EISCAT VHF Research Articles

Ion upflow and downflow at the topside ionosphere observed by the EISCAT VHF radar

<strong class="journal-contentHeaderColor">Abstract.</strong> We have determined the MLT distribution and <i>K_P</i> dependence of the ion upflow and downflow of the thermal bulk oxygen ion population based on a data analysis using the EISCAT VHF radar CP-7 data obtained at Tromsø during the period between 1990 and 1996: (1) both ion upflow and downflow events can be observed at any local time (MLT), irrespective of dayside and nightside, and under any magnetic disturbance level, irrespective of quiet and disturbed levels; (2) these upflow and downflow events are more frequently observed in the nightside than in the dayside; (3) the upflow events are more frequently observed than the downflow events at any local time except midnight and at any <i>K_P</i> level and the difference of the occurrence frequencies between the upflow and downflow events is smaller around midnight; and (4) the occurrence frequencies of both the ion upflow and downflow events appear to increase with increasing <i>K_P</i> level, while the occurrence frequency of the downflow appears to stop increasing at some <i>K_P</i> level.<br><br><b>Key words:</b> Ionosphere (particle acceleration; plasma waves and instabilities) - Space plasma physics (transport processes)

Simultaneous EISCAT Svalbard and VHF radar observations of ion upflows at different aspect angles

A simultaneous EISCAT Svalbard and VHF radar experiment has shown that field‐aligned (FA) ion upflows observed at an altitude of 665 km in the dayside cusp are associated with significant anisotropy of ion temperature, isotropic increases of electron temperature and enhancements of electron density. There is no clear correspondence between the enhancements of the electric field strength and the occurrence of the ion upflows. This suggests that the upflow is driven primarily by precipitation. The data support that in addition to “direct” precipitation effects, namely enhanced ambipolar diffusion and heat flux, also wave‐particle interaction, like wave‐induced transverse ion heating, which causes a hydrodynamic mirror force, may play a role.

EISCAT radar reflection from the vicinity of a noctilucent cloud

The backscattered signal profile received with the EISCAT VHF radar during a noctilucent cloud event of 3–4 August 1997 is discussed. The processing of the radar signal was standard. A strong signal reflection from 83–88 km altitude was observed for three hours without abnormal ionization above this layer. The layer descended with a velocity of 0.5–3 km/hr. Sometimes it was stratified into two layers, the ionosphere between the layers being undisturbed. The scattered signal strength was as strong as would be expected from an electron density of 1011 el/cm3. Simultaneously the MST radar at Esrange revealed the occurrence of PMSE at the same heights. A case of strongly reflected signals for the MST radar from 62 km altitude during the Solar Proton Event of 2 May 1998 is also discussed.

Effect of suprathermal electrons on the intensity and Doppler frequency of electron plasma lines

Abstract. In an incoherent scattering radar experiment, the spectral measurement of the so-called up- and downshifted electron plasma lines provides information about their intensity and their Doppler frequency. These two spectral lines correspond, in the backscatter geometry, to two Langmuir waves travelling towards and away from the radar. In the daytime ionosphere, the presence of a small percentage of photoelectrons produced by the solar EUV of the total electron population can excite or damp these Langmuir waves above the thermal equilibrium, resulting in an enhancement of the intensity of the lines above the thermal level. The presence of photo-electrons also modifies the dielectric response function of the plasma from the Maxwellian and thus influences the Doppler frequency of the plasma lines. In this paper, we present a high time-resolution plasma-line data set collected on the Eiscat VHF radar. The analysed data are compared with a model that includes the effect of a suprathermal electron population calculated by a transport code. By comparing the intensity of the analysed plasma lines data to our model, we show that two sharp peaks in the electron suprathermal distribution in the energy range 20-30 eV causes an increased Landau damping around 24.25 eV and 26.25 eV. We have identified these two sharp peaks as the effect of the photoionisation of N2 and O by the intense flux of monochromatic HeII radiation of wavelength 30.378 nm (40.812 eV) created in the chromospheric network and coronal holes. Furthermore, we see that what would have been interpreted as a mean Doppler drift velocity for a Maxwellian plasma is actually a shift of the Doppler frequency of the plasma lines due to suprathermal electrons.Key words. Ionosphere (electric fields and currents; solar radiation and cosmic ray effects)

Effect of suprathermal electrons on the intensity and Doppler frequency of electron plasma lines

