Inhomogeneities of plasma density and electric field as sources of electrostatic turbulence in the auroral region

Abstract

Inhomogeneities of plasma density and non-uniform electric fields are compared as possible sources of a sort of electrostatic ion cyclotron waves that can be identified with broadband extremely low frequency electrostatic turbulence in the topside auroral ionosphere. Such waves are excited by inhomogeneous energy-density-driven instability. To gain a deeper insight in generation of these waves, computational modeling is performed with various plasma parameters. It is demonstrated that inhomogeneities of plasma density can give rise to this instability even in the absence of electric fields. By using both satellite-observed and model spatial distributions of plasma density and electric field in our modeling, we show that specific details of the spatial distributions are of minor importance for the wave generation. The solutions of the nonlocal inhomogeneous energy-density-driven dispersion relation are investigated for various ion-to-electron temperature ratios and directions of wave propagation. The relevance of the solutions to the observed spectra of broadband extremely low frequency emissions is shown.

Influences of shear in the ion parallel drift velocity and of inhomogeneous perpendicular electric field on generation of oblique ion acoustic waves

“…Since different (i.e., electrostatic ion cyclotron and oblique ion acoustic) shear modified modes have broadband spectra and may be a part of broadband electrostatic noise, the question arises of relative significance of these modes. We note that the growth rate of shear modified ion cyclotron mode obtained by Ilyasov et al [] and that of ion acoustic mode calculated in the present study are close to each other, so that from this point of view the contributions of the two modes to broadband electrostatic turbulence are expected to be similar. However, it is known that for instance, shear modified ion cyclotron mode is damped in some configurations with special distributions of inhomogeneous electric field and field‐aligned current [ Golovchanskaya et al , ].…”

“…It can be represented in the matrix form: where h is the vector composed of the coefficients B j and , the matrix M is defined by matching and boundary conditions. Exact expression of matrix M , boundary and matching conditions, and computational methods are described in detail in our article [ Ilyasov et al , ].…”

“…Here the model parabolic profile of electric field is used. It was shown in the studies [ Ilyasov et al , ; Chernyshov et al , ] that the detailed form of distributions of electric field, as well as of plasma density, is of minor importance for instability generation. Parameters of the background plasma are the following: τ = 0.5, u = 0.1, and ρ i =20 m. As one can see from the plots, unstable solutions are localized at small frequencies.…”

“…In the nonlocal formulation, the oblique ion acoustic waves are not excited in the domain where ω r >0.13Ω i . As in the case of ion cyclotron waves, the growth rate of instability increases with parameter b [ Ilyasov et al , ], but the difference in the growth rate of ion acoustic waves for small and large values of b is less pronounced. It is interesting to note that the ion acoustic waves are excited in a wide range of frequencies and there is no clear maximum at any frequency.…”

Laboratory generation of broadband ELF waves by inhomogeneous plasma flow

“…This is an interesting result because the previous attempts could not produce such low‐frequency IEDDI, although it was known theoretically to be possible. By extension, the observations made by FAST, Freja, and THEMIS satellites might be explainable in terms of the reported experimental results [ Golovchanskaya et al , ; Ilyasov et al , ; Tang et al , ]. The broad bandwidth of these modes ensure that the resonance condition is easier to meet.…”