Data reduction of incoherent scatter plasma line parameters

In the ionosphere, a sustained population of suprathermal electrons is generated due to photoionization or electron precipitation. At resonance, the phase velocity of the electrons matches the Langmuir phase velocity observed by the Incoherent Scatter Radar (ISR). As a result, the scattered power in the plasma line spectrum is enhanced, thus making it possible to detect them. Plasma lines can be used to improve the accuracy of the estimates of electron density and temperature, study features in the electron velocity distribution of the suprathermal electrons and provide an independent method to calculate ionospheric currents.   We analyzed the data collected with the EISCAT Tromsø UHF radar on 27 January 2022. We present a novel technique of data reduction to detect plasma lines and extract parameters. We use the method developed by Ivchenko (2017) to determine the times with enhanced plasma lines. For those times, we model the spectrum with a Gaussian function, where the plasma line intensity, frequency and bandwidth correspond to the amplitude, mean and variance, respectively. We observe photoelectron-enhanced plasma lines between 09:16:15 LT – 13:56:15 LT. All the detected plasma lines are field-aligned, except for 11:29:30 LT - 12:51:30 LT, when they are also detected in the vertical direction (i.e. at 11.67° to the magnetic field). The detection of the plasma lines is accompanied by an increase in the electron density estimates from the ion line. 

In the spring and fall of 1978 we made an extensive series of plasma line and correlative observations with the Chatanika incoherent scatter radar. To make these measurements, we greatly modified the radar receiving system. In addition to enlarging the plasma line filter bank the most significant change was the incorporation of a high‐speed correlator provided by the French. This was the first use of a correlator in a monostatic radar to obtain the intensity spectra of naturally occurring plasma lines. In this paper we develop the signal‐processing theory that we use to obtain the plasma line intensities from these measurements; we also show that these intensities compare well with those obtained from the filter bank. To show the richness of the phenomena and to explore the capabilities of the correlator, we examine a wide variety of spectra that have been enhanced by secondary electrons in the auroral E layer. From the other simultaneous measurements we are able to relate these spectra and their variations to the auroral situation. We also obtained the first measurements in the auroral region of photoelectron‐excited plasma lines in the E and F layers. Perhaps most significant, in the plasma line spectra we detected a Doppler shift that we then used to determine the Birkeland current carried by ambient electrons. Although there is a large estimated uncertainty for this first determination, we obtained a downward Birkeland current of 10 μA/m² in the diffuse aurora in what is, most likely, the equatorward portion of the evening sector auroral oval.

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.

Measurements of plasma and ion lines induced during HF ionospheric interaction experiments have been made with the European Incoherent Scatter (EISCAT) facility at Tromsø with sufficiently high‐altitude resolution to compare with theories of Langmuir turbulence. Recent Langmuir turbulence models predict a change from broad structureless spectra to line or cascade spectra within a few hundred meters for VHF (224 MHz) observations assuming typical ionospheric density gradients. In a campaign in May 1994 we found VHF spectra that were grouped into two regions separated in altitude by ∼2 km, with broad, unstructured plasma line spectra in the upper region and cascade type spectra in the lower region. The ion line channels showed detectable spectra mainly in the upper altitude region, which corresponds to that which had the broad plasma lines. The background ionospheric density profile showed an unusually low plasma density gradient near the HF reflection heights, thus allowing the two regions, which are normally so close together that one only sees a transition from one type of spectra to the other, to be clearly separated in height. Thus, in the high‐latitude ionosphere there can, at times, be a simultaneous existence in spatially separate regions of cavitation (often referred to as strong turbulence) and cascading (normally associated with saturated parametric decay) as predicted by some simulations. Another new feature is a height variation in the plasma line cascades with the highest‐order cascades strongest at the lowest heights, in accordance with expectations based on the parametric decay instability.

Abstract. Photo-electrons and secondary electrons from particle precipitation enhance the incoherent scatter plasma line to levels sufficient for detection. When detectable the plasma line gives accurate measure of the electron density and can potentially be used to constrain incoherent scatter estimates of electron temperature. We investigate the statistical occurrence of plasma line enhancements with data from the high-latitude EISCAT Svalbard Radar obtained during the International Polar Year (IPY, 2007–2008). A computationally fast method was implemented to recover the range-frequency dependence of the plasma line. Plasma line backscatter strength strongly depends on time of day, season, altitude, and geomagnetic activity, and the backscatter is detectable in 22.6 % of the total measurements during the IPY. As expected, maximum detection is achieved when photo-electrons due to the Sun's EUV radiation are present. During summer daytime hours the occurrence of detectable plasma lines at altitudes below the F-region peak is up to 90 %. During wintertime the occurrence is a few percent. Electron density profiles recovered from the plasma line show great detail of density variations in height and time. For example, effects of inertial gravity waves on the electron density are observed.

We present a statistical study of anomalous radar echoes observed in the auroral ionosphere thought to be signatures of Langmuir turbulence (LT). Data obtained with the European Incoherent Scatter Svalbard radar during the international polar year (IPY) were searched for these anomalous echoes in the auroral F region. In incoherent scatter radar experiments LT may in certain circumstances be observed as enhanced backscattered radar power at the ion line frequencies, plasma line frequencies, and at zero Doppler shift. The power enhancement at zero Doppler shift could arise due to Bragg scattering from nonpropagating density fluctuations caused by strong LT. In the IPY data set, around 0.02% of the data comply with our search criteria for altitudes above 190 km based on the ion line spectrum including enhancement at zero Doppler shift. The occurrence frequency of the identified events peaks in the premidnight sector and increases with local geomagnetic disturbance. Enhanced backscattered power is observed with limited altitude extent (below 20 km in 70% of the events), and the altitude distribution of identified radar signatures in the ion line channel has a peak at about 220 km. Enhancement of the plasma line is observed with the ion line enhancements in more than 60% of the events. Two classes of enhanced plasma lines occur. In the first class, plasma lines are limited in frequency and altitude and occur at altitudes of ion line enhancements. In the second class, the plasma lines are spread in frequency and range and are observed at lower altitudes than the first class (at about 170 km) with frequencies close to 3 MHz. Available optical data available indicate that the identified events to occur during auroral breakup with high‐energy electron precipitation.

