[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 main goal of the Cluster mission, consisting of four identical spacecraft, is the spatial resolution of plasma structures. For the determination of the wave vectors of a wave field from four positions, classical Fourier analysis is inappropriate. We develop a generalized minimum variance technique which gives a high wave vector resolution though the spatial grid is restricted to only a few sampling positions. This technique uses the amplitude and phase information of the magnetic field from the four satellite positions and determines the optimum wave field corresponding to the measured data. The components of the magnetic field are assumed to be normally distributed. The divergence-free nature of the magnetic field is used as a constraint. Using the magnetic data measured at four positions allows up to seven different wave vectors at one frequency to be uniquely resolved.

We report the first direct observations of parallel electric fields (E_{ parallel}) carried by double layers (DLs) in the plasma sheet of Earth's magnetosphere. The DL observations, made by the THEMIS spacecraft, have E_{ parallel} signals that are analogous to those reported in the auroral region. DLs are observed during bursty bulk flow events, in the current sheet, and in plasma sheet boundary layer, all during periods of strong magnetic fluctuations. These observations imply that DLs are a universal process and that strongly nonlinear and kinetic behavior is intrinsic to Earth's plasma sheet.

[1] We present the first systematic observational evidence for a traveling periodic structure in the pre-onset optical aurora -the longitudinally propagating arc wave (LPAW) -associated with flapping oscillations in the magnetotail. The LPAW is characterized by azimuthally moving intensity enhancements inside auroral arcs as seen by THEMIS ground-based all-sky imagers. It travels westward in the pre-midnight auroral sector during the 10-20 minutes preceding auroral breakup with a velocity of 2 -10 km/s, time period 40-110 s, and wavelength 250 -420 km. Magnetically conjugate measurements by THEMIS satellites show low frequency plasma oscillations consistent with the parameters of the arc wave in the course of current sheet thinning. When mapped into tail, wavelength (4800 -9400 km) and velocity (70 -190 km/s) of the LPAW are compatible with observations and theoretical predictions for current sheet flapping motions. Our results strongly suggest that LPAW is an auroral footprint of the drift wave mode (kink, sausage, ballooning, etc.) in a stretched magnetotail.Citation: Uritsky, V. M.,

The four‐satellite array of the Cluster mission will allow the separation of spatial and temporal variations in the Earth's magnetosphere and magnetosheath and in the solar wind. We are using a general linear filter formulation to construct a mathematical formalism to determine not only the frequency spectra but also directional distribution and mode separation in a general wave field. This formalism is completely model independent and requires knowledge of the basic dispersion and polarization properties of the participating waves only. The four‐satellite array combined with a suitable formalism is called a wave telescope. We show that at a given frequency the transmission properties expressed by a transmission function can be shaped by 72 independent parameters for a three‐component vector output of the wave telescope filter. Considering the periodicity of the transmission function in wave vector k space, we are led to the concept of an elementary cell in k space given by the geometric properties of the array such that outside the elementary cell the k ‐ vectors are subject to spatial aliasing analogously to aliasing in time at frequencies above the Nyquist frequency. We show that for MHD waves the fast mode is particularly benign as far as spatial aliasing is concerned. The aliasing properties of other wave modes are also discussed. As an example a filter is constructed which suppresses waves propagating in the magnetic field direction but allows passage of waves propagating antiparallel to the magnetic field.

Using four-point measurements of the Cluster spacecraft, the energy distribution was determined for magnetic field fluctuations in the solar wind directly in the three-dimensional wave-vector domain in the range |k|

[1] Cluster provides us with a unique possibility to study ULF waves. We analyze ULF wave activity in the near-Earth upstream solar wind. Using Cluster as a wave telescope we investigate in detail wave propagation directions and wave numbers for various frequencies, obtaining, for the first time, three dimensional dispersion relations experimentally. After Doppler shift correction, we find that the dispersion relations are not linear and the waves are propagating in the sunward direction in the plasma rest frame. Comparison of the experimentally derived dispersion relation with that one for a beam plasma system shows good agreement. We suggest that the ULF waves in the foreshock are generated by a proton population backstreaming from the shock.

[1] Collisionless shocks in space are accompanied by the low-frequency waves of the magnetic field both upstream and downstream of the shock. However, characteristics of these waves in the plasma rest frame are poorly understood, since spatial scale such as wavelength is hardly determined using single or at best two spacecraft observations. Taking advantage of multipoint measurements of the Cluster spacecraft, we present a statistical study to reveal the rest frame properties of the waves in the terrestrial bow shock environment such as frequencies, wave numbers, phase velocities, propagation directions, polarization and transport ratios for the quasi-parallel and the quasi-perpendicular shock regimes and in the upstream region, the outer, the middle, and the inner magnetosheath. We find that the quasi-parallel shock upstream waves (or the foreshock waves) exhibit properties of the fast mode wave excited by the ion beam instability. Upstream of the quasi-perpendicular shock the waves also exhibit similar properties to the foreshock waves, but the wave mode identification needs further investigations. In the magnetosheath the waves exhibit properties which are persistent from the outer to the inner magnetosheath. They can be interpreted as the mirror mode coupled to the plasma inhomogeneity or as the slow mode. We also find an organization in the wave propagation pattern that the upstream waves propagate toward upstream, while the magnetosheath waves propagate toward the flank region and toward the magnetopause. It is concluded that wave properties are different between the upstream and the downstream regions, but they are similar between the quasi-parallel and the quasi-perpendicular shock regimes, suggesting that the upstream waves are not transmitted to the downstream region across the shock and that the downstream waves do not depend on the shock angle (between the upstream magnetic field and the shock normal direction).