Downshifted Plasma Lines Research Articles

Incoherent Scatter Radar Studies of Daytime Plasma Lines

First results from wideband (electron phase energies of 5–51 eV), high-resolution (0.1 eV) spectral measurements of photoelectron–enhanced plasma lines made with the 430 MHz radar at Arecibo Observatory are presented. In the F region, photoelectrons produced by solar EUV line emissions (He II and Mg IX) give rise to plasma line spectral peaks/valleys. These and other structures occur within an enhancement zone extending from electron phase energies of 14–27 eV in both the bottomside and topside ionosphere. However, photoelectron–thermal electron Coulomb energy losses can lead to a broadened spectral structure with no resolved peaks in the topside ionosphere. The plasma line energy spectra obtained in the enhancement zone exhibit a unique relation in that phase energy is dependent on pitch angle; this relation does not exist in any other part of the energy spectrum. Moreover, large fluctuations in the difference frequency between the upshifted and downshifted plasma lines are evident in the 14–27 eV energy interval. At high phase energies near 51 eV the absolute intensities of photoelectron-excited Langmuir waves are much larger than those predicted by existing theory. The new measurements call for a revision/improvement of plasma line theory in several key areas.

Effect of the Heating Flow on the Determination of the Field-aligned Currents

给出了任意速度分布函数条件下复频率的介电函数和色散关系的求解方法. 讨论了16矩近似条件下, 场向热流对电子速度分布函数, 介电函数实部与虚部, 以及离子与电子谐振频率和阻尼率的影响, 并与平衡态时麦克斯韦分布等离子体的计算结果进行了比较. 忽略场向热流效应, 正向电子漂移速度条件下, 上行等离子线探测, 会过高估计场向电流; 负向电子漂移速度条件下, 上行等离子线探测会过低估计场向电流. 对上下行等离子线同时探测情况, 忽略热流效应同样会过高估计场向电流.

Auroral field-aligned currents by incoherent scatter plasma line observations in the E region

The aim of the Swedish-Japanese EISCAT campaign in February 1999 was to measure the ionospheric parameters inside and outside the auroral arcs. The ion line radar experiment was optimised to probe the E-region and lower F-region with as high a speed as possible. Two extra channels were used for the plasma line measurements covering the same altitudes, giving a total of 3 upshifted and 3 downshifted frequency bands of 25 kHz each. For most of the time the shifted channels were tuned to 3 (both), 4 (up), 5.5 (down) and 6.5 (both) MHz. Weak plasma line signals are seen whenever the radar is probing the diffuse aurora, corresponding to the relatively low plasma frequencies. At times when auroral arcs pass the radar beam, significant increases in return power are observed. Many cases with simultaneously up and down shifted plasma lines are recorded. In spite of the rather active environment, the highly optimised measurements enable investigation of the properties of the plasma lines. A modified theoretical incoherent scatter spectrum is used to explain the measurements. The general trend is an upgoing field-aligned suprathermal current in the diffuse aurora, There are also cases with strong suprathermal currents indicated by large differences in signal strength between up- and downshifted plasma lines. A full fit of the combined ion and plasma line spektra resulted in suprathermal electron distributions consistent with models.

Electron velocity distribution function in a plasma with temperature gradient and in the presence of suprathermal electrons: application to incoherent-scatter plasma lines

Abstract. The plasma dispersion function and the reduced velocity distribution function are calculated numerically for any arbitrary velocity distribution function with cylindrical symmetry along the magnetic field. The electron velocity distribution is separated into two distributions representing the distribution of the ambient electrons and the suprathermal electrons. The velocity distribution function of the ambient electrons is modelled by a near-Maxwellian distribution function in presence of a temperature gradient and a potential electric field. The velocity distribution function of the suprathermal electrons is derived from a numerical model of the angular energy flux spectrum obtained by solving the transport equation of electrons. The numerical method used to calculate the plasma dispersion function and the reduced velocity distribution is described. The numerical code is used with simulated data to evaluate the Doppler frequency asymmetry between the up- and downshifted plasma lines of the incoherent-scatter plasma lines at different wave vectors. It is shown that the observed Doppler asymmetry is more dependent on deviation from the Maxwellian through the thermal part for high-frequency radars, while for low-frequency radars the Doppler asymmetry depends more on the presence of a suprathermal population. It is also seen that the full evaluation of the plasma dispersion function gives larger Doppler asymmetry than the heat flow approximation for Langmuir waves with phase velocity about three to six times the mean thermal velocity. For such waves the moment expansion of the dispersion function is not fully valid and the full calculation of the dispersion function is needed.Key words. Non-Maxwellian electron velocity distribution · Incoherent scatter plasma lines · EISCAT · Dielectric response function

