Low‐frequency wave characteristics in the upstream and downstream regime of the terrestrial bow shock

Abstract

[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).

Anisotropy evolution of magnetic field fluctuation through the bow shock

“…Luhmann et al (1986) argued that one of the major sources of fluctuations downstream of the quasi-parallel shock is the foreshock activity, but our result suggests that the property of the foreshock fluctuations is lost across the shock. Our result is consistent with the statistical analysis of Cluster data that the foreshock fluctuation property is lost in the magnetosheath (Narita et al, 2006). It is also worthwhile to note that the perpendicular wave vector geometry may be interpreted not only as the mirror mode but also as quasi-two-dimensional turbulence.…”

Ultra‐low‐frequency waves and associated wave vectors observed in the plasma sheet boundary layer by Cluster

“…When its four spacecraft are in the correct configuration, Cluster can measure the full wave vector ( k ), which provides both the wavelength and the propagation direction of a wave. Previous wave telescope work has been used to determine dispersion relations [ Narita and Glassmeier , 2005], propagation patterns [ Narita et al , 2006a], energy spectra [ Narita et al , 2006b] and cross‐helicity [ Narita et al , 2007]. Using Cluster, we present measurements of k for waves with frequencies near the harmonics of the local proton cyclotron frequency (Ω p ).…”

Mirror mode structures near Venus and Comet P/Halley

“…They are frequently observed in planetary magnetosheaths of Earth (e.g., Tsurutani et al, 1982;Czaykowska et al, 1998;Baumjohann et al, 1999;Lucek et al, 1999a;Narita et al, 2006), Venus (e.g., Volwerk et al, 2008;Zhang et al, 2009), Jupiter (e.g., Tsurutani et al, 1982;Erdõs and Balogh, 1993;Bavassano-Cattaneo et al, 1998), Saturn (e.g., Tsurutani et al, 1982;Violante et al, 1995), and even near the magnetic pileup boundary of a comet (e.g., Glassmeier et al, 1993).…”