Low-frequency electrostatic waves in the ionospheric E-region: a comparison of rocket observations and numerical simulations (original) (raw)
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Structure functions and intermittency in ionospheric plasma turbulence
Nonlinear Processes in Geophysics, 2008
Low frequency electrostatic turbulence in the ionospheric E-region is studied by means of numerical and experimental methods. We use the structure functions of the electrostatic potential as a diagnostics of the fluctuations. We demonstrate the inherently intermittent nature of the low level turbulence in the collisional ionospheric plasma by using results for the space-time varying electrostatic potential from two dimensional numerical simulations. An instrumented rocket can not directly detect the one-point potential variation, and most measurements rely on records of potential differences between two probes. With reference to the space observations we demonstrate that the results obtained by potential difference measurements can differ significantly from the one-point results. It was found, in particular, that the intermittency signatures become much weaker, when the proper rocket-probe configuration is implemented. We analyze also signals from an actual ionospheric rocket experiment, and find a reasonably good agreement with the appropriate simulation results, demonstrating again that rocket data, obtained as those analyzed here, are unlikely to give an adequate representation of intermittent features of the low frequency ionospheric plasma turbulence for the given conditions.
Spectral properties of low-frequency electrostatic waves in the ionospheric E region
Journal of Geophysical Research: Space Physics, 2000
The spectral properties of low frequency electrostatic waves in the polar cap E region over northern Scandinavia were studied experimentally by instruments on the Rocket and Scatter Experiment (ROSE) rockets. Fluctuations in plasma density were detected as well as potential differences between boom-mounted probes. By comparison of the spectral index for fluctuations in the potential signal and plasma density, evidence is obtained for deviations from Boltzmann distributions in the electron dynamics, which would predict fluctuations in density and potential to be proportional, with the same constant of proportionality at all frequencies. Investigations of the cross correlation between density and potential signals demonstrate that the phase between the two increases approximately linearly with frequency. Empirical relations are obtained for the frequency dependence of the amplitude and phase relations between fluctuations in density and potential. 1. Introduction Low-frequency electrostatic fluctuations are often observed in the ionospheric E region and have been extensively studied experimentally and theoretically. In a linearized analysis the growth rate of these fluctuations has, in general, contributions from a gradient and an electrojet current term, the latter giving rise to the Farley-Buneman, or "two-stream," instability [Farley, 1963; Buneman, 1963]. Provided the scalar product of the density gradient and the dc electric field is positive, a gradient instability can be excited without any significant threshold value for the electric field strength, while the excitation of the two-stream waves requires the dc electric field to exceed intensities of the order of 20 mV/m, somewhat depending on plasma parameters. These wave types were observed first in the ionosphere by radar scattering and subsequently studied in detail by rocket-borne instruments. Although known and studied for a long time, the electrojet fluctuations present some puzzles for an interpretation and a detailed understanding. A simple linear stability analysis for homogeneous plasma conditions will always give phase velocities for the Farley-Buneman unstable waves at or above the sound speed, with velocities being close to the electron E x B drift,
Journal of Geophysical Research, 1989
We present experimental evidence on a new category of meter wavelength radio auroral irregularities characterized by a unique Doppler spectrum signature. The new echo type has been identified with the Scandinavian Twin Auroral Hadar Experiment radars which are capable of observing simultaneously full Doppler spectra from the same scattering voltune along two widely different azimuths. The new echoes were seen only by the Finland radar which observes at large angles to the electrojet flow and at times when the Norway radar observed very stable type 1 echoes with phase velocities near the ion acoustic speed. The new spectrum, named type 5, is composed of three main frequency bands, two being symmetric about zero Doppler shift near 4-450 m/s and a third always at very large phase velocities near 1000 m/s, this resulting to a characteristic asymmetry in the spectrum. The Doppler polarity of the large phase velocity component is always at negative shifts in the eastward electrojet and at positive shifts in the westward electrojet. These echoes, which are not strong and last from a few minutes to tens of minutes, were observed during strong but also moderate geomagnetic conditions but, apparently, always in the presence of large and stable electric fields. The evidence suggested that the new echoes cannot be due to direct generation by the two-stream instability and, most likely, a secondary generation mechanism is involved. Although there are some similarities with 50 MHz type 4 spectral observations, type 5 echoes have unique characteristics which we believe are sufficient to justify their classification into a different echo category. We have no satisfactory theoretical explanation for the new echoes. That the fast moving irregularities have frequencies about two times the frequency of the simultaneously observed ion acoustic waves led us to suggest that a nonlinear resonant wave-wave interaction process might be in operation under some appropriate conditions in the plasma. 