Electron velocity distribution function in a plasma with temperature gradient and in the presence of suprathermal electrons: application to incoherent-scatter plasma lines

The plasma dispersion function and the reduced velocity distribution function are calculated numerically for any arbitrary velocity distribution function with cylindrical symmetry along the magnetic field. The electron velocity distribution is separated into two distributions representing the distribution of the ambient electrons and the suprathermal electrons. The velocity distribution function of the ambient electrons is modelled by a near-Maxwellian distribution function in presence of a temperature gradient and a potential electric field. The velocity distribution function of the suprathermal electrons is derived from a numerical model of the angular energy flux spectrum obtained by solving the transport equation of electrons. The numerical method used to calculate the plasma dispersion function and the reduced velocity distribution is described. The numerical code is used with simulated data to evaluate the Doppler frequency asymmetry between the up- and downshifted plasma lines of the incoherent-scatter plasma lines at different wave vectors. It is shown that the observed Doppler asymmetry is more dependent on deviation from the Maxwellian through the thermal part for high-frequency radars, while for low-frequency radars the Doppler asymmetry depends more on the presence of a suprathermal population. It is also seen that the full evaluation of the plasma dispersion function gives larger Doppler asymmetry than the heat flow approximation for Langmuir waves with phase velocity about three to six times the mean thermal velocity. For such waves the moment expansion of the dispersion function is not fully valid and the full calculation of the dispersion function is needed.Key words. Non-Maxwellian electron velocity distribution · Incoherent scatter plasma lines · EISCAT · Dielectric response function

Bistatic measurements of incoherent scatter plasma lines

According to the classical theory of incoherent scatter plasma lines, it is possible to determine the electron drift velocity from the difference in offset frequencies between the up and downshifted plasma lines. Together with the electron density and the ion drift velocity derived from the ion line it should then be possible to determine the field aligned current. However, measurements interpreted with theory assuming a Maxwellian electron distribution have indicated far too large currents. A recent theory by Kofman et al. (1993) J. geophys. Res. 98, 6079–6085, takes into account the temperature-gradient induced heat flow which significantly influences the result. We show measurements of plasma lines, for both high and low solar activity, measured with both the monostatic, F-region cutoff (Tromsø) and the bistatic, intersection-volume (Kiruna) techniques. These are compared to the standard Maxwellian theory as well as to the theory including heat flow. The Kiruna data give the same results as do the Tromsø measurements, within the measurement accuracy, showing that the discrepancy between theory and measurements is not an artefact of the technique of measuring the plasma line cutoff at the peak of the F region. The additional heat flow term makes up for most of the discrepancy between the theoretical prediction and the measurements, and the latter indicate that it may be slightly overestimated in the high solar activity case. For the high solar activity conditions the surrounding neutral atmosphere is much denser, which might reduce the heat flow. For low solar activity there is still a remaining discrepancy between theory and measurements after accounting for the heat flow term. This may be explained by an enhanced heat flow caused by runaway electrons, as proposed by Mishin and Hagfors (1994) J. geophys. Res. 99, 6537–6539, or by a thermal electron drift balancing a net photoelectron escape flux.

