Co-operative UK Twin Located Auroral Sounding System Research Articles

First observations of electron gyro-harmonic effects under X-mode HF pumping the high latitude ionospheric F-region

We provide the first experimental evidence of the sensitivity of phenomena induced by extraordinary (X-mode) polarized HF high power radio waves to pump frequency stepping across the fifth electron gyro-harmonic (5fce) from below to above. The results were obtained at the EISCAT (European Incoherent Scatter Scientific Association) HF heater facility near Tromsø under effective radiated powers of 456–715MW, when the HF pump wave was transmitted into the magnetic zenith. We have analyzed the behavior and intensities of various spectral lines in the narrowband stimulated electromagnetic emission (SEE) spectra observed far from the heater, HF-enhanced plasma and ion lines (HFPL and HFIL) from EISCAT UHF incoherent scatter radar spectra, and artificial field-aligned irregularities from CUTLASS (Co-operative UK Twin Located Auroral Sounding System) observations, depending on the frequency offset of the pump field relative to the 5fce. At pump frequencies below 5fce the narrowband SEE spectra exhibited very intense so-called stimulated ion Bernstein scatter (SIBS), accompanied by other spectral components, associated with stimulated Brillouin scatter (SBS), which are greatly suppressed and disappeared in the vicinity of 5fce and did not reappear at fH>5fce. As the pump frequency reached 5fce, the abrupt enhancements of the HFPL and HFIL power, the appearance of cascade lines in the plasma line spectra, and the onset of increasing CUTLASS backscatter power occurred. That is opposite to the ordinary mode (O-mode) effects in the vicinity of 5fce. The X-mode pumping at frequencies below and in the vicinity of the fifth electron gyro-harmonic clearly demonstrated an ascending altitude of generation of induced plasma and ion lines from the initial interaction height, whereas for O-mode heating the region of interaction descended. The observations are consistent with the coexistence of the electron acceleration along and across the geomagnetic field at fH<5fce, while only very strong electron acceleration along the magnetic field was observed at fH≥5fce.

Aspect angle sensitivity of pump-induced optical emissions at EISCAT

Abstract We investigate the aspect angle sensitivity of the pump-induced artificial optical emissions in the ionosphere over the European Incoherent Scatter Scientific Association (EISCAT) high-frequency transmitter facility at Ramfjord, Norway, as a function of the pump beam launch angle relative to the magnetic field line direction. The highest intensity optical emissions occur when the pump beam pointing direction is in the magnetic zenith (approximately 12° S of local zenith). For pump beam directions further north from field aligned, the optical emission intensity decreases for the same pump power. In addition, the primary photon-emitting region becomes displaced towards the magnetic zenith relative to the pump beam and for larger aspect angles, the brightest emissions were found to be outside the −3-dB pump beam width. The Cooperative UK Twin-Located Auroral Sounding System (CUTLASS) coherent scatter high-frequency (HF) radar detected a quasi-constant level of backscatter power from the pumped ionosphere, indicating that saturated striations were formed for all pump beam directions. This indicates that the presence of upper-hybrid resonance is not sufficient to explain the angular sensitivity of the optical emissions.

Observations of unusually broadened HF radar spectra from heater-induced artificial plasma irregularities

[1] Cooperative UK Twin-Located Auroral Sounding System (CUTLASS) HF backscatter targets may be artificially produced using the European Incoherent Scatter (EISCAT) HF ionospheric modification facility at Tromso, Norway. Plasma irregularities created in this way are known to be highly field aligned and usually give rise to radar echoes possessing spectral widths that are much lower than those seen in natural radar auroral data. In this study we present CUTLASS Finland and Iceland observations of heater-induced irregularities; these irregularities gave rise to enhanced spectral width in the Iceland radar backscatter signal, while the width observed by the Finland radar, pointing almost orthogonal to the Iceland beams, remained low. The observations coincided with a period of disturbed ionospheric conditions when faster-flowing natural irregularities were present, poleward of the heated volume, as well as a flow gradient across the heated volume itself. A latitudinal variation of the Doppler velocity measurements was exhibited in both the Finland and Iceland radar observations. We suggest that this flow inhomogeneity, taken together with the geometry of the observing radar beams, would explain not only the broad spectrum of flow components across the heated volume detected by the Iceland radar but also the asymmetry between the spectral widths measured at Iceland and Finland.

