Abstract
AbstractWe report observations of the ion dynamics inside an Alfvén branch wave that propagates near the reconnecting dayside magnetopause. The measured frequency, wave normal angle and polarization are consistent with the predictions of a dispersion solver. The magnetospheric plasma contains hot protons (keV), cold protons (eV), plus some heavy ions. While the cold protons follow the magnetic field fluctuations and remain frozen‐in, the hot protons are at the limit of magnetization. The cold protons exchange energy back and forth, adiabatically, with the wave fields. The cold proton velocity fluctuations contribute to balance the Hall term fluctuations in Ohm's law, and the waveEfield has small ellipticity and right‐handed polarization. The dispersion solver indicates that increasing the cold proton density facilitates propagation and amplification of these waves at oblique angles, as for the observed wave.
Highlights
Electromagnetic ion cyclotron (EMIC) waves are generated in various regions of the Earth's magnetosphere when hot ions have T⊥ > T‖ (e.g., Gary, 1992; Gary & Winske, 1990; Kennel &TOLEDO-REDONDO ET AL.Geophysical Research LettersPetschek, 1966)
EMIC waves are thought to grow at parallel wave normal angles and exhibit left-handed polarization (LHP), but it is common to observe them propagating with large θBk, and this is associated with a departure from LHP (e.g., Allen et al, 2015; Min et al, 2012)
The solver indicates that the wave is not strongly damped at the observed k vector, the maximum growth rate is expected for k close to parallel, suggesting that the observation may be out of the source region
Summary
Electromagnetic ion cyclotron (EMIC) waves are generated in various regions of the Earth's magnetosphere when hot (keV to tens of keV) ions have T⊥ > T‖ EMIC waves are thought to grow at parallel wave normal angles (θBk) and exhibit left-handed polarization (LHP), but it is common to observe them propagating with large θBk, and this is associated with a departure from LHP (e.g., Allen et al, 2015; Min et al, 2012). Anderson et al (1992) observed that most EMIC waves in the dawn-sector exhibited small ellipticity that could not be explained only by propagation near the crossover frequency along a magnetic field gradient. Hu et al (2010) showed, using 2.5D hybrid simulations, that the waves could be generated at oblique angles, in particular when there is a small amount of heavy ions and a large amount of cold protons, in addition to hot anisotropic protons which provide the energy source. The cold protons have a gyroradius well below the observed perpendicular wavelength, allowing them to remain frozen-in and follow the fluctuations imposed by the slowly varying fields of the waves, exchanging energy adiabatically and favoring wave propagation at oblique angles
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