EISCAT Incoherent Scatter Radar Research Articles

Reconnection electric field estimates and dynamics of high-latitude boundaries during a substorm

Abstract. The dynamics of the polar cap and the auroral oval are examined in the evening sector during a substorm period on 25 November 2000 by using measurements of the EISCAT incoherent scatter radars, the north-south chain of the MIRACLE magnetometer network, and the Polar UV Imager. The location of the polar cap boundary (PCB) is estimated from electron temperature measurements by the mainland low-elevation EISCAT VHF radar and the 42 m antenna of the EISCAT Svalbard radar. A comparison to the poleward auroral emission (PAE) boundary by the Polar UV Imager shows that in this event the PAE boundary is typically located 0.7° of magnetic latitude poleward of the PCB by EISCAT. The convection reversal boundary (CRB) is determined from the 2-D plasma drift velocity extracted from the dual-beam VHF data. The CRB is located 0.5–1° equatorward of the PCB indicating the existence of viscous-driven antisunward convection on closed field lines. East-west equivalent electrojets are calculated from the MIRACLE magnetometer data by the 1-D upward continuation method. In the substorm growth phase, electrojets together with the polar cap boundary move gradually equatorwards. During the substorm expansion phase, the Harang discontinuity (HD) region expands to the MLT sector of EISCAT. In the recovery phase the PCB follows the poleward edge of the westward electrojet. The local ionospheric reconnection electric field is calculated by using the measured plasma velocities in the vicinity of the polar cap boundary. During the substorm growth phase, values between 0 and 10 mV/m are found. During the late expansion and recovery phase, the reconnection electric field has temporal variations with periods of 7–27 min and values from 0 to 40 mV/m. It is shown quantitatively, for the first time to our knowledge, that intensifications in the local reconnection electric field correlate with appearance of auroral poleward boundary intensifications (PBIs) in the same MLT sector. The results suggest that PBIs (typically 1.5 h MLT wide) are a consequence of temporarily enhanced longitudinally localized magnetic flux closure in the magnetotail.

EISCAT and Cluster observations in the vicinity of the dynamical polar cap boundary

Abstract. The dynamics of the polar cap boundary and auroral oval in the nightside ionosphere are studied during late expansion and recovery of a substorm from the region between Tromsø (66.6° cgmLat) and Longyearbyen (75.2° cgmLat) on 27 February 2004 by using the coordinated EISCAT incoherent scatter radar, MIRACLE magnetometer and Cluster satellite measurements. During the late substorm expansion/early recovery phase, the polar cap boundary (PCB) made zig-zag-type motion with amplitude of 2.5° cgmLat and period of about 30 min near magnetic midnight. We suggest that the poleward motions of the PCB were produced by bursts of enhanced reconnection at the near-Earth neutral line (NENL). The subsequent equatorward motions of the PCB would then represent the recovery of the merging line towards the equilibrium state (Cowley and Lockwood, 1992). The observed bursts of enhanced westward electrojet just equatorward of the polar cap boundary during poleward expansions were produced plausibly by particles accelerated in the vicinity of the neutral line and thus lend evidence to the Cowley-Lockwood paradigm. During the substorm recovery phase, the footpoints of the Cluster satellites at a geocentric distance of 4.4 RE mapped in the vicinity of EISCAT measurements. Cluster data indicate that outflow of H+ and O+ ions took place within the plasma sheet boundary layer (PSBL) as noted in some earlier studies as well. We show that in this case the PSBL corresponded to a region of enhanced electron temperature in the ionospheric F region. It is suggested that the ion outflow originates from the F region as a result of increased ambipolar diffusion. At higher altitudes, the ions could be further energized by waves, which at Cluster altitudes were observed as BBELF (broad band extra low frequency) fluctuations. The four-satellite configuration of Cluster revealed a sudden poleward expansion of the PSBL by 2° during ~5 min. The beginning of the poleward motion of the PCB was associated with an intensification of the downward FAC at the boundary. We suggest that the downward FAC sheet at the PCB is the high-altitude counterpart of the Earthward flowing FAC produced in the vicinity of the magnetotail neutral line by the Hall effect (Sonnerup, 1979) during a short-lived reconnection pulse.

