Generation of ELF electromagnetic waves in the ionosphere by localized transverse dc electric fields: Subcyclotron frequency regime

Abstract

Abstract. It is commonly believed that Alfv•nic waves observed in the ionosphere originated at trans ionospheric altitudes. However, recent observations of waves localized over skin depth scales, which are prone to Landau damping, and upward going Poynting flux suggest an ionospheric source. This theoretical study establishes the generation of electromagnetic waves at subcyclotron frequencies by localized static electric fields of the type commonly observed in the auroral ionosphere. The problem is formulated in terms of an eigenvalue system of equations which can describe all cold plasma normal modes, ion acoustic waves, and the various mode couplings induced by nonuniform electric fields. It is found that the veloc.i.ty shear associated with the background electric field can significantly affect the observable properties of Alfv•n waves. In particular, Alfv•n waves can be destabilized when the magnitude of the velocity shear frequency exceeds the ion cyclotron frequency. Velocity shear can also significantly modify the ratio of E/B for these waves. The analysis further reveals electromagnetic Kelvin-Helmholtz instabilities, which are non-Alfv•nic, with lower velocity shear thresholds. We model conditions encountered by several rockets and satellites, for example, FAST, Freja, AMICIST, AT2, to analyze wave properties and find that for typical ionospheric parameters the KelvinHelmholtz instabilities can generate electromagnetic waves with broadbanded spectra and other physical characteristics similar to observations. It is also shown that these waves are capable of resonant interaction with electrons over localized regions and hence may be important to understanding the generation of suprathermal electron bursts.

Low frequency oscillations in a plasma with spatially variable field-aligned flow

“…The model will have to be generalized to include electromagnetic effects. However, our experience with electromagnetic effects associated with transverse flow 55,56 indicates that while the modes have magnetic signatures associated with them, the electrostatic model can explain the bulk of the physics at smaller scales. We also note that recent statistical study of solar plasmas indicate the existence of multiple harmonics of electromagnetic ion cyclotron waves in the solar plasma environment.…”

Generation of electromagnetic ion cyclotron waves in the ionosphere by localized transverse dc electric fields

“…We utilize our previous model [ Peñano and Ganguli , 2000] describing small‐amplitude electromangetic waves in the presence of a localized transverse dc electric field, E 0 = E 0 ( x ) , and a uniform magnetic field, B 0 = B 0 . Assuming linear fluctuations of the form where the wave numbers ( k y , k z ) are real while the frequency ω and amplitude A 1 ( x ) can be complex.…”

“…Assuming electrostatic fields ( = − ∇) and quasi‐neutrality (∇ · = 0), it was shown [ Peñano and Ganguli , 2000] that the same model yields the following equation [ Ganguli , 1997] for the electrostatic wave potential ϕ 1 : where α denotes the particle species and Q α is defined by with D α = η α − (ω 1 /Ω α ) 2 , η α = 1 + ∂ x V E /Ω α , ω 1 = ω − k y V E , and , where is the thermal speed. and, in the electrostatic limit, support solutions describing ion cyclotron waves driven unstable by the shear in the zero order E × B flow.…”

“…Earlier theoretical [ Ganguli et al , 1988; Gavrishchaka et al , 1996] and experimental [ Amatucci , 1999] investigations have shown that such dc fields are capable of generating a broadband spectrum of electrostatic ion cyclotron‐like waves. Recently, we have extended these theoretical models to the electromagnetic regime and studied unstable eigenmodes at subcyclotron frequencies [ Peñano and Ganguli , 1999, 2000]. These modes were characterized by long wavelengths parallel to the background magnetic field, i.e., when the electron response could be characterized as cold.…”

Multi-spacecraft observations of broadband waves near the lower hybrid frequency at the Earthward edge of the magnetopause

“…In addition, shear in the plasma flow usually occurs across the magnetopause. One type of generation mechanism causing broadband waves at frequencies of the order of the ion gyrofrequency depends on shear due to inhomogeneous plasma flow, either caused by changes in the flow parallel to the ambient magnetic field (Gavrishchaka et al, 1999) or by inhomogeneous perpendicular flows (Amatucci, 1999;Koepke et al, 1999;Peñano and Ganguli, 2000). Recent investigations of Freja data show that such perpendicular shear may well contribute to the generation of waves at altitudes around 2000 km in the auroral region (Hamrin et al, 2001).…”