In an incoherent scattering radar experiment, the spectral measurement of the so-called up- and downshifted electron plasma lines provides information about their intensity and their Doppler frequency. These two spectral lines correspond, in the backscatter geometry, to two Langmuir waves travelling towards and away from the radar. In the daytime ionosphere, the presence of a small percentage of photoelectrons produced by the solar EUV of the total electron population can excite or damp these Langmuir waves above the thermal equilibrium, resulting in an enhancement of the intensity of the lines above the thermal level. The presence of photo-electrons also modifies the dielectric response function of the plasma from the Maxwellian and thus influences the Doppler frequency of the plasma lines. In this paper, we present a high time-resolution plasma-line data set collected on the Eiscat VHF radar. The analysed data are compared with a model that includes the effect of a suprathermal electron population calculated by a transport code. By comparing the intensity of the analysed plasma lines data to our model, we show that two sharp peaks in the electron suprathermal distribution in the energy range 20-30 eV causes an increased Landau damping around 24.25 eV and 26.25 eV. We have identified these two sharp peaks as the effect of the photoionisation of N2 and O by the intense flux of monochromatic HeII radiation of wavelength 30.378 nm (40.812 eV) created in the chromospheric network and coronal holes. Furthermore, we see that what would have been interpreted as a mean Doppler drift velocity for a Maxwellian plasma is actually a shift of the Doppler frequency of the plasma lines due to suprathermal electrons.Key words. Ionosphere (electric fields and currents; solar radiation and cosmic ray effects)

Mesospheric observations with the EISCAT UHF radar during polar cap absorption events: 3. Comparison with simultaneous EISCAT VHF measurements

Abstract. Mesospheric observations were obtained by the EISCAT UHF and VHF radars during the solar proton event of March 1990. We present the first comparison of incoherent-scatter spectral measurements from the middle mesosphere using simultaneous, co-located observations by the two radars. VHF spectra observed with a vertical antenna were found to be significantly narrower than model predictions, in agreement with earlier UHF results. For antenna pointing directions that were significantly away from the vertical, the wider VHF radar beam gave rise to broadening of the observed spectra due to vertical shears in the horizontal wind. In this configuration, UHF spectral measurements were found to be more suitable for aeronomical applications. Both radar systems provide consistent and reliable estimates of the neutral wind. Spectral results using both the multipulse and pulse-to-pulse schemes were intercompared and their suitability for application to combined mesosphere – lower thermosphere studies investigated.Key words. Mesophere · Lower thermosphere · EISCAT UHF radar · EISCAT VHF radar

Vertical velocities associated with gravity waves measured in the mesosphere and lower thermosphere with the EISCAT VHF radar

Abstract. The EISCAT VHF radar (69.4°N, 19.1°E) has been used to record vertical winds at mesopause heights on a total of 31 days between June 1990 and January 1993. The data reveal a motion field dominated by quasi-monochromatic gravity waves with representative apparent periods of ~30–40 min, amplitudes of up to ~2.5 m s–1 and large vertical wavelength. In some instances waves appear to be ducted. Vertical profiles of the vertical-velocity variance display a variety of forms, with little indication of systematic wave growth with height. Daily mean variance profiles evaluated for consecutive days of recording show that the general shape of the variance profiles persists over several days. The mean variance evaluated over a 10 km height range has values from 1.2 m2s–2 to 6.5 m2s–2 and suggests a semi-annual seasonal cycle with equinoctial minima and solsticial maxima. The mean vertical wavenumber spectrum evaluated at heights up to 86 km has a slope (spectral index) of –1.36 ± 0.2, consistent with observations at lower heights but disagreeing with the predictions of a number of saturation theories advanced to explain gravity-wave spectra. The spectral slopes evaluated for individual days have a range of values, and steeper slopes are observed in summer than in winter. The spectra also appear to be generally steeper on days with lower mean vertical-velocity variance.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides)

Modelling turbulent energy dissipation in the high-latitude mesosphere

Abstract The global numerical model of the Earth's thermosphere, ionosphere and protonosphere constructed at the Kaliningrad Observatory of IZMIRAN and Polar Geophysical Institute in Murmansk (Namgaladze et al., 1991), hereafter referred to as PGI97, is being extended to encompass modelling of the mesosphere. Here we report the first predictions of turbulent intensities in the height regime 80 to 90 km. Recently, Hall (1997) reported estimates of the turbulent energy dissipation rate, e, using the EISCAT VHF radar located in Northern Norway (69°N, 19°E), which has, in turn, been compared to in situ measurements. Thus initial testing of PGI97 has concentrated on the same region. The agreements between PGI97 and EISCAT results for summer and winter solstice mesospheres are good. The general seasonal variation has been investigated, again showing good agreement with the EISCAT results. However, when examining the average energy dissipation in the 80–90 km height regime, the model shows less variability than the observations.

