Cavitating Langmuir Turbulence in the Terrestrial Aurora

Langmuir cavitons have been artificially produced in Earth’s ionosphere, but evidence of naturally occurring cavitation has been elusive. By measuring and modeling the spectra of electrostatic plasma modes, we show that natural cavitating, or strong, Langmuir turbulence does occur in the ionosphere, via a process in which a beam of auroral electrons drives Langmuir waves, which in turn produce cascading Langmuir and ion-acoustic excitations and cavitating Langmuir turbulence. The data presented here are the first direct evidence of cavitating Langmuir turbulence occurring naturally in any space or astrophysical plasma.

Figure 1

Background ionospheric parameters measured during the most prominent auroral event. The panels show, from top to bottom, electron density, electron temperature, ion temperature, and vertical ion velocity (positive indicates motion away from the observer) at 1-min time resolution. Typical auroral plasma signatures can be seen, namely, a sharp increase in electron density localized in time and space, a corresponding increase in electron temperature, localized small increases in ion temperature, and, in the velocity plot, high-altitude ion outflow to space [28, 29].

Figure 2

Incoherent scatter intensity profiles from up- and down-going Langmuir and ion-acoustic waves recorded during the auroral event discussed in Fig. 1. Four distinct sources of scattering can be identified. (i) The dark background in the ion-acoustic channel is backscatter from thermal-level waves. (ii) The relatively faint bands in the down-going and, more prominently, up-going Langmuir channels correspond to 3 and 5 MHz Langmuir waves weakly enhanced by low energy, direct and backscattered diffuse electron precipitation associated with the aurora. (iii) The repeated 10-s-long enhancements seen in all three channels at about 225 and 125 km before and after 18:18:30, respectively, are backscatter from waves enhanced by experimental 4.04-MHz high-power radio-wave transmissions [17]. (iv) The intense features occurring between 18:18:30 and 18:20:30 UT near 300 km and at 18:23:30 UT near 250 km are backscatter associated with the aurora. The top edges of the Langmuir enhancements give the approximate altitudes of the enhanced backscatter. Uncoded 420- and $25\mathrm{\text{−}}\mu \mathrm{s}$ pulses were used to measure the Langmuir and ion-acoustic backscatter, respectively.

Figure 3

Power spectral densities of the naturally enhanced ion-acoustic backscatter showing enhanced shoulders and enhanced central peaks at 2-kHz frequency and 3.75-km range resolution. A $475\mathrm{\text{−}}\mu \mathrm{s}$ phase-coded pulse with a $25\mathrm{\text{−}}\mu \mathrm{s}$ baud was used. The change in enhancement height with time is a result of the vertical pointing of the radar, while the auroral structure is oriented along the geomagnetic field, tilted 13° south of vertical, and drifting south; this effect can also be seen in Fig. 2. The progression from higher to lower heights with time corresponds to a drift velocity of about $80\mathrm{m}/\mathrm{s}$.

Figure 4

Results from a 1D simulation made for plasma parameters matching those during this observation and with a downward-going beam of electrons at beam energies of 18, 42, 75, and 170 eV. The beam creates a bump-on-tail distribution which excites Langmuir waves according to the resonance condition $v_b\approx {\lambda }_{\mathrm{L}}{f}_{\mathrm{L}}$, where $v_b$ is the beam velocity and ${\lambda }_{\mathrm{L}}$ and $f_{\mathrm{L}}$ are the Langmuir wavelength and frequency, respectively. The spectra of up- and down-going Langmuir and ion-acoustic waves were calculated for a wavelength of 0.67 m, matching the Bragg backscatter condition of the radar. Both the precipitating electron beam and the thermal background were included in the driving terms. The beam velocity spread ratio $\Delta v_b/v_b$ is 0.3 in all cases, where $\Delta v_b$ is the velocity spread. The beam density ratio $n_b/n_p$ is $2\times {10}^{-5}$, where $n_b$ is the beam density and $n_p$ is the density of the surrounding plasma, $3\times {10}^{11}{\mathrm{m}}^{-3}$, as determined from the 5-MHz frequency of the enhanced Langmuir modes. The electron collision frequency is $200{\mathrm{s}}^{-1}$. These parameters are well within the ranges known to occur in the auroral ionosphere [19]. The peak value of each spectrum is given at the top right of each panel. The Langmuir spectra are shown on identical log scales with arbitrary but equal reference levels and with a minimum value ${10}^{-6}$ that of the maximum; the ion-acoustic spectra are shown on a linear scale in arbitrary units with a minimum value of zero.