The CRRES AA 2 Release: HF wave‐plasma interactions in a dense Ba+ cloud

An ionospheric chemical release, designated AA 2, was performed on July 12, 1992, as part of the NASA Combined Release and Radiation Effects Satellite (CRRES) El Coqui rocket campaign. The purpose of the AA 2 experiment was to study the interaction between a powerful radio wave and a high ion mass (Ba+), “collisionless” plasma. Approximately 35 kg of Ba were explosively released near the center of the Arecibo high‐frequency (HF) beam at 253 km altitude. This was the largest Ba release of the CRRES experiments; it yielded a distinctive ionospheric layer having a maximum plasma frequency of 11 MHz. At early times (< 1 min after the release) the HF beam produced the strongest Langmuir waves ever detected with the Arecibo 430‐MHz radar. Resonantly enhanced Langmuir waves were observed to be excited principally at the upshifted plasma line (i.e., near 430 MHz + ƒHF, where ƒHF is the frequency of the modifying HF wave), and only weakly excited waves were apparent at the downshifted plasma line (430 MHz − ƒHF). The upshifted plasma‐line spectrum contained a dominant peak at the “decay line,” that is, at the frequency 430 MHz + ƒHF − δ, where δ is close to the Ba+ ionacoustic frequency (∼2 kHz). Downshifted plasma‐line echoes occurred at frequencies near 430 MHz − ƒHF and 430 MHz − ƒHF − 1 kHz and exhibited little or no signal strength at the decay line (430 MHz − ƒHF + δ). During an initial period of intense upshifted plasma‐line excitation, the power asymmetry between the upshifted and downshifted plasma lines was of the order of 105 at the decay line. The upshifted plasma line was accompanied by strong HF‐enhanced ion waves that were present only at the downshifted acoustic sideband. After geomagnetic field‐aligned irregularities formed in the plasma the amplitudes of the upshifted and downshifted plasma lines equalized, and each exhibited spectra characteristic of the parametric decay instability. At early times in the Ba+ plasma the symmetry of wave excitation anticipated for a parametric instability in a stationary, homogeneous plasma was absent. The experimental results indicate that the development of the parametric decay instability needs to be reexamined for a smooth plasma having a small (∼5 km) vertical scale length. Moreover, ion flow down geomagnetic field lines appears to suppress instabilities responsible for the formation of field‐aligned irregularities and may also have an impact on the way parametric instabilities are excited. New theoretical approaches are needed to resolve many of the issues raised by this experiment.

Incoherent scatter plasma lines at angles with the magnetic field

Photoelectron‐enhanced plasma lines at angles with the geomagnetic field are investigated numerically and experimentally with the EISCAT UHF radar. Numerical simulations of enhanced plasma line intensities are carried out for a power law photoelectron distribution and a set of different temperatures and angles with the magnetic field. The calculations indicate that plasma line measurements may be successful at large angles even in the E region, because of the low electron temperature in this region. A filter bank power profile experiment was set up to measure upshifted and downshifted plasma line intensities as a function of angle with the field, from small to intermediate angles, and also for a fixed large angle (74°) with the field. As predicted by the simulations, plasma lines were detected at large angles also in the E region at 120 km altitude. To study the angular variation of the plasma line intensities, the photoelectron flux was calculated from a model based on the Boltzmann transport equation with parameters describing the ionospheric conditions present during the experiment. A power law flux was fitted to the calculated data and used in the numerical calculations to model the plasma line intensities for comparison with the experimental results. When allowing for a pitch angle dependence in the flux, the plasma line temperatures can be predicted to within a very good accuracy at altitudes where remnants of the N2 excitation dip are no longer present in the photoelectron distribution.

On the frequency of Langmuir waves in the ionosphere

Diverse geophysical applications exploit the existence of Langmuir waves in the natural ionosphere and magnetosphere. The frequency of these waves is often used as an absolute measure of plasma concentration in both remote sensing and in‐situ diagnostics. It is often wrongly assumed, however, that the frequency of Langmuir waves is directly interpretable in terms of the natural oscillation frequency of the thermal electron gas. This is only approximately true. Studies requiring more precision include the effects of the plasma temperature on the dispersion relation for these waves. Because this temperature dependence is also a function of the k vector of the waves, care must be taken when comparing measurements at different probing wavelengths (e.g., ionosondes and incoherent scatter radars). Herein, we point out that even further considerations are important in determining the exact frequency of Langmuir waves. The velocity distribution function of the entire electron gas must be accounted for properly. In particular, the presence of photoelectrons or auroral secondary electrons can affect the frequency of these waves. Failure to account properly for these effects may explain the unphysically large currents previously estimated by incoherent scatter measurements of the frequency asymmetry in upshifted and downshifted plasma line echoes.

