Parametric instabilities of Langmuir waves observed by Freja

Abstract

We have analyzed complete spectrum of waves (0–4 MHz) and electron distributions during events of modulated Langmuir waves observed by Freja in the topside polar ionosphere. Modulated Langmuir waves are observed with amplitudes 1–1000 mV/m in association with beams of electrons with energies 50–5000 eV streaming through cold ionospheric plasma. We find overwhelming evidence for interactions of Langmuir (L) waves with lower hybrid (LH) waves. The interactions are consistent with parametric decay of beam induced L waves (L ↝ L′ + LH) as well as with scattering of L waves on preexisting LH waves (L + LH ↝ L′). These interactions are observed at all wave amplitudes ∼ 1 – 1000 mV/m but are sensitive to the k vectors of the primary waves (i.e., electron beam energy). In association with modulated Langmuir waves we also observe ion acoustic‐like signals which appear to be produced by rectification of the high‐frequency waves in the probe sheath through nonlinearities in the current‐voltage characteristic. The Langmuir wave events occur frequently in localized regions which are usually related to the phase of magnetic perturbation of an Alfvén wave. The observations show that Langmuir waves with amplitudes > 50 mV/m are very common in the auroral zone, and they represent the strongest electrostatic wave mode at altitudes of ∼ 1500 km. Langmuir waves appear to be an important element in the energy exchange chain in which part of the energy carried by Alfvén waves is transferred to energetic electrons and finally deposited to thermal plasma in a sequence: Alfvén wave ↝ electron beam ↝ Langmuir wave ↝ LH decays ↝ heating of thermal plasma.

Cavitating Langmuir Turbulence in the Terrestrial Aurora

“…The data presented here provide the first direct evidence of naturally occurring cavitating Langmuir turbulence, thought to be important in space and astrophysical plasmas as varied as pulsar magnetospheres and the Earth's ionosphere [6][7][8][9][10][11][12][13][14]. Further observations of Langmuir turbulence in the ionosphere may yield advances in our understanding of supra-thermal electron distributions [26], naturally-enhanced ion-acoustic waves [13,22], natural ionospheric radio emissions [27], anomalous resistivity [28,29], and auroral currents and dark aurora [18].…”

Ion dynamics in electron beam–plasma interaction: particle-in-cell simulations

“…Among them are spectra with power at ω pe only (e.g., Sigsbee et al, 2010), a double peak in absence of ion acoustic activity (e.g., Sigsbee et al, 2010), multiple spectral peaks (e.g., Graham and Cairns, 2013a), a single peak with side-band structure (e.g., Gurnett at al., 1981;Thejappa et al, 2013), Langmuir eigenmodes localized in density wells (Ergun et al, 2008;Graham and Cairns, 2013b) and waveforms resulting from the interaction of Langmuir waves with electrostatic whistler/lower hybrid waves instead of ion sound (Stasiewicz et al, 1996).…”

Vlasov simulation of Langmuir wave packets

“…Cairns and Robinson, 1992;Cairns et al, 1998;Pottelette et al, 1999;Matsukiyo et al, 2004;Soucek et al, 2005) or lower hybrid waves (e.g. Stasiewicz et al, 1996;Bonnell et al, 1997;Lizunov et al, 2001). Note that Cairns et al (1998) demonstrated that electron-beam-driven Langmuir wave packets observed in the magnetosphere lie well outside the region of parameter space for which the modulational instability can proceed.…”

“…Correspondence to: T. Umeda (umeda@stelab.nagoya-u.ac.jp) waves were also observed in various regions of the magnetosphere, such as in the auroral ionosphere (Ergun et al, 1991;Stasiewicz et al, 1996;Bonnell et al, 1997;Pottelette et al, 1999;Lizunov et al, 2001), in the solar wind (Gurnett et al, 1993;Bale et al, 1996;Kellogg et al, 1999b), in the electron foreshock region (Kellogg et al, , 1999aSoucek et al, 2005), and in the magnetotail (Kojima et al, 1997).…”