CUTLASS HF Radar Research Articles

Ion heating in high-speed flow channel within the duskside cell of the polar cap ion convection under large IMF-Bycondition

[1] F region strong sunward ion flow embedded in the duskside cell of expanding polar cap ion convection and coincidental increase in the ion temperature were observed from about 73° to 69° in geomagnetic latitude between 16:00 and 16:48 MLT on 19 August, 2006 from an experiment using the European Incoherent Scatter (EISCAT) radars at Tromso, Sodankyla and Longyearbyen together with the CUTLASS HF radars. The spatiotemporal structure of the convection map obtained by the SuperDARN HF radars showed that the high-speed flow channel elongated in 14–17 MLT was moving equatorward. The flow channel appeared during an interval of negative interplanetary magnetic field Bz associated with large positive By. In order to evaluate ion frictional heating and resultant ion temperature increase quantitatively, the experiment was conducted to estimate the ion velocity vector and the ion temperature at the common scattering volume of the EISCAT radars. While the flow channel moved equatorward, the ion temperature increase became less evident at higher part of latitudes within the flow channel, which means that ion frictional heating was depressed where the neutral gases had already responded to the high-speed ions. The increase in the ion temperature at particular geomagnetic latitude continued for about 20 min, and then ceased while the ion velocity was still enhanced. Ion frictional heating lasted until the neutral wind was accelerated to the equivalent level of the surrounded ion flow.

Bistatic observations of large and small scale ULF waves in SPEAR-induced HF coherent backscatter

Abstract. HF radar backscatter which has been artificially-induced by a high power RF facility has been demonstrated to provide ionospheric electric field data of unprecedented temporal resolution and accuracy. Here such data, induced by the SPEAR high power radar on Svalbard, are used to investigate ULF wave processes observed by the CUTLASS HF radars. Observations are presented of both waves with a large-scale nature, driven externally to the magnetosphere and those with small azimuthal scale lengths, driven by wave-particle interactions. For ULF wave events with large azimuthal scale lengths an excellent agreement in the observed wave polarisation ellipse is found between the radar observations and ground-based magnetometer data. In contrast, for the small scale events, no ground-based magnetic counterpart is observed. Indeed the data from the two CUTLASS radars seem inconsistent, and each radar must be interpreted separately, as the spatial resolution of the radars is sufficient to resolve the wave characteristics along the radar beams, but insufficient to resolve the wave characteristics across the beams. A high azimuthal wave number (m) wave with a period of 300 s and m~−60 is observed to occur over Svalbard at ~14:00 magnetic local time. This confirms the existence of waves driven by wave-particle interactions with trapped particle populations in the outer magnetosphere. A comparison of the observed wave characteristics with previous, lower latitude, observations suggests that these high latitude waves have a similar azimuthal scale size to those generated in the inner magnetosphere; the azimuthal wave number of −60 observed in the present study is comparable to previous values of −20– −50, but suggests an increase of m with latitude. A similar energy source in drifting proton populations is also suggested, but with lower characteristic proton energies of 10 keV implicated at high latitude, compared to the 20–60 keV energies invoked for previous lower latitude observations.

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.

Simultaneous optical, CUTLASS HF radar, and FAST spacecraft observations: signatures of boundary layer processes in the cusp

Abstract. In this paper we discuss counterstreaming electrons, electric field turbulence, HF radar spectral width enhancements, and field-aligned currents in the southward IMF cusp region. Electric field and particle observations from the FAST spacecraft are compared with CUTLASS Finland spectral width enhancements and ground-based optical data from Svalbard during a meridional crossing of the cusp. The observed 630nm rayed arc (Type-1 cusp aurora) is associated with stepped cusp ion signatures. Simultaneous counterstreaming low-energy electrons on open magnetic field lines lead us to propose that such electrons may be an important source for rayed red arcs through pitch angle scattering in collisions with the upper atmosphere. The observed particle precipitation and electric field turbulence are found to be nearly collocated with the equatorward edge of the optical cusp, in a region where CUTLASS Finland also observed enhanced spectral width. The electric field turbulence is observed to extend far poleward of the optical cusp. The broad-band electric field turbulence corresponds to spatial scale lengths down to 5m. Therefore, we suggest that electric field irregularities are directly responsible for the formation of HF radar backscatter targets and may also explain the observed wide spectra. FAST also encountered two narrow highly structured field-aligned current pairs flowing near the edges of cusp ion steps. Key words. Ionosphere (electric fields and currents). Magnetosphere physics (magnetopause, cusp, and boundary layers; auroral phenomena)

