EISCAT Heating Facility Research Articles

Modulation of polar mesospheric summer echoes (PMSEs) with high-frequency heating during low solar illumination

Abstract. Polar mesospheric summer echo (PMSE) formation is linked to charged dust/ice particles in the mesosphere. We investigate the modulation of PMSEs with radio waves based on measurements with EISCAT VHF radar and EISCAT heating facility during low solar illumination. The measurements were made in August 2018 and 2020 around 20:02 UT. Heating was operated in cycles with intervals of 48 s on and 168 s off. More than half of the observed heating cycles show a PMSE modulation with a decrease in PMSE when the heater is on and an increase when it is switched off again. The PMSE often increases beyond its initial strength. Less than half of the observed modulations have such an overshoot. The overshoots are small or nonexistent at strong PMSE, and they are not observed when the ionosphere is influenced by particle precipitation. We observe instances of very large overshoots at weak PMSE. PMSE modulation varies strongly from one cycle to the next, being highly variable on spatial scales smaller than a kilometer and timescales shorter than the timescales assumed for the variation in dust parameters. Average curves over several heating cycles are similar to the overshoot curves predicted by theory and observed previously. Some of the individual curves show stronger overshoots than reported in previous studies, and they exceed the values predicted by theory. A possible explanation is that the dust-charging conditions are different either because of the reduced solar illumination around midnight or because of long-term changes in ice particles in the mesosphere. We conclude that it is not possible to reliably derive the dust-charging parameters from the observed PMSE modulations.

СПУТНИКОВЫЕ НАБЛЮДЕНИЯ ЭФФЕКТОВ ОТ НАГРЕВА АВРОРАЛЬНОЙ ИОНОСФЕРЫ КОМПЛЕКСОМ EISCAT

The measurements are presented of the dynamics of energetic electrons on the Meteor-M No. 2 orbit during the experiment on the ionosphere heating by strong shortwave radiation with the EISCAT heating facility (Tromso, Norway) on October 12, 2018. It is revealed that the counting rate of electrons with energy >40 keV increased by five times in 30 s after the moment of the facility switch-off, and sharp (by 3–4 times) short counting rate peaks were registered as compared to the background. The peaks were repeated approximately every 6 seconds. The results of observations of variations in total electron content of the high-latitude ionosphere reconstructed from ionospheric delays of GLONASS R21 satellite signals during the same heating experiment are also presented. The analysis of these data revealed small exceeding of TEC values over the trend at the boundary of the heating area.

Polarization analysis of ELF/VLF waves generated by beating of two HF waves in the polar ionosphere

Modulated high frequency heating of the polar ionosphere in the presence of a polar electrojet current (PEJ) can generate extremely/very low frequency (ELF/VLF, 3 Hz–30 kHz). In order to overcome the dependence of the radiation intensity on the variable PEJ, researchers have focused on electrojet-independent approaches. One potential method is so-called “beat-wave” (BW) ELF/VLF generation invoking nonlinearity of the F-region ionospheric plasma, but this F-region source mechanism remains controversial. We perform polarization analysis of BW generated ELF/VLF waves using EISCAT heating facility near Tromsø. The results show that ELF/VLF waves can be decomposed into right- (R) and -left (L) handed circularly polarized waves with comparable strengths, and the polarization ellipse tends to be parallel to the well-known dominant background electric field, which is fully consistent with the recognized features of ELF/VLF waves generated by PEJ modulation. Hence, the results demonstrate that the BW generated current is located in the D/E region, and not the F region.

On the Accuracy of the Conjugation of High-Orbit Satellites with Small-Scale Regions in the Ionosphere

The degree of uncertainty that arises when mapping high-orbit satellites of the Cluster type into the ionosphere using three geomagnetic field models (T89, T98, and T01) has been estimated. Studies have shown that uncertainty is minimal in situations when a satellite in the daytime is above the equatorial plane of the magnetosphere at the distance of no more than 5 RE from the Earth’s surface and is projected into the ionosphere of the northern hemisphere. In this case, the dimensions of the uncertainty region are about 50 km, and the arbitrariness of the choice of the model for projecting does not play a decisive role in organizing satellite support based on optical observations when studying such large-scale phenomena as, e.g., WTS, as well as heating experiments at the EISCAT heating facility for the artificial modification of the ionosphere and the generation of artificial fluctuations in the VLF band. In all other cases, the uncertainty in determining the position of the base of the field line on which the satellite is located is large, and additional information is required to correctly compare the satellite with the object in the ionosphere.

