Parametric instabilities of Langmuir waves observed by Freja

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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.