First observations of 140‐MHZ plasma line backscatter during heating experiments at Tromsø

In September 1983 VHF plasma line backscatter was obtained at Kiruna, Sweden, from the ionosphere illuminated by strong HF radiation from the German‐Norwegian heating facility located near Tromsø, Norway. The downshifted plasma line was observed during two periods when the heater operated on a frequency that was lower than critical. The plasma line was never observed earlier than about 15 s after heater turn‐on and the intensity was very variable in time. A plausible assumption is therefore that the plasma line scattering observed is caused by Langmuir waves generated by direct conversion of the heating wave due to interactions with striations built up slowly in the modified ionospheric region.

HF‐enhanced ion and plasma line spectra with two pumps

Spectra of the HF‐enhanced ion line and upshifted and downshifted plasma lines were obtained by a multiple‐pulse technique (9 pulses) and by a technique using pulse‐to‐pulse correlation (512 pulses) with a short (about 1 ms) interpulse period for high frequency resolution. Observations using the multiple‐pulse technique suggest the presence of two different types of spectral features. One type has a spectral width typically greater than 1 kHz, and another type a width much less than 400 Hz, the frequency resolution of the multiple‐pulse technique used. Observations using pulses with a short interpulse period show the width of the narrow spectral features to be in the 20‐ to 60‐Hz range. The results are in good qualitative agreement with predictions of the linear theory of parametric instabilities excited by two pumps. Observations of the narrow spectral features can be used to measure the line‐of‐sight component of ionospheric drift velocities of the electron gas with an accuracy of about 2 m/s.

Frequency asymmetry in the upshifted and downshifted plasma lines induced by an HF wave at Arecibo

The Langmuir waves responsible for the HF‐wave‐induced plasma lines observed during ionospheric heating experiments at Arecibo are generated and propagate to the point of detection, in field‐aligned iontzation ducts. These Langmuir waves grow parametrically in the ducts at the height in the ionosphere at which the HF wave electric field is strongest. If it is assumed that there is a positive density gradient upward along the duct, then the observed ion acoustic frequency asymmetry between the upshifted and downshifted plasma lines can be explained. Agreement can be obtained between experimental and theoretical values for the frequency asymmetry and for the growth rates of the plasma line for a scale length along the magnetic field direction of about 40 km. A 40‐km‐scale length is less than that which would be expected during the day in the ambient plasma. However, it is probably not an unreasonable value for a duct under the influence of HF heating. A 40‐km scale length also leads to a height differential of about 55 m between the points at which the waves responsible for the upshifted and downshifted plasma lines are detected by the Arecibo radar. The model also implies that the intensity of the upshifted plasma line would be greater than that for the downshifted plasma line.

The spectral measurement of plasma lines

A method for measuring the critical frequencies of the ionospheric layers to accuracies better than 1 kHz is presented. The method depends on the fact that the intensity of the incoherent backscatter plasma line is a function of the electron number density scale height. At the peak or valley of a layer, more electrons are resonant within a specific frequency resolution cell. When fine‐frequency measurements are made of the echo from a long radar pulse, the abrupt signal intensity variations as a function of the frequency permit accurate determination of the critical frequencies. Measurements with the Arecibo Observatory 430‐MHz radar of the peak densities show that fluctuations of the order of 0.005% can be detected. The integration time must not be longer than a few seconds or frequency smearing caused by ionospheric changes occurs. Comparison of the upshifted and downshifted plasma line frequencies demonstrates the effect of different reception wavelengths on the resonant frequency and also permits determination of the electron drift velocity.