The effective altitude range of the ionospheric Alfvén resonator studied by high-altitude EISCAT measurements

The altitude profiles of ionospheric plasma obtained by the EISCAT incoherent scatter radar can be used to study the effect of the ionospheric Alfvén resonator (IAR) upon the formation of the wave signal on the ground. We examine here a fortunate case of a multiband wave Pc1 event on March 7, 2001, at a time when the EISCAT radar was operated in two modes simultaneously, thus covering an exceptionally wide altitude range. This made it possible to test the IAR model and its effect upon the wave signal over a larger altitude range than earlier, and to determine the effective altitude range of the resonator (the IAR domain), which has to be taken into account in full-wave numerical modeling. This case study demonstrates that the IAR domain that affects the wave signal on the ground is essentially localized below the height of about 1200–1500 km.

Sunset transition of negative charge in the D-region ionosphere during high-ionization conditions

Abstract. The solar proton event of October 1989 and especially the sunset of 23 October is examined in this study of negative ion chemistry, which combines measurements of nitric oxide, electron density, and cosmic radio noise absorption with ion and neutral chemistry modelling. Model results show that the negative charge transition from electrons to negative ions during sunset occurs at altitudes below 80 km and is dependent on both ultraviolet and visible solar radiation. The ultraviolet effect is mostly due to rapid changes in atomic oxygen and O2(1Δg), while the decrease in NO3- photodetachment plays a minor role. The effect driven by visible wavelengths is due to changes in photodissociation of CO3- and the subsequent electron photodetachment from O-, and at higher altitudes is also due to a decrease in the photodetachment of O2-. The relative sizes of the ultraviolet and visible effects vary with altitude, with the visible effects increasing in importance at higher altitudes, and they are also controlled by the nitric oxide concentration. These modelling results are in good agreement with EISCAT incoherent scatter radar and Kilpisjärvi riometer measurements.

Comparison of EISCAT and ionosonde electron densities: application to a ground-based ionospheric segment of a space weather programme

Abstract. Space weather applications require real-time data and wide area observations from both ground- and space-based instrumentation. From space, the global navigation satellite system - GPS - is an important tool. From the ground the incoherent scatter (IS) radar technique permits a direct measurement up to the topside region, while ionosondes give good measurements of the lower part of the ionosphere. An important issue is the intercalibration of these various instruments. In this paper, we address the intercomparison of the EISCAT IS radar and two ionosondes located at Tromsø (Norway), at times when GPS measurements were also available. We show that even EISCAT data calibrated using ionosonde data can lead to different values of total electron content (TEC) when compared to that obtained from GPS.

Calibration of the TEC derived from GPS measurements and from ionospheric models using the EISCAT radar

This work aims to introduce the EISCAT incoherent scatter radar as a calibration tool for one of the most important parameter in ionospheric space weather, the vertical total electron content (TEC). Five successful EISCAT experiments in 1998 and 1999 have been used to calculate the vertical integrated total electron content (ITEC) values up to 498 km . They cover more than 257 h in 15 days during different solar and magnetic conditions. We compare these values with those TEC values evaluated from GPS measurements at Troms ø. We show that in some cases, the comparison reveals an underestimation of the TEC deduced from the GPS system. By exploring the geophysical conditions (magnetic field, electric field, solar wind, solar activity, IMF) during those cases we find that the under estimates are a result of a non-adequate consideration of the protonosphere. Further comparisons are being made with corresponding TEC values obtained from IRI and COSTprof ionospheric models. We show that the models can reproduce well the electron density profiles measured by EISCAT when they are fed with NmF2 and hmF2. Of the five experiments, the comparison reveals a better accuracy with COSTprof.