High resolution observations of spectral width features associatedwith ULF wave signatures in artificial HF radar backscatter

Abstract. The EISCAT high power heating facility at Tromsø, northern Norway, has been utilised to generate artificial radar backscatter in the fields of view of the CUTLASS HF radars. It has been demonstrated that this technique offers a means of making very accurate and high resolution observations of naturally occurring ULF waves. During such experiments, the usually narrow radar spectral widths associated with artificial irregularities increase at times when small scale-sized (high m-number) ULF waves are observed. Possible mechanisms by which these particle-driven high-m waves may modify the observed spectral widths have been investigated. The results are found to be consistent with Pc1 (ion-cyclotron) wave activity, causing aliasing of the radar spectra, in agreement with previous modelling work. The observations also support recent suggestions that Pc1 waves may be modulated by the action of longer period ULF standing waves, which are simultaneously detected on the magnetospheric field lines. Drifting ring current protons with energies of ∼ 10keV are indicated as a common plasma source population for both wave types. Key words. Magnetospheric physics (MHD waves and instabilities) – Space plasma physics (wave-particle interactions) – Ionosphere (active experiments)

<i>Foreword by the Chairman of the EISCAT Council</i>

This volume contains papers presented at the 10th biennial International EISCAT workshop, held in Tokyo, Japan, in July 2001. The quality and excitement of the papers clearly demonstrate that the vitality and diversity of the science made possible by these radars is stronger than ever and that the facilities have maintained their pre-eminence in the field. For more than 20 years, the EISCAT radars have been producing world-leading data for research into space plasmas. The majority of the observations are of the ionosphere; however, because the behaviour of the ionospheric plasma is strongly linked by collisions to the upper neutral atmosphere and, via electomagnetic coupling and auroral particle precipitation, to the magnetosphere, EISCAT is able to study both these regions of geospace as well. The location of the EISCAT Svalbard Radar under the cusp allows studies of how Earth’s magnetosphere-ionosphere system interacts with the solar wind; the locations of the mainland systems are ideal for studying the internal dynamics of the magnetospheric tail. In addition, EISCAT is unique in having a high-power HF modification (“heater”) facility supported by an incoherent scatter radar. In fact, in addition to having both the UHF and VHF radars as diagnostics, the heater field-of-view is also monitored by a digital ionosonde and the CUTLASS HF radar system as well as optical instrumentation and an imaging riometer. This combination really does turn the ionosphere over Tromso into a natural laboratory for studying the mechanisms and behaviour of large-scale plasma. As an example, studies of “artificial aurora” induced by the heater have given new insights into plasma-wave interactions. In addition to this broad diversity of science, the interplanetary scintillation technique has been applied to EISCAT with great success, making it possible to observe uniquely close to the Sun and, consequently, to study the acceleration of the solar wind. Further developments will allow yet more diversification into areas such as space debris and interplanetary particles.

Radar ACFs and turbulence characteristics from artificially generated field‐aligned irregularities

The plasma in the auroral ionosphere may be considered as a flowing turbulent fluid and the radar auto‐correlation function (ACF), calculated from received radar signals, is used to derive turbulence characteristics, such as diffusion coefficients and correlation times. Artificial backscatter generated by the Tromsø heater has been diagnosed in this way using the CUTLASS HF radars. Turbulence characteristics were extracted from the ACFs using the method of Villain et al. [1996] for naturally occurring irregularity backscatter. In this study, it was found that the distribution of diffusion coefficients for artificially generated backscatter centered on a value that was about an order of magnitude less than that corresponding to the return from naturally occurring irregularities. Also, the modal correlation length value was about twice that corresponding to the natural case.

