these waves can provide diffusive transport of magnetosheath plasmas across the magnetopause at up to the Bohm rate. Observations[2] The presence of broadband magnetic noise at the magnetopause has been reported since the first traversals of this boundary by the ISEE spacecraft [Rezeau et al., 1989]. It has been suggested that these oscillations are kinetic Alfvén waves (KAWs) Stasiewicz et al., 2001]. Particle observations in these regions reveal anisotropic ion [Nishino et al., 2007] and field-aligned electron (e À ) distributions [Chaston et al., 2007]. Just inside the magnetopause, spacecraft observe a boundary layer of mixed magnetosheath/magnetospheric like plasmas [Song et al., 2003]. The role that KAWs may play in particle acceleration/heating and transport across the magnetopause to account for these observations has been considered [Lee et al., 1994; Cheng, 1997, 2001;Chaston et al., 2007]. In this report we exploit the multi-point measurements provided by the THEMIS spacecraft to characterize the low frequency electromagnetic oscillations in the near magnetosheath/magnetopause/ boundary layer and discuss how these oscillations may drive particle acceleration and cross-field transport in these regions.[3] Figure 1 presents observations recorded during a THEMIS [Angelopoulos, 2008] traversal of the magnetopause. Measurements from the ion plasma instrument [McFadden et al., 2008] shown in Figure 1a reveal the shocked solar wind plasmas of the magnetosheath transitioning to the more energetic plasmas of the magnetosphere after 0300 UT via a region of mixed magnetosheath/magnetospheric plasmas comprising the low latitude boundary layer. Figure 1b reveals a significant density gradient across this transition which provides increasing Alfvén speed (V A ) with decreasing flow speed (Figure 1c) into the boundary layer. Measurements from the fluxgate magnetometer experiment [Auster et al., 2008] shown in Figure 1d indicate that at those times in the magnetosheath when B ZGSE is negative we find enhanced ion energies in Figure 1a. While not especially clear in this case, during these times in the ...

The dawn–dusk asymmetry of ion density in the dayside magnetosheath and its annual variability measured by THEMIS

“…To complicate matters, the means of transporting mass, momentum and energy across the magnetopause involves a synthesis of magnetohydrodynamic (MHD), kinetic, and numerous wave-particle interaction processes. In spite of that, several candidates capable of facilitating transport across the magnetopause have been identified: magnetic reconnection (Dungey, 1961), impulsive penetration (Karlsson et al, 2012;Archer and Horbury, 2013;Plaschke et al, 2013), Kinetic Alfvén waves (Johnson and Cheng, 1997;Chaston et al, 2008) the Kelvin Helmholtz Instability (KHI) (Miura and Pritchett, 1982;Miura, 1992;Chen et al, 1997;Otto and Fairfield, 2000;Nykyri and Otto, 2001;Nykyri et al, 2006;Hasegawa et al, 2009) and flux transfer events (Russell and Elphic, 1978). It is noteworthy that each of these processes do not strictly operate independently, and during certain conditions, plasma transport may be a result of a combination of these processes operating simultaneously.…”

Simulation of mode conversion at the magnetopause

“…It has been suggested that mode conversion from compressional to Alfvén modes provides a source of Alfvén waves at *Corresponding author (email: ylin@physics.auburn.edu) the magnetopause boundary [30][31][32][33][34][35]. Multi-point measurements have verified that the dispersion of the broadband waves is consistent with the kinetic Alfvén waves (KAWs) [35,36].…”

“…Multi-point measurements have verified that the dispersion of the broadband waves is consistent with the kinetic Alfvén waves (KAWs) [35,36]. Such mode conversion may lead to effective heating and transport of plasma at the magnetopause [30,32,33,37].…”

ARTEMIS Science Objectives

“…ARTEMIS will determine whether or not a quasi-steady inertial range of turbulence is attained as a result of energy injection via reconnection. The two-point measurements permit estimations of the wavelength (instead of or in addition to the frequency) of the associated waves (Chaston et al 2008). A spacecraft in the magnetosheath can provide information about the nature of magnetosheath turbulence and whether conditions favor high-latitude reconnection, thereby helping discriminate between the effects of magnetosheath and reconnection-induced turbulence.…”