EISCAT Svalbard Radar Research Articles

Equatorward propagating auroral arcs driven by ULF wave activity: Multipoint ground‐ and space‐based observations in the dusk sector auroral oval

AbstractObservations of multiple equatorward propagating arcs driven by a resonant Alfvén wave on closed field lines are presented. Data sets from the European Incoherent Scatter Svalbard Radar (ESR) and Meridian Scanning Photometer in Longyearbyen, All‐Sky Camera in Ny Ålesund, ground magnetometer data in Svalbard, and Defense Meteorological Satellite Program (DMSP) F16 satellite were utilized to study the arc structures. The arcs had an equatorward phase propagation of ~0.46 km s−1 and were observed in the dusk ionosphere from 1800 to 2030 magnetic local time. Analysis of the optical data indicates that the Alfvén wave had a frequency of 1.63 mHz and an azimuthal wave number, m ~ −20 (the negative sign indicating a westward propagation). Inverted‐V electron populations associated with field‐aligned currents of between 0.5 and 0.8 μA m−2 are observed by DMSP F16 inside the arc structures. In addition to electron density enhancements associated with the arcs, the ESR data show elevated ion temperatures in between the arcs consistent with electric field enhancements and ionospheric heating effects. The combination of ESR and DMSP F16 data indicates that the wave energy was dissipated through ionospheric Joule and/or ion frictional heating and acceleration of particles into the ionosphere, generating the auroral displays. The fine‐scale structuring, in addition to the propagation direction and scale size, would suggest that the auroral features are the signatures of a field line resonance driven by an interaction with a compressional fast mode wave propagating earthward from the magnetotail.

Horizontal profile of a moving red line cusp aurora

AbstractAuroral emission at a wavelength of 630.0 nm (red line) has a long radiative time. In this study, we show how this long radiative time controls the horizontal extent of a moving mesoscale aurora, which is typical of the cusp. Using high time resolution (4 s) observations by an all‐sky imager at Longyearbyen, Svalbard, and observations by the European Incoherent Scatter Svalbard Radar (ESR), we examined the auroral emission enhancements obtained in the cusp during an interval of southward interplanetary magnetic field on 27 November 2011. Simultaneous observations from the all‐sky imager and the ESR clearly show the manner in which auroral emission regions passed through the radar's field of view. When the front edge of the moving auroral structure intersected the radar's field of view, the ESR electron temperature was enhanced. A few minutes later, the ESR electron temperature dropped to the background level, indicating that the mesoscale electron precipitation region shifted away from the radar's field of view. At that time, the auroral emission in the radar's field of view decreased but still had adequate intensities. These results provide evidence demonstrating that moving cusp auroral emission occurs behind the electron precipitation region as well as inside that region. We interpreted this feature semiquantitatively by using the equation of continuity of the density of the excited atomic oxygen. Our model indicates that the maximum intensity in the moving auroral structure occurs at a point along the trailing edge of the electron precipitation region and that the velocity of that region is important for determining how the 630.0 nm aurora emissions extend horizontally.

GPS scintillations associated with cusp dynamics and polar cap patches

This paper investigates the relative scintillation level associated with cusp dynamics (including precipitation, flow shears, etc.) with and without the formation of polar cap patches around the cusp inflow region by the EISCAT Svalbard radar (ESR) and two GPS scintillation receivers. A series of polar cap patches were observed by the ESR between 8:40 and 10:20 UT on December 3, 2011. The polar cap patches combined with the auroral dynamics were associated with a significantly higher GPS phase scintillation level (up to 0.6 rad) than those observed for the other two alternatives, i.e., cusp dynamics without polar cap patches, and polar cap patches without cusp aurora. The cusp auroral dynamics without plasma patches were indeed related to GPS phase scintillations at a moderate level (up to 0.3 rad). The polar cap patches away from the active cusp were associated with sporadic and moderate GPS phase scintillations (up to 0.2 rad). The main conclusion is that the worst global navigation satellite system space weather events on the dayside occur when polar cap patches enter the polar cap and are subject to particle precipitation and flow shears, which is analogous to the nightside when polar cap patches exit the polar cap and enter the auroral oval.

