Model of electromagnetic ion cyclotron waves in the inner magnetosphere

Abstract

Citation:Gamayunov, K. V., M. J. Engebretson, M. Zhang, and H. K. Rassoul (2014) 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. 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.

Predicting electromagnetic ion cyclotron wave amplitude from unstable ring current plasma conditions

“…Inside geosynchronous orbit, enhanced EMIC waves are most frequently observed in the afternoon sector in geomagnetically active times and are believed to be associated with enhanced cold plasma densities near the plasmapause [ Fraser and Nguyen , ; Jordanova et al , ] or in plasmaspheric plumes [ Jordanova et al , ; Chen et al , ]. EMIC waves can grow from background fluctuations [ Gamayunov et al , ] and be amplified to an observable level as they propagate through a source region close to the equator [ Loto'aniu et al , ].…”

“…Inside geosynchronous orbit, enhanced EMIC waves are most frequently observed in the afternoon sector in geomagnetically active times and are believed to be associated with enhanced cold plasma densities near the plasmapause [Fraser and Nguyen, 2001;Jordanova et al, 2001] or in plasmaspheric plumes [Jordanova et al, 2007;Chen et al, 2014a]. EMIC waves can grow from background fluctuations [Gamayunov et al, 2014] and be amplified to an observable level as they propagate through a source region close to the equator [Loto'aniu et al, 2005]. Due to their low-frequency long-wavelength nature ( ≲ Ω i , kc∕ pi ≲ 1), EMIC waves can resonate with ring current ions (∼keV) and relativistic electrons (in the reference frame of relativistic electrons, the electromagnetic fields of EMIC waves are right-hand polarized) so that they play an important role in precipitation of ring current ions and MeV electrons [Jordanova et al, 2008].…”

Propagation of EMIC Waves Inside the Plasmasphere: A Two‐Event Study

“…Therefore, the observed right-hand He + band waves cannot originate from some left-hand H + band waves near the equatorial plane in the same L-shell. As the orientations of Poynting fluxes are bidirectional in Region C, one reasonable source region of the waves is inside Region C. Based on the observed hot proton fluxes, we have calculated the local growth rates, and proven that no right-hand waves can be generated by hot protons in Region C. In addition to hot proton, hot helium, and hot oxygen (Gamayunov et al, 2014;Lee & Lee, 2016) can also generate EMIC waves. However, ECT⧵MagEIS does not have the observations of He + and O + , and the ECT⧵HOPE flux data maybe inadequate for the calculation of growth rates.…”

Generation of He+ and O+ EMIC waves by the bunch distribution of O+ ions associated with fast magnetosonic shocks in the magnetosphere

“…However, the growth rates of He + and O + waves are nearly zero for Case B and of the order of 10 À 3 Ω H for Case A. It is noted that Gamayunov et al [2014] examined the generation of He + EMIC waves by loss-cone distribution of O + ions. In the circumstances that the density of heavy ions is comparable to that of protons, e.g., in the plasmasphere, the H + EMIC waves may be suppressed and waves in the heavy ion bands may prevail [Kozyra et al, 1984;Price et al, 1986;Thorne and Horne, 1993].…”

“…Moreover, the observed He + and O + waves exhibit medium intensity with quasiperpendicular propagation (θ k > 45°) and low wave ellipticity (|ε| ≤ 0.3) [Min et al, 2012;Saikin et al, 2015], where the wave ellipticity is defined as the ratio of minimum to maximum magnetic fluctuation in the plane perpendicular to wave normal. It is noted that Gamayunov et al [2014] examined the generation of He + EMIC waves by loss-cone distribution of O + ions.…”