Electric field and plasma density measurements in the strongly driven daytime equatorial electrojet: 2. Two‐stream waves

Abstract

Both primary and secondary two‐stream (Farley‐Buneman) waves have been detected by in situ electric field and plasma density probes in the strongly driven daytime equatorial electrojet over Peru. Simultaneous Jicamarca radar observations showed strong vertical and oblique 3‐m type 1 echoes, also indicative of the two‐stream mechanism. The rocket data show the two‐stream region on the topside of the unstable layer to be situated between 103 and 111 km where the electron current was the strongest. This region was characterized by broadband plasma oscillations extending past 1 kHz in the rocket frame. Furthermore, above 106.5 km, where the electron density gradient was stable, a layer of laminarlike, horizontally propagating two‐stream waves was detected. These waves were strongest near 108 km, the altitude where previous measurements have shown the electrojet current over Peru to be strongest. The peak rocket frame frequency of these waves was 25 Hz, and the observed broadband electric field and plasma density amplitudes were 2 mV/m and 1–2% rms, respectively. The electric field amplitudes were likely attenuated by a spatial filtering effect and may have been several times higher. The data suggest that these waves had phase velocities comparable to the electron drift velocity (≃500 m/s) and peak wavelengths (2–3 m) that agree with kinetic theory predictions. Distinct vertically oriented waves which may have been generated by a mode coupling process were also observed by the rocket in this region, in agreement with the simultaneous radar observations. Below the laminar two‐stream layer, in the region of the large‐scale, gradient drift driven horizontal electric fields, secondary two‐stream waves were observed that were driven by localized δE × B plasma drifts. The amplitudes of the horizontally propagating kilometer scale waves were clearly strong enough (10–15 mV/m) to drive vertically oriented secondary two‐stream (type 1) waves, as proposed by Sudan et al. (1973) and observed by the Jicamarca radar between roughly 103 and 106 km. Below 103 km, secondary two‐stream waves could not be generated because ψ (the ratio of the ion and electron collision frequencies to their gyro frequencies) becomes large, despite the fact that the observed large‐scale wave amplitudes remained strong at the lower altitudes. Observations of “flat‐topped” waveforms of the large horizontal electric field structures suggest that secondary two‐stream phase velocities may be saturated because of a limitation of the driving electric fields rather than a process intrinsic to the two‐stream instability itself.

Ionospheric turbulence induced in the lower part of the E region by the turbulence of the neutral atmosphere

“…It should be mentioned that the observational data for large scales in the lower part of the E region obtained at various latitudes and various times show generally similar spectra [Fejer and Kelley, 1980;Pfaff et al, 1987;Rinnert, 1992]. That could be considered as the indication at the unique background of these phenomena and easily understood if the excitation of a large-scale electric field and density fluctuations is strongly affected by the neutral atmosphere turbulence, which has a unique spectrum (4).…”

Auroral <i>E</i>-region electron density gradients measured

“…The variations in the pro®les are controlled by ionization production and loss mechanisms relating to direct solar ultraviolet radiation and/or energetic particle precipitation during magnetically disturbed conditions. Photoionization-produced vertical gradients are more intense at lower latitudes, particularly at the equator where they are known to play a key role in destabilizing the equatorial electrojet plasma through the gradient drift instability Pfa et al, 1987). Also, strong vertical gradients can exist at midlatitude during sporadic E-layer conditions.…”

Evidence and effects of a wave-driven nonlinear current in the equatorial electrojet

“…This depends on the unknown, nonlinearly determined quantity, j nj. For two-stream waves in the equatorial electrojet, evidence from in situ experiments and simulations suggest that the average n=n 0 is 0 to 10 percent and magnitude of the perturbed electric field, h Ei, is similar to E 0 (see Pfaff et al, 1987b;. For gradient-drift waves, the h Ei is more difficult to estimate, though it has been measured to reach approximately 15 mV/m, corresponding to the largest measured values of E 0 (Pfaff et al, 1987a;Prakash et al, 1974).…”