An evaluation of International Reference Ionosphere electron density in the polar cap and cusp using EISCAT Svalbard radar measurements

Abstract. Incoherent scatter radar measurements are an important source for studies of ionospheric plasma parameters. In this paper the EISCAT Svalbard radar (ESR) long-term database is used to evaluate the International Reference Ionosphere (IRI) model. The ESR started operations in 1996, and the accumulated database up to 2012 thus covers 16 years, giving an overview of the ionosphere in the polar cap and cusp during more than one solar cycle. Data from ESR can be used to obtain information about primary plasma parameters: electron density, electron and ion temperature, and line-of-sight plasma velocity from an altitude of about 50 and up to 1600 km. Monthly averages of electron density and temperature and ion temperature and composition are also provided by the IRI model from an altitude of 50 to 2000 km. We have compared electron density data obtained from the ESR with the predicted electron density from the IRI-2016 model. Our results show that the IRI model in general fits the ESR data well around the F2 peak height. However, the model seems to underestimate the electron density at lower altitudes, particularly during winter months. During solar minimum the model is also less accurate at higher altitudes. The purpose of this study is to validate the IRI model at polar latitudes.

Joule heating hot spot at high latitudes in the afternoon sector

AbstractThe afternoon Joule heating hot spot has been studied statistically by using the EISCAT Svalbard Radar (ESR) measurements at 75.4° Corrected Geomagnetic latitude (CGMLAT) and the OMNI solar wind data base. For a small subset of events, the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) field‐aligned current distributions have been available. The main results are as follows. Afternoon Joule heating hot spots are associated with high values of ionospheric electric fields and slightly enhanced Pedersen conductances. The Joule heating hot spot values are larger in summer than in winter, which can be explained by the higher Pedersen conductances during summer than winter. The afternoon Joule heating hot spots are located close to the reversals of the large‐scale field‐aligned current systems. The most common location is close to the Region 1/Region 2 boundary and those events are associated with sunward convecting F region plasma. In a few cases, the hot spots take place close to the Region 1/Region 0 boundary and then the ionospheric plasma is convecting antisunward. The hot spots may occur both during slow (<450 km/s) and high (>450 km/s) speed solar wind conditions. During slow‐speed solar wind events, the dominant interplanetary magnetic field (IMF) direction is southward, which is the general requirement for the low‐latitude magnetic merging at the dayside magnetopause. During high‐speed solar wind, also northward IMF conditions appear, but those are associated with large values of the IMF |By| component, making again the dayside magnetopause merging possible. Finally, the measured afternoon hot spot Joule heating rates are not a linear function of the solar wind energy coupling function.

Statistical study of the GNSS phase scintillation associated with two types of auroral blobs

AbstractThis study surveys space weather effects on GNSS (Global Navigation Satellite System) signals in the nighttime auroral and polar cap ionosphere using scintillation receivers, all‐sky imagers, and the European Incoherent Scatter Svalbard radar. We differentiate between two types of auroral blobs: blob type 1 (BT 1) which is formed when islands of high‐density F region plasma (polar cap patches) enter the nightside auroral oval, and blob type 2 (BT 2) which are generated locally in the auroral oval by intense particle precipitation. For BT 1 blobs we have studied 41.4 h of data between November 2010 and February 2014. We find that BT 1 blobs have significantly higher scintillation levels than their corresponding polar cap patch; however, there is no clear relationship between the scintillation levels of the preexisting polar cap patch and the resulting BT 1 blob. For BT 2 blobs we find that they are associated with much weaker scintillations than BT 1 blobs, based on 20 h of data. Compared to patches and BT 2 blobs, the significantly higher scintillation level for BT 1 blobs implies that auroral dynamics plays an important role in structuring of BT 1 blobs.