Remote sensing of ELF/VLF radiation induced in experiments on artificial modification of the ionosphere

In 2012, remote measurements of electromagnetic signals in the ELF/VLF band were taken at different points in Russia during experiments on artificial ionospheric modification with the powerful HF wave at the EISCAT heating facility (Tromso, Norway). The use of the new, highly sensitive magnetometric equipment allowed signals with an amplitude of a few femtoteslas to be recorded at a distance of up to 2000 km from the source. Analysis of the measurement results discovered substantial differences in the amplitude-phase characteristics of the signals, which were caused by a change in helio-geophysical conditions in the region of heating and along the signal passage route, and features of signal propagation, which are related to their mode of guided propagation, the directivity of the source, and angles of reception.

High latitude artificial periodic irregularity observations with the upgraded EISCAT heating facility

We present a recently developed ionospheric modification experiment that produces artificial periodic irregularities in the ionosphere and uses them to make observations of the spatiotemporal behaviour of the irregularities. In addition, the method can be used to measure Faraday rotation and vertical velocities. We also introduce a novel experiment that allows monitoring the formation of the irregularities during heating, in addition to observing their decay after heating. The first measurements indicate, contrary to existing theory, that the amplitude of the radar echoes from the periodic irregularities grows faster than they decay. We focus on the API effects in the D- and E-region of the ionosphere.

Plasma modifications induced by an X-mode HF heater wave in the high latitude F region of the ionosphere

We presented experimental results of strong plasma modifications induced by X-mode powerful HF radio waves injected towards the magnetic zenith into the high latitude F region of the ionosphere. The experiments were conducted in 2009–2011 using the EISCAT Heating facility, UHF incoherent scatter radar and the EISCAT ionosonde at Tromsø, Norway; and the CUTLASS SuperDARN HF coherent radar at Hankasalmi, Finland. The results showed that the X-mode HF pump wave can generate strong small-scale artificial field aligned irregularities (AFAIs) in the F region of the high-latitude ionosphere. These irregularities, with spatial scales across the geomagnetic field of the order of 9–15m, were excited when the heater frequency (fH) was above the ordinary-mode critical frequency (foF2) by 0.1–1.2MHz. It was found that the X-mode AFAIs appeared between 10s and 4min after the heater is turned on. Their decay time varied over a wide range between 3min and 30min. The excitation of X-mode AFAIs was accompanied by electron temperature (Te) enhancements and an increase in the electron density (Ne) depending on the effective radiated power (ERP). Under ERPs of about 75–180MW the Te enhances up to 50% above the background level and an increase in Ne of up to 30% were observed. Dramatic changes in the Te and Ne behavior occurred at effective radiated powers of about 370–840MW, when the Ne and Te values increased up to 100% above the background ones. It was found that AFAIs, Ne and Te enhancements occurred, when the extraordinary-mode critical frequency (fxF2) lied in the frequency range fH–fce/2≤fxF2≤fH+fce/2, where fce is the electron gyrofrequency. The strong Ne enhancements were observed only in the magnetic field-aligned direction in a wide altitude range up to the upper limit of the UHF radar measurements. In addition, the maximum value of Ne is about 50km higher than the Te enhancement peak. Such electron density enhancements (artificial ducts) cannot be explained by temperature-dependent reaction rates. They can be attributed to HF-induced ionization production by accelerated electrons. The possible mechanisms for plasma modifications induced by powerful X-mode HF radio waves were discussed.

The spatial features of the up- and downshifted maxima in stimulated electromagnetic emissions

The determination of the source regions of some of the spectral components of stimulated electromagnetic radio emissions (SEE) is reported. The investigation was performed using observations made in 2005 during heating campaigns with the EISCAT heating facility near Tromsø, Norway. A four-channel high-frequency interferometer, designed by the Polar Geophysical Institute (PGI), was used to determine the arrival direction of the SEE signal. Three channels of the interferometer were connected to three antennas of the receiving antenna array. The antennas were installed in the meridional (north-south) and zonal (east-west) planes. The distance between the heating facility and receiver antenna array was 12.6km. The measurements allowed determination of the azimuth and elevation angles of the intensity-weighted mean location of the SEE source regions by the phase difference method. In order to increase accuracy and eliminate the effects of interferences, data were selected based on the coherence of the signals. The results show that different parts of the down- and up-shifted maxima (DM, UM) features in the SEE spectrum are generated in different regions of the ionosphere, possibly in an elongated region along the magnetic field.