Cusp ion steps, field‐aligned currents and poleward moving auroral forms

We predict the field‐aligned currents around cusp ion steps produced by pulsed reconnection between the geomagnetic field and an interplanetary magnetic field (IMF) with a BY component that is large in magnitude. For BY > 0, patches of newly opened flux move westward and eastward in the Northern and Southern Hemispheres, respectively, under the influence of the magnetic curvature force. These flow directions are reversed for BY < 0. The speed of this longitudinal motion initially grows with elapsed time since reconnection, but then decays as the newly opened field lines straighten. We predict sheets of field‐aligned current on the boundaries between the patches produced by successive reconnection pulses, associated with the difference in the speeds of their longitudinal motion. For low elapsed times since reconnection, near the equatorward edge of the cusp region where the field lines are accelerating, the field‐aligned current sheets will be downward or upward in both hemispheres for positive or negative IMF BY, respectively. At larger elapsed times since reconnection, as events slow and evolve from the cusp into the mantle region, these field‐aligned current directions will be reversed. Observations by the Polar spacecraft on August 26, 1998, show the predicted upward current sheets at steps seen in the mantle region for IMF BY > 0. Mapped into the ionosphere, the steps coincide with poleward moving events seen by the CUTLASS HF radar. The mapped location of the largest step also coincides with a poleward moving arc seen by the UVI imager on Polar. We show that the arc is consistent with a region of upward field‐aligned current that has become unstable, such that a potential drop of about 1 kV formed below the spacecraft. The importance of these observations is that they confirm that the poleward moving events, as seen by the HF radar and the UV imager, are due to pulsed magnetopause reconnection. Milan et al. [2000] noted that the great longitudinal extent of these events means that the required reconnection pulses would have contributed almost all the voltage placed across the magnetosphere at this time. The observations also show that auroral arcs can form on open field lines in response to the pulsed application of voltage at the magnetopause.

CUTLASS HF radar observations of high-velocity E-region echoes

Abstract. A short event of high-velocity E-region echo observations by the Pykkvibaer HF radar is analysed to study echo parameters and the echo relation to the Farley-Buneman plasma instability. The echoes were detected in several beams aligned closely to the magnetic L-shell direction. Two echo groups were identified: one group corresponded to the classical type 1 echoes with velocities close to the nominal ion-acoustic speed of 400 ms–1 , while the other group had significantly larger velocities, of the order of 700 ms–1 . The mutual relationship between the echo power, Doppler velocity, spectral width and elevation angles for these two groups was studied. Plotting of echo parameters versus slant range showed that all ~700 ms–1 echoes originated from larger heights and distances of 500–700 km, while all ~400 ms–1 echoes came from lower heights and from farther distances; 700–1000 km. We argue that both observed groups of echoes occurred due to the Farley-Buneman plasma instability excited by strong ( ~70 mVm–1 ) and uniformly distributed electric fields. We show that the echo velocities for the two groups were different because the echoes were received from different heights. Such a separation of echo heights occurred due to the differing amounts of ionospheric refraction at short and large ranges. Thus, the ionospheric refraction and related altitude modulation of ionospheric parameters are the most important factors to consider, when various characteristics of E-region decametre irregularities are derived from HF radar measurements.Key words. Ionosphere (ionospheric irregularities; plasma waves and instabilities; polar ionosphere)

ULF waves with drift resonance and drift-bounce resonance energy sources as observed in artificially-induced HF radar backscatter

Abstract. HF radar backscatter which has been artificially-induced by a high power RF facility such as the EISCAT heater at Tromsø has been demonstrated to provide ionospheric electric field data of unprecedented temporal resolution and accuracy. Here such data are used to investigate ULF wave processes observed by the CUTLASS HF radars. Within a short period of time during a single four hour experiment three distinct wave types are observed with differing periods, and latitudinal and longitudinal phase evolution. Combining information from the three waves allows them to be divided into those with a large-scale nature, driven externally to the magnetosphere, and those with small azimuthal scale lengths, driven by wave-particle interactions. Furthermore, the nature of the wave-particle interactions for two distinct small-scale waves is revealed, with one wave interpreted as being driven by a drift resonance process and the other by a drift-bounce resonance interaction. Both of these mechanisms with m ≈ -35 and proton energies of 35–45 keV appear to be viable wave energy sources in the postnoon sector.Key words. Ionosphere (active experiments; wave-particle interactions) – Magnetospheric physics (MHD waves and in-stabilities).

On auroral dynamics observed by HF radar: 1. Equatorward edge of the afternoon-evening diffuse luminosity belt

Abstract. Observations and modelling are presented which illustrate the ability of the Finland CUTLASS HF radar to monitor the afternoon-evening equatorward auroral boundary during weak geomagnetic activity. The subsequent substorm growth phase development was also observed in the late evening sector as a natural continuation of the preceding auroral oval dynamics. Over an 8 h period the CUTLASS Finland radar observed a narrow (in range) and persistent region of auroral F- and (later) E-layer echoes which gradually moved equatorward, consistent with the auroral oval diurnal rotation. This echo region corresponds to the subvisual equatorward edge of the diffuse luminosity belt (SEEL) and the ionospheric footprint of the inner boundary of the electron plasma sheet. The capability of the Finland CUTLASS radar to monitor the E-layer SEEL-echoes is a consequence of the nearly zero E-layer rectilinear aspect angles in a region 5–10° poleward of the radar site. The F-layer echoes are probably the boundary blob echoes. The UHF EISCAT radar was in operation and observed a similar subvisual auroral arc and an F-layer electron density enhancement when it appeared in its antenna beam.Key words: Ionsophere (ionospheric irregularities) · Magnetospheric physics (auroral phenomena; magnetosphere–ionosphere interactions)