High-latitude ion temperature climatology during the International Polar Year 2007–2008

This article presents the results of an ion temperature climatology study that examined ionospheric measurements from the European Incoherent SCATter (EISCAT) Svalbard Radar (ESR: 78.2° N, 16.0° E) and the Poker Flat Incoherent Scatter Radar (PFISR: 65.1° N, 212.6° E) during the year-long campaign of the International Polar Year (IPY) from March 2007 to February 2008. These observations were compared with those of the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIE-GCM), as well as the International Reference Ionosphere 2012 (IRI-2012). Fairly close agreement was found between the observations and TIE-GCM results. Numerical experiments revealed that the daily variation in the high-latitude ion temperature, about 100–200 K, is mainly due to ion frictional heating. The ion temperature was found to increase in response to elevated geomagnetic activity at both ESR and PFISR, which is consistent with the findings of previous studies. At ESR, a strong response occurred during the daytime, which was interpreted as a result of dayside-cusp heating. Neither TIE-GCM nor IRI-2012 reproduced the strong geomagnetic activity response at ESR, underscoring the need for improvement in both models at polar latitudes.

Ion upflow dependence on ionospheric density and solar photoionization

AbstractMotivated by rocket observations showing a variety of different ionospheric responses to precipitation, this paper explores the influence of the background ionospheric density on upflow resulting from auroral precipitation. Simulations of upflow driven by auroral precipitation were conducted using a version of the Varney et al. (2014) model driven by precipitation characterized by observations made during the 2012 Magnetosphere‐Ionosphere Coupling in the Alfvén resonator rocket mission and using a variety of different initial electron density profiles. The simulation results show that increased initial density before the onset of precipitation leads to smaller electron temperature increases, longer ionospheric heating timescales, weaker ambipolar electric fields, lower upflow speeds, and longer upflow timescales but larger upflow fluxes. The upflow flux can increase even when the ambipolar electric field strength decreases due to the larger number of ions that are accelerated. Long‐term observations from the European Incoherent Scatter (EISCAT) Svalbard radar taken during the International Polar Year support the effects seen in the simulations. This correlation between ionospheric density and ion upflows emphasizes the important role of photoionization from solar ultraviolet radiation, which the EISCAT observations show can increase ionospheric density by as much as an order of magnitude during the summer months.

Three-dimensional imaging of the plasma parameters of a moving cusp aurora

During a period of negative IMF BZ on 13 January 2013, an all-sky imager at Longyearbyen, Svalbard observed a mesoscale aurora moving towards the east-northeast in the cusp, passing through the field of view of the EISCAT Svalbard Radar (ESR) elevation scan. The elevation scans that were being performed at that time have a horizontal coverage of approximately 300km, at an altitude of 300km. The plasma data obtained from the elevation scans and the 630-nm aurora emission data from the all-sky imager have shown that ion temperature enhanced 50–60s earlier than electron density, and that the maximum auroral intensity in the ESR's field of view occurred about 40s after the electron density enhancement. On the basis of these results we have constructed three-dimensional images of elevated ion temperatures and enhanced electron density associated with the mesoscale moving cusp aurora. The three-dimensional image shows that the enhancement of the ion temperature is prominent in the horizontal area of ~160km×~80km below an altitude of ~300km, and that this volume forms on the forward side of the enhanced electron density region. We interpret these configurations as being a result of a mesoscale twin-cell convection, which is embedded in the background flow such that the symmetrical axis of the twin-cell is inclined from the background flow direction by several tens of degrees. Our method for visualizing three-dimensional features such as these could be an effective approach to understanding the mesoscale dynamics of the cusp, which is usually located in latitudes that are difficult for the currently-operated radars that permit three-dimensional, simultaneous measurements to investigate.