We report here a series of experiments conducted at the Sondre Stromfjord incoherent scatter radar, aimed at detecting enhanced plasma lines associated with midnight sector auroral arcs. Using different receivers, we detected both ion and plasma lines simultaneously. The plasma line signal was recorded with the use of a filter bank of eight frequencies. Plasma lines were found to originate mainly from the topside of the particle‐produced E layer. The enhanced plasma lines are sometimes a factor of 100 times larger than the thermal level. Our data show a rapid decay of the plasma lines, however. In some cases, only a 30‐s integration time was needed in order to unambiguously detect both upshifted and downshifted lines. The level of the plasma lines reaches values of, for the largest cases, up to 40°K above the noise temperature. These are considerably higher than results from prior auroral zone plasma line experiments. In situ observations of enhanced plasma waves in this same region are reported in a companion paper.

During an experiment involving the alternating O / X mode pump, the Incoherent Scatter Radar (ISR) observation demonstrated that the high frequency enhanced ion line (HFIL) and plasma line (HFPL) did not immediately appear, but were delayed by a few seconds after the pump onset. By examining the initial behaviors of the ion line, plasma line and electron temperature as well as ionosphere condition, it is shown that (1) the HFIL and HFPL are delayed not only in the X mode pump but also in the O mode pump; (2) the HFIL can not be observed until the electron temperature is enhanced. The analysis suggests that (1) the leakage of the X mode to the O mode pump may not be ignored; (2) the spatiotemporal uncertainty, the spatiotemporal change in the profiles of ion mass and electron density, may play an important role in the lack of the Bragg condition; (3) nevertheless, the absence of parametric decay instability can not be ruled out due to the lack of matching condition caused by the spatiotemporal uncertainty.

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.

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.

We present the observations of the artificial ionospheric modification experiment of EISCAT on 18 October 2012 in Tromsø, Norway. When the pump of alternating O mode and X mode is switched on, the UHF radar observation shows some strong enhancements in electron density, ion lines and plasma lines. Based on some existing theories, we find the following: First, during the experiment, the frequency of plasma line (fL), ion line (fia) and pump (fh) matches fL = fh − 3fia and = fh − 5fia occasionally demonstrated that the cascade process occurred. Second, through quantitative calculation, we found that the O-mode component mixed in X-mode wave satisfies the thresholds of the parametric decay instability and the oscillation two-stream instability, from which we infer that the HF-induced plasma lines (HFPLs) and HF-enhanced ion lines (HFILs) observed in X-mode pulse could have been caused by the O-mode component mixed in X-mode wave. Third, the UHF radar observation shows some apparent enhancements over a wide altitude range (from approximately the reflection altitude to ~670 km) in electron density during X-mode pulse, which also does not, in fact, correspond to a true increase in electron density, but due to the enhancement in ion line or the enhancement in radar backscatter induced by some unknown mechanism.

Bistatic measurements of incoherent scatter plasma lines

Incoherent Scatter Radar (ISR) measurements provide an independent method to estimate ionospheric currents. We can estimate the ion and electron mean drift velocities from the ion and plasma line data respectively. These velocities together with the electron density can be used to calculate the ionospheric currents along the radar beam direction. Usually, the electron drift velocity is calculated from the Doppler offset between up- and downshifted plasma lines. Here, we propose an alternative method to estimate the electron drift velocity when only up- or downshifted plasma line data is available. We will assume that the electron drift is mainly due to the bulk motion of thermal electrons and use The New Hampshire Dispersion Relation Solver (NHDS) to get an estimate of the theoretical linear resonance frequency at an arbitrary angle to the magnetic field. We will use the ionospheric parameters estimated from the ion line as inputs to NHDS and as the output obtain the mean electron drift velocity. We will also compare the estimated ionospheric current with the one obtained using approximate analytical expression for the resonance frequency.  

Incoherent scatter radar observations of ionospheric plasmas rely on echoes from electron density fluctuations with properties governed by the dispersion relations for ion acoustic and Langmuir waves. Radar observations of echoes associated with Langmuir waves (plasma lines) from thermal plasma are weak, and only a few near‐thermal level measurements have been reported. Plasma line echoes are typically only observed with existing radars only when the Langmuir waves are enhanced by suprathermal electrons. A new observation technique has been developed which is sensitive enough to allow observations of these echoes without the presence of suprathermal electrons up to at least 1000 km. This paper presents recent observations from the Arecibo Observatory 430 MHz incoherent scatter radar which show plasma line echoes during the night when no suprathermal enhancement is expected to be present. The observations are compared with theory, and the results are found to be in agreement with classical incoherent scatter theory for thermal plasmas. The theoretical ratio of the ion line and plasma line power spectral density is within approximately 3 dB of the predicted value. The finding adds a new observational capability, allowing electron density to also be observed at night using the plasma line well into the top side of the ionosphere, increasing the accuracy of the electron density measurement.

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.