Model of electromagnetic ion cyclotron waves in the inner magnetosphere
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Abstract The evolution of He + ‐mode electromagnetic ion cyclotron (EMIC) waves is studied inside the geostationary orbit using our global model of ring current (RC) ions, electric field, plasmasphere, and EMIC waves. In contrast to the approach previously used by Gamayunov et al. (2009), however, we do not use the bounce‐averaged wave kinetic equation but instead use a complete, nonbounce‐averaged, equation to model the evolution of EMIC wave power spectral density, including off‐equatorial wave dynamics. The major results of our study can be summarized as follows. (1) The thermal background level for EMIC waves is too low to allow waves to grow up to the observable level during one pass between the “bi‐ion latitudes” (the latitudes where the given wave frequency is equal to the O + –He + bi‐ion frequency) in conjugate hemispheres. As a consequence, quasi‐field‐aligned EMIC waves are not typically produced in the model if the thermal background level is used, but routinely observed in the Earth's magnetosphere. To overcome this model‐observation discrepancy we suggest a nonlinear energy cascade from the lower frequency range of ultralow frequency waves into the frequency range of EMIC wave generation as a possible mechanism supplying the needed level of seed fluctuations that guarantees growth of EMIC waves during one pass through the near equatorial region. The EMIC wave development from a suprathermal background level shows that EMIC waves are quasi field aligned near the equator, while they are oblique at high latitudes, and the Poynting flux is predominantly directed away from the near equatorial source region in agreement with observations. (2) An abundance of O + strongly controls the energy of oblique He + ‐mode EMIC waves that propagate to the equator after their reflection at bi‐ion latitudes, and so it controls a fraction of wave energy in the oblique normals. (3) The RC O + not only causes damping of the He + ‐mode EMIC waves but also causes wave generation in the region of highly oblique wave normal angles, typically for θ > 82°, where a growth rate γ > 10 −2 rad/s is frequently observed. The instability is driven by the loss cone feature in the RC O + distribution function, where ∂ F / ∂ v ⟂ >0 for the resonating O + . (4) The oblique and intense He + ‐mode EMIC waves generated by RC O + in the region L ≈2–3 may have an implication to the energetic particle loss in the inner radiation belt.Keywords:
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The plasmapause is a highly dynamic boundary between different magnetospheric particle populations and convection regimes. Some of the most important space weather processes involve wave-particle interactions in this region, but wave properties may also be used to remote sense the plasmasphere and plasmapause, contributing to plasmasphere models. This paper discusses the use of existing ground magnetometer arrays for such remote sensing. Using case studies we illustrate measurement of plasmapause location, shape and movement during storms; refilling of flux tubes within and outside the plasmasphere; storm-time increase in heavy ion concentration near the plasmapause; and detection and mapping of density irregularities near the plasmapause, including drainage plumes, biteouts and bulges. We also use a 2D MHD model of wave propagation through the magnetosphere, incorporating a realistic ionosphere boundary and Alfvén speed profile, to simulate ground array observations of power and cross-phase spectra, hence confirming the signatures of plumes and other density structures.
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Enhancements of convection electric fields during two substorms have been analyzed using CRRES satellite data measured in the premidnight inner magnetosphere. The electric field, related to subauroral polarization streams (SAPS), begins to increase within 30 sec after the substorm onset, indicating a quicker response of convection in the inner magnetosphere to substorms than has been reported (∼10 min) before. A prompt response of the ion pressure and the following decrease in the cold plasma density supports the fact that the electric field enhances just after the substorm onset and drives accelerations of energetic ions and plasmapause erosion. The SAPS electric field enhances between the earthward edges of the ring current and plasmasheet, and the plasmapause coincides with the earthward edge of the electron plasmasheet. The plasmapause location deviates from the stagnation point, and the SAPS electric field penetrates into the plasmasphere, driving a sunward plasma drift of the plasmaspheric plasma.
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Both the solar wind and the ionosphere contribute to Earth's magnetospheric plasma environment. However, it is not widely appreciated that the plasmasphere is a large reservoir of ionospheric ions that can be tapped to populate the plasma sheet. We employ empirical models of high‐latitude ionospheric convection and the geomagnetic field to describe the transport of outer plasmasphere flux tubes from the dayside, over the polar cap and into the magnetotail during the early phases of a geomagnetic storm. We calculate that this process can give rise to high densities of cold plasma in the magnetotail lobes and in the near‐Earth plasma sheet during times of enhanced geomagnetic activity, and especially during storms. This model can help explain both polar cap ionization patches and the presence of cold flowing ions downtail.
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Effects of northward IMF on the structure of the magnetosphere were studied by using a global MHD model. The model suggests a mechanism for creating high latitude sunward convection. The mechanism is not due to magnetic merging over the cusp region. Instead, the model indicates that the northward IMF field lines, which move around the magnetosphere, tend to squeeze the magnetotail at the boundary. This leads to formation of vortex flows in the tail and the observed sunward convection in the polar cap.
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