Effect of neutral particle density in the upper atmosphere on the generation of artificial magnetic pulsations in the Pc1 range

A series of experiments on modification of the ionosphere by a powerful ground HF transmitter was performed using the EISCAT heating facility in order to generate artificial magnetic pulsations in the frequency range 0.1–3 Hz. In several cases, the ionospheric electric field and the electron density vertical profile were measured with the EISCAT incoherent scatter radar. The measurement of the background values of the ionospheric parameters made it possible to verify the numerical model for generating artificial emissions. The calculated amplitudes of magnetic pulsations correspond to the values measured on the Earth’s surface. However, the model cannot explain the sporadic nature of artificial signals, which indicates that this model is incomplete. Disturbances of the neutral particle density in the upper atmosphere are one of the possible causes explaining a difference between the calculations and the experimental values. The numerical simulation indicated that the amplitude variations caused by such disturbances can be 20%. For artificial emissions whose intensity is comparable with the intensity of artificial noise, variations in the neutral components can result in the disappearance of an artificial signal on the spectrogram.

Electromagnetic and plasma perturbations induced by radio emission of the EISCAT high-frequency heating facility in the outer ionosphere of the earth

We present the results of measuring the characteristics of electromagnetic and plasma perturbations at the altitudes of the Earth’s outer ionosphere, which were obtained by the onboard equipment of the French microsatellite DEMETER when it passed through the magnetic flux tube based on a region modified by high-power HF radio emission of the EISCAT heating facility. Physical mechanisms which allow one to explain the observed phenomena are considered.

First observation of the overshoot effect for polar mesosphere winter echoes during radiowave electron temperature modulation

Polar mesosphere winter echoes (PMWE), observed with the EISCAT VHF radar located near Tromsø, Norway on October 24, 2006, were modulated using the EISCAT Heating facility. The heater was turned on for 20 s and then off for 160 s. The experiment confirmed previous results that when the heater was switched on, the PMWE power dropped significantly compared to the undisturbed level measured before heating. The experiment showed for the first time that, after the heater was switched off, the PMWE strength overshot the value that it had before heating. By using data from 6 heating cycles we find that the overshoot effect is about 50% of the pre‐heating value, which is rather weak compared to 3–5 times for PMSE. No, or little recovery of PMWE strength during heating periods was observed. This may indicate the presence of very small dust particles, such as meteoric smoke particles.

Determining the ionospheric irregularity velocity vector based on doppler measurements in the artificially modified F 2 region of the polar ionosphere

The method for estimating the behavior of the ionospheric irregularity motion vector in the artificially disturbed HF ionospheric region has been proposed, and this behavior has been analyzed based on the simultaneous Doppler observations performed on several paths using the method of bi-static backscatter of diagnostic HF signals by small-scale artificial ionospheric irregularities. The Doppler measurements were performed during the modification of the auroral ionosphere by powerful HF radiowaves emitted by the EISCAT heating facility (Tromso, Norway). It has been obtained that the dynamics of the ionospheric irregularity directions in the F region, calculated based on the Doppler measurements of the total vector of the ionospheric irregularity velocity above the Tromso EISCAT radar at a frequency of 931 MHz, is in satisfactory agreement with such calculations performed using the three-position method.

Saturation and hysteresis effects in ionospheric modification experiments observed by the CUTLASS and EISCAT radars

Abstract. The results of high latitude ionospheric modification experiments utilising the EISCAT heating facility at Tromsø are presented. As a result of the interaction between the high power pump waves and upper hybrid waves in the ionosphere, field-aligned electron density irregularities are artificially excited. Observations of these structures with the CUTLASS coherent HF radars and the EISCAT incoherent UHF radar exhibit hysteresis effects as the heater output power is varied. These are explained in terms of the two-stage mechanism which leads to the growth of the irregularities. Experiments which involve preconditioning of the ionosphere also indicate that hysteresis could be exploited to maximise the intensity of the field-aligned irregularities, especially where the available heater power is limited. In addition, the saturation of the irregularity amplitude is considered. Although, the rate of irregularity growth becomes less rapid at high heater powers it does not seem to fully saturate, indicating that the amplification would continue beyond the capabilities of the Tromsø heater - currently the most powerful of its kind. It is shown that the CUTLASS radars are sensitive to irregularities produced by very low heater powers (effective radiated powers <4 MW). This fact is discussed from the perspective of a new heating facility, SPEAR, located on Spitzbergen and capable of transmitting high frequency radio waves with an effective radiated power ~10% of that of the Tromsø heater (28MW).