Ionospheric electron heating, optical emissions, and striations induced by powerful HF radio waves at high latitudes: Aspect angle dependence

In recent years, large electron temperature increases of 300% (3000 K above background) caused by powerful HF‐radio wave injection have been observed during nighttime using the EISCAT incoherent scatter radar near Tromsø in northern Norway. In a case study we examine the spatial structure of the modified region. The electron heating is accompanied by ion heating of about 100 degrees and magnetic field‐aligned measurements show ion outflows increasing with height up to 300 m s−1 at 582 km. The electron density decreases by up to 20%. When the radar antenna was scanned between three elevations from near field‐aligned to vertical, the strongest heating effects were always obtained in the field‐aligned position. When the HF‐pump beam was scanned between the same three positions, the heating was still almost always strongest in the field‐aligned direction. Simultaneous images of the 630 nm O(1D) line in the radio‐induced aurora showed that the enhancement caused by the HF radio waves also remained localized near the field‐aligned position. Coherent HF radar backscatter also appeared strongest when the pump beam was pointed field‐aligned. These results are similar to some Langmuir turbulence phenomena which also show a strong preference for excitation by HF rays launched in the field‐aligned direction. The correlation of the position of largest temperature enhancement with the position of the radio‐induced aurora suggests that a common mechanism, upper‐hybrid wave turbulence, is responsible for both effects. Why the strongest heating effects occur for HF rays directed along the magnetic field is still unclear, but self‐focusing on field‐aligned striations is a candidate mechanism, and possibly ionospheric tilts may be important.

SuperDARN radar HF propagation and absorption response to the substorm expansion phase

Abstract. Coherent scatter HF ionospheric radar systems such as SuperDARN offer a powerful experimental technique for the investigation of the magnetospheric substorm. However, a common signature in the early expansion phase is a loss of HF backscatter, which has limited the utility of the radar systems in substorm research. Such data loss has generally been attributed to either HF absorption in the D-region ionosphere, or the consequence of regions of very low ionospheric electric field. Here observations from a well-instrumented isolated substorm which resulted in such a characteristic HF radar data loss are examined to explore the impact of the substorm expansion phase on the HF radar system. The radar response from the SuperDARN Hankasalmi system is interpreted in the context of data from the EIS-CAT incoherent scatter radar systems and the IRIS Riometer at Kilpisjarvi, along with calculations of HF absorption for both IRIS and Hankasalmi and ray-tracing simulations. Such a study offers an explanation of the physical mechanisms behind the HF radar data loss phenomenon. It is found that, at least for the case study presented, the major cause of data loss is not HF absorption, but changes in HF propagation conditions. These result in the loss of many propagation paths for radar backscatter, but also the creation of some new, viable propagation paths. The implications for the use of the characteristics of the data loss as a diagnostic of the substorm process, HF communications channels, and possible radar operational strategies which might mitigate the level of HF radar data loss, are discussed.Key words. Ionosphere (ionosphere-magnetosphere interactions). Magnetospheric physics (storms and substorms). Radio science (radio wave propagation)

On the factors controlling occurrence of F-region coherent echoes

Abstract. Several factors are known to control the HF echo occurrence rate, including electron density distribution in the ionosphere (affecting the propagation path of the radar wave), D-region radio wave absorption, and ionospheric irregularity intensity. In this study, we consider 4 days of CUTLASS Finland radar observations over an area where the EISCAT incoherent scatter radar has continuously monitored ionospheric parameters. We illustrate that for the event under consideration, the D-region absorption was not the major factor affecting the echo appearance. We show that the electron density distribution and the radar frequency selection were much more significant factors. The electron density magnitude affects the echo occurrence in two different ways. For small F-region densities, a minimum value of 1 × 1011 m-3 is required to have sufficient radio wave refraction so that the orthogonality (with the magnetic field lines) condition is met. For too large densities, radio wave strong "over-refraction" leads to the ionospheric echo disappearance. We estimate that the over-refraction is important for densities greater than 4 × 1011 m-3. We also investigated the backscatter power and the electric field magnitude relationship and found no obvious relationship contrary to the expectation that the gradient-drift plasma instability would lead to stronger irregularity intensity/echo power for larger electric fields.Key words. Ionosphere (ionospheric irregularities; plasma waves and instabilities; auroral ionosphere)

