Cavitating Langmuir Turbulence in the Terrestrial Aurora

Abstract

Langmuir cavitons have been artificially produced in the 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.PACS numbers: 94.05. Lk, 94.05.Pt, 94.05.Fg, 94.20.wj Langmuir turbulence is known to occur in controlled laboratory [1,2] and space plasma experiments [3][4][5] and is thought to occur naturally in a variety of space and astrophysical plasmas, including pulsar magnetospheres [6], the solar corona [7], the interplanetary medium [8], planetary foreshocks [9], the terrestrial magnetosphere [10], and the ionosphere [11][12][13]. In its most developed form, this turbulence contains electron Langmuir modes trapped in dynamic density depressions known as cavitons [14][15][16]. Cavitons have been shown to be artificially produced in the earth's ionosphere during high-power radiowave pumping experiments as deduced from radar spectra containing simultaneously-excited up and downshifted Langmuir and ion-acoustic lines plus a central peak due to cavitation [3][4][5], but evidence of naturally occurring cavitation has until now been elusive.Between 18 and 21 UT on 11 and 12 November 1999 a measurement program designed to detect both ionacoustic and Langmuir modes was run on the European Incoherent Scatter Scientific Association (EISCAT) 224-MHz radar located near Tromsø in northern Norway (local standard time in Norway is UT plus one hour). The principal objectives were to observe enhanced waves stimulated by high-power radiowave pumping, and, in the event of auroral activity, to gather data on natural energetic waves [17]. On both nights conditions were disturbed, and enhanced echoes were detected, the strongest being on 11 November between 18:18:30 and 18:21:30 UT, during the passage of an aurora through the vertically-directed radar beam. Fig. 1 presents parameters derived from the ion-acoustic backscatter between 18:15 and 18:28 UT, during the most intense auroral event. Fig. 2 shows the intensities of Langmuir and ion-acoustic backscatter as a function of height and time. The prominent features occurring between 18:18:30 and 18:20:30 UT and at 18:23:30 UT near 300 and 250 km altitude, respectively, are backscatter associated with the aurora, and are the most energetic natural events observed on either night. Two other events occurred later that evening and two more on the following evening. Weak ion-acoustic enhancements occurred during each event; the Langmuir enhancements, however, are always stronger. Fig. 3 shows up and down-shifted spectral lines, or "shoulders", which are produced by Doppler-...

Enhanced EISCAT UHF backscatter during high-energy auroral electron precipitation

“…The observations show NEIAL with a feature at zero Doppler shift from a limited-altitude region and simultaneously enhanced plasma lines (see also Isham et al, 2012). Geometrical effects need to be considered for these observations as the VHF radar was pointing vertically during their experiment.…”

“…Rietveld et al, 1991;Forme, 1999;Sedgemore-Schulthess and St. Maurice, 2001;Isham et al, 2012). Another model recently proposed by Ekeberg et al (2010) describes NEIAL observations from limited-altitude regions with almost flat ion line spectra, i.e.…”

Modulation of Observed Thomson Scattering Spectra in a Plasma Density Irregularity

“…At the same time, we suppose that the plasma wave damping is heavy in a larger interval (0, d). This condition means that the plasma wave trapping effect [10][11][12] will be negligible. Therefore, under these conditions the dependence of δ n 2 on ω in Eq.…”

“…The most significant influence of the plasma irregularities on the plasma density fluctuation spectrum takes place in plasma density cavities where the Langmuir waves can be trapped, enhanced, and acquire a discrete frequency spectrum [10][11][12]. These effects depend essentially on the collision frequency of electrons and the spatial scale of the irregularities.…”

Weak Langmuir optical turbulence in a fiber cavity

“…Furthermore, evidence of cavitating Langmuir turbulence has been recently shown to occur in natural Earth's aurora driven by solar wind [71]. To our knowledge however, besides preliminary experiments in [49], a clear experimental signature of weak Langmuir turbulence has not been reported in the context of nonlinear optics.…”