Mesospheric observations with the EISCAT UHF radar during polar cap absorption events: 3. Comparison with simultaneous EISCAT VHF measurements

<strong class="journal-contentHeaderColor">Abstract.</strong> Mesospheric observations were obtained by the EISCAT UHF and VHF radars during the solar proton event of March 1990. We present the first comparison of incoherent-scatter spectral measurements from the middle mesosphere using simultaneous, co-located observations by the two radars. VHF spectra observed with a vertical antenna were found to be significantly narrower than model predictions, in agreement with earlier UHF results. For antenna pointing directions that were significantly away from the vertical, the wider VHF radar beam gave rise to broadening of the observed spectra due to vertical shears in the horizontal wind. In this configuration, UHF spectral measurements were found to be more suitable for aeronomical applications. Both radar systems provide consistent and reliable estimates of the neutral wind. Spectral results using both the multipulse and pulse-to-pulse schemes were intercompared and their suitability for application to combined mesosphere – lower thermosphere studies investigated.<br><br><b>Key words. </b>Mesophere · Lower thermosphere · EISCAT UHF radar · EISCAT VHF radar

Vertical velocities associated with gravity waves measured in the mesosphere and lower thermosphere with the EISCAT VHF radar

Vertical velocities associated with gravity waves measured in the mesosphere and lower thermosphere with the EISCAT VHF radar

An improved technique to determine neutral winds in the auroral mesosphere using the EISCAT VHF incoherent scatter radar

Abstract The EISCAT VHF radar can be operated to obtain simultaneous measurements of the zonal and meridional components of the mesospheric neutral wind with an altitude resolution of the order of 1 km. We describe the application of this technique and introduce some initial results from an interval near the autumn equinox. These include 21 hours of continuous observations of the zonal wind in the mesosphere, showing predominantly semi-diurnal tidal characteristics in the observed altitude range of 75–95 km. Mean winds, and the amplitude and phase of the 12-h zonal component as a function of altitude, are deduced. Significant differences from published climatological results are found. Evidence of shorter period wave activity is also apparent. The vertical wind component is demonstrated to have a negligible effect on the derived horizontal velocities in this data set, using 5-min averages. Clockwise rotation of the horizontal wind vector with increasing altitude, with a vertical wavelength of 8–12 km, is a fundamental property of the wind field at these heights.

Enhanced incoherent scatter plasma lines

Abstract. Detailed model calculations of auroral secondary and photoelectron distributions for varying conditions have been used to calculate the theoretical enhancement of incoherent scatter plasma lines. These calculations are compared with EISCAT UHF radar measurements of enhanced plasma lines from both the E and F regions, and published EISCAT VHF radar measurements. The agreement between the calculated and observed plasma line enhancements is good. The enhancement from the superthermal distribution can explain even the very strong enhancements observed in the auroral E region during aurora, as previously shown by Kirkwood et al. The model calculations are used to predict the range of conditions when enhanced plasma lines will be seen with the existing high-latitude incoherent scatter radars, including the new EISCAT Svalbard radar. It is found that the detailed structure, i.e. the gradients in the suprathermal distribution, are most important for the plasma line enhancement. The level of superthermal flux affects the enhancement only in the region of low phase energy where the number of thermal electrons is comparable to the number of suprathermal electrons and in the region of high phase energy where the suprathermal fluxes fall to such low levels that their effect becomes small compared to the collision term. To facilitate the use of the predictions for the different radars, the expected signal- to-noise ratios (SNRs) for typical plasma line enhancements have been calculated. It is found that the high-frequency radars (Søndre Strømfjord, EISCAT UHF) should observe the highest SNR, but only for rather high plasma frequencies. The VHF radars (EISCAT VHF and Svalbard) will detect enhanced plasma lines over a wider range of frequencies, but with lower SNR.