1. the Doppler spectrum to characterize different irregularity types and identify the existence of different plasma instability mechanisms. Also, it is true to say that a large part example, see reviews by Greenwald et al. [1975], Fejer and Kelley [1980], Hanuise [1983], Fejer and Providakes [1987], and Haldoupis [1989]). The last few years, two newly discovered echo types (identified as type 3 and type 4) with characteristic Doppler spectra were observed so far at 50 MHz. Type 3 echoes have narrow spectra at subion acoustic velocities near the gyrofrequencies of the main ionic constituents (NO + , O2 + , O +); this suggested the possibility of electrostatic ion cyclotron waves generated by strong field aligned currents [e.g., Fejer et al., 1984; Providakes et al., 1985; Haldoupis et al., 1985]. Type 4 echoes, which are very short lived, were observed during extremely disturbed geomagnetic conditions [Haldoupis and of the existing theoretical work has been triggered from the Sofko, 1979; Fejer et al., 1986]. These echoes are associspectral measurements and in particular, the observation of ated with a distinct spectrum composed of a broad and a distinct spectrum signatures. There are several spectral studies of E region auroral irregularities in both the VHF and UHF frequency range. In spite of the diversity of auroral spectra, the bulk of the observations can be grouped into two main echo categories, labeled type I and type 2, which are reminiscent of those in the equatorial electrojet; the corresponding irregularities are believed to be generated by the combined action of Farley-Buneman two stream and gradient drift instabilities for direct and indirect meter scale wave generation (for
Statistical Studies of Low-Frequency Electrostatic Waves in the Ionospheric e Region
The spectral properties of low frequency electrostatic waves in the polar cap E region were studied experimentally by instrumented rockets. By comparison of the spectral index for fluctuations in the potential signal and plasma density, evidence is obtained for deviations from Boltzmann distributions in the electron dynamics. Investigations of the cross correlation between density and potential signals demonstrate that the phase between the two increases approximately linearly with frequency. The characteristics of the fluctuations were analyzed with particular attention to non-Gaussian effects and phase coherent mode couplings of the fluctuations. Short-time phase coherent effects are analyzed and quan- tified by means of the squared wavelet-bicoherence.
Observation of the modified two-stream plasma instability in the midlatitude E region ionosphere
Journal of Geophysical Research, 1994
We present the first clear evidence of the occurrence of the modified two stream (Farley-Buneman) instability and excitation of pure type 1 irregularities in the midlatitude ionospheric E region. The observations are made with a bistatic 50-MHz Doppler radio experiment set up recently in Crete, Greece. The system can perform high-frequency resolution coherent backscatter measurements along a fixed direction, from 3-m magnetic aspect sensitive irregularities inside a limited ionospheric volume in the E layer at the invariant geomagnetic latitude of 30.8 ø (L = 1.35). The observations presented here are from an event of backscatter characterized by large Doppler motions caused, presumably, by an impulsive electric field reaching a magnitude at least 14 mV/m. Apparently, the unusually high electron drifts along the radar viewing direction were sufficient in this case to excite pure Farley-Buneman waves. This had been manifested convincingly by the measured power Doppler spectra, which are reminiscent of the typical spectral signature of type 1 echoes observed regularly in the equatorial 50-MHz backscatter. Further, the spectral data confirmed the anticipation that in the midlatitude E region plasma exist two irregularity types corresponding to those of type 1 and type 2 echoes in the equatorial electrojet. The important difference with the equatorial results, however, is that the threshold conditions for the two-stream instability are seldom met at moderate latitudes, thus the medium is highly suitable for studying secondarily generated short-scale plasma turbulence. 1.
Propagation and dispersion of electrostatic waves in the ionospheric E region
Annales Geophysicae, 1997
Low-frequency electrostatic¯uctuations in the ionospheric E region were detected by instruments on the ROSE rockets. The phase velocity and dispersion of plasma waves in the ionospheric E region are determined by band-pass ®ltering and cross-correlating data of the electric-®eld¯uctuations detected by the probes on the ROSE F4 rocket. The results were con®rmed by a dierent method of analysis of the same data. The results show that the waves propagate in the Hall-current direction with a velocity somewhat below the ion sound speed obtained for ionospheric conditions during the¯ight. It is also found that the waves are dispersive, with the longest wavelengths propagating with the lowest velocity.
