Enhanced ion acoustic lines due to strong ion cyclotron wave fields

Abstract

Abstract. The Fast Auroral Snapshot Explorer (FAST) satellite detected intense and coherent 5-20 m electric field structures in the high-latitude topside auroral ionosphere between the altitudes of 350 km and 650 km. These electric fields appear to belong to electrostatic ion cyclotron (EIC) waves in terms of their frequency and wavelengths. Numerical simulations of the response of an electron plasma to the parallel components of these fields show that the waves are likely to excite a wave-driven parallel ion acoustic (IA) instability, through the creation of a highly non-Maxwellian electron distribution function, which when combined with the (assumed) Maxwellian ion distribution function provides inverse Landau damping. Because the counter-streaming threshold for excitation of EIC waves is well below that for excitation of IA waves (assuming Maxwellian statistics) our results suggest a possible two step mechanism for destabilization of IA waves. Combining this simulation result with the observational fact that these EIC waves share a common phenomenology with the naturally enhanced IA lines (NEIALS) observed by incoherent scatter radars, especially that they both occur near field-aligned currents, leads to the proposition that this two-step mechanism is an alternative path to NEIALS.

Enhanced EISCAT UHF backscatter during high-energy auroral electron precipitation

“…Observations by Michell and Samara (2010) and Ekeberg et al (2010) indicate the presence of high electric fields during the reported events as well. Bahcivan and Cosgrove (2008) suggested a mechanism for ion cyclotron waves, in the vicinity of arcs with strong electric fields and shear, to excite enhanced levels of ion acoustic fluctuations and produce enhanced backscatter in incoherent scatter experiments.…”

“…spectrally uniform radar power enhancements, caused by increased incoherent backscatter cross section driven by ionsolitary waves. Bahcivan and Cosgrove (2008) proposed a model for NEIALs based on observations of ion cyclotron waves in the vicinity of arcs with strong electric fields observed with the FAST satellite. In this model the ion acoustic instability is driven by the ion cyclotron waves.…”

Analysis of beam plasma instability effects on incoherent scatter spectra

“…Another different mechanism to enhance the ion acoustic line is proposed by Forme (1993), where through a two step wave-wave interaction process a beam driven instability generates growing Langmuir waves that can decay into ion acoustic waves and opposite propagating Langmuir waves. A more recent theory proposed by Bahcivan and Cosgrove (2008) postulates that ion acoustic waves can be destabilized through a twostep process, where Electrostatic Ion Cyclotron (EIC) waves can produce non-Maxwellian electron distribution function, which creates inverse Landau damping (assuming the ions are Maxwellian). Only the Langmuir decay (Forme, 1993) and the EIC-electron distribution interaction (Bahcivan and Cosgrove, 2008) models can explain the simultaneous enhancement of both shoulders in the ion acoustic line.…”

“…A more recent theory proposed by Bahcivan and Cosgrove (2008) postulates that ion acoustic waves can be destabilized through a twostep process, where Electrostatic Ion Cyclotron (EIC) waves can produce non-Maxwellian electron distribution function, which creates inverse Landau damping (assuming the ions are Maxwellian). Only the Langmuir decay (Forme, 1993) and the EIC-electron distribution interaction (Bahcivan and Cosgrove, 2008) models can explain the simultaneous enhancement of both shoulders in the ion acoustic line.…”

Auroral ion acoustic wave enhancement observed with a radar interferometer system

“…The enhanced backscatter can reach several orders of magnitude above the thermal backscatter and is thought to occur due to enhanced levels of ion acoustic wave activity (e.g., Sedgemore-Schulthess and St. Maurice, 2001). Explanations of the physical cause of NEIALs include streaming instabilities (Rietveld et al, 1991;Wahlund et al, 1992), decay from electrostatic ion-cyclotron waves (Bahcivan and Cosgrove, 2008), and Langmuir turbulence (e.g., Forme, 1993;Guio and Forme, 2006) driven by electron beams. Observations of NEIALs reported in recent years favors the Langmuir turbulence as a model to describe the echoes.…”