CUTLASS HF radar observations of high-latitude azimuthally propagating vortical currents in the nightside ionosphere during magnetospheric substorms

Abstract. High-time resolution CUTLASS observations and ground-based magnetometers have been employed to study the occurrence of vortical flow structures propagating through the high-latitude ionosphere during magnetospheric substorms. Fast-moving flow vortices (~800 m s-1) associated with Hall currents flowing around upward directed field-aligned currents are frequently observed propagating at high speed (~1 km s-1) azimuthally away from the region of the ionosphere associated with the location of the substorm expansion phase onset. Furthermore, a statistical analysis drawn from over 1000 h of high-time resolution, nightside radar data has enabled the characterisation of the bulk properties of these vortical flow systems. Their occurrence with respect to substorm phase has been investigated and a possible generation mechanism has been suggested.Key words: Ionosphere (auroral ionosphere; electric fields and currents) · Magnetospheric physics (storms and substorms)

CUTLASS HF radar observations of high-latitude azimuthally propagating vortical currents in the nightside ionosphere during magnetospheric substorms

High-time resolution CUTLASS observations and ground-based magnetometers have been employed to study the occurrence of vortical flow structures propagating through the high-latitude ionosphere during magnetospheric substorms. Fast-moving flow vortices (~800 m s-1) associated with Hall currents flowing around upward directed field-aligned currents are frequently observed propagating at high speed (~1 km s-1) azimuthally away from the region of the ionosphere associated with the location of the substorm expansion phase onset. Furthermore, a statistical analysis drawn from over 1000 h of high-time resolution, nightside radar data has enabled the characterisation of the bulk properties of these vortical flow systems. Their occurrence with respect to substorm phase has been investigated and a possible generation mechanism has been suggested.Key words: Ionosphere (auroral ionosphere; electric fields and currents) · Magnetospheric physics (storms and substorms)

Dynamic cusp aurora and associated pulsed reverse convection during northward interplanetary magnetic field

We report a study of ionospheric signatures of plasma entry and momentum transfer at the dayside magnetopause during northward oriented interplanetary magnetic field (IMF), combining ground observations of the dayside aurora and ionospheric ion drift (CUTLASS HF radar) with simultaneous particle precipitation data obtained from three overflights by the Defence Meteorological Satellite Program (DMSP) F12, F13 and F14 spacecraft. The observations were taken during a 37‐min long interval of strongly northward IMF (Bz=7 nT; clock angle ∼10°–15°) after a rapid northward turning. The meridan scanning photometer at the ground station recorded a long stepwise poleward retraction and latitudinal widening of the band of auroral emission in the cusp region. Thus the activity includes a series of episodes which are characterized by an initial 1–2 min poleward “step” of the auroral poleward boundary, followed by a ∼3–4 min period of relatively steady auroral latitude. The auroral events were accompanied by bursts of “reverse” two‐cell convection characterized by equatorward flow across the cusp poleward boundary. The three DMSP spacecraft, which traversed the poleward boundary of the cusp aurora from north to south, entered into a region of auroral precipitation where electrons and ions of magnetosheath origin were present, together with equatorward convection. The observations are found to be consistent with a theoretical description of a sequence of bursts of lobe reconnection involving both hemispheres. This process results in the capture of magnetosheath flux tubes and thereby closed flux is added to the dayside magnetosphere.

A flux transfer event observed at the magnetopause by the Equator-S spacecraft and in the ionosphere by the CUTLASS HF radar

Abstract. Observations of a flux transfer event (FTE) have been made simultaneously by the Equator-S spacecraft near the dayside magnetopause whilst corresponding transient plasma flows were seen in the near-conjugate polar ionosphere by the CUTLASS Finland HF radar. Prior to the occurrence of the FTE, the magnetometer on the WIND spacecraft ~226 RE upstream of the Earth in the solar wind detected a southward turning of the interplanetary magnetic field (IMF) which is estimated to have reached the subsolar magnetopause ~77 min later. Shortly afterwards the Equator-S magnetometer observed a typical bipolar FTE signature in the magnetic field component normal to the magnetopause, just inside the magnetosphere. Almost simultaneously the CUTLASS Finland radar observed a strong transient flow in the F region plasma between 78° and 83° magnetic latitude, near the ionospheric region predicted to map along geomagnetic field lines to the spacecraft. The flow signature (and the data set as a whole) is found to be fully consistent with the view that the FTE was formed by a burst of magnetopause reconnection.Key words. Interplanetary physics (ionosphere-magnetosphere interaction) · Magnetospheric physics (magnetopause · cusp · and boundary layers; solar wind-magnetosphere interactions)

