10 research outputs found
Cluster observations of non-time-continuous magnetosonic waves
Equatorial magnetosonic waves are normally observed as temporally
continuous sets of emissions lasting from minutes to hours. Recent
observations, however, have shown that this is not always the case. Using
Cluster data, this study identifies two distinct forms of these non-temporallycontinuous
emissions. The first, referred to as rising tone emissions, are characterised
by the systematic onset of wave activity at increasing proton gyroharmonic
frequencies. Sets of harmonic emissions (emission elements) are
observed to occur periodically in the region ±10◦ off the geomagnetic equator.
The sweep rate of these emissions maximises at the geomagnetic equator.
In addition, the ellipticity and propagation direction also change systematically
as Cluster crosses the geomagnetic equator. It is shown that the
observed frequency sweep rate is unlikely to result from the sideband instability
related to nonlinear trapping of suprathermal protons in the wave field.
The second form of emissions is characterised by the simultaneous onset of
activity across a range of harmonic frequencies. These waves are observed
at irregular intervals. Their occurrence correlates with changes in the spacecraft
potential, a measurement that is used as a proxy for electron densit
Space Technology 5 Multi-point Measurements of Near-Earth Magnetic Fields: Initial Results
The Space Technology 5 (ST-5) mission successfully placed three micro-satellites in a 300 x 4500 km dawn-dusk orbit on 22 March 2006. Each spacecraft carried a boom-mounted vector fluxgate magnetometer that returned highly sensitive and accurate measurements of the geomagnetic field. These data allow, for the first time, the separation of temporal and spatial variations in field-aligned current (FAC) perturbations measured in low-Earth orbit on time scales of approximately 10 sec to 10 min. The constellation measurements are used to directly determine field-aligned current sheet motion, thickness and current density. In doing so, we demonstrate two multi-point methods for the inference of FAC current density that have not previously been possible in low-Earth orbit; 1) the "standard method," based upon s/c velocity, but corrected for FAC current sheet motion, and 2) the "gradiometer method" which uses simultaneous magnetic field measurements at two points with known separation. Future studies will apply these methods to the entire ST-5 data set and expand to include geomagnetic field gradient analyses as well as field-aligned and ionospheric currents
Energy Transfer Between Hot Protons and Electromagnetic Ion Cyclotron Waves in Compressional Pc5 Ultra-low Frequency Waves
The Magnetospheric Multiscale (MMS) spacecraft observed many enhancements of electromagnetic ion cyclotron (EMIC) waves in an event in the late afternoon outer magnetosphere. These enhancements occurred mainly in the troughs of magnetic field intensity associated with a compressional ultralow frequency (ULF) wave. The ULF wave had a period of ∼2–5 min (Pc5 frequency range) and was almost static in the plasma rest frame. The magnetic and ion pressures were in antiphase. They are consistent with mirror-mode type structures. We apply the Wave-Particle Interaction Analyzer method, which can quantitatively investigate the energy transfer between hot anisotropic protons and EMIC waves, to burst-mode data obtained by the four MMS spacecraft. The energy transfer near the cyclotron resonance velocity was identified in the vicinity of the center of troughs of magnetic field intensity, which corresponds to the maxima of ion pressure in the compressional ULF wave. This result is consistent with the idea that the EMIC wave generation is modulated by ULF waves, and preferential locations for the cyclotron resonant energy transfer are the troughs of magnetic field intensity. In these troughs, relatively low resonance velocity due to the lower magnetic field intensity and the enhanced hot proton flux likely contribute to the enhanced energy transfer from hot protons to the EMIC waves by cyclotron resonance. Due to the compressional ULF wave, regions of the cyclotron resonant energy transfer can be narrow (only a few times of the gyroradii of hot resonant protons) in magnetic local time
EMIC waves converted from equatorial noise due to M/Q=2 ions in the plasmasphere: Observations from Van Allen Probes and Arase
Equatorial noise (EN) emissions are observed inside and outside the plasmapause. EN emissions are referred to as magnetosonic mode waves. Using data from Van Allen Probes and Arase, we found conversion from EN emissions to electromagnetic ion cyclotron (EMIC) waves in the plasmasphere and in the topside ionosphere. A low frequency part of EN emissions becomes EMIC waves through branch splitting of EN emissions, and the mode conversion from EN to EMIC waves occurs around the frequency of M/Q=2 (deuteron and/or alpha particles) cyclotron frequency. These processes result in plasmaspheric EMIC waves. We investigated the ion composition ratio by characteristic frequencies of EN emissions and EMIC waves and obtained ion composition ratios. We found that the maximum composition ratio of M/Q=2 ions is ~10% below 3000 km. The quantitative estimation of the ion composition will contribute to improving the plasma model of the deep plasmasphere and the topside ionosphere
Comparison between Liouville's theorem and observed latitudinal distribution of trapped ions in the plasmapause region
Journal of Geophysical Research, Volume 99, pp. 2191-2203The presence of anisotropic plasma distributions, trapped at the Earth's magnetic equator, has
consequences for the electric field structure which must exist in equilibrium along the magnetic field line.
Data from SCATHA and Dynamics Explorer 1indicated that the core ion distributions at the magnetic
equator can be well described as bi-Maxwellian distributions, with a perpendicular temperature an order of
magnitude larger than the parallel temperature
Plasma Sources in Planetary Magnetospheres: Mercury
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