Feasibility study on the derivation of the O+‐O collision frequency from ionospheric field‐aligned observations

AbstractRecently, Vickers et al. (2013) used a simplified ion momentum equation to derive the ion‐neutral (O+‐O) collision frequency and thermospheric atomic oxygen density from the European Incoherent Scatter Svalbard Radar data. It was assumed that the vertical neutral wind at high latitudes is generally small under geomagnetically quiet conditions so that it was neglected in their derivation. Although the vertical neutral wind may indeed be small, the field‐aligned neutral wind component, which needs to be taken into consideration in the ion momentum equation, can be large. Simulations with the Thermosphere‐Ionosphere Electrodynamics Global Circulation Model have been carried out to evaluate the effect of the neutral wind on the estimation of the ion‐neutral collision frequency from the ion momentum equation. Our results reveal that, even at high latitudes, the effect of the neutral wind can be significant in the derivation of the ion‐neutral collision frequency from ionospheric field‐aligned observations, especially at night.

Swarm and ESR observations of the ionospheric response to a field‐aligned current system in the high‐latitude midnight sector

AbstractWe present a conjunction between the Swarm fleet and the European Incoherent Scatter Svalbard Radar (ESR) on 9 January 2014. The Swarm orbit in the early phase of the mission gives us the unique opportunity of sequencing the temporal evolution of the observed field‐aligned current system in the nightside, near magnetic local midnight. These field‐aligned currents are seen to move poleward through the radar field of view and to affect the observed ionosphere. The upward field‐aligned current (FAC) is responsible, at least in part, for the heating of the ionospheric electrons. It is less clear whether the downward FAC cools the ionosphere. We use the TRANSCAR model of the ionosphere to quantify the thermoelectric effect that comes into play. Finally, we compare the plasma parameters measured by the Langmuir probe on board Swarm and the ESR and conclude on an agreement within the errors.

GPS scintillation and irregularities at the front of an ionization tongue in the nightside polar ionosphere

AbstractIn this paper we study a tongue of ionization (TOI) on 31 October 2011 which stretched across the polar cap from the Canadian dayside sector to Svalbard in the nightside ionosphere. The TOI front arrived over Svalbard around 1930 UT. We have investigated GPS scintillation and irregularities in relation to this TOI front. This is the first study presenting such detailed multi‐instrument data of scintillation and irregularities in relation to a TOI front. Combining data from an all‐sky imager, the European Incoherent Scatter Svalbard Radar, the Super Dual Auroral Radar Network Hankasalmi radar, and three GPS scintillation and total electron content (TEC) monitors in Longyearbyen and Ny‐Ålesund, we observe bursts of phase scintillation and no amplitude scintillation in relation to the leading gradient of the TOI. Spectrograms of 50Hz phase measurements show highly localized and variable structuring of the TOI leading gradient, with no structuring or scintillation within the TOI or ahead of the TOI.

On the relation of Langmuir turbulence radar signatures to auroral conditions

AbstractWe 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.

Strong E region ionization caused by the 1767 trail during the 2002 Leonids

AbstractIntensive E region ionization extending up to 140 km altitude and lasting for several hours was observed with the European Incoherent Scatter (EISCAT) UHF radar during the 2002 Leonids meteor shower maximum. The level of global geomagnetic disturbance as well as the local geomagnetic and auroral activity in northern Scandinavia were low during the event. Thus, the ionization cannot be explained by intensive precipitation. The layer was 30–40 km thick, so it cannot be classified as a sporadic E layer which are typically just a few kilometers wide. Incoherent scatter radars have not to date reported any notable meteor shower‐related increases in the average background ionization. The 2002 Leonids storm flux, however, was so high that it might have been able to induce such an event. The Chemical Ablation Model is used to estimate deposition rates of individual meteors. The resulting electron production, arising from hyperthermal collisions of ablated atoms with atmospheric molecules, is related to the predicted Leonid flux values and observed ionization on 19 November 2002. The EISCAT Svalbard Radar (ESR) located at some 1000 km north of the UHF site did not observe any excess ionization during the same period. The high‐latitude electrodynamic conditions recorded by the SuperDARN radar network show that the ESR was within a strongly drifting convection cell continuously fed by fresh plasma while the UHF radar was outside the polar convection region maintaining the ionization.