A flux transfer event observed at the magnetopause by the Equator-S spacecraft and in the ionosphere by the CUTLASS HF radar

A flux transfer event observed at the magnetopause by the Equator-S spacecraft and in the ionosphere by the CUTLASS HF radar

<i>Letter to the Editor:</i> First complementary observations by ionospheric tomography, the EISCAT Svalbard radar and the CUTLASS HF radar

Abstract. Experimental results are presented from ionospheric tomography, the EISCAT Svalbard radar and the CUTLASS HF radar. Tomographic measurements on 10 October 1996, showing a narrow, field-aligned enhancement in electron density in the post-noon sector of the dayside auroral zone, are related to a temporal increase in the plasma concentration observed by the incoherent scatter radar in the region where the HF radar indicated a low velocity sunwards convection. The results demonstrate the complementary nature of these three instruments for polar-cap ionospheric studies.Key words. Ionosphere · Auroral ionosphere · Polar ionosphere · Radio science (ionospheric physics)

Validation of the CUTLASS HF radar gravity wave observing capability using EISCAT CP-1 data

Abstract. Quasi-periodic fluctuations in the returned ground-scatter power from the SuperDARN HF radars have been linked to the passage of medium-scale gravity waves. We have applied a technique that extracts the first radar range returns from the F-region to study the spatial extent and characteristics of these waves in the CUTLASS field-of-view. Some ray tracing was carried out to test the applicability of this method. The EISCAT radar facility at Tromsø is well within the CUTLASS field-of-view for these waves and provides a unique opportunity to assess independently the ability of the HF radars to derive gravity wave information. Results from 1st March, 1995, where the EISCAT UHF radar was operating in its CP-1 mode, demonstrate that the radars were in good agreement, especially if one selects the electron density variations measured by EISCAT at around 235 km. CUTLASS and EISCAT gravity wave observations complement each other; the former extends the spatial field of view considerably, whilst the latter provides detailed vertical information about a range of ionospheric parameters.Key words. Ionosphere (ionosphere – atmosphere interactions) · Meteorology and atmospheric dynamics (thermospheric dynamics) · Radio science (ionospheric propagations)

Validation of the CUTLASS HF radar gravity wave observing capability using EISCAT CP-1 data

Quasi-periodic fluctuations in the returned ground-scatter power from the SuperDARN HF radars have been linked to the passage of medium-scale gravity waves. We have applied a technique that extracts the first radar range returns from the F-region to study the spatial extent and characteristics of these waves in the CUTLASS field-of-view. Some ray tracing was carried out to test the applicability of this method. The EISCAT radar facility at Tromso is well within the CUTLASS field-of-view for these waves and provides a unique opportunity to assess independently the ability of the HF radars to derive gravity wave information. Results from 1st March, 1995, where the EISCAT UHF radar was operating in its CP-1 mode, demonstrate that the radars were in good agreement, especially if one selects the electron density variations measured by EISCAT at around 235 km. CUTLASS and EISCAT gravity wave observations complement each other; the former extends the spatial field of view considerably, whilst the latter provides detailed vertical information about a range of ionospheric parameters.

Initial backscatter occurrence statistics from the CUTLASS HF radars

Abstract. A statistical study of the occurrence of ground and ionospheric backscatter within the fields-of-view of the CUTLASS HF radars, at an operating frequency of 10 MHz, during the first 20 months of operation has been undertaken. The diurnal variation of the occurrence of backscatter and the range at which such backscatter is observed is found to be highly dependent on seasonal changes of the ionospheric electron density in both the E and F region, determined from ionosonde observations. In general, ionospheric backscatter is observed at far ranges during the local day in winter months and at near ranges during the local night in summer months. The Iceland radar observes more near-range E region backscatter than the Finland radar as a consequence of its more zonal look-direction. The dependence of the occurrence of backscatter on geomagnetic activity and radar operating frequency are also investigated. The occurrence of ground and ionospheric backscatter is discussed in terms of HF propagation modes and ionospheric electron densities as well as geophysical processes. A brief assessment of the possible impact of solar cycle variations on the observations is made and frequency management is discussed. Such a study, with its focus on the `instrumental' aspect of backscatter occurrence, is essential for a full interpretation of HF coherent radar observations.