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Klimushkin D.Yu.,Russian Academy of Sciences | Mager P.N.,Russian Academy of Sciences | Glassmeier K.-H.,Institute For Geophysik Und Extraterrestrische Physik
Annales Geophysicae

This paper is concerned with the spatial structure and temporal evolution of the azimuthally small scale Alfvén wave generated by a sudden impulse concentrated on a given magnetic shell. At the outset, both poloidal and toroidal components are present in the wave's magnetic field. The oscillation in the poloidal component on a given magnetic shell is a superposition of two monochromatic oscillations, one with the local resonance frequency on this shell, and the other with the frequency corresponding to the resonance frequency on the source surface. The superposition of these two oscillations leads to beating. Due to phase mixing, the poloidal component of the oscillation decreases with time down to zero, transferring its energy to the toroidal component. Beating in the toroidal component is less pronounced. As time elapses, energy concentration near the source magnetic shell occurs with the frequency of the oscillation corresponding to the Alfvénic resonance frequency on this. © 2012 Author(s). CC Attribution 3.0 License. Source

Zhang H.,University of California at Los Angeles | Kivelson M.G.,University of California at Los Angeles | Angelopoulos V.,University of California at Los Angeles | Khurana K.K.,University of California at Los Angeles | And 4 more authors.
Journal of Geophysical Research: Space Physics

Between May and October in 2007 and 2008, the five THEMIS spacecraft recorded a total of 3701 instances of bipolar magnetic variations in the magnetopause normal direction associated with enhancements of field magnitude that are interpreted as flux transfer events (FTEs) on the magnetopause and/or associated perturbations in the background magnetosphere and magnetosheath. When spacecraft traversed the FTE structures, the velocity components tangential to the magnetopause were generally antisunward, consistent with the sheath flow direction. On the other hand, when the spacecraft were located within the low-latitude boundary layer (LLBL) in the magnetosphere and remotely sensed the perturbations related to FTEs on the magnetopause, the velocity tangential to the magnetopause was found to be antisunward near the magnetopause but sunward further in from the magnetopause. The normal component variations for both groups had the same bipolar structure with inward flows followed by outward flows. This pattern has the form of a flow vortex just inside the magnetopause associated with an FTE moving in an antisunward direction at or outside of the magnetopause. A 2-dimensional magnetohydrodynamic (MHD) simulation code has been developed to understand the flow perturbations outside an FTE. Our simulation starts from an explicit solution, in which it is assumed that the plasma is inviscid and incompressible and no flow vortex is present. Only when we impose finite viscosity near the FTEs do flow vortices develop. However, the origin of this viscosity remains unknown. Copyright © 2011 by the American Geophysical Union. Source

Cully C.M.,Swedish Institute of Space Physics | Angelopoulos V.,University of California at Los Angeles | Auster U.,Institute For Geophysik Und Extraterrestrische Physik | Bonnell J.,University of California at Berkeley | Le Contel O.,French National Center for Scientific Research
Geophysical Research Letters

Chorus emissions are a striking feature of the electromagnetic wave environment in the Earth's magnetosphere. These bursts of whistler-mode waves exhibit characteristic frequency sweeps (chirps) believed to result from wave-particle trapping of cyclotron-resonant particles. Based on the theory of Omura et al. (2008), we predict the sweep rates of chorus elements observed by the THEMIS satellites. The predictions use independent observations of the electron distribution functions and have no free parameters. The predicted chirp rates are a function of wave amplitude, and this relation is clearly observed. The predictive success of the theory lends strong support to its underlying physical mechanism: cyclotron-resonant wave-particle trapping. Copyright 2011 by the American Geophysical Union. Source

Halekas J.S.,University of California at Berkeley | Halekas J.S.,NASA | Angelopoulos V.,University of California at Los Angeles | Sibeck D.G.,NASA | And 11 more authors.
Space Science Reviews

We present observations from the first passage through the lunar plasma wake by one of two spacecraft comprising ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun), a new lunar mission that re-tasks two of five probes from the THEMIS magnetospheric mission. On Feb 13, 2010, ARTEMIS probe P1 passed through the wake at ∼3.5 lunar radii downstream from the Moon, in a region between those explored by Wind and the Lunar Prospector, Kaguya, Chandrayaan, and Chang'E missions. ARTEMIS observed interpenetrating proton, alpha particle, and electron populations refilling the wake along magnetic field lines from both flanks. The characteristics of these distributions match expectations from self-similar models of plasma expansion into vacuum, with an asymmetric character likely driven by a combination of a tilted interplanetary magnetic field and an anisotropic incident solar wind electron population. On this flyby, ARTEMIS provided unprecedented measurements of the interpenetrating beams of both electrons and ions naturally produced by the filtration and acceleration effects of electric fields set up during the refilling process. ARTEMIS also measured electrostatic oscillations closely correlated with counter-streaming electron beams in the wake, as previously hypothesized but never before directly measured. These observations demonstrate the capability of the comprehensively instrumented ARTEMIS spacecraft and the potential for new lunar science from this unique two spacecraft constellation. © 2011 The Author(s). Source

Baumjohann W.,Austrian Academy of Sciences | Matsuoka A.,Japan Aerospace Exploration Agency | Magnes W.,Austrian Academy of Sciences | Glassmeier K.-H.,Institute For Geophysik Und Extraterrestrische Physik | And 31 more authors.
Planetary and Space Science

The Mercury magnetospheric orbiter (MMO) of the Japanese-European BepiColombo mission carries a dual-sensor magnetometer, MMO/MGF. The sensors are of the classical fluxgate type mounted on a boom. For redundancy, each sensor carries its own electronics and is connected to a different data processing unit. MMO/MGF can sample the magnetic field at a rate of up to 128 Hz. The resulting comparatively high time resolution of the magnetic field measurements, i.e., down to 8 ms, will be necessary when studying the dynamics of and processes within the Hermean magnetosphere, since the Mariner 10 observations have shown that their typical time scales are much shorter than in the Earth's magnetosphere, by about a factor of 30. The high time resolution will also be very useful for studying the evolution of the still young solar wind plasma as well as interplanetary shocks at 0.3-0.46 AU. Of course, MMO/MGF is also well-prepared to assist the sister magnetometer aboard the Mercury planetary orbiter, MPO/MAG, in measuring Mercury's intrinsic magnetic field, in particular by helping to distinguish between temporal fluctuations and spatial variations. © 2009 Elsevier Ltd. All rights reserved. Source

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