Particle-in-cell simulation of incoherent scatter radar spectral distortions related to beam-plasma interactions in the auroral ionosphere (original) (raw)
Related papers
On incoherent scatter plasma lines in aurorae
Journal of Geophysical Research, 1994
We consider a new mechanism for the generation of short-scale, •Xll • 10 cm, electron density fluctuations giving rise to plasma line enhancements measured by incoherent scatter radars in the auroral E region. It is explained as a coupling (conversion) of long-scale Langrnuir waves generated by precipitated electron beams with short-scale ion-acoustic waves. The latter are supposed to be driven by field-aligned currents associated with the precipitation. This mechanism is most effective inside the thin layer of the enhanced plasma turbulence, named the plasma turbulence layer. 1. Introduction Observations of the electronic, or plasma line, component of the backscatter signal from the auroral ionosphere by matins of incoherent scatter radars (ISR) were performed many times [e.g.,
Incoherent scatter ion line enhancements and auroral arc-induced Kelvin–Helmholtz turbulence
Journal of Atmospheric and Solar-Terrestrial Physics, 2015
We present two cases of incoherent-scatter ion line enhancements in conjunction with auroral arcs drifting through the radar beam. The up-and downshifted ion line shoulders as well as the spectral region between them are enhanced equally and simultaneously. The power enhancements are one order of magnitude above the thermal level and are concentrated in less than 15 km wide altitude ranges at the ionospheric F region peak. The auroral arc passages are preceded by significantly enhanced ion temperatures in the E region, assumed to be caused by transient electric fields associated with velocity shears. We use a Hall MHD model of velocity shears perpendicular to the geomagnetic field and show that a Kelvin-Helmholtz instability will grow for the two presented cases.
Nonlinearly generated plasma waves as a model for enhanced ion acoustic lines in the ionosphere
Geophysical Research Letters, 2007
Observations from the EISCAT Svalbard Radar, for instance, demonstrate that the symmetry of the naturally occurring ion line can be broken by an enhanced, nonthermal, level of fluctuations, i.e., Naturally Enhanced Ion-Acoustic Lines (NEIALs). In a significant number of cases, the entire ion spectrum can be distorted, with the appearance of a third line, corresponding to a propagation velocity significantly below the ion acoustic sound speed. By numerical simulations, we consider one possible model accounting for the observations, suggesting that a primary process can be electron acoustic waves excited by a cold electron beam. Subsequently, an oscillating two-stream instability excites electron plasma waves which in turn decay to asymmetric ion lines. Our code solves the full Vlasov equation for electrons and ions, with the dynamics coupled through the electrostatic field derived from Poisson's equation.
Journal of Geophysical Research, 1984
Emissions that appear to have been electrostatic ion cyclotron (EIC) waves have been observed at low altitude in the diffuse aurora by a sounding rocket payload. The rocket was launched from Poker Flat, Alaska, at •2030 MLT. The flight successively traversed •70 km of the diffuse aurora, a dark region, and a quiet 40 kR auroral arc. In the diffuse aurora, peaks were observed in the power spectrum of the electric field at frequencies near the hydrogen and oxygen ion cyclotron frequencies. Doppler shift and polarization analyses have been performed using EIC wave spectrum parameters derived from linear theory. Both analyses indicated that these emissions had properties consistent with those VLF covering a frequency range from 2.5 Hz to 8 kHz. Emphasis in the analysis and discussion will be placed on emission features that had properties similar to those expected for electrostatic hydrogen and oxygen cyclotron waves [Bering, 1983b]. Electrostatic ion cyclotron (EIC) waves are considered to be one of the more important plasma wave modes present in and near auroral arcs. First discussed in detail by Drummond and Rosenblurb [1962], the EIC wave mode has the lowest threshold for instability among the various possible current driven instabilities to which auroral Birkeland currents might be subject [Kindel and Kennel, 1971]. Among other effects, expected for H + and O + EIC waves. Taken together, EIC waves have been suggested as the mechanism the two analyses indicated that both emission bands were due to waves propagating both up and down the field line and eastward parallel to the poleward boundary of the diffuse aurora. The large local cold plasma density and resulting large Landau damping require that the source be responsible for the selective perpendicular ion heating which produces "ion conics." Ion conic is the term used to describe ions flowing up into the magnetosphere with a minimum in their distribution function at 180 ø pitch angle and broad maximum between 90 ø and 130 ø pitch angle local. Free energy for the waves was apparently [Sharp et al., 1977, 197q; Ghielmetti et al., available in the 5 BA/m 2 downward parallel current 1978; Croley et al., 1978; Whalen et al., 1978; density which was inferred from the magnetometer data. The presence of the waves indicates that this current was being carried by less than 2% of the plasma, presumably in the form of a field aligned beam of electrons with energies of a few eV. Introduct ion This paper is the first in a series of three papers about an extensive set of electric field Klumpar,
Plasma parameter analysis of the Langmuir decay process via Particle-in-Cell simulations
2012
Abstract. The beam-plasma mechanism, based on the Langmuir decay process, has been proposed to explain naturally enhanced ion-acoustic lines (NEIALs), which are spectral distortions in incoherent scatter radar (ISR) data frequently observed in the vicinity of auroral arcs. In this work the effect of the Langmuir decay process on the ISR spectrum is studied and compared with an analytical model for different plasma parameters by using an electrostatic parallel particle-in-cell (EPPIC) code.