A statistical study of the spatial distribution of Co‐operative UK Twin Located Auroral Sounding System (CUTLASS) backscatter power during EISCAT heater beam‐sweeping experiments

High‐power electromagnetic waves can excite a variety of plasma instabilities in Earth's ionosphere. These lead to the growth of plasma waves and plasma density irregularities within the heated volume, including patches of small‐scale field‐aligned electron density irregularities. This paper reports a statistical study of intensity distributions in patches of these irregularities excited by the European Incoherent Scatter (EISCAT) heater during beam‐sweeping experiments. The irregularities were detected by the Co‐operative UK Twin Located Auroral Sounding System (CUTLASS) coherent scatter radar located in Finland. During these experiments the heater beam direction is steadily changed from northward to southward pointing. Comparisons are made between statistical parameters of CUTLASS backscatter power distributions and modeled heater beam power distributions provided by the EZNEC version 4 software. In general, good agreement between the statistical parameters and the modeled beam is observed, clearly indicating the direct causal connection between the heater beam and the irregularities, despite the sometimes seemingly unpredictable nature of unaveraged results. The results also give compelling evidence in support of the upper hybrid theory of irregularity excitation.

Electromagnetic energy deposition rate in the polar upper thermosphere derived from the EISCAT Svalbard radar and CUTLASS Finland radar observations

Abstract. From simultaneous observations of the European incoherent scatter Svalbard radar (ESR) and the Cooperative UK Twin Located Auroral Sounding System (CUTLASS) Finland radar on 9 March 1999, we have derived the height distributions of the thermospheric heating rate at the F region height in association with electromagnetic energy inputs into the dayside polar cap/cusp region. The ESR and CUTLASS radar observations provide the ionospheric parameters with fine time-resolutions of a few minutes. Although the geomagnetic activity was rather moderate (Kp=3+~4), the electric field obtained from the ESR data sometimes shows values exceeding 40 mV/m. The estimated passive energy deposition rates are also larger than 150 W/kg in the upper thermosphere over the ESR site during the period of the enhanced electric field. In addition, enhancements of the Pedersen conductivity also contribute to heating the upper thermosphere, while there is only a small contribution for thermospheric heating from the direct particle heating due to soft particle precipitation in the dayside polar cap/cusp region. In the same period, the CUTLASS observations of the ion drift show the signature of poleward moving pulsed ionospheric flows with a recurrence rate of about 10–20 min. The estimated electromagnetic energy deposition rate shows the existence of the strong heat source in the dayside polar cap/cusp region of the upper thermosphere in association with the dayside magnetospheric phenomena of reconnections and flux transfer events.

First observations of SPEAR-induced artificial backscatter from CUTLASS and the EISCAT Svalbard radars

Abstract. Results are presented from the first two active experimental campaigns undertaken by the new SPEAR (Space Plasma Exploration by Active Radar) high-power system that has recently become operational on Spitzbergen, in the Svalbard archipelago. SPEAR's high-power beam was used to excite artificial enhancements in the backscatter detected by the ESR (EISCAT Svalbard Radar) parallel to the geomagnetic field, as well as coherent backscatter detected by both of the CUTLASS (Co-operative UK Twin Located Auroral Sounding System) coherent radars, in directions orthogonal to the geomagnetic field. The ESR detected both enhanced ion-lines as well as enhanced plasma-lines, that were sustained for the whole period when SPEAR was transmitting ordinary mode radio waves, at frequencies below the maximum F-region plasma frequency. On a number of occasions, coherent backscatter was also observed in one or in both of the CUTLASS radars, in beams that intersected the heated volume. Although the levels of enhanced backscatter varied considerably in time, it appeared that ion-line, plasma-line and coherent backscatter were all excited simultaneously, in contrast to what has typically been reported at Tromsø, during EISCAT heater operations. A description of the technical and operational aspects of the new SPEAR system is also included.

Motion of the dayside polar cap boundary during substorm cycles: II. Generation of poleward-moving events and polar cap patches by pulses in the magnetopause reconnection rate

Abstract. Using data from the EISCAT (European Incoherent Scatter) VHF and CUTLASS (Co-operative UK Twin-Located Auroral Sounding System) HF radars, we study the formation of ionospheric polar cap patches and their relationship to the magnetopause reconnection pulses identified in the companion paper by Lockwood et al. (2005). It is shown that the poleward-moving, high-concentration plasma patches observed in the ionosphere by EISCAT on 23 November 1999, as reported by Davies et al. (2002), were often associated with corresponding reconnection rate pulses. However, not all such pulses generated a patch and only within a limited MLT range (11:00-12:00 MLT) did a patch result from a reconnection pulse. Three proposed mechanisms for the production of patches, and of the concentration minima that separate them, are analysed and evaluated: (1) concentration enhancement within the patches by cusp/cleft precipitation; (2) plasma depletion in the minima between the patches by fast plasma flows; and (3) intermittent injection of photoionisation-enhanced plasma into the polar cap. We devise a test to distinguish between the effects of these mechanisms. Some of the events repeat too frequently to apply the test. Others have sufficiently long repeat periods and mechanism (3) is shown to be the only explanation of three of the longer-lived patches seen on this day. However, effect (2) also appears to contribute to some events. We conclude that plasma concentration gradients on the edges of the larger patches arise mainly from local time variations in the subauroral plasma, via the mechanism proposed by Lockwood et al. (2000).