The effect of electron bite-outs on artificial electron heating and the PMSE overshoot

Abstract. We have considered the effect that a local reduction in the electron density (an electron bite-out), caused by electron absorption on to dust particles, can have on the artificial electron heating in the height region between 80 to 90km, where noctilucent clouds (NLC) and the radar phenomenon PMSE (Polar Mesospheric Summer Echoes) are observed. With an electron density profile without bite-outs, the heated electron temperature Te,hot will generally decrease smoothly with height in the PMSE region or there may be no significant heating effect present. Within a bite-out Te,hot will decrease less rapidly and can even increase slightly with height if the bite-out is strong. We have looked at recent observations of PMSE which are affected by artificial electron heating, with a heater cycling producing the new overshoot effect. According to the theory for the PMSE overshoot the fractional increase in electron temperature Te,hot/Ti, where Ti is the unaffected ion temperature=neutral temperature, can be found from the reduction in PMSE intensity as the heater is switched on. We have looked at results from four days of observations with the EISCAT VHF radar (224 MHz), together with the EISCAT heating facility. We find support for the PMSE overshoot and heating model from a sequence of observations during one of the days where the heater transmitter power is varied from cycle to cycle and where the calculated Te,hot/Ti is found to vary in proportion to the transmitter power. We also looked for signatures of electron bite-outs by examining the variation of Te,hot/Ti with height for the three other days. We find that the height variation of Te,hot/Ti is very different on the three days. On one of the days we see typically that this ratio can increase with height, showing the presence of a bite-out, while on the next day the heating factor mainly decreases with height, indicating that the fractional amount of dust is low, so that the electron density is hardly affected by it. On the third day there is little heating effect on the PMSE layer. This is probably due to a sufficiently high electron density in the atmosphere below the PMSE layer, so that the transmitted heater power is absorbed in these lower layers. On this day the D-region, as given by the UHF (933MHz) observations, extends deeper down in the atmosphere than on the other two days, indicating that the degree of ionization in and below the PMSE layers is higher as well.

On the influence of plasma absorption by dust on the PMSE overshoot effect

The radar phenomenon PMSE (Polar Mesospheric Summer Echo), which is associated with the charged dust in the noctilucent clouds (NLCs), can be affected by artificial electron heating. If the heater is run in a cycle with equal, and comparatively short (10–20 s) off and on periods, the PMSE strength is observed to weaken when the heater is on, and recover to approximately the same strength when the heater is switched off [Chilson, P.B., Belova, E., Rietveld, M.T., Kirkwood, S., Hoppe U.P., 2000]. First artificially induced modulation of PMSE using the EISCAT heating facility. Geophys. Res. Lett. 27, 3801]. We have adopted a new heater cycling, during which the heater is on for 20 s and then off for a period of 160 s. With this cycling we very often observe an “overshoot” effect where the PMSE strength, when the heater is switched off, increases by a factor of several compared to what it was directly before the heater was switched on [Havens, O., LaHoz, C., Naesheim, L.I., Rietveld, M.T., 2003. Chilson, P. B., Belova, E., Rietveld, M. T., Kirkwood, S., Hoppe U. P., 2000. First artificially induced modulation of PMSE using the EISCAT heating facility. Geophys. Res. Lett. 30, 2229]. By analysing the time-dependant reflected power showing that the PMSE weakens when the heater is switched on, overshoot when it is switched off and subsequently relaxes back to the original strength, we should be able to obtain a considerable amount of information about the PMSE dusty plasma. Until now, the only model presented for the overshoot is based on the assumption that the dust density is low so that absorption of plasma by the dust does not seriously affect the overall electron and ion density throughout the overshoot cycle. Observations indicate that this probably is true in many, possibly most cases, but it will not always be the case. Here, we present a model for the overshoot in which we include the absorption of plasma due to the dust. If the dust density is comparatively high, this will influence the equilibrium density of the plasma, since the dust is a sink for plasma. Further, when the electrons are heated, the flux of electrons to the dust increases and the background electron density will also decrease during the time the heater is on. We will present model results and compare with observations and discuss the possible information which can be gained from the overshoot effect.