A solar cycle of upper thermosphere density observations from the EISCAT Svalbard Radar

AbstractWe exploit a recently developed technique, based on ion‐neutral coupling, which allows estimations of the upper thermospheric neutral density using measurements of ionospheric plasma parameters made by the European Incoherent Scatter (EISCAT) Svalbard Radar (ESR). The technique is applied to a 13 year long data set of measurements for the purpose of studying and quantifying the effect of solar activity on the upper thermospheric density inside the polar cap. We concentrate on the effect of solar activity at 350 km altitude and find a strong linear correlation between the ESR estimates for the atomic oxygen density and the solar irradiance proxy F10.7 index. We use the relationship to isolate variations in the thermospheric density that are present after solar activity influences are removed. Our results show a decrease in the density of a few percent over the 13 year period, which is nevertheless smaller than the uncertainty associated with the decline. We anticipate that the statistical significance of this result will only increase by studying a longer data set. Conjunctions with the CHAMP satellite that show very good agreement is achieved at 350 km especially during low solar activity.

Which cusp upflow events can possibly turn into outflows?

AbstractTwo sequences, before and after magnetic noon, respectively, of poleward moving auroral forms with associated upflows situated above the European Incoherent Scatter Svalbard Radar allowed close study of ion upflow dynamics. We find that flux intensity is correlated with plasma temperature and that upflowing plasma undergoes acceleration proportional to the slope of the velocity profile and to the velocity at each altitude. The potential for upflows to lift thermal plasma to regions where broadband extremely low frequency electric field activity can cause nonthermal acceleration leading to outflow is examined. Equations for estimating the travel time of upflowing plasma are presented. We find that around 40% of the observed upflow profiles with a unit number flux greater than 1 × 1013 m−2 s−1 can transport plasma from 500 to 800 km altitude in less than 10 min, approximately the typical lifetime of pulsed upflow events. Almost all such profiles can transport plasma from 600 to 800 km in the same time span. Typical transport times for other altitude ranges are also presented. Post magnetic noon the background electron density was somewhat higher than prenoon due to transport of EUV‐enhanced plasma, and the postnoon ion flux was somewhat weaker than prenoon.

Extreme ion heating in the dayside ionosphere in response to the arrival of a coronal mass ejection on 12 March 2012

Abstract. Simultaneous measurements of the polar ionosphere with the European Incoherent Scatter (EISCAT) ultra high frequency (UHF) radar at Tromsø and the EISCAT Svalbard radar (ESR) at Longyearbyen were made during 07:00–12:00 UT on 12 March 2012. During the period, the Advanced Composition Explorer (ACE) spacecraft observed changes in the solar wind which were due to the arrival of coronal mass ejection (CME) effects associated with the 10 March M8.4 X-ray event. The solar wind showed two-step variations which caused strong ionospheric heating. First, the arrival of shock structures in the solar wind with enhancements of density and velocity, and a negative interplanetary magnetic field (IMF)-Bz component caused strong ionospheric heating around Longyearbyen; the ion temperature at about 300 km increased from about 1100 to 3400 K over Longyearbyen while that over Tromsø increased from about 1050 to 1200 K. After the passage of the shock structures, the IMF-Bz component showed positive values and the solar wind speed and density also decreased. The second strong ionospheric heating occurred after the IMF-Bz component showed negative values again; the negative values lasted for more than 1.5 h. This solar wind variation caused stronger heating of the ionosphere in the lower latitudes than higher latitudes, suggesting expansion of the auroral oval/heating region to the lower latitude region. This study shows an example of the CME-induced dayside ionospheric heating: a short-duration and very large rise in the ion temperature which was closely related to the polar cap size and polar cap potential variations as a result of interaction between the solar wind and the magnetosphere.