Numerical modelling of the thermospheric and ionospheric effects of magnetospheric processes in the cusp region

Abstract. The thermospheric and ionospheric effects of the precipitating electron flux and field-aligned-current variations in the cusp have been modelled by the use of a new version of the global numerical model of the Earth's upper atmosphere developed for studies of polar phenomena. The responses of the electron concentration, ion, electron and neutral temperature, thermospheric wind velocity and electric-field potential to the variations of the precipitating 0.23-keV electron flux intensity and field-aligned current density in the cusp have been calculated by solving the corresponding continuity, momentum and heat balance equations. Features of the atmospheric gravity wave generation and propagation from the cusp region after the electron precipitation and field-aligned current-density increases have been found for the cases of the motionless and moving cusp region. The magnitudes of the disturbances are noticeably larger in the case of the moving region of the precipitation. The thermospheric disturbances are generated mainly by the thermospheric heating due to the soft electron precipitation and propagate to lower latitudes as large-scale atmospheric gravity waves with the mean horizontal velocity of about 690 m s–1. They reveal appreciable magnitudes at significant distances from the cusp region. The meridional-wind-velocity disturbance at 65° geomagnetic latitude is of the same order (100 m s–1) as the background wind due to the solar heating, but is oppositely directed. The ionospheric disturbances have appreciable magnitudes at the geomagnetic latitudes 70°–85°. The electron-concentration and -temperature disturbances are caused mainly by the ionization and heating processes due to the precipitation, whereas the ion-temperature disturbances are influence strongly by Joule heating of the ion gas due to the electric-field disturbances in the cusp. The latter strongly influence the zonal- and meridional-wind disturbances as well via the effects of ion drag in the cusp region. The results obtained are of interest because of the location of the EISCAT Svalbard Radar in the cusp region and the associated observations at lower latitudes that will be possible using the existing EISCAT UHF and VHF radars. The paper makes predictions for both these regions, and these predictions will be tested by joint observations by ESR, EISCAT UHF/VHF and other ground-based ionosphere/thermosphere observations.

Calibration of a numerical ionospheric model with EISCAT observations

Abstract. A set of EISCAT UHF and VHF observations is used for calibrating a coupled fluid-kinetic model of the ionosphere. The data gathered in the period 1200- 2400 UT on 24 March 1995 had various intervals of interest for such a calibration. The magnetospheric activity was very low during the afternoon, allowing for a proper examination of a case of quiet ionospheric conditions. The radars entered the auroral oval just after 1900 UT: a series of dynamic events probably associated with rapidly moving auroral arcs was observed until after 2200 UT. No attempts were made to model the dynamical behaviour during the 1900–2200 UT period. In contrast, the period 2200–2400 UT was characterised by quite steady precipitation: this latter period was then chosen for calibrating the model during precipitation events. The adjustment of the model on the four primary parameters observed by the radars (namely the electron concentration and temperature and the ion temperature and velocity) needed external inputs (solar fluxes and magnetic activity index) and the adjustments of a neutral atmospheric model in order to reach a good agreement. It is shown that for the quiet ionosphere, only slight adjustments of the neutral atmosphere models are needed. In contrast, adjusting the observations during the precipitation event requires strong departures from the model, both for the atomic oxygen and hydrogen. However, it is argued that this could well be the result of inadequately representing the vibrational states of N2 during precipitation events, and that these factors have to be considered only as ad hoc corrections.

Alternating-code experiment for plasma-line studies

Abstract. We present results of the first plasma-line measurement of the incoherent spectrum using the alternating-code technique with the EISCAT VHF radar. This technique, which has earlier mostly been used to measure high-resolution E-region ion-line spectra, turned out to be a very good alternative to other techniques for plasma-line measurements. The experiment provides simultaneous measurement of the ion line and downshifted and upshifted plasma-line spectra with an altitude resolution of 3 km and a temporal resolution of 10 s. The measurements are taken around the peak of the F region, but not necessarily at the peak itself, as is the case with the long-pulse technique. The condition for success is that the scale height should be large enough such that the backscattered signal from the range extent of one gate falls inside the receiver filter. The data are analysed and the results are combined with the results of the ion-line data analysis to estimate electron mean drift velocity and thereafter electric currents along the line of sight of the radar using both the standard dispersion relation assuming a Maxwellian electron velocity distribution and the more recent model including a heat-flow correction term.