Langmuir wave propagation and the enhanced plasma line in sporadic E

Observations of HF‐induced plasma lines in blanketing sporadic E at Arecibo have been reported in the literature. The purely growing instability was found to be the probable source of the plasma lines, although the parametric decay instability could not be ruled out. It was also observed that the upshifted plasma lines tend to be much stronger than the downshifted plasma lines. A calculation of the purely growing and decay thresholds, using typical ray paths of the type thought to be responsible for the Arecibo observations, indicates that the purely growing instability is most likely responsible for the observed Es plasma lines. The purely growing threshold is smallest when it is determined by electron collisions rather than by the density gradient and when the wave normal of the unstable waves is nearly parallel to the earth's magnetic field. It is thus likely that the unstable Langmuir waves are generated where these conditions are satisfied and subsequently propagate to where they are observed by the radar. In order that these waves not be highly damped, the density gradient must be greater than about 10° off the vertical. This indicates that these waves propagate in ionization irregularities embedded in the Es layer. For irregularities with an upward component of density gradient directed north of the antimagnetic field direction, upshifted Es plasma lines would occur; otherwise, downshifted Es plasma lines would result. Since the former case covers a much larger range of angles than the latter case, it is not surprising that normally the upshifted Es plasma line is stronger than the downshifted Es plasma line.

Photoelectron distribution determination from plasma line intensity measurements obtained at Nancay (France)

The plasma oscillations that can be observed by the French incoherent scatter system have small phase velocities and are excited by low energy photoelectrons, typically 2–5 eV. Consequently, the method used to determine the energy photoelectron distribution from plasma line measurements made at other observatories (e.g. Cicerone, 1974) cannot be applied here: it is necessary to chose a model energy distribution with a small number of parameters. The energy shape of the flux is assumed Maxwellian and the angular shape is assumed linear with the cosine of the pitch angle. Total flux values and mean energies are obtained as a function of altitude, in agreement with other determinations, and the difference between upshifted and downshifted plasma line intensities lead to the determination of the anisotropy of the photoelectron flux.

Analysis of the backscatter spectrum in an ionospheric modification experiment

Predictions of the backscatter spectrum, including effects of ionospheric inhomogeneity, are compared with experimental observations of incoherent backscatter from an artificially heated region. Our calculations show that the strongest backscatter echo received is not from the reflection level, but from a region some distance below. Certain asymmetrical features are explained of the up-shifted and down-shifted plasma lines in the backscatter spectrum, and the several satellite peaks accompanying them.

Arecibo heating experiments

Enhancements of various features of the incoherent scatter spectrum are observed when the ionosphere is illuminated with powerful, high frequency radio waves. The radio waves excite plasma instabilities producing lines or more complex spectral features near the local plasma frequency, at the local ion acoustic frequency, near the local gyrofrequency, and near twice the gyrofrequency. The enhancements occur in a thin slab as observed by the incoherent scatter radar and at both upshifted and downshifted frequencies with respect to the probing radar frequency. The enhancements are observed to vary with time when the excitation is held constant and is turned on or off.The high power radio waves are produced by a 160 kw transmitter feeding a log‐periodic pair of curtains mounted at the focus of the 1000‐ft reflector and covering the frequency range from 5 to 12 MHz. The effects are observed with the incoherent scatter radar using the same reflector and with ionosondes and photometers.The frequencies of the enhanced plasma line and the ion line and their relation to the pump (high frequency radio wave) frequency are predictable from available parametric instability theory. Other spectral features are being explained as the theory develops with the help of the observations. There remain some discrepancies, in particular the asymmetries in intensity, width, and fluctuations of the upshifted compared to the downshifted plasma lines.

High frequency induced enhancements of the incoherent scatter spectrum at Arecibo, 2

When the ionospheric F region is illuminated with a strong O mode HF radio wave, excitations of longitudinal plasma and acoustic waves are observed by 430-MHz radar scattering. The frequency structure of the excited waves and their variability with local parameters, time, and space are described. The observations are interpreted in terms of parametric instability theory. The spectra of the observed features and the correlation of their amplitudes are shown. The upshifted and downshifted plasma lines exhibit some marked asymmetries. Absolute as well as relative amplitudes of the enhancement are presented.