Comparative studies on ionospheric climatological features of NmF2 among the Arctic and Antarctic stations

Climatological features of the maximum electron density of the ionospheric F2 region (NmF2) are investigated and compared among the Arctic and Antarctic stations using long-term observations from the dynasonde measurements at Tromso, the EISCAT Svalbard Radar (ESR) measurements at Longyearbyen, and the digisonde DPS-4 measurements at Zhongshan, Antarctica. The NmF2 data are sorted as monthly medians of NmF2 for each hour, month, and solar activity. Results are presented as follows: (1) On the diurnal variability, the maximum value of NmF2 mainly occurs at local noon at the auroral latitude station Tromso, but at magnetic noon at the cusp latitude station Longyearbyen and between local noon and magnetic noon at Zhongshan, indicating the importance of soft particle precipitation and ionospheric convection in the cusp region. (2) In addition to the daytime peak, there is another peak just before magnetic midnight at Longyearbyen in winter during solar maximum years, which is attributed to the cross-polar transport of EUV ionization from day side to night side. (3) An enhancement of NmF2 is detected at magnetic midnight in winter at Tromso. It may be caused by the nighttime substorms. (4) On the seasonal variability, there are semi-annual anomaly at Tromso and normal variability at both Longyearbyen and Zhongshan during solar minimum years. (5) During solar maximum years there is always semi-annual anomaly for all three stations. The well-known winter anomaly is evident at both Tromso and Zhongshan, but does not exist at Longyearbyen. These anomalies are mainly due to the differences in photoionization and chemical compositions between the three stations.

Incoherent scatter radar‐FAST satellite common volume observations of upflow‐to‐outflow conversion

AbstractIncoherent scatter radar measurements from the Sondrestrom Research Facility and the European Incoherent Scatter Svalbard radar have been combined with all‐sky images, polar convection measurements, and FAST particle and field measurements to quantify the contribution of different magnetosphere‐ionosphere coupling processes to the extraction efficiency of ions from the ionosphere. Upflowing ions are traced from their source vertically and horizontally to determine where and when they are likely to intersect the acceleration region observed by FAST. The duration and location of auroral emissions are used to estimate the size and duration of the acceleration region. The upflow‐to‐outflow efficiency is estimated for three periods of polar cap boundary intensifications and streamers during substorm recovery and steady magnetospheric convection. The extraction efficiency of conics ranges between 0.1%, for the lowest amplitude of broadband extremely low frequency waves, and 5%, for the highest‐amplitude waves sampled. Simultaneous measurements of all‐sky images and magnetic field‐aligned radar measurements show that the most intense ion upflux occurs adjacent to the boundary of intense electron precipitation characteristic of polar cap boundary intensifications and streamers, suggesting that the most efficient acceleration mechanisms couple ionospheric heating at F region altitude with dispersive Alfvén waves that grow from horizontal gradients in electric field and conductivity.

Storm time enhancements of 630.0 nm airglow associated with polar cap patches

AbstractWe examined the brightness of 630.0 nm airglow, I630, associated with polar cap patches observed during a magnetic storm that occurred on 22 January 2012. Brightness was measured using an all‐sky imager (ASI) located at Longyearbyen, Svalbard. The observed I630 was compared with the F region electron density observed by the European Incoherent Scatter Svalbard Radar (ESR). The I630 was positively correlated with the F2 layer peak electron density, NmF2, and inversely correlated with the altitude of the F2layer peak electron density, hmF2, as expected from the known relationship between these parameters. To estimate the altitude of the peak emission of the airglow, we performed model calculations of the volume emission rate, V630, under quiet and disturbed conditions, using Mass Spectrometer Incoherent Scatter modeled neutral gas profiles and the electron density profile obtained from the ESR data. In order to validate the V630 calculation, I630 was calculated by integrating the V630 along altitude and then compared with the ASI‐observed I630. During the observation periods the measured brightness frequently exceeded the calculated I630; we infer that in most cases, low‐energy particle precipitation is responsible for the extra brightness. However, when there was less particle precipitation, the observed values were in good agreement with the calculated values. Under the magnetically disturbed conditions during our observations, the model calculation showed that the altitude of V630 peak increases, the thickness of the emission layer increases, and patch brightness increases.