PMSE observations at three different frequencies in northern Europe during summer 1994

Abstract. Simultaneous observations of polar mesosphere summer echoes (PMSE) have been carried out during summer 1994 in northern Norway using three radars on different frequencies: the ALOMAR SOUSY radar at Andenes on 53.5 MHz, the EISCAT VHF radar at Tromsø on 224 MHz and the MF radar at Tromsø on 2.78 MHz. During the common measuring period in July/August 1994, PMSE could be detected at 224 and 53.5 MHz, and there are strong hints that PMSE also occur at 2.78 MHz. Reliable correlations between hourly backscattered power values indicate that the PMSE structures have zonal extensions of more than 130 km and can be detected at very different scales (half wavelength) between 0.67 (EISCAT VHF radar) and 54 m (MF radar). Using the wind values derived by the MF radar it can be shown that the mesospheric wind field influences the structure of PMSE. The diurnal variation of PMSE is strongly connected with tidal-wind components, whereas spatial differences of PMSE can partly be explained by the mean wind field.

Coherent echoes during EISCAT UHF Common Programmes

Coherent echoes, whatever their origin, appear as narrow spectra superimposed on the normal incoherent scatter spectra recorded by the EISCAT UHF and VHF radars. Their narrow width, higher than normal amplitude and the induced spectral asymmetry in the overall spectrum enable them to be easily found. by a simple computer algorithm which has routinely been used to pick out such spectra from Common-Programme (CP) data, usually in real time. The UHF CP-1 data have been searched back to 1986 for such echoes. Most of the echoes found in this way are caused by noise spikes, but radio interference, satellite echoes, and, of most interest, enhanced ion acoustic echoes caused by plasma processes are all found. We present a summary of the findings of this search, with special emphasis on the CP-1 data set. Examples of remote site enhanced ion acoustic shoulders are presented, as well as some unexplained echoes from Tromsø.

Variability of dayside high latitude convection associated with a sequence of auroral transients

10 second resolution ionospheric convection data covering the invariant latitude range from 71° to 76°, obtained by using the EISCAT UHF and VHF radars, are combined with optical data from Ny Ålesund during a sequence of auroral transients in the post-noon sector (∼ 15 MLT). Satellite observations of polar cap convection patterns suggest negative B Z and B Y components of the interplanetary magnetic field. Burst-like enhancements of westward (sunward) post-noon convection were accompanied by eastward moving auroral forms at higher latitudes, above the convection reversal boundary. In this case the background convection was weak, whereas the integrated potential drop across the radar field-of-view associated with the westward flow bursts was typically ∼ 20-35 kV. The auroral phenomenon consists of a series of similar events with a mean repetition period of 8 min. A close correlation between the auroral activity and convection enhancements in the cleft ionosphere is demonstrated.

A statistical study of polar mesosphere summer echoes observed by EISCAT

A comprehensive survey of data on ‘Polar Mesosphere Summer Echoes’, observed by the EISCAT VHF radar during 1988–1993, confirms that (1) these echoes are a summer phenomenon, with a season lasting from June to August; (2) PMSE occur most frequently around noon and midnight, and thus follow a semidiurnal pattern; (3) PMSE occur at a mean height of 85±2 km; (4) there is often a good correlation between the vertical Doppler velocity and the rate of change of echo height, which suggests that the echoing structures move bodily, perhaps in response to gravity waves. Previous results on the lack of correlation between the occurrence of PMSE and noctilucent clouds are reinforced.

Spatial interferometry measurements of polar mesosphere summer echoes with the EISCAT VHF radar

Spatial domain interferometry is a useful technique for studying the dynamics of both the ionosphere and the neutral atmosphere. In the ionospheric case the scatterers are highly localized in space and can be tracked as they drift through the beam to determine the horizontal winds. However, in the atmospheric case the scatterers can fill most of the resolution volume of the radar. For the EISCAT VHF radar in Tromsø, Norway, with its single E‐W baseline, this volume scattering only allows determination of the trace velocity parallel to the baseline. With this limitation we have applied this technique at EISCAT to study the dynamics of the measosphre. On August 11, 1992, a thin sheet of strong scatterers during a polar mesospheric summer echo (PMSE) event, initially located at 83 km, was lifted upward by 300 m within a couple of minutes. The vertical velocity at this time, determined from the measured Doppler shifts, jumped from 2 m/s downward to more than 4 m/s upward. From the cross‐spectral analysis we also see a change in the direction of the zonal wind from westward to eastward during the discontinuity in the vertical velocity. The behavior of the two velocity components is qualitatively similar to the wind corner observations of Widdel and von Zahn (1990). We also consider the possibility that a solitary wave may cause this event, but conclude that there is no compelling evidence to support this explanation.