Langmuir wave structures registered by FREJA: analysis and modeling

Abstract

A large set of observations of Langmuir emissions generated by super-thermal electron beams in the topside polar ionosphere has been obtained during the recent FREJA mission. Langmuir emissions appeared as a chain of strongly modulated wave packets; in many cases the packet envelopes were modulated periodically. In present report we suggest the theory which explains these observations. We suggest that packet-like waveforms are the beatings of several Langmuir modes. One of modes (the primary mode) appears as a result of beam-plasma instability, other modes as a result of parametric decay of primary mode to the secondary Langmuir wave and lower-hybrid wave (or as a result of decay cascade). We present the quantitative theory of three-wave interaction LoL+LH which explain many details of FREJA observations.

Statistics of auroral Langmuir waves

“…Modulational instabilities and strong turbulence wave collapse might also alter the field statistics at high E, likely causing power-laws (Robinson, 1997;Cairns, 2004). Despite evidence for such processes in other auroral Langmuir wave data sets (Ergun et al, 1991b;Khotyaintsev et al, 2001), the distributions of Langmuir wave electric fields analyzed here show no evidence for such power-law deviations from pure SGT. Nevertheless, in the Appendix we consider the constraints on collapsing wave packets and find that collapse cannot be ruled out.…”

“…Indeed, Cairns and Menietti (2001) and Burinskaya et al (2000) analyse the statistics of Langmuir-like waves driven by electron beams in the more strongly magnetized plasma of the polar cap, finding agreement with SGT. On the other hand, nonlinear three-wave processes involving Langmuir and lower hybrid waves arguably cause modulation of auroral Langmuir waves observed in satellite and rocket experiments (Ergun et al, 1991b;Bonnell et al, 1997;Khotyaintsev et al, 2001;Lizunov et al, 2001).…”

Electron Energization by Alfv?n Waves: Freja and Sounding Rocket Observations

“…The observed distribution functions (Fig. 2) are almost ¢eld aligned (a < 20 ) in pitch-angle and are close to a plateau or a weak``bump in tail'' distribution.Through a beam-plasma instability they may excite Langmuir waves which would quasilinearly thermalize the beam [12]. On the other hand the perpendicular temperature of this population is about several eV and remains constant.…”

Magnetic field-aligned plasma currents in gravitational fields

“…10-20 % of the ion sound speed, C s . At higher altitudes, 1500 km for instance, the plasma density decreases to approximately 5 × 10 8 m −3 , as observed by the Freja satellite (Khotyaintsev et al, 2001), and the electron flow velocity has to increase correspondingly in order to maintain a constant net vertical current.…”