Radio Science, 1980
Two rocket payloads carrying plasma density probes with high spatial resolution have been flown in the auroral zone during active conditions. Simultaneous Wideband satellite scintillation and Chatanika incoherent scatter radar observations were made in order to study the properties of high-latitude irregularities and their effects on radio wave transmission. Unlike barium cloud striations and bottomside equatorial spread F, the observed power law dependence of the irregularities does not seem to be due to steepening of kilometer-scale structures, rather, a turbulent process seems to occur. In addition the power law indexes determined both from the probe and from the scintillation measurements indicates an in situ one-dimensional spectrum less steep than the k-2 value often reported. Both the probe and the scintillation data indicate absolute electron density fluctuations (An2•) •/2 of several times 109 m-3 during the expansion phase of an auroral substorm, with a layer thickness of several hundred kilometers. The observed S4 levels at VHF were in the range of 0.1-0.4. This level of scintillation, as well as the absolute density fluctuation levels and the power spectral density at the kilometer scale, are shown to be comparable with bottomside equatorial spread F. It is suggested that differences between the power spectral index in the present data set and the other ionospheric experiments mentioned above may be due to a highly conductive E layer and its effects upon the nonlinear evolution of irregularities. During another flight with lower magnetic activity but several bright auroral areas in the trajectory, much lower levels of absolute and relative density fluctuations were observed with a corresponding lower value for S 4. Two very sharp changes in electron density were observed (e-folding scales of 1.45 and 0.7 km) near the field line projected position of the auroral arcs. The associated density spectra were peaked at short wavelengths. The detection of very structured plasma within minutes of the poleward expansion phase of a substorm suggests that the Flayer irregularities were formed in the precipitation event. On the other hand, evidence is also presented for production or enhancement of irregularities in the presence of horizontal density gradients which suggests that plasma instabilities also play a role in the production of auroral zone irregularities. 1. INTRODUCTION New information concerning the phenomenology of equatorial and auroral scintillation has accrued from the successful operation of the Wideband satellite [Frernouw et al., 1977, 1978; Rino et al., Copyright ¸ 1980 by the American Geophysical Union. 1978]. These new data have encouraged further developments in the theory of ionospheric radio wave scintillations [Rino, 1980]. In addition, with the operation of Wideband and correlative groundbased measurements, notably the Chatanika radar, progress has begun toward understanding questions of physical mechanisms. In this paper we describe the results of rocket flights which probed the auroral 0048-6604 / 80/0506-1472501.00 491 492 KELLEY, BAKER, ULWICK, RINO, AND BARON plasma nearly simultaneous to an overpass of the Wideband satellite. Two of the rockets were launched in the midnight time period in November 1976 to altitudes of nearly 500 km. A third rocket in the series was launched at 0900 local time in March 1978 with an apogee of 320 km. All three were flown from the Poker Flat Research Range in Alaska, and the Chatanika radar was operated during each flight. In this paper we concentrate on the nighttime data. The daytime results will be discussed in a future publication. Details of the rocket payload have been presented elsewhere [Baker et al., 1978]. In brief the measurements were obtained by a 6-cm-diameter cylindrical probe extending 1 m along the forward spin axis of the rocket. The electron density of the plasma was derived by two techniques. The absolute density was determined from the RF impedance of the antenna. In addition, the variation in current to the hemispherical tip of this probe was measured while the probe was held at a fixed dc potential. The frequency response of both systems was high enough to provide measurements of electron density variations down to scale sizes of about a meter.
Journal of Geophysical Research, 2002
1] Low-frequency electrostatic fluctuations detected by the instruments on the Rocket and Scatter Experiment (ROSE) rockets in the E region over northern Scandinavia are analyzed. It is demonstrated that the time-resolved statistical properties of the fluctuations deviate significantly from those associated with a Gaussian process. The characteristics of the fluctuations are analyzed with particular attention to non-Gaussian effects and phase-coherent mode couplings of electrostatic fluctuations. Non-Gaussian effects are analyzed by means of the kernel estimator for probability density for short data segments. Short-time phase-coherent effects are analyzed and quantified by means of the squared wavelet bicoherence, which has desirable statistical properties in the frequency regions of interest. The results are compared with similar results from rocket data obtained over Søndre Strømfjord in Greenland. The results show a remarkable similarity, indicating that the observed phase coherence is a robust feature of the saturated stage of the E region instabilities.