Nightside studies of coherent HF Radar spectral width behaviour

Abstract. A previous case study found a relationship between high spectral width measured by the CUTLASS Finland HF radar and elevated electron temperatures observed by the EISCAT and ESR incoherent scatter radars in the post-midnight sector of magnetic local time. This paper expands that work by briefly re-examining that interval and looking in depth at two further case studies. In all three cases a region of high HF spectral width (>200 ms-1) exists poleward of a region of low HF spectral width (<200 ms-1). Each case, however, occurs under quite different geomagnetic conditions. The original case study occurred during an interval with no observed electrojet activity, the second study during a transition from quiet to active conditions with a clear band of ion frictional heating indicating the location of the flow reversal boundary, and the third during an isolated sub-storm. These case studies indicate that the relationship between elevated electron temperature and high HF radar spectral width appears on closed field lines after 03:00 magnetic local time (MLT) on the nightside. It is not clear whether the same relationship would hold on open field lines, since our analysis of this relationship is restricted in latitude. We find two important properties of high spectral width data on the nightside. Firstly the high spectral width values occur on both open and closed field lines, and secondly that the power spectra which exhibit high widths are both single-peak and multiple-peak. In general the regions of high spectral width (>200 ms-1) have more multiple-peak spectra than the regions of low spectral widths whilst still maintaining a majority of single-peak spectra. We also find that the region of ion frictional heating is collocated with many multiple-peak HF spectra. Several mechanisms for the generation of high spectral width have been proposed which would produce multiple-peak spectra, these are discussed in relation to the data presented here. Since the regions of high spectral width are observed both on closed and open field lines the use of the boundary between low and high spectral width as an ionospheric proxy for the open/closed field line boundary is not a simple matter, if indeed it is possible at all.Key words. Ionosphere (auroral ionosphere; ionospheric irregularities)

High resolution general purpose D-layer experiment for EISCAT incoherent scatter radars using selected set of random codes

Abstract. The ionospheric D-layer is a narrow bandwidth radar target often with a very small scattering cross section. The target autocorrelation function can be obtained by transmitting a series of relatively short coded pulses and computing the correlation between data obtained from different pulses. The spatial resolution should be as high as possible and the spatial side lobes of the codes used should be as small as possible. However, due to the short pulse repetition period (in the order of milliseconds) at any instant, the radar receives detectable scattered signals not only from the pulse illuminating the D-region but also from 3–5 ambiguous-range pulses, which makes it difficult to produce a reliable estimate near zero lag of the autocorrelation function. A new experimental solution to this measurement problem, using a selected set of 40-bit random codes with 4 µs elements giving 600 m spatial resolution is presented. The zero lag is approximated by dividing the pulse into two 20-bit codes and computing the correlation between those two pulses. The lowest altitudes of the E-layer are measured by dividing the pulse into 5 pieces of 8 bits, which allows for computation of 4 lags. In addition, coherent integration of data from four pulses is used for obtaining separately the autocorrelation function estimate for the lowest altitudes and in cases when the target contains structures with a long coherence time. Design details and responses of the experiment are given, and analysed test data are shown.Key words. Radio science (signal processing); Ionosphere (plasma temperature and density; instruments and techniques)

Multi-instrument observations of the dynamics of auroral arcs: a case study

We present results from a multi-instrument study of events on February 13–14, 1996. The data were taken by the EISCAT incoherent-scatter radar, the Sussex, MPI and FMI all-sky cameras, the IRIS riometer and the IMAGE and SAMNET magnetometer networks. Observations from the various instruments are compared to establish the concurrency of the radar, optical and absorption signatures of arcs, and to make a detailed study of their dynamics. The distribution of electric field strength near each arc is determined and the drift velocity of each arc is compared with EISCAT measurements of F-region plasma velocity. Two multiple-onset substorms are studied in detail, the behaviour of each arc depends on the phase of the substorm in which it is observed. Our major points are: (1) All arcs observed during the growth or recovery phases are seen to drift at velocities close to the average convection velocity. In contrast, arcs observed during the expansion phase travel at a significantly different velocity. (2) There is strong qualitative evidence that arcs which drift with the convection are optically less bright than arcs with a large relative velocity with respect to the surrounding plasma so that arc brightening at substorm onset is clearly associated with a sharp increase in relative velocity. (3) The observations confirm the current electrodynamics model of arcs and, moreover, we notice a difference in the conductivity behaviour with respect to the direction of the arc movement. It is generally true that when the band of enhanced electric field is in advance of a moving arc the gradient in conductivity is very sharp at the leading edge of the arc and hence the largest electric fields are seen very close to the arc. When, however, the band of enhanced field follows the moving arc the increase in Pedersen conductivity is more gradual, and in such cases too the maximum electric field strength is observed at greater distances from the optical arc itself.