Determination of the parameters of travelling ionospheric disturbances in the high-latitude ionosphere using CUTLASS coherent scatter radars

A multi-frequency experiment at thermospheric heights using skip observations (METSO) has been designed for the co-operative UK twin located auroral sounding system (CUTLASS) radars for studying travelling ionospheric disturbances (TIDs). This mode uses both frequency and azimuthal scanning to build a two-dimensional phase map of the wave field independently in the two fields of view. On February 21, 2004, a superposition of three disturbances was observed and their parameters have been determined by a two-dimensional least-squares fitting method. Two of the disturbances were found to be consistent with models of TIDs caused by acoustic gravity waves (AGWs) from a point source: a medium-scale Earth-reflected mode and a large-scale direct mode. The propagation directions of the third disturbance, on the other hand, were found to differ in the two fields of view in a way inconsistent with a single point source, but rather a longitudinally extended one. All the waves were found to originate north of the radars.

Stereo CUTLASS - A new capability for the SuperDARN HF radars

Abstract. The Super Dual Auroral Radar Network (SuperDARN), which consists of networks of HF radars surrounding the northern and southern poles, has proven to be an extremely successful experimental technique in a wide range of scientific areas. The basic design of the radars, which contributes to SuperDARN, has remained virtually unchanged since the first radar was built in the early 1980s. This paper presents the first results of a significant new development of the basic system. Termed "Stereo", this development involves the addition of a duplicate receive channel and makes use of the spare duty cycle available in the current transmitters. The Stereo system has been implemented on the two radars that form the Co-operative UK Twin Located Auroral Sounding System (CUTLASS), which are the easternmost pair of radars in the Northern Hemisphere SuperDARN chain. Instead of the standard 7 pulse sequence normally employed by the radars, two such pulse sequences are interleaved and transmitted at different frequencies separated by more than 15kHz. This development allows for the radar to run two entirely different experimental modes simultaneously. Here we describe the basic Stereo system and some of the early results. We also identify potential new experiments which could be run with Stereo to complement the existing standard SuperDARN radar. Key words. Ionosphere (ionosphere-magnetosphere interactions; ionospheric irregularities; instruments and techniques)

Convection and auroral response to a southward turning of the IMF: Polar UVI, CUTLASS, and IMAGE signatures of transient magnetic flux transfer at the magnetopause

We present the first spacecraft‐borne imager observations of the auroral manifestation of transient magnetic flux transfer at the magnetopause. During an interval of interplanetary magnetic field Bz ≈ −10 nT, By ≈ 10 nT, and solar wind dynamic pressure and velocity Psw ≈ 5 nPa and vsw ≈ 650 km s−1, Polar Ultraviolet Imager (UVI) images show a sequence of events, each of which begins as a bifurcation of the main auroral oval in the 14 to 16 magnetic local time (MLT) sector which subsequently progresses antisunward (eastward) at 2 km s−1 toward the 19 MLT sector. The poleward portion of the bifurcation is interpreted as a poleward‐moving auroral form (PMAF) as has previously been observed by ground‐based optical instrumentation and identified as the auroral signature of flux transfer events. Ground‐based measurements of the associated plasma drift, made with the Cooperative U.K. Twin Located Auroral Sounding System (CUTLASS) Finland HF radar, show poleward (1 km s−1) and westward (1 km s−1) convection flow, consistent with the By tension force, as well as poleward‐moving regions of backscatter. International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers within the radar field of view observe poleward‐progressing, 10 min period, X component magnetic deflections, which are consistent with the effect of Hall currents associated with the plasma flow. The combined radar and optical observations suggest that the PMAFs can be 3500 km or 7 hours of MLT in length. The antisunward motion of the bifurcation of the auroral oval is interpreted as an expansion of the reconnection X line along the flank of the magnetopause.

