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Abstract. High temporal resolution electron detectors aboard the PHAZE II rocket flight have shown that the energy-dispersed, field-aligned bursts (FABs) are time coincident with pitch angle-dispersed electrons having energies at the maximum voltage of the inverted-V potential. This modulation of the energetic inverted-V electrons is superimposed upon an energy-diffused background resulting in a peak-to-valley ratio of -2 for the pitch angle-dispersed electrons. Since the characteristic energy of the FABs, the order of an eV, is considerably less than that of the plasma sheet electrons (the order of a keV) presumably falling through the inverted-V potential to create the discrete aurora, the modulation mechanism has to be independent of the electron temperature. The mechanism must accelerate the cold electrons over a range of energies from the inverted-V energy down to a few tens of eV. It must do this at the same time it is creating a population of hot, pitch angle-dispersed electrons at the inverted-V energy. Both the energy dispersion of the FABs and the pitch angle dispersion of the inverted-V electrons can be used to determine a source height assuming both populations start from the same source region at the same time. These calculations give source heights between 3500 and 5300 km for various events and disagreement between the two methods the order of 20%, which is within the rather substantial error limits of both calculations. A simple mechanism of providing a common start time for both populations of electrons would be a turning on/off of a spatially limited (vertically), inverted-V potential. The energy-dispersed FABs can be reconstructed at rocket altitudes if one assumes that cold electrons are accelerated to an energy determined by how much of the inverted-V potential they fall through when it is turned on. Similarly, the pitch angle-dispersed, inverted-V electrons can be modeled at rocket altitudes if one assumes that the plasma sheet electrons falling through the entire potential drop all start to do so at the same time when the potential is turned on. The FABs seem to fluctuate at either -10 Hz or near 100 Hz. An important constraint of the on/off mechanism is whether cold electrons (1 eV) can fill the inverted-V volume during the off cycle. The maximum vertical height of the 10 kV potential region for the 10 Hz events would be the order of 100 and 10 km for the 100 Hz events. To get 10 kV, these heights require parallel electric fields of 0.1 and 1 V/m respectively for the 10 and 100 Hz events assuming that the filling is along B from below the inverted-V potential. Alternative mechanisms are also discussed in the light of the data presented. IntroductionIt is generally accepted that there is parallel electric field acceleration of auroral electrons occurring at altitudes the order of 1 R E. The FAST spacecraft at altitudes of -4000 km is either below or within an upward directed electric field region responsible for the acceleration of the electrons producing the discrete aurora within the upw...

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.

Very short duration, large amplitude bursts of monochromatic waves (“spikelets”) were detected by the electric field experiment on the sounding rocket MARIE, launched in February 1985 from Churchill, Manitoba. About 35 events were detected, with an average time scale of 5 ms and an average amplitude of 100–150 mV/m. Their frequency varied between 7 and 18 kHz, and there is some evidence that the frequency is a decreasing function of altitude. The bursts are not correlated with any events on the payload, and their occurrence is not related to the rocket's spin or coning. The events were confined to the altitude range 450–650 kilometers. This coincides exactly with the altitude range for which perpendicular (90°) ion conics were detected by the particle experiment on the same payload. The “spikelets” were also correlated one‐to‐one with small (10–100 mV/m) double‐layerlike or shocklike features of similar time scale in the dc electric field data.