The TEC and F2 parameters as tracers of the ionosphere and thermosphere

The more common measurements of the ionosphere are the total electron content (TEC) (measured with the positioning systems) and the parameters of the F-region peak, namely its altitude (hmF2) and its density (NmF2). In this paper, we address the question of using these reduced parameters as tracers of the ionosphere and thermosphere. We make use of a 19 h daylight experiment by the EISCAT incoherent scatter radar based in Troms ø, Norway. We reduce the full profile measurement of the electron density (between 80 and 425 km ) to the TEC, hmF2 and NmF2. Using a 1-D fluid/kinetic model, we reproduce these parameters. Then, by inversion, we retrieve the ionospheric profiles of the electron density, electron and ion temperature and, to a smaller extent, of the ion velocity. With this method, we show that it is in principle possible to evaluate a correction factor for the atomic oxygen. This factor evolves with time. We study the different uncertainties (heat flow, molecular densities, limitations of the model). Finally, we evaluate the contribution of the upper ionosphere (up to 3000 km) to the calculation of the TEC and show that 90% of the TEC is reached when the integration is made up to 1200 km .

The variability of Joule heating, and its effects on the ionosphere and thermosphere

Abstract. A considerable fraction of the solar wind energy that crosses the magnetopause ends up in the high-latitude thermosphere-ionosphere system as a result of Joule heating, the consequences of which are very significant and global in nature. Often Joule heating calculations use hourly averages of the electric field, rather than the time-varying electric field. This leads to an underestimation of the heating. In this paper, we determine the magnitude of the underestimation of Joule heating by analysing electric field data from the EISCAT Incoherent Scatter Radar, situated at the 67° E magnetic latitude. We find that the underestimation, using hourly-averaged electric field values, is normally ~20%, with an upper value of about 65%. We find that these values are insensitive to changes in solar flux, magnetic activity and magnetic local time, implying that the electric field fluctuations are linear related to the amplitude of the electric field. Assuming that these changes are representative of the entire auroral oval, we then use a coupled ionosphere-thermosphere model to calculate the local changes these underestimations in the heating rate cause to the neutral temperature, mean molecular mass and meridional wind. The changes in each parameter are of the order of a few percent but they result in a reduction in the peak F-region concentration of ~20% in the summer hemisphere at high latitudes, and about half of this level in the winter hemisphere. We suggest that these calculations could be used to add corrections to modelled values of Joule heating.Key words. Ionosphere (eletric fields and currents; ionospheric disturbances; polar ionosphere)

First EISCAT measurement of electron-gas temperature in the artificially heated D-region ionosphere

Abstract. The ionospheric electron gas can be heated artificially by a powerful radio wave. According to our modeling, the maximum effect of this heating occurs in the D-region where the electron temperature can increase by a factor of ten. Ionospheric plasma parameters such as Ne, Te and Ti are measured by EISCAT incoherent scatter radar on a routine basis. However, in the D-region the incoherent scatter echo is very weak because of the low electron density. Moreover, the incoherent scatter spectrum from the D-region is of Lorentzian shape which gives less information than the spectrum from the E- and F-regions. These make EISCAT measurements in the D-region difficult. A combined EISCAT VHF-radar and heating experiment was carried out in November 1998 with the aim to measure the electron temperature increase due to heating. In the experiment the heater was switched on/off at 5 minute intervals and the integration time of the radar was chosen synchronously with the heating cycle. A systematic difference in the measured autocorrelation functions was found between heated and unheated periods.Key words: Ionosphere (active experiments; plasma temperature and density; wave propagation)

Ion-neutral coupling in the high-latitude F-layer from incoherent scatter and Fabry-Perot interferometer measurements

