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

Abstract

It is well known that the broadband electrostatic turbulence observed in the topside auroral ionosphere can be identified with electrostatic ion cyclotron and/or oblique ion acoustic waves. Under certain conditions generation of the ion cyclotron modes is inhibited, so that the oblique ion acoustic waves become the prevailing part of the broadband noise. While generation of ion cyclotron waves by the inhomogeneous distribution of energy density (IEDD) instability has been actively studied in recent years, much less attention was paid to the excitation of ion acoustic waves by means of the IEDD instability. In this work, influence of shear in the ion parallel drift velocities and of inhomogeneous perpendicular electric field on generation of nonlocal oblique ion acoustic mode is studied. It is demonstrated that the shear of the ion parallel drift velocities can generate ion acoustic waves. It is shown that this mechanism of instability development provides broadband spectrum in the frequency range around 0.1 of ion gyrofrequency, and thus, this instability can be invoked to explain the observed broadband electrostatic turbulence in the auroral region. Effect of the main background plasma parameters on excitation of oblique ion acoustic waves is analyzed.

Studies of small-scale plasma inhomogeneities in the cusp ionosphere using sounding rocket data

“…26 and 27, and references herein). Numerical stability analysis using FAST or FREJA satellite data was carried out in recent papers, 20,21,[28][29][30] and it was confirmed that the IEDD instability could play an important role in the creation of electrostatic broadband turbulence (or noise) in the auroral zone and explain the experimental observations.…”

“…17,[19][20][21]. It was shown that inhomogeneous structures of electric field and field-aligned current produce broadband spectra for several wave modes such as EIC 21 and IA 30 waves. The role of the IEDD instability could also be broader than the production of broadband waves in the auroral zone.…”

“…For the ion velocity, we adopt V di ðnÞ ¼ 0:02V de ðnÞ, which is approximately consistent with the ratio of ion mobility to electron mobility in the Earth's ionosphere. 30 Since it appeared that IEDD waves are destabilized more readily for Oþ ions, that is, for a wider range of the input parameters, 56 we present the results for this particular species.…”

“…The IEDD instability can excite several types of waves, such as EIC and IA waves. 19,20,28,30 An essential property of the IEDD instability is the broadband spectrum, so the application of the IEDD instability mechanism is of particular interest for ionospheric problems related to observations of broadband electrostatic waves (e.g., Refs. 16 and 27).…”