Journal of Geophysical Research, 1995
The results in this paper were obtained with SAPPHIRE, a new auroral Doppler radar experiment designed to study meter-scale E region irregularities. SAPPHIRE is a dual 50-MHz continuous wave, phased array, multibeam, bistatic system which is capable of performing cross-beam measurements from two widely different directions. There are two transmitters, each of which probes the auroral electrojet plasma over a large spatial t•rget grid area of multiple intersections that determine 16 scattering regions or cells. Initial observations using untapered antenna •rrays showed a class of scatter characterized by a narrow power spectrum peaking •t the same Doppler shift in all, or several, observing cells simultaneously. These are strong echoes ranging in lifetime from a few tens of seconds to a few minutes and occurring preferentially in the midnight and morning magnetic time sectors. The analysis showed that this scatter is strongly anisotropic in azimuth and comes from localized regions of spatially coherent large-amplitude plasma waves that produce mostly type III, but also type I and the rare type IV, radar auroras. By using many events and analyzing a large number of Doppler spectra, we found that type III echoes are the strongest observed, having on the average relative intensities at least 15 dB higher than the type I echoes. The observations relate to the "short discrete radar auroras" which are known to originate in spatially confined, dynamic plasma regions. The possibility exists that the large free energy for instability in these active regions is provided from intense electric fields and/or very sharp electron density gradients, both expected to occur at times near the edges of discrete auroral arcs. Finally, the present results confirm that, because of the l•rge dynamic range of radio auroral echoes, strong scattering regions lead to the complete domination, at times, by backscatter through antenna sidelobes. For the localized regions of strong type III and type I echoes, this means that the conventional 3-dB antenna beam width scale size of the scattering region is unrealistic. Obviously, this has important implications for the radar auroral experiments and the interpretation of observations. when the backscatter echoes were classified in two main categories, that is, "diffuse" and discrete, and several sub categories according to the range-time-intensity
Nonlinear beam generated plasma waves as a source for enhanced plasma and ion acoustic lines
Physics of Plasmas, 2011
Observations by, for instance, the EISCAT Svalbard Radar (ESR) demonstrate that the symmetry of the naturally occurring ion line in the polar ionosphere can be broken by an enhanced, nonthermal, level of fluctuations (naturally enhanced ion-acoustic lines, NEIALs). It was in many cases found that the entire ion spectrum can be distorted, also with the appearance of a third line, corresponding to a propagation velocity significantly slower than the ion acoustic sound speed. It has been argued that selective decay of beam excited primary Langmuir waves can explain some phenomena similar to those observed. We consider a related model, suggesting that a primary nonlinear process can be an oscillating two-stream instability, generating a forced low frequency mode that does not obey any ion sound dispersion relation. At later times, the decay of Langmuir waves can give rise also to enhanced asymmetric ion lines. The analysis is based on numerical results, where the initial Langmuir waves are excited by a cold dilute electron beam. By this numerical approach, we can detect fine details of the physical processes, in particular, demonstrate a strong space-time intermittency of the electron waves in agreement with observations. Our code solves the full Vlasov equation for electrons and ions, with the dynamics coupled through the electrostatic field derived from Poisson's equation. The analysis distinguishes the dynamics of the background and beam electrons. This distinction simplifies the analysis for the formulation of the weakly nonlinear analytical model for the oscillating two-stream instability. The results have general applications beyond their relevance for the ionospheric observations. V
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.
Dynamic rayed aurora and enhanced ion-acoustic radar echoes
Annales Geophysicae, 2005
The generation mechanism for naturally enhanced ion-acoustic echoes is still debated. One important issue is how these enhancements are related to auroral activity. All events of enhanced ion-acoustic echoes observed simultaneously with the EISCAT Svalbard Radar (ESR) and with high-resolution narrow field-of-view auroral imagers have been collected and studied. Characteristic of all the events is the appearance of very dynamic rayed aurora, and some of the intrinsic features of these auroral displays are identified. Several of these identified features are directly related to the presence of low energy (10-100eV) precipitating electrons in addition to the higher energy population producing most of the associated light. The low energy contribution is vital for the formation of the enhanced ion-acoustic echoes. We argue that this type of aurora is sufficient for the generation of naturally enhanced ion-acoustic echoes. In one event two imagers were used to observe the auroral rays simultaneously, one from the radar site and one 7km away. The data from these imagers shows that the auroral rays and the strong backscattering filaments (where the enhanced echoes are produced) are located on the same field line, which is in contrast to earlier statements in the litterature that they should be separated.