Abstract. Since the auroral ionosphere provides an important energy sink for the magnetosphere, ionosphere-thermosphere coupling must be investigated when considering the energy budget of the ionosphere-magnetosphere coupling. We present the first Scandinavian ground-based study of high-latitude F-region ion-neutral frictional heating where ion velocity and temperature are measured by the EISCAT incoherent scatter radar as well as neutral wind and temperature being measured simultaneously by a Fabry-Perot interferometer. A geomagnetically active period (Kp = 7– – 5–) and quiet period (Kp = 0+ – 0) were studied. Neglecting the neutral wind can result in errors of frictional heating estimates of 60% or more in the F-layer. About 96% of the local ion temperature enhancement over the neutral temperature is accounted for by ion-neutral frictional heating.Key words: Ionosphere (auroral ionosphere; ionosphere-atmosphere interactions)

Irregular structures of the F layer at high latitudes during ionospheric heating

Abstract. Results on heating the ionospheric F region above Tromsø, Norway are presented. The ionosphere was monitored by satellite tomography and amplitude scintillation methods as well as the EISCAT incoherent scatter radar. No effect of heating was observed in the daytime. In the evening and in the pre-midnight sector, noticeable tilts of the F region were observed during heating periods. The tilts overlapped the heating cone, where the electron density decreased and irregularities exceeding 10 km in size appeared. Between the heating periods the F layer was restored to its horizontal shape. The anisotropic parameters of small-scale irregularities with scale lengths of hundreds of metres were also determined. It was found that the perpendicular anisotropy points in the direction of F region plasma flow. In some cases the results can be explained by assuming that the small-scale irregularities were generated within the heating cone and drifted out of the heating region where they were subsequently observed.Key words: Ionosphere (active experiments; auroral ionosphere; ionospheric irregularities)

Irregular structures of the F layer at high latitudes during ionospheric heating

Results on heating the ionospheric F region above Tromsø, Norway are presented. The ionosphere was monitored by satellite tomography and amplitude scintillation methods as well as the EISCAT incoherent scatter radar. No effect of heating was observed in the daytime. In the evening and in the pre-midnight sector, noticeable tilts of the F region were observed during heating periods. The tilts overlapped the heating cone, where the electron density decreased and irregularities exceeding 10 km in size appeared. Between the heating periods the F layer was restored to its horizontal shape. The anisotropic parameters of small-scale irregularities with scale lengths of hundreds of metres were also determined. It was found that the perpendicular anisotropy points in the direction of F region plasma flow. In some cases the results can be explained by assuming that the small-scale irregularities were generated within the heating cone and drifted out of the heating region where they were subsequently observed.Key words: Ionosphere (active experiments; auroral ionosphere; ionospheric irregularities)

Comparison of the orientation of small-scale electron density irregularities and F region plasma flow direction

Abstract. Results are shown from an experimental campaign where satellite scintillation was observed at three sites at high latitudes and, simultaneously, the F region plasma flow was measured by the nearby EISCAT incoherent scatter radar. The anisotropy parameters of field-aligned irregularities are determined from amplitude scintillation using a method based on the variance of the relative logarithmic amplitude. The orientation of the anisotropy in a plane perpendicular to the geomagnetic field is compared with the direction of F region plasma flow. The results indicate that in most cases a good agreement between the two directions is obtained.Key words: Ionosphere (auroral ionosphere; ionospheric irregularities)

Direct calculation of f-region joule heating from simultaneous ion and neutral measurements at high latitudes

We study a set of simultaneous and coincident ionospheric and thermospheric measurements made by the EISCAT incoherent scatter radar and a nearby Fabry-Perot interferometer, located within the Scandinavian auroral zone. Combined observations of ion and neutral temperatures as well as ion drifts and neutral winds in the F-region are presented for the geomagnetically active night (Kp = 5−-7−) of 9–10 November 1998 and the following quiet night (Kp = 0-0+) of 10–11 November 1998. These data enable a direct calculation of ion temperature enhancements over the neutral temperature, resulting from Joule heating effects, which can be compared directly with the observation. The importance of the neutral wind contribution to the total electric field and Joule heating rates, often assumed to be insignificant, is estimated. Furthermore, MSISE-90 model predictions for neutral temperature are compared to the measurements.