Observations of time dependence and aspect sensitivity of regions of enhanced UHF backscatter associated with RF heating

Abstract. The EISCAT incoherent radar system, which is collocated with the EISCAT heating facility, is used to diagnose the ionosphere while heating experiments are conducted. In late September 2002, an experiment was performed in which the heater transmitted a 2-min-on/2-min-off cycle while its pointing direction was kept fixed and the UHF beam was cycled through five pointing directions. This UHF cycle was used for three heater beam-pointing directions. For field-aligned heater beam and UHF pointing, UHF data indicated a gradual decrease, with time, in the altitude at which enhanced ion-line scatter occurred. This was accompanied by a reduction in the intensity of the scatter. For field-aligned heater pointing and the UHF elevation angle of 6° in the field-aligned direction, a persistent high-amplitude signature was observed, which remained at a fairly constant altitude throughout the period that the heater remained switched on. Different time histories of the backscatter amplitude were observed in other UHF pointing directions, including the "ion-line overshoot", which is characterized by an increase and subsequent decrease in the heater-enhanced backscatter just after heater switch-on. It is suggested that these signatures may be caused by the presence or absence of field-aligned irregularities and reduced recombination caused by heating. The CUTLASS coherent radar system, which operated simultaneously with the UHF radar and the heater, observed backscatter from field-aligned irregularities created by the heater. The intensity of this backscatter was highest from the regions of the ionosphere that were excited by the central part of the heater beam.

Multi-frequency HF radar measurements of artificial F-region field-aligned irregularities

Abstract. We present radar backscatter power measurements using the CUTLASS HF radar at Hankasalmi, Finland from F-region field-aligned irregularities induced by HF radio pumping with the EISCAT Heating facility. A novel radar operating mode is used in which the radar frequency is rapidly swept through a number of bands, making use of the varying ionospheric refraction to probe different heights within the heated region. We obtain height profiles of backscatter power which correspond to e-folding scale lengths of around 20km for the mean-square electron density perturbations for pump wave interaction heights in the region of 240-250km in daytime conditions. The results are in agreement with previous measurements made by other techniques. We discuss some problems with the method and suggest improvements for future experiments.

First observations of the PMSE overshoot effect and its use for investigating the conditions in the summer mesosphere

We report from the first campaign, with the EISCAT radars and heating facility, which looked for a proposed new PMSE overshoot effect resulting when PMSE is being affected by a specific cycling of artificial electron heating. The overshoot was predicted to appear after unaffected PMSE dusty plasma had been acted upon, for a comparatively short time, by the EISCAT Heating facility. We show model results and observational examples of the overshoot effect. For the overshoot to be clearly observable, a long relaxation time after the heater is switched off is necessary to get the dust charges back to those of dust unaffected by the heated electrons or, to bring unaffected PMSE dusty plasma into the radar beams by horizontal wind transport. The overshoot characteristic curve (OCC) contain a substantial amount of information on the conditions during PMSE and must lead to new possibilities in the study of PMSE and the conditions close to the summer mesopause.

First artificially induced modulation of PMSE using the EISCAT Heating Facility

A coordinated experiment involving ionospheric heating and VHF observations of polar mesosphere summer echoes (PMSE) has recently been conducted at the EISCAT facility near Tromsø, Norway. We have demonstrated for the first time that ionospheric heating can influence VHF radar returns associated with PMSE. Artificially elevating the electron temperatures within the PMSE layer has been shown to reduce the echo power. Based on this and other results from the experiment, it is suggested that the observed reduction in PMSE power is related to an enhancement of the electron diffusivity through the heating.

Unambiguous evidence of HF pump‐enhanced airglow at auroral latitudes

Simultaneous observations by up to three low‐light imaging stations belonging to the Auroral Large Imaging System (ALIS) have provided the first strong evidence of high‐frequency (HF) pump‐enhanced airglow at auroral latitudes. The airglow was enhanced by an ordinary mode 4.04 MHz electromagnetic wave with an effective radiated power (ERP) of about 210 MW that was transmitted from the EISCAT‐Heating facility near Tromsø, Norway. While often observed at low or mid‐latitudes, and despite numerous earlier experiments, no unambiguous observations of pump‐enhanced airglow have been reported at auroral latitudes. On February 16, 1999, the first successful results were obtained, and this paper concentrates on discussing optical data from this event. Triangulated estimations of the altitude and position of the enhanced airglow are also presented. Auroral‐latitude observations of HF pump‐enhanced airglow are important in order to better understand the underlying excitation mechanisms.