Two different types of enhanced ion acoustic fluctuations observed in the upper ionosphere
Journal of Geophysical Research, 1995
We present European incoherent scatter observations of several events that show enhanced ion acoustic fluctuations in the altitude region 400-1500 km. The incoherently scattered spectra have one or both the ion acoustic lines enhanced, and sometimes a peak close to the zero frequency is observed. It is shown that these observations can be separated into two types depending on the surrounding physical conditions. The first type is related to strongly enhanced electron temperatures, ion outflows with large vertical velocities, an altitudinal extend of 300-700 km, and auroral arcs and precipitating particles of 100 eV to 10 keV. The spectra are in that case strongly asymmetric. The second type corresponds to shghtly enhanced electron temperatures, no ion outflows, spatial extends of 100-200 kin, and an apparent lack of precipitating particles of less than 1 keV. The corresponding spectra show mostly both the ion lines enhanced. These observations are compared with the existing interpretations. Introduction Enhanced ion acoustic fluctuations were observed for the first time in the auroral ionosphere by Foster et al. [1988a, b] with the Millstone Hill incoherent scatter radar. Thereafter, similar observations with the European incoherent scatter radar (EISCAT) have been reported by Collis et al. [1991], Rietveld et al. [1991], and Wahlund et al. [1992a, 1993]. Enhanced ion acoustic spectra have been shown to occur over a large altitude range (140-1700 kin)for UHF and VHF respectively, and to be bursty with typical timescale of less than a few tens of seconds to several minutes. Such spectra show enhancements of 1 or 2 orders of magnitude compared to the thermal level; however, much smaller values are also possible. Rietveld et al. [1991] have shown that enhanced ion acoustic lines often occurred with huge electron temperatures of about 6000 K and large fluxes of precipitating particles of less than 500 eV, responsible for the simultaneous observation of the very intense red line in the upper ionosphere. Afterward Wahlund et al. [1992a] showed that such events were inbedded in regions with strong field-aligned bulk ion outflows. The effects of enhanced ion acoustic fluctuations on incoherent radar spectra have been treated theoretically by Rosenblurb and Rostoker [1962]. They showed that such spectra can become enhanced and asymmetric if
Journal of Geophysical Research, 1989
Observations of very large poleward directed electric fields were obtained with a clustered set of instrumentation that included the Millstone Hill incoherent scatter radar, the Boston University Mobile Ionospheric Observatory, and the HILAT and Defense Meteorological Satellite Program (DMSP) F6 and F7 satellites. In this paper we concentrate on data from the Millstone Hill incoherent scatter radar which was operated on selected evenings in a rapid azimuthal scan, centered on magnetic west. The mode was designed with the express purpose of measuring line-of-sight drift velocity and electron density as a function of latitude during events with large localized electric fields. During this same period, the Defense Nuclear Agency HILAT satellite made northem hemisphere measurements every 100 min of ion drift, density, and field-aligned currents across the equatorial boundary of the auroral oval. A detailed study of optical data in this region is provided in a companion paper. On the evenings of April 20 and 21, 1985, during an intense magnetic storm (K•, > 8+), large ionospheric electric fields (E > 80 mV/m) were detected along the edge of the auroral oval with the Millstone Hill incoherent scatter radar. These constitute the first definitive incoherent scatter observations of this phenomenon. An L shell-aligned (zero order) deep trough in electron density was colocated with these large electric fields at L shells as low as L = 2.8. These data indicate that the trough develops much more quickly than present theories predict, at least near the F peak. We also report elevated ion and electron temperatures in the trough and conjecture that these may contribute to the rapid decay. We also show that the associated field-aligned currents are very weak, as predicted by Banks and Yasuhara (1978) but that it is the F region structure which dominates the conductivity gradient rather than the E region emphasized by the earlier work. We also discuss the data set in light of competing theories for the production of large electric fields and for undulations of the edge of the diffuse aurora. In particular we discuss the importance of large radial ion temperature gradients indicated by the DMSP data we present. INTRODUCrlON In situ observations of large electric fields in the premidnight, high-latitude ionosphere were first published by Srniddy et al. [1977] using S3-2 satellite data, and subsequently by Maynard et al. [ 1978], Spiro et al. [1979], and Rich et al. [1980] using satellite-borne electric field detectors or particle drift meters. These observations have shown that very large localized poleward directed electric fields form along the edge of the auroral oval with strengths as high as 350 mV/m perpendicular to the magnetic field at altitudes around 1300 km, corresponding to westward directed flows of up to 10 km/s. Maynard et al. [1980] have also detected large localized electric fields directed radially outward in the equatorial plane near L = 4. Labelle et al. [ 1988] have reported a localized intense plasma flow region at 1500 local time at similar L values. To our Copyright 1989 by the American Geophysical Union. Paper number 88JA03874. 0148-0227/89/88JA-03874505.00 knowledge no examples of coincident large localized flow and large electric fields have been reported at magnetospheric altitudes. All of the above observations have been based on satellite data which typically consist of a few minutes of data spaced hours or days apart. In mid-April 1985, a multi-instrumented experiment was conducted at the Millstone Hill incoherent scatter radar facility in Westford, Massachusetts, for the purpose of obtaining the first continuous and extensive ground-based radar and optical measurements of these large electric fields and their effects on the ionospheric and magnetospheric plasma. HILAT and Defense Meteorological Satellite Program (DMSP) satellite data were also obtained during this period, which provided the first simultaneous measurements of these large localized electric fields by both satellite and groundbased incoherent scatter radar. This paper and the companion paper by Mendillo et al. [this issue] describe the radar and optical measurements and the geophysical conditions leading to the large fields, and provide some insights into the various mechanisms involved. These data provide information on magnetosphere-ionosphere 535O PROVIDAKES ET AL.: ]:•OU)• AND OPTICAl., i¾[EASURElVi•S OF IONOSPHERIC PROCESSF.,