&amp;lt;i&amp;gt;Letter to the editor&amp;lt;/i&amp;gt;CUTLASS observations of a high-m ULF wave and its consequences for the DOPE HF Doppler sounder

Abstract. The CUTLASS (Co-operative UK Twin Located Auroral Sounding System) Finland HF radar, whilst operating in a high spatial and temporal resolution mode, has measured the ionospheric signature of a naturally occurring ULF wave in scatter artificially generated by the Tromsù Heater. The wave had a period of 100 s and exhibited curved phase fronts across the heated volume (about 180 km along a single radar beam). Spatial information provided by CUTLASS has enabled an m-number for the wave of about 38 to be determined. This high-m wave was not detected by the IMAGE (International Monitor for Auroral Geomagnetic Efects) network of ground magnetometers, as expected for a wave of a small spatial scale size. These observations over the first independent confirmation of the existence of the ground uncorrelated ULF wave signatures previously reported in measurements recorded from an HF Doppler sounder located in the vicinity of Tromsö. These results both demonstrate a new capability for geophysical exploration from the combined CUTLASS-EISCAT ionospheric Heater experiment, and provide a verification of the HF Doppler technique for the investigation of small scale ULF waves.Key words. Ionosphere (ionosphere – magnetosphere interactions) . Magnetospheric physics (magnetosphere – ionosphere interactions; MHD waves and instabilities)

Coherent HF radar backscatter characteristics associated with auroral forms identified by incoherent radar techniques: A comparison of CUTLASS and EISCAT observations

Backscatter from decameter‐wavelength field‐aligned F region irregularities, as measured by the Cooperative UK Twin Located Auroral Sounding System (CUTLASS) Finland HF coherent radar, is compared with common volume plasma parameters and the electric field deduced by the European Incoherent Scatter (EISCAT) UHF incoherent radar system, for a 12 hour period from June 18 to June 19, 1996. During this interval we find an excellent agreement between irregularity Doppler velocity and bulk ion drift resolved along the CUTLASS beam. Backscatter is found to exist only in regions of nonzero electric field, as the E×B instability growth rate is dependent on E. Following a substorm expansion phase onset, backscatter largely disappears for a period of several hours, thought to be a consequence of nondeviative absorption of the HF radio wave in the D region or a quenching of the F region instability mechanism by enhanced E region Pedersen conductivity. Finally, the presence of auroral arcs within the scatter volume increases the intensity of backscatter returns and introduces a subsidiary peak, displaced from the preexisting peak, in the backscatter spectra; this subsidiary peak results in an increase in the apparent spectral width of the backscatter. We show how this allows the location of precipitation features within the field of view to be determined.

The phase speed of artificial field‐aligned irregularities observed by CUTLASS during HF modification of the auroral ionosphere

The RF heater facility at Ramfjordmoen, Tromsø can generate field‐aligned plasma irregularities in the field of view of the Cooperative UK Twin‐Located Auroral Sounding System (CUTLASS) coherent backscatter radar at Hankasalmi, Finland. In a recent set of experiments conducted in April 1996, the phase speed of the generated plasma irregularities has been compared with independent measurements of the plasma drift velocity observed by the European Incoherent Scatter radar facility. The phase speed of the plasma irregularities is found to be equal to the component of the plasma drift velocity in the direction of propagation of the plasma wave. This result is a further verification of the high performance of the CUTLASS radar and also demonstrates how artificially generated plasma irregularities can be employed to detect small plasma drift velocities and contribute to geophysical research. The characteristics of the Doppler spectrum of the artificial plasma waves are also described and discussed.

Spatial observations by the CUTLASS coherent scatter radar of ionospheric modification by high power radio waves

Abstract. Results are presented from an experimental campaign in April 1996, in which the new CUTLASS (Co-operative UK twin-located Auroral Sounding System) coherent scatter radar was employed to observe artificial field aligned irregularities (FAI) generated by the EISCAT (European Incoherent SCATter) heating facility at Tromsø, Norway. The distribution of backscatter intensity from within the heated region has been investigated both in azimuth and range with the Finland component of CUTLASS, and the first observations of artificial irregularities by the Iceland radar are also presented. The heated region has been measured to extend over a horizontal distance of 170±50km, which by comparison with a model of the heater beam pattern corresponds to a threshold electric field for FAI of between 0.1 and 0.01V/m. Differences between field-aligned and vertical propagation heating are also presented.

Spatial observations by the CUTLASS coherent scatter radar of ionospheric modification by high power radio waves

Results are presented from an experimental campaign in April 1996, in which the new CUTLASS (Co-operative UK twin-located Auroral Sounding System) coherent scatter radar was employed to observe artificial field aligned irregularities (FAI) generated by the EISCAT (European Incoherent SCATter) heating facility at Tromsø, Norway. The distribution of backscatter intensity from within the heated region has been investigated both in azimuth and range with the Finland component of CUTLASS, and the first observations of artificial irregularities by the Iceland radar are also presented. The heated region has been measured to extend over a horizontal distance of 170±50km, which by comparison with a model of the heater beam pattern corresponds to a threshold electric field for FAI of between 0.1 and 0.01V/m. Differences between field